DRUG DISPOSITION

Clin Pharmacokinet 1998 Jun; 34 (6): 429-455 0312-5963/98/0006-0429/$13.50/0 © Adis International Limited. All rights reserved.

Clinical Pharmacokinetics of Mycophenolate Mofetil Roy E.S. Bullingham, Andrew J. Nicholls and Barbara R. Kamm CS Associates, Palo Alto, and Roche Global Development – Palo Alto, Palo Alto, California, USA

Contents Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Protein Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Single Dose Pharmacokinetics in Healthy Individuals and Patients with Rheumatoid Arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Pharmacokinetics in Renal Transplant Patients . . . . . . . . . . . . . . . . . . . . . . . 5.1 First Dose Pharmacokinetics in Patients and Healthy Individuals . . . . . . . . . . 5.2 Single Dose Pharmacokinetics in Patients . . . . . . . . . . . . . . . . . . . . . . . 5.3 Multiple Dose Pharmacokinetics in Patients . . . . . . . . . . . . . . . . . . . . . . 6. Pharmacokinetics in Renal Impairment . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Single Dose Pharmacokinetics in Renal Impairment . . . . . . . . . . . . . . . . . 6.2 Multiple Dose Pharmacokinetics in Renal Impairment . . . . . . . . . . . . . . . . 6.3 Effect of Haemodialysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Pharmacokinetics in Hepatic Impairment . . . . . . . . . . . . . . . . . . . . . . . . . 8. Enterohepatic Circulation Following Administration . . . . . . . . . . . . . . . . . . . . 9. Pharmacokinetic Interactions with Mycophenolate Mofetil . . . . . . . . . . . . . . . 9.1 Effect of Food . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Effect of Concomitant Antacids . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Drug Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.Clinical Pharmacokinetic-Pharmacodynamic Correlations . . . . . . . . . . . . . . . 10.1 Correlations of Plasma Mycophenolic Acid Concentration with Adverse Events 10.2 Correlations of Mycophenolic Acid Concentration with Immunosuppressive Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Correlations of Plasma Mycophenolic Acid Pharmacokinetic Parameters with Clinical Effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Therapeutic Drug Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Summary

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The pharmacokinetics of the immunosuppressant mycophenolate mofetil have been investigated in healthy volunteers and mainly in recipients of renal allografts. Following oral administration, mycophenolate mofetil was rapidly and completely absorbed, and underwent extensive presystemic de-esterification. Systemic plasma clearance of intravenous mycophenolate mofetil was around 10 L/min in healthy individuals, and plasma mycophenolate mofetil concentrations fell below the quantitation limit (0.4 mg/L) within 10 minutes of the cessation of infusion. Similar plasma mycophenolate mofetil concentrations were seen after intrave-

430

Bullingham et al.

nous administration in patients with severe renal or hepatic impairment, implying that the de-esterification process had not been substantially affected. Mycophenolic acid, the active immunosuppressant species, is glucuronidated to a stable phenolic glucuronide (MPAG) which is not pharmacologically active. Over 90% of the administered dose is eventually excreted in the urine, mostly as MPAG. The magnitude of the MPAG renal clearance indicates that active tubular secretion of MPAG must occur. At clinically relevant concentrations, mycophenolic acid and MPAG are about 97% and 82% bound to albumin, respectively. MPAG at high (but clinically realisable) concentrations reduced the plasma binding of mycophenolic acid. The mean maximum plasma mycophenolic acid concentration (Cmax) after a mycophenolate mofetil 1g dose in healthy individuals was around 25 mg/L, occurred at 0.8 hours postdose, decayed with a mean apparent half-life (t1⁄2) of around 16 hours, and generated a mean total area under the plasma concentration-time curve (AUC∞ ) of around 64 mg • h/L. Intra- and interindividual coefficients of variation for the AUC∞ of the drug were estimated to be 25% and 10%, respectively. Intravenous and oral administration of mycophenolate mofetil showed statistically equivalent MPA AUC∞ values in healthy individuals. Compared with mycophenolic acid, MPAG showed a roughly similar Cmax about 1 hour after mycophenolic acid Cmax, with a similar t1⁄2 and an AUC∞ about 5-fold larger than that for mycophenolic acid. Secondary mycophenolic acid peaks represent a significant enterohepatic cycling process. Since MPAG was the sole material excreted in bile, entrohepatic cycling must involve colonic bacterial deconjugation of MPAG. An oral cholestyramine interaction study showed that the mean contribution of entrohepatic cycling to the AUC∞ of mycophenolic acid was around 40% with a range of 10 to 60%. The pharmacokinetics of patients with renal transplants (after 3 months or more) compared with those of healthy individuals were similar after oral mycophenolate mofetil. Immediately post-transplant, the mean Cmax and AUC∞ of mycophenolic acid were 30 to 50% of those in the 3-month post-transplant patients. These parameters rose slowly over the 3-month interval. Slow metabolic changes, rather than poor absorption, seem responsible for this nonstationarity, since intravenous and oral administration of mycophenolate mofetil in the immediate post-transplant period generated comparable MPA AUC∞ values. Renal impairment had no major effect on the pharmacokinetic of mycophenolic acid after single doses of mycophenolate mofetil, but there was a progressive decrease in MPAG clearance as glomerular filtration rate (GFR) declined. Compared to individuals with a normal GFR, patients with severe renal impairment (GFR 1.5 L/h/1.73m2) showed 3- to 6-fold higher MPAG AUC values. In renal transplant recipients during acute renal impairment in the early post-transplant period, the plasma MPA concentrations were comparable to those in patients without renal failure, whereas plasma MPAG concentrations were 2- to 3-fold higher. Haemodialysis had no major effect on plasma mycophenolic acid or MPAG. Dosage adjustments appear to not be necessary either in renal impairment or during dialysis. In a single dose study with graded impairment of hepatic oxidative function in patients with cirrhosis, only minor effects on plasma mycophenolic acid or MPAG pharmacokinetics were seen. Other states of hepatic impairment may have different effects, given the central role of the liver in the pharmacokinetics of mycophenolic acid. © Adis International Limited. All rights reserved.

Clin Pharmacokinet 1998 Jun; 34 (6)

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431

No significant pharmacokinetic interactions were seen with cyclosporin, cotrimoxazole, aciclovir, ganciclovir or a combined oral contraceptive. General mechanisms for potential drug interactions are interference with entrohepatic cycling (as seen with cholestyramine), and competition between MPAG and other drugs for excretion at the renal tubule. Mycophenolic acid acts through specific enzyme inhibition with a consequent concentration-dependent suppression of mitogen-induced lymphocyte transformation. Consistent with this, a logistic correlation between the interdosage interval MPA AUC∞ and the probability of acute rejection was found in renal transplant patients. No correlation could be found for adverse events. Therapeutic drug monitoring seems unnecessary given the low mycophenolic acid variability and the small mycophenolate mofetil clinical dose range.

Mycophenolate mofetil is the 2-morpholinoethyl ester of mycophenolic acid, with a structural formula shown in figure 1. It is a prodrug immunosuppressant, shown to be effective in the suppression of acute allograft rejection following cadaveric renal transplantation when given orally on a twice daily schedule in combination with cyclosporin and steroids.[1-3] Mycophenolic acid, a fermentation product of Penicillium stoloniferum, is the active immunosuppressant species. Mycophenolate mofetil was specifically developed to increase the oral bioavailability of mycophenolic acid.[4] Over 20 years ago, mycophenolic acid was shown capable of inhibiting cell division in mammalian cell cultures, and underwent clinical investigation in the treatment of psoriasis and advanced malignant tumours.[5] The in vitro cytostatic activity of mycophenolic acid is relatively selective for lymphocytes. Mitogen-induced proliferation of both

O

CH3

OH

O N

O

O OCH3

O

CH 3

Mycophenolic acid portion Morpholino portion

Fig. 1. Structural formula of mycophenolate mofetil (molecular weight 433.50).

© Adis International Limited. All rights reserved.

T and B cells is inhibited.[6] The action of mycophenolic acid is mediated through specific uncompetitive binding to inosine monophosphate dehydrogenase (IMPDH),[7] an enzyme critical for the de novo synthesis of guanine nucleotides. Lymphocytes are primarily dependent on this pathway, whereas other cell types, including polymorphonuclear leucocytes and neurones, depend primarily on the alternative salvage pathway. In addition to the effects on lymphocyte proliferation, a reduction in the lymphocyte guanine nucleotide pool may lead to secondary effects, such as reduction in the affinity of adhesion molecules, which may also have a role in the immunosuppressive activity of mycophenolic acid.[8] The actions of mycophenolic acid do not primarily involve interleukin-2 (IL-2) and can affect both T and B lymphocytes. In contrast, the action of the cyclophilins, such as cyclosporin and tacrolimus (FK 506), is mainly mediated through IL-2 and, consequently, the major effect is on T lymphocytes and not the B lymphocytes. In distinction to azathioprine, mycophenolic acid has specific inhibitory activity on the enzyme IMPDH, and is not incorporated into nucleic acids. These properties suggest that mycophenolate mofetil might usefully serve as an adjunct to cyclophilins, with a different profile than that of azathioprine. Large scale multicentre double-blind randomised controlled studies of oral mycophenolate mofetil in renal allograft transplantation have shown its effectiveness in the suppression of acute biopsyClin Pharmacokinet 1998 Jun; 34 (6)

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proven rejection when used in combination with standard doses of cyclosporin and corticosteroids. Relative to placebo,[1] or azathioprine with[2] or without[3] antithymocyte globulin induction therapy, mycophenolate mofetil at doses of 2 or 3 g/day on a twice daily schedule consistently reduced the incidence of acute rejection by almost 50% over the first 6 months post-transplant. Although the 3 g/day dose appeared to show somewhat more effect, it was less well tolerated than the lower dose and so overall did not offer any additional benefit. The recommended dose is thus 2 g/day. Azathioprine and mycophenolate mofetil at 2 g/day had comparable clinical safety, so the reduction in acute rejection was not gained at the expense of an increase in adverse events. Mycophenolate mofetil has subsequently found wide use as adjunctive therapy in renal transplantation instead of azathioprine. This article reviews the pharmacokinetics of mycophenolate mofetil, primarily when administered by the oral route. In many countries, mycophenolate mofetil is marketed as both a 250mg capsule formulation and a 500mg tablet formulation; unless explicitly indicated, all oral data in this review refer to the capsule formulation only. The intravenous formulation is not commercially available at present, but intravenous pharmacokinetic data are included where this contributes to mechanistic understanding. The relationship between plasma mycophenolic acid concentrations and the pharmacodynamic actions of the drug is also reviewed, and the potential value of therapeutic drug monitoring for adjustment of mycophenolate mofetil dosage is briefly discussed.

Bullingham et al.

MPAG in plasma,[10] the quantification limit of the method was reported to be 0.100 mg/L for mycophenolic acid and 4.00 mg/L for MPAG, with linear ranges extending from the quantification limit up to 40 mg/L for mycophenolic acid and up to 400 mg/L for MPAG. Inter- and intra-assay coefficients of variation were less than 5% except at the lower quantification limit. Mycophenolic acid and MPAG were stable in plasma at clinically observed concentrations for at least 4 hours at room temperature, at least 8 hours at 1 to 4°C, and at least 11 months at –20°C: thus, samples may be routinely processed for these analytes. Throughout this review, MPAG concentrations are reported as ‘mycophenolic acid equivalents’, obtained by multiplying MPAG concentrations by 0.594 (ratio of the molecular weight of mycophenolic acid to that of MPAG). For mycophenolate mofetil in plasma,[9] the quantification limit was reported as 0.400 mg/L with a linear range extending from this limit to 4.00 mg/L. Inter- and intra-assay coefficients of variation were less than 5%. Mycophenolate mofetil in blood and plasma showed temperature-dependent degradation to mycophenolic acid. In whole blood and plasma, 10% of the drug was lost after 2.0 and 3.2 hours at room temperature, respectively, and after 7.6 hours and 6.8 hours, respectively, at 1 to 4°C. Mycophenolate mofetil was stable in plasma at –80°C for at least 4 months, but there was slow degradation at –20°C, with 10% being lost by approximately 6 days. Samples taken for analysis of mycophenolate mofetil thus require special treatment, with immediate storage on ice, rapid, cold processing, frozen storage of plasma at –80°C, and analysis within months of sample collection.

1. Analytical Methods High performance liquid chromatography (HPLC) methods have been used for the analysis of mycophenolate mofetil,[9] and of mycophenolic acid and its stable phenolic glucuronide metabolite (MPAG) in plasma.[10] Ultraviolet detection at 254nm was used for all these species. Mycophenolic acid and MPAG have been similarly analysed in urine[11] and in bile.[12] For mycophenolic acid and © Adis International Limited. All rights reserved.

2. Metabolism Two separate radiolabelled absorption, distribution, metabolism and excretion (ADME) studies have been performed with single dose oral administration of 14C-labelled mycophenolate mofetil in humans.[12] In 1 study mycophenolate mofetil was labelled in the mycophenolic acid portion of the molecule, and in the other the label was in the Clin Pharmacokinet 1998 Jun; 34 (6)

Mycophenolate Mofetil

morpholino portion (fig. 1) so that the dispositions of each portion could be separately characterised. In each study, 4 healthy fasting men were given mycophenolate mofetil 1g in solution, containing 74 and 100 μCi of 14C for the mycophenolic acid and morpholino-labelled material respectively. Urine, faeces and blood samples were collected at timed intervals for 7 days after drug administration. The total radioactivity of collected samples was determined by liquid scintillation counting. The radioactive metabolites of [morpholine-14C]mycophenolate mofetil were determined following HPLC separation. After [mycophenolate-14C]mycophenolate mofetil administration, radioactivity was rapidly absorbed with the mean plasma maximum concentration (Cmax) of radioactivity occurring 45 minutes after administration. Mean plasma elimination half-life (t1⁄2) for the total radioactivity was 17.6 hours, compared with 16.0 and 17.1 hours, respectively, determined for mycophenolic acid and MPAG. Based on the area under the plasma concentration-time curves over 24 hours postdose (AUC24), mycophenolic acid and MPAG averaged 17% and 76%, respectively, of the total radioactivity in plasma. Blood-to-plasma total radioactivity concentration ratios remained relatively constant throughout the study and averaged 0.59, implying minimal distribution of mycophenolic acid and MPAG into the cellular fractions of blood. Of the total radioactivity administered, an average of 90.4% was recovered during the first 72 hours after administration. A mean of 96.3% of the recovered radioactivity was in urine, with 55.7% of the administered mycophenolate mofetil dose being recovered within the 12 hours post-dosage interval. Total recovery of the dose in the faeces averaged 5.5%. Material recovered in urine was almost exclusively MPAG. Mycophenolate mofetil itself was not detected in urine. Small amounts of mycophenolic acid (mean 0.6%; range 0 to 1.4% of the administered dose) and an acyl glucuronide conjugate of mycophenolic acid (mean 0.3%; range 0 to 1.1% of the administered dose) were also detected as minor urinary components. © Adis International Limited. All rights reserved.

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Following [morpholine-14C]mycophenolic acid, the plasma Cmax of radioactivity occurred at a mean of 53 minutes post-dose, and declined with an average t1⁄2 of 4.34 hours. Concentrations of total radioactivity in whole blood paralleled, and were slightly lower than, those in plasma. HPLC analysis of samples of plasma collected between 0.25 and 6 hours after administration indicated that 4 metabolites were present in plasma. Three of the metabolites were identified as N-(2-carboxymethyl)morpholine (CMM), N-(2-hydroxyethyl)-morpholine (HEM) and an N-oxide of HEM (HEMNO). CMM, HEM and HEMNO accounted for an average of 76%, 2.9% and 10%, respectively, of the total radioactivity in plasma based upon AUC6 values. The remaining unidentified metabolite represented an average of 2.7% of the total radioactivity in plasma based upon AUC6 values. Over the 0 to 6 hours interval these 4 metabolites accounted for 92% of the total radioactivity in plasma. The major route of excretion of radioactivity was in the urine. Recovery of total administered radioactivity in urine averaged 77.2% at 12 hours after administration, 92.1% at 24 hours after administration and 94.4% at 168 hours after administration. Faecal samples (analysed from only 1 person) were found to contain less than 1% of the administered dose. Urine contained CMM as the major metabolite and 4 other minor metabolites (HEM, HEMNO and 2 unidentified metabolites of which one was identical to the unidentified metabolite recovered from plasma). Together, these metabolites in the 0 to 24 hours urine samples accounted for a mean total of 88.9% of the administered dose, with CMM alone accounting for a mean total of 80.8% of the administered dose. In summary, mycophenolate mofetil is rapidly absorbed and rapidly de-esterified in healthy fasting individuals. Based on the recovery of the administered dose in urine, mycophenolate mofetil is essentially completely absorbed. Mycophenolic acid is almost completely glucuronidated to MPAG, which is excreted in urine and represents almost all of the administered dose. The hydroxyethylmorpholine moiety of mycophenolate mofetil is Clin Pharmacokinet 1998 Jun; 34 (6)

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rapidly and extensively metabolised to form CMM and, to a lesser extent, HEMNO. These 2 small molecular weight products and small amounts of other metabolites are excreted almost entirely in the urine, within 24 hours of administration. 3. Protein Binding Mycophenolic acid binding to plasma proteins has been determined both by ultrafiltration and by equilibrium dialysis, with comparable results.[13] In pooled plasma or human serum albumin (HSA) at a concentration of 44 g/L, the mycophenolic acid free fraction was 1.25 ± 0.08% (mean ± standard deviation, SD). Over the mycophenolic acid total concentration range of 1 to 60 mg/L (which encompasses the therapeutic range), binding did not change. The free fraction was dependent on HSA concentration, decreasing from 53.3 to 0.92% as the HSA concentration increased from 0.7 to 69 g/L. Mycophenolic acid did not bind significantly to α1-acid glycoprotein. The binding was unaltered by normal therapeutic plasma concentrations of warfarin, digoxin, phenytoin, cyclosporin, tacrolimus or prednisone. Mycophenolic acid free fraction showed a 6- to 8-fold progressive increase with increase in sodium salicylate concentration. More significantly, the free fraction increased around 3fold as the concentration of the metabolite MPAG increased to 475 mg/L mycophenolic acid equivalents. As will be discussed (section 6), MPAG concentrations higher than this can be seen in the presence of renal impairment. The in vivo effect of these in vitro changes in mycophenolic acid free fraction depend on whether elimination is restrictive or non-restrictive.[14] The hepatic clearance of mycophenolic acid is around 24 L/h from plasma in healthy volunteers given cholestyramine (see section 8), showing elimination falls between the restrictive and non-restrictive categories. A priori, these binding displacements could, therefore, lead to some degree of change both in total plasma pharmacokinetics and in the free concentration of plasma mycophenolic acid. © Adis International Limited. All rights reserved.

Bullingham et al.

The potential for mycophenolic acid to displace other therapeutic agents has also been evaluated.[12] Mycophenolic acid concentrations as high as 100 mg/L had little effect on the binding of warfarin, digoxin or propranolol, but caused small decreases in the binding of theophylline and phenytoin. The mean free fraction of mycophenolic acid in plasma from patients with moderate to severe compensated hepatic cirrhosis was 2.80% at a concentration of 40 mg/L, thus being approximately 2-fold higher than in normal plasma.[15] MPAG is about 82% bound to human plasma and HSA at MPAG concentration ranges normally seen in stable renal transplant patients.[12] Higher MPAG concentrations lead to a progressive increase in the free fraction of MPAG. In whole blood, 99.99% of mycophenolic acid is found in the plasma with only 0.01% in cellular elements,[13, 16] a result consistent with the blood distribution of radioactivity seen in the [14C]mycophenolic acid ADME study. 4. Single Dose Pharmacokinetics in Healthy Individuals and Patients with Rheumatoid Arthritis Because of the nature of the drug, only single doses of mycophenolate mofetil have been administered to healthy volunteers. Table I gives the reported pharmacokinetic data of mycophenolic acid and MPAG for administration of single doses of mycophenolate mofetil to healthy individuals both orally[11,15] and intravenously,[11] and oral data for a group of patients with stable rheumatoid arthritis[17] who were otherwise healthy. Table I also shows analyses of the combined sets of data for all available healthy individuals given oral mycophenolate mofetil.[12] This pooled set includes data from the studies[11,15] shown individually in the table, as well as from other unpublished studies. Mycophenolate mofetil itself is not quantifiable in plasma after oral administration, but is measurable after intravenous infusion,[11] implying almost complete first pass de-esterification of the drug. During intravenous infusion of mycophenolate mofClin Pharmacokinet 1998 Jun; 34 (6)

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Table I. Pharmacokinetic parameters (mean ± SD) for single dose administration of mycophenolate mofetil to fasting healthy individuals or fasting patients with rheumatoid arthritis Group

Healthy volunteers[15]

Healthy volunteers[11]

Healthy volunteers[11]

RA patients[17]

Healthy volunteers[12]

Healthy volunteers[12]

Dose (g)

1.0

1.5

1.5

2.0

1.0

1.5

Route

PO

PO

IVa

PO

PO

PO

n

6

12

12

10

129

36 32.8 ± 8.2

Mycophenolic acid parameters Cmax (mg/L) 24.3 ± 5.7

34.0 ± 7.1

47.2 ± 9.3

23.8 ± 11.6

24.5 ± 9.5

tmax (h)

0.6 ± 0.1

1.0 ± 0.04

0.99 ± 0.41

1.0 (0.5-2.0)b

0.8 ± 0.4

0.9 ± 0.4

t1⁄2 (h)

NCd

17.9 ± 6.5

16.6 ± 5.8

NCe

16c ± 6

17 ± 6

AUC∞ (mg/L • h)

29.0d ± 5.8

101 ± 23.4

108 ± 26.0

79.9e ± 23.0

63.9c ± 16

84.2 ± 26

CLR (ml/min)

1.5f ± 0.8

1.17g (0-4.6)b

0.56g (0-2.1)b

NCe

NR

1.2h ± 1.4 43.3 ± 13.1

Mycophenolic glucuronide parameters 31.4 ± 3.3 Cmax (mg/L)

43.1 ± 6.8

39.3 ± 9.1

52.7 ± 20.0

28.0 ± 6.72

tmax (h)

1.3 ± 0.3

1.81 ± 0.47

1.65 ± 0.28

2.5 (2-4)b

1.7 ± 0.60

1.7 ± 0.5

t1⁄2 (h)

12.5 ± 5.7

16.1 ± 5.2

21.8 ± 19.0

NCe

15.2c ± 5.55

15.1 ± 4.5

AUC∞ (mg/L • h)

261 ± 32.4

480 ± 105

442 ± 102

508e ± 168

301c ± 57.1

475 ± 150

CLR (ml/min)

25.0f ± 3.8

33.7g ± 7.35

42.5g ± 14.9

NR

NR

33.7h ± 7.3

a

Dose was given as constant rate infusion over a 60-minute interval.

b

Range.

c

n = 117.

d

The plasma mycophenolic acid concentration fell below the quantitation limit of 2.5 mg/L too quickly to allow estimation of t1⁄2 or AUC∞ in this study. The AUC is for 0 to 96 hours, not ∞.

e

Study duration of 24 hours was too short to allow estimation of the t1⁄2 or AUC∞ of mycophenolic acid or MPAG in this study. The AUC is for 0 to 24 hours, not ∞.

f

Urine collection over 0 to 24 hours post-administration.

g

Urine collection over 0 to 48 hours post-administration.

h

n = 12.

Abbreviations: AUC = area under the concentration-time curve; CLR = renal clearance; Cmax = maximum drug concentration; IV = intraveously; MPAG = mycophenolic glucuronide; n = number of participants; NC = not calculable; NR = not reported; PO = orally; RA = rheumatoid arthritis; tmax = time to reach maximum concentration; t1⁄2 = half-life.

etil 1.5g at a constant rate over 1 hour, a mean plateau concentration around 2.5 to 3 mg/L was rapidly reached. The mean plasma clearance of intravenous mycophenolate mofetil was in the range of 8.5 to 11.6 L/min, values which exceed plasma cardiac output. Following termination of the mycophenolate mofetil infusion, rapid decline in the plasma concentration occurred; t1⁄2 could not be estimated, but the drug was not detected in plasma 10 minutes after the cessation of the infusion, indicating that the t1⁄2 was probably less than 2 minutes. Since degradation of mycophenolate mofetil in the blood or plasma[9] is relatively slow in comparison, the tissue de-esterification must be widespread and rapid. © Adis International Limited. All rights reserved.

The plasma concentration-time profile after oral mycophenolate mofetil is characterised by a very sharp initial peak around 1 hour and, with these single doses, the occurrence of secondary maxima usually around 6 to 12 hours postdose. Secondary maxima are also seen after single intravenous doses.[11] Enterohepatic cycling appears to be responsible for the occurrence of these secondary peaks, as will be described later. The t1⁄2 values of mycophenolic acid in table I include the effect of recirculation, and, therefore, do not reflect metabolic elimination alone. Similarly, only the AUCs, and not clearances, are reported in this review, since clearance calculated from AUC where recirculation is present does not reflect the intrinsic Clin Pharmacokinet 1998 Jun; 34 (6)

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ability of the liver to remove the drug from the blood.[18] Mycophenolic acid is the active species, so a twice daily administration schedule is supported by the decay t1⁄2 of around 16 hours for mycophenolic acid. For the pooled healthy volunteer data, the mean Cmax and AUC∞ of mycophenolic acid was somewhat less than the 50% higher expected for the 1.5g dose relative to the 1.0g dose on a dose proportional basis. The difference was not to an extent likely to have major clinical consequences. Maximum plasma MPAG concentrations occurred just under 1 hour after the peak mycophenolic acid concentrations, consistent with a precursorsuccessor relationship of mycophenolic acid to MPAG. Plasma MPAG concentrations were higher than plasma mycophenolic acid concentrations from about 1 hours onwards, and were around 10fold higher by 8 hours.[11,15] Total MPAG AUC was about 5 times higher than that of mycophenolic acid. The t1⁄2 of MPAG was similar to that of mycophenolic acid, reflecting the formation-limited kinetics of MPAG with respect to mycophenolic acid. The 1.5g dose parameter values for MPAG Cmax and AUC∞ appeared to be proportionally higher relative to the 1.0g dose. Interindividual coefficients of variation for the pharmacokinetic parameters of mycophenolic acid were under 50%, with AUC showing the lowest coefficients of variation at around 25 to 30%. The pharmacokinetic parameters of MPAG showed somewhat lower interindividual coefficients of variation than for the corresponding mycophenolic acid parameters. In a single dose study in healthy individuals comparing capsule and tablet formulations, equivalent AUC∞ values for mycophenolic acid were found,[12] allowing an estimation of the inter- and intra-individual components of variability for AUC∞. The intraindividual coefficients of variation for mycophenolic acid AUC∞ were estimated to be 10%, and the interindividual coefficients of variation to be 25%. As a prodrug of mycophenolic acid, mycophenolate mofetil appears to be a drug of low variability. © Adis International Limited. All rights reserved.

Bullingham et al.

In the intravenous study[11] the mean Cmax and AUC∞ of mycophenolic acid from the intravenous dose were somewhat higher than for the same dose given orally, but the differences were not large. The mean AUC∞ for oral mycophenolic acid was 93.5% (range 60.8 to 167%) of that from the intravenous dose, and showed statistically equivalent bioavailability (80 to 120 rule) between the intravenous and oral routes. Effectively, oral mycophenolate mofetil is 100% bioavailable as mycophenolic acid in healthy individuals. As discussed later, a significant proportion of the plasma AUC for mycophenolic acid derives from the entrohepatic cycling of MPAG. Total plasma AUC for mycophenolic acid is thus the sum of 2 components, one derived from initial systemic availability (after any first pass effect), and the other from recirculation. Oral administration of mycophenolate mofetil may lead to lower initial systemic availability of mycophenolic acid through first pass removal, but subsequent entrohepatic cycling may largely compensate for that initial removal. Equivalent bioavailability of mycophenolic acid for the same mycophenolate mofetil doses given intravenous or oral cannot be assumed in patients who may show alteration of first pass effect and/or entrohepatic cycling. Mean urine output of MPAG over the 48 hours postdose period represented 71.3% and 72.1% of administered dose for the oral and intravenous doses, respectively. The 90% confidence interval of the mean value of the ratio of these outputs was contained within the 80 to 120% range. This equivalence shows that the absorption of orally administered mycophenolate mofetil is close to 100%. These results may be compared with serum data reported in an early study in 2 patients where mycophenolic acid itself was given intravenously and orally.[5] With an hourly sampling schedule, peak oral concentrations measured spectrofluorimetrically occurred at 1 hour and the peak plasma glucuronide concentration (determined by measurement of mycophenolic acid following β-glucuronidase treatment) was at 2 hours. Numerical data were not presented, but by inspection of the concentration-time profiles, Cmax was in the range of Clin Pharmacokinet 1998 Jun; 34 (6)

Mycophenolate Mofetil

15 to 20 mg/L and 20 to 30 mg/L for mycophenolic acid and MPAG, respectively. Urine recovery after oral mycophenolic acid was 60%, almost all of which was glucuronide conjugate. These early data are in substantial agreement with the data for mycophenolate mofetil. Since 1g of mycophenolate mofetil is equivalent to 0.74g of mycophenolic acid, the Cmax values are somewhat lower than might be predicted from the data on the administration of mycophenolate mofetil 1g to healthy individuals in table I, an observation consistent with the improvement in systemic bioavailability of mycophenolic acid seen in animals given mycophenolate mofetil.[4] The renal clearances of mycophenolic acid and MPAG following mycophenolate mofetil administration indicate different renal excretory mechanisms for these species. Assuming a glomerular filtration rate (GFR) of 7.5 L/h and a mean plasma mycophenolic acid free fraction of 1.25% (see section 3), the filtration clearance of mycophenolic acid is about 0.096 L/h. This value is comparable to the measured mycophenolic acid mean renal clearance, suggesting the renal excretion of mycophenolic acid is largely through passive filtration. MPAG filtration clearance estimated in a similar fashion is around 1.32 L/h, which is appreciably lower than the measured value and implies tubular secretion of MPAG is occurring. The renal clearance of MPAG after oral mycophenolate mofetil administration is statistically significantly lower than that after intravenous mycophenolate mofetil in the same individuals, although plasma AUC values of MPAG are comparable.[11] This has been interpreted as evidence of renal glucuronidation of mycophenolic acid, with direct tubular excretion of the generated MPAG. The mean and median values of the Cmax and AUC of mycophenolic acid and MPAG in the single dose healthy population (combined 1 and 1.5g data, dose adjusted to 1g) were similar, with the maximum percentage difference for any parameter being 7.5%.[12] Overall, the distribution of these parameters was best described by a log-normal function. Women had slightly higher Cmax and AUC © Adis International Limited. All rights reserved.

437

values for both mycophenolic acid and MPAG compared with men, but not after adjustments for bodyweight. This finding is consistent with an inverse relationship between AUC, Cmax and bodyweight. A multivariate analysis was performed which included age, bodyweight, albumin, hepatic enzymes and creatinine clearance. Age, creatinine clearance, and particularly bodyweight, were found to be important factors for the AUC of mycophenolic acid, yielding reasonable correlations (r2 = 0.5). The AUC of MPAG was poorly correlated with any variable. Almost all of the individuals were Caucasian in origin so that the effect of race could not be examined, and the age correlation was obtained in individuals almost all of whom were less than 65 years old. 5. Pharmacokinetics in Renal Transplant Patients Definitive determination of the pharmacokinetics of the drug in renal allograft recipients after transplantation is not without difficulty. In principle, substantial changes in pharmacokinetics could be produced by changes following transplantation, both in the immediate post-transplant period (reflecting rapid alterations in drug therapy, renal function, haemodynamics and gastrointestinal motility) as well as more gradual changes (reflecting change in bodyweight, plasma proteins and organ function). The influence of one of these variables, that of renal function, on the pharmacokinetics of mycophenolate mofetil will be described later (section 6). The risk of acute allograft rejection is greatest in the early post-transplant period (the first 3 months). During this time, the interruption of therapy carries the risk of a loss of therapeutic benefit so it is difficult to justify single dose studies. A more complex strategy is then needed to determine the effect of the changes between the early and late post-transplant periods with a twice daily schedule of mycophenolate mofetil. Comparisons can be made of the Cmax, time to maximum drug concentration (tmax) and AUC12 for the first dose of a course of mycophenolate mofetil given to patients, Clin Pharmacokinet 1998 Jun; 34 (6)

438

or for the first 12 hours of a single dose administration in healthy individuals. Under these circumstances, t1⁄2 comparisons are not possible. In the later stable post-transplant period, single dose studies are feasible, and pharmacokinetics parameters may be compared with those obtained from single dose evaluations in healthy individuals. Most easily obtained in patients are AUC12 and Cmax determined over the interdosage interval after multiple administration. These parameters can be validly compared at different times post-transplant, provided steady state has been reached. The latter may only be a quasi-steady state because of the early post-transplant changes listed above, and the final steady may not occur until some time into the late post-transplant period. At true steady state, interdosage interval AUC12 may also be appropriately compared with AUC∞ from single dose studies in both late post-transplant patients and healthy individuals. 5.1 First Dose Pharmacokinetics in Patients and Healthy Individuals

Table II shows first dose pharmacokinetics parameters of mycophenolic acid and MPAG at various times after renal transplantation. In an early dose ranging study,[19] the pharmacokinetics were also reported for some investigational once daily administration regimens (not shown here), and the measured AUC12 for twice daily administration was doubled to allow comparison with the once daily regimens. In table II, these reported values for twice daily regimens have been halved to obtain the first dose AUC12 parameter. The day 1 combined group consists of all renal transplant patients who received 1.5g twice daily administration,[12] and includes the 5 patients in the table from the reported study.[19] The late posttransplant data are from a single study in a group of patients at least 3 months after transplantation, all of whom had a serum creatinine greater than 200 μmol/L (2.25 mg/dl) and a haemoglobin of greater than 10 g/ml, and all of whom were receiving cyclosporin.[12] For comparison, the pharmacokinetic parameters of combined single dose myco© Adis International Limited. All rights reserved.

Bullingham et al.

phenolate mofetil 1.5g in the healthy individuals cohort in table I are shown, with AUC12 calculated for the first 12 hours only. Comparison of the late renal transplant patients with the healthy volunteers shows the pharmacokinetics of mycophenolic acid and MPAG are very similar, except for a higher AUC12 for MPAG in the renal transplant population. The coefficients of variation for mycophenolic acid AUC12 and the pharmacokinetics of MPAG are about the same, although the Cmax and tmax of mycophenolic acid are somewhat more variable. These results suggest that in patients who are stable and have a well-functioning graft, the pharmacokinetics after oral mycophenolate mofetil are generally quite comparable with those at in healthy individuals. The raised AUC12 of MPAG is consistent with a mild degree of renal impairment (see section 6), such as that that usually accompanies successful transplantation of renal cadaveric allografts. Comparisons of the pharmacokinetics of mycophenolic acid in late and day 1 renal transplant patients, on the other hand, shows substantially lower Cmax and AUC12 values with a prolonged tmax in the early (day 1) patients. The tmax of MPAG is also markedly prolonged in the early patients, as expected for the precursor-successor sequence, although the Cmax and AUC12 of MPAG are relatively less affected than the corresponding mycophenolic acid parameters. The lengthened tmax values are consistent with delayed absorption in the immediate post-transplant state. At first sight, the lower AUC12 values for mycophenolic acid appear to suggest poor bioavailability: discussion of this issue is deferred until after review of the multiple dose pharmacokinetics data. The day 1 pharmacokinetic parameters for mycophenolic acid have coefficients of variation of around 100%, and thus show considerably more variability than in the late renal transplant patients. The day 1 Cmax and AUC12 values show an approximate dose proportionality,[19] although the increased variability can obscure this with small numbers of patients. Clin Pharmacokinet 1998 Jun; 34 (6)

Mycophenolate Mofetil

439

Table II. Pharmacokinetic parameters (mean ± SD) for first dose administration of oral mycophenoate mofetil to patients with renal transplants and healthy individuals Groupa

Day 1 RT patients[19]

Day 1 RT patients[19]

Day 1 RT patients[19]

Day 1 RT combined[12]

Late RT patients[12]

Healthy volunteers[12]

BID dose (g)

1.0

1.5

1.75

1.5

1.5

1.5b

n

6

5

6

47

10

36

Mycophenolic acid parameters Cmax (mg/L) 2.6 ± 2.5

5.1 ± 6.2

3.2 ± 3.5

6.6 ± 6.6

29.7 ± 14.2

32.8 ± 8.2

tmax (h)

6.1 ± 4.8

9.6 ± 3.6

4.8 ± 4.3

4.5 ± 5.4

1.2 ± 0.8

0.90 ± 0.4

AUC12 (mg/L • h)

12.3c ± 5.8

19.5c ± 13.9

18.8c ± 9.45

21.4 ± 17.5

52.1 ± 17.7

51.5b ± 15.1

Mycophenolic glucuronide parameters NR NR Cmax (mg/L)

NR

32.9 ± 17.4

59.8 ± 19.7

43.3 ± 13.1

tmax (h)

NR

NR

NR

8.65 ± 6.8

2.8 ± 0.8

1.7 ± 0.5

AUC12 (mg/L • h)

NR

NR

NR

220 ± 140

420 ± 140

234b ± 87

a

Day 1 patients are on the first day post-transplant; late patients were at least 3 months post-transplant. The combined group on day 1 consists of all patients with renal transplants who received bid 1.5g,[12] and includes the 5 patients in the table from the reported study.[19]

b

Healthy volunteers given single dose and truncated AUC12 is shown for comparison.

Reported values halved,[19] see text. Abbreviations: AUC12 = area under the concentration-time curve at 12 hours; BID = twice daily doses; Cmax = maximum drug concentration; NR = not reported; RT = renal transplant; tmax = time to reach the maximum drug concentration. c

5.2 Single Dose Pharmacokinetics in Patients

A 3-period randomised crossover study[20] of the interaction of mycophenolate mofetil with ganciclovir used a single oral dose of mycophenolate mofetil 1.5g in 12 males with renal transplants; most of the patients were in the late transplant period. The mean (± SD) Cmax, tmax, and AUC∞ of mycophenolic acid was 30.9 ± 11 mg/L, 0.9 ± 0.3 hours, and 80.3 ± 16.4 mg/h • L, respectively. These values are very similar to those for mycophenolate mofetil 1.5g in healthy individuals (see table I). The single and first dose data thus agree with the conclusion that the late post-transplant patient has pharmacokinetic parameters for mycophenolic acid comparable with those in healthy individuals. The Cmax, tmax and AUC∞ values for MPAG were 54.0 ± 15.9 mg/L, 2.4 ± 0.9 hours and 1109 ± 538 mg/L • h, respectively. Compared with healthy individuals, the MPAG parameters after a dose of mycophenolate mofetil 1.5g in the patients showed a somewhat increased mean Cmax and tmax, but the mean AUC∞ was more than than twice the size. The first dose data showed a similar difference when comparing renal transplant patients with healthy individuals. The plasma © Adis International Limited. All rights reserved.

concentration of MPAG depends directly on renal glomerular filtration rate (see section 6), and this difference between healthy individuals and patients arises from residual post-transplant renal functional impairment in the latter. 5.3 Multiple DosePharmacokinetics in Patients

The pharmacokinetic parameters of multiple doses of mycophenolate mofetil administered on a twice daily schedule are given in table III. The late set of data is from a study in Caucasian patients who were at least 3 months posttransplant, with stable renal function, and who had been on twice daily doses of mycophenolate mofetil 1.5g for at least 2 weeks.[12] The 3 week post-transplant set of data is from a dose ranging study in Japanese patients.[21] No pharmacokinetic data were presented in the literature but samples were taken and analysed.[12] At 3 weeks, the Cmax and interdosage interval AUC12 of mycophenolic acid had increased relative to day 1. In one of the studies,[19] mean values on day 20 post-transplant were still almost 50% lower for the same dose than in the late patient group. Clin Pharmacokinet 1998 Jun; 34 (6)

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Table III. Pharmacokinetic parameters (mean ± SD) for multiple twice daily administration of oral mycophenolate mofetil to patients with renal transplants Time from transplant

Day 20[19]

Day 20[19]

Day 20[19]

3 weeks[12,21]

3 weeks[12,21]

Dose (g)

1.0

1.5

1.75

1.0

1.5

1.5

n

6

5

6

6

8

13 23.2 ± 11.9

Mycophenolic acid parameters Cmax (mg/L) 8.2 ± 3.6

Late[12]a

13.0 ± 8.4

18.7 ± 12.2

12.0 ± 4.9

13.9 ± 8.3

tmax (h)

1.6 ± 1.3

1.1 ± 0.6

1.3 ± 0.8

2.1 ± 1.5

2.25 ± 1.2

0.9 ± 0.2

AUCb (mg/L • h)

32.7c ± 5.45

36.6c ± 5.15

59.5c ± 6.95

47.0 ± 15.3

60.3 ± 25.3

61.3 ± 28.7

Mycophenolic glucuronide parameters NR Cmax (mg/L)

NR

NR

90.2 ± 34.4

116 ± 43.1

111 ± 26.5

tmax (h)

NR

NR

NR

4.0 ± 1.3

4.8 ± 1.8

3.0 ± 1.2

AUCb (mg/L • h)

NR

NR

NR

820 ± 400

1000 ± 450

1040 ± 290

a

Late patients were at least 3 months post-transplant.

b

AUC12 over the interdosage interval.

Reported values halved,[19] see text. Abbreviations: AUC = area under the concentration-time curve; Cmax = maximum drug concentration; n = number of participants; NR = not reported; tmax = time to reach the maximum drug concentration. c

In the other study,[21] the values appear comparable with those in the late patient group, in part because the Japanese population in which this study was performed appear to show a systematically higher plasma mycophenolic acid interdosage interval AUC12 for a given dose than do Caucasians (see below). Overall, the review of all available data indicates that in the early post-transplant period the mean plasma mycophenolic acid interdosage interval AUC12 is some 30 to 50% lower for the same dose than in the late transplant period. The rise in the mean mycophenolic acid interdosage interval AUC12 takes place slowly, over several months. Changes in mycophenolate mofetil dose will be superimposed on this nonstationary pharmacokinetics. Based on the mycophenolic acid t1⁄2, changes in the dosage of mycophenolate mofetil are expected to reach a new quasi-steady state for plasma mycophenolic acid concentration in a few days. The mechanism of this nonstationarity is uncertain, but probably multifactorial. It does not appear to be the result of an absorptive defect since intravenous administration of mycophenolate mofetil on day 1 gives low mycophenolic acid AUC12 values, comparable with that from the same dose given orally.[12] Mycophenolic acid is highly bound to albumin. Serum albumin concentrations systemati© Adis International Limited. All rights reserved.

cally rise about one-third over the same time scale in these patients (fig. 2), so that at least part of the change may simply reflect increased drug bound to plasma albumin. The nonstationary pharmacokinetics of the plasma mycophenolic acid and MPAG after the administration of mycophenolate mofetil was also observed in other solid organ transplants. It is most pronounced in hepatic transplants, perhaps because these patients experience the greatest degree of metabolic derangement, and so is correspondingly less pronounced in cardiac transplant recipients. These changes are of some interest because acute rejection occurs in all transplants, mostly during the same period of the pharmacokinetic changes. Furthermore, total plasma mycophenolic acid concentrations correlate with clinical outcome,[22] although free mycophenolic acid is the pharmacodynamically active component.[13] Similarly to healthy individuals, the population distribution Cmax and interdosage interval AUC12 of mycophenolic acid and MPAG was best described by a log-normal function. Nonetheless, these parameters were not very far from being symmetrically distributed about the mean. The maximum percentage deviation between mean and median for any parameter was 17.0%.[12] A combined dose-normalised pharmacokinetics data set from Clin Pharmacokinet 1998 Jun; 34 (6)

Mycophenolate Mofetil

6. Pharmacokinetics in Renal Impairment The majority of renal transplant patients show slow progressive renal impairment, usually over a time scale of years, and mostly related to angiopathic changes in the kidney attributed to the pathological process called chronic rejection. Approximately 25% of patients receiving a cadaveric donor kidney require dialysis early in the posttransplant period for an acute, severe, but usually transient, renal impairment, commonly called delayed graft function (DGF). The pharmacokinetics of mycophenolate mofetil in renal impairment is therefore of considerable practical importance. Studies have been performed using both single doses of mycophenolate mofetil in patients with varying degrees of renal impairment and multiple doses of mycophenolate mofetil in patients with DGF. The effect of haemodialysis has also been investigated. © Adis International Limited. All rights reserved.

10 9 8 7 6 Serum albumin (g/dl)

all renal transplant patients receiving the drug twice daily (n = ~200, dependent on parameter[12]) showed gender had no effect on the pharmacokinetics of either mycophenolic acid or MPAG. Asians had higher interdosage interval AUC and Cmax values relative to Caucasians, but this apparent racial difference may have been confounded by differences in bodyweight; the Asian subgroup weighed substantially less on average than the Caucasian group. In contrast to healthy individuals, the pharmacokinetic parameters of the transplant recipients had little correlation with bodyweight,[22] perhaps reflecting the effect of pathophysiology and concomitant medications in the patients. In practical terms, administration on a bodyweight basis is likely to provide little decrease in the pharmacokinetics of mycophenolic acid variability relative to a fixed dosage schedule.[22] For MPAG, the interdosage interval AUC was correlated to renal function. Multivariate analysis contributed little more to these findings.

441

5 4 3

2

1 0

100

300 200 Days post-transplant

400

Fig. 2. Serum albumin vs days post-transplant; a total of 2076

values are presented with a polynomial fit of these data.[2]

6.1 Single Dose Pharmacokinetics in Renal Impairment

Oral administration of single doses of mycophenolate mofetil 1g was studied in 25 patients with established, stable impaired renal function spanning a range of renal function from mild to severe with maintenance haemodialysis.[23] 20 of the 25 patients had received renal allografts. An additional group of 6 healthy individuals with normal renal function were studied with them. GFR was determined by iohexol clearance. Mild renal impairment was defined by a GFR of 3 to 4.8 L/h/1.73m2, moderate renal impairment by a GFR of 1.5 to 2.94 L/h/1.73m2, and severe renal impairment by a GFR of < 1.5 L/h/1.73m2. Even in severe renal impairment mycophenolate mofetil was not quantifiable in any of the plasma samples, indicating that renal failure had little effect on the deesterification of mycophenolate mofetil to mycophenolic acid. Figure 3 shows the relation of the plasma clearance of mycophenolic acid and MPAG to the GFR for the 19 patients not on haemodialysis together with the 6 healthy individuals.[12] The mycophenoClin Pharmacokinet 1998 Jun; 34 (6)

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30

Severe

Moderate

Mild

Normal

4

Severe

Moderate

Mild

Normal

MPAG clearance (L/h)

MPA clearance (L/h)

25

20

15

10

3

2

1 5

0

0 0

20

40

60

80

100

0

120

20

40

60

80

100

120

GFR (ml/min/1.73m2)

Fig. 3. (left) The glomerular filtration rate (GFR) vs mycophenolic acid (MPA) clearance following single doses of mycophenolate

mofetil in patients with renal impairment (but not on haemodialysis) and healthy individuals,[23] total number of participants is 25. The linear correlation coefficient (r2) was 0.04 (p > 0.05, n = 25). The slope of the linear regression equation (mycophenolic acid clearance (L/h) = 12.0 + 0.06 • GFR, where GFR is in ml/min/1.73m2) was not statistically significantly different from zero (p = 0.17), and is not shown. The intercept was statistically significant (p < 0.0001). (right) The GFR vs mycophenolic glucuronide (MPAG) clearance following single doses of mycophenolate mofetil in patients with renal impairment (but not on haemodialysis) and healthy individuals,[23] total n = 25. The r2 was 0.86 (p < 0.001). The slope of the linear regression equation [mycophenolic glucuronide (MPAG) clearance (L/h) = 0.10 + 0.026 • GFR, where GFR is in ml/min/1.73m2] was statistically significantly different from zero (p < 0.0001), and is shown as a solid line. The intercept was not statistically different from zero (p = 0.44).

lic acid plasma clearance appeared to decrease slightly with decreasing GFR, but the slope of the linear regression was not statistically significantly different from zero. Thus there is no, or minimal, effect of renal impairment on the plasma AUC of mycophenolic acid, as would be expected if the majority of mycophenolic acid metabolism occurred in the liver. In contrast, MPAG plasma clearance showed a steep and highly statistically significant decline as GFR decreased (fig. 3), with a linear correlation coefficient (r2) of 0.86. The plasma AUC96 of MPAG was 3- to 6-fold higher in patients with severe renal impairment than in patients with mild renal impairment or in healthy individuals. The mean renal clearance of MPAG in healthy individuals, patients with mild renal impairment, patients with moderate renal impairment or patients with severe renal impairment (but not on haemodialysis) were respectively 1.848, 1.248, © Adis International Limited. All rights reserved.

0.612 and 0.258 L/h.[23] The accumulation of MPAG in patients with renal impairment is consistent with the major role of the kidneys in the excretion of this metabolite. Single doses of mycophenolate mofetil 1g were given intravenous over 40 minutes to 4 patients with severe renal impairment; these patients had previously received the same dose of mycophenolate mofetil orally. The plasma concentrations of mycophenolate mofetil during infusion were around 5.5 mg/L, closely comparable with the values seen with the same infusion rate in healthy individuals.[11] De-esterification of mycophenolate mofetil seems little affected by severe renal impairment. The plasma AUC96 of mycophenolic acid was also similar for the 2 routes, suggesting that severe renal insufficiency does not grossly affect the bioavailability of mycophenolic acid from oral mycophenolate mofetil.[12] Clin Pharmacokinet 1998 Jun; 34 (6)

Mycophenolate Mofetil

© Adis International Limited. All rights reserved.

MPA clearance (L/h)

The pharmacokinetics of mycophenolate mofetil after multiple administration in patients with chronic stable renal impairment has not been reported. Some information is provided by combining renal transplant patients from clinical studies in which multiple doses of mycophenolate mofetil were used and interdosage interval plasma profiles were taken.[12] The creatinine clearance of each patient was estimated from their age, gender and bodyweight using the Cockcroft-Gault formula.[24] The clearances of mycophenolic acid and MPAG are plotted against calculated creatinine clearance in figure 4. In agreement with the single dose pharmacokinetics, mycophenolic acid appeared to be minimally affected by renal impairment, whereas plasma MPAG clearance was linearly related to creatinine clearance. However, almost all the patients had renal impairment of no more than moderate severity. The pharmacokinetic effects of DGF have been investigated in 8 patients receiving mycophenolate mofetil 1.5g twice daily who required haemodialysis within the first week of transplantation.[12] Plasma sample profiles were obtained on day 1 after the first dose, and then at weekly intervals up to 4 weeks. The mean plasma interdosage interval AUC12 of mycophenolic acid and MPAG are shown plotted against time in figure 5. During the course of the study, patients received haemodialysis and renal function usually improved with time. In patients with DGF, the mean plasma interdosage interval AUC12 values of mycophenolic acid were similar to those in patients without DGF (see tables II and III), and showed comparable increases of around 2-fold between the day 1 first dose and week 4. Mean interdosage interval MPAG AUC12 increased as much as 6- to 8-fold in patients with DGF, to achieve concentrations 2 to 3 times higher than in patients without DGF. The decline in MPAG over time reflects the improving renal function as DGF resolves. MPAG has no known pharmacological activity. In the reported studies[1-3] around 25% of the pa-

100

50

0 4

3

MPAG clearance (L/h)

6.2 Multiple Dose Pharmacokinetics in Renal Impairment

443

2

1

0 0

20

40 60 80 100 Creatinine clearance (ml/min)

120

Fig. 4. (top) Creatinine clearance vs mycophenolic acid (MPA) clearance from clinical studies in which multiple doses of mycophenolate mofetil were used (n = 179). The linear correlation coefficient (r2) was 0.03 (p < 0.05). The slope of the linear regression equation (mycophenolic acid clearance (L/h) = 37.9 – 0.19 • GFR, where GFR is in ml/min) was shallow relative to the intercept and is not shown. Both the slope and the intercept were statistically significantly different from zero (p < 0.05 and p < 0.0001, respectively). (bottom) Creatinine clearance vs mycophenolic glucuronide (MPAG) clearance from clinical studies in which multiple doses of mycophenolate mofetil were used (n = 183). The r2 was 0.26 (p < 0.0001). The slope of the linear regression equation [mycophenolic glucuronide (MPAG) clearance (L/h) = 0.59 – 0.01 • GFR, where GFR is in ml/min] was steep relative to the intercept and is shown by the dotted line. Both the slope and the intercept were statistically significantly different from zero (p < 0.0001 and p < 0.0001 respectively).

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MPA AUC (mg/L · h)

100

the short term. Mycophenolic acid is the active immunosuppressant entity, and since renal impairment (including the severe renal impairment of DGF) had no major effect on its pharmacokinetics, dose adjustment of mycophenolate mofetil appears not to be necessary in these circumstances. The plasma concentrations of MPAG seen in patients with DGF, which are also expected to occur in severe chronic renal impairment, are higher than those reported to displace mycophenolic acid in vitro from its plasma protein binding sites.[13] Thus, in principle, free concentrations of mycophenolic acid, which correlate with pharmacodynamic activity,[13] may be altered by severe renal impairment.

50

0

MPAG AUC (mg/L · h)

5000

6.3 Effect of Haemodialysis

2500

0 1

7 14 21 Time post-transplant (days)

28

Fig. 5. (top) Days post-transplant vs mycophenolic acid area under the concentration-time curve for 12 hours (AUC12) in patients with delayed graft function. Mean (± SD) n = 8, 8, 5, 2 and 7 on days 1, 7, 14, 21 and 28, respectively. (bottom) Days posttransplant vs mycophenolic glucuronide (MPAG) AUC12 in patients with delayed graft function. Mean (± SD) n = 8, 8, 5, 2 and 7 on days 1, 7, 14, 21 and 28, respectively.

tients receiving mycophenolate mofetil had DGF, and presumably then accumulated MPAG in plasma. Clinically, no excess of adverse events were noted in this subgroup. Exposure to these concentrations of MPAG does not appear deleterious, at least in © Adis International Limited. All rights reserved.

Removal of mycophenolic acid and MPAG across the dialysis coil was measured in patients with severe impairment given a single dose of mycophenolate mofetil, and in the patients undergoing haemodialysis during DGF. There was no difference in mycophenolic acid concentrations across the coil, whereas MPAG concentrations were a mean 15.8% lower at the output coil than at the input coil.[12] The drop in MPAG concentrations across the coil was highest in patients who had high MPAG concentrations, which may reflect saturation of MPAG protein binding sites at these high plasma MPAG concentrations. Plasma concentrations of mycophenolic acid were not reduced by haemodialysis. In some patients, plasma concentrations of mycophenolic acid rose slightly, presumably the result of haemoconcentration. Plasma MPAG concentrations sometimes did decrease, but only in patients with a pre-dialysis plasma concentrations in excess of approximately 100 mg/L (fig. 6). The amounts of mycophenolic acid and MPAG removed by haemodialysis were very small, and consistent with the high protein binding of these species. Mycophenolate mofetil doses need not be adjusted if haemodialysis is performed. The effect of peritoneal dialysis has not been reported, but the minimal effects of haemodialysis on the removal Clin Pharmacokinet 1998 Jun; 34 (6)

Difference pre/post [MPAG] (mg/L)

Mycophenolate Mofetil

445

150

100

50

0

–50

–100 0

50

100

150

200

250

300

350

400

Pre-dialysis (MPAG) [mg/ml]

Fig. 6. Pre-dialysis mycophenolic glucuronide (MPAG) concentration vs MPAG concentration difference (before and after di-

alysis).

of mycophenolic acid and MPAG would indicate that a similar lack of effect would be seen there also. 7. Pharmacokinetics in Hepatic Impairment The effect of varying degrees of hepatic oxidative impairment has been studied in a oral single dose study with mycophenolate mofetil 1g.[15] Eighteen patients with compensated alcoholic cirrhosis were divided into groups of 6, each with mild, moderate or severe hepatic oxidative impairment, defined by the antipyrine breath test (APBT). None of the test participants had clinically severe symptoms of hepatic disease. Stratification by APBT may not be optimal since the metabolism of mycophenolic acid is by glucuronidation, but no test is available to stratify by conjugative capacity and the APBT correlated well with overall hepatic impairment. Six healthy individuals were included as a control group. There was no gross effect on the plasma pharmacokinetics of either mycophenolic acid or MPAG (table IV), although there were group-to-group differences which suggested this result might be the net result of factors which acted in opposite directions. © Adis International Limited. All rights reserved.

In comparison with healthy individuals, patients with cirrhosis had a less marked trough in the plasma mycophenolic acid profile at 4 hours postdose, and plasma concentrations were statistically significantly higher in the patients than in the healthy individuals between 2 to 4 hours post-dose. The secondary peak at around 6 hours also appeared less distinct in the patients with cirrhosis, although this was not statistically different from the healthy group. A consistent pattern was seen as hepatic oxidative impairment increased, in which the mean Cmax, AUC6 and AUC96 of mycophenolic acid and MPAG decreased in the mildly impaired group relative to the healthy group; increased in the moderately impaired group relative to the mildly impaired group; and then decreased in the severely impaired group relative to the moderately impaired group. As a result, no consistent linear trend of these parameters with degree of hepatic oxidative impairment was seen. The parameters for the moderately impaired group were consistently statistically significantly higher than those in the mildly impaired group. The renal clearance of MPAG was similar in healthy individuals and patients with cirrhosis with mild and moderate impairment, but in the severely impaired group, the mean renal clearance was almost twice that of the other groups. The latter result may arise from increased renal metabolism of mycophenolic acid by induction of the glucuronidation process[25] in the severely impaired cirrhotic patients. To explain the observed biphasic relationship of the plasma pharmacokinetics of mycophenolic acid and MPAG with worsening hepatic function, progressive impairment of hepatic metabolism and biliary excretion was suggested to counteract the effect of enhanced renal mycophenolic acid metabolism. If renal glucuronidation is induced in patients with severe hepatic impairment, it may possibly be a homeostatic response to unconjugated bilirubin: both APBT and plasma bilirubin were reasonably correlated with renal MPAG clearance. If confirmed, such induction of glucuronidation would be of general interest for Clin Pharmacokinet 1998 Jun; 34 (6)

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Table IV. Pharmacokinetic parameters (mean ± SD) for a single dose of orally administered mycophenolate mofetil 1g to healthy individuals or patients with impaired hepatic function[15] Degree of impaired functiona

None (healthy individuals)

Mild

Moderate

Severe

n

6

6

6

6

Mycophenolic acid parameters Cmax (mg/L)

24.3 ± 5.7

15.1 ± 8.9

30.8 ± 6.0

21.3 ± 9.0

tmax (h)

0.6 ± 0.1

1.1 ± 1.0

0.7 ± 0.2

0.8 ± 0.3

AUCb (mg/L • h)

29.0 ± 5.8

21.8 ± 6.7

36.6 ± 2.9

31.0 ± 14.4

CLRc (ml/min)

0.6 ± 0.7

0.3 ± 0.2

1.0 ± 1.2

1.7 ± 1.5 26.8 ± 5.5

Mycophenolic glucuronide parameters 31.4 ± 3.3 Cmax (mg/L)

24.8 ± 9.5

43.0 ± 14.7

tmax (h)

1.3 ± 0.3

2.4 ± 1.3

1.8 ± 0.4

1.7 ± 0.5

t1⁄2 (h)

12.5 ± 5.7

8.6 ± 4.2

8.5 ± 2.7

6.3 ± 2.0

AUC∞ (mg/L • h)

261 ± 32.4

223 ± 49.0

418 ± 122

225 ± 44.4

CLRc (ml/min)

25.0 ± 3.8

28.7 ± 8.9

23.2 ± 6.2

49.8 ± 25.2

a

Determined by APBT in patients (mild = 0.4 to 0.6%, moderate = 0.2 to 0.39%, and severe = <0.2% of radiolabelled aminopyrine excreted in 30 minute period); healthy individuals had normal hepatic function tests.

b

Over 96 hours.

c

Renal clearance determined over a 24 hour period post-dose. Abbreviations: AUC = area under the concentration-time curve; Cmax = maximum drug concentration; n = number of participants; NR = not reported; tmax = time to reach the maximum drug concentration.

the pharmacokinetics of the drug in patients with hepatic impairment. Use of mycophenolate mofetil can be expected in hepatic transplantation, as well as in autoimmune hepatic diseases. If, as suggested, plasma pharmacokinetics of mycophenolic acid are the summed resultant of several processes operating in different directions, the lack of apparent effect of hepatic impairment in the patients with compensated cirrhosis may not be directly applicable to these other situations where different pathophysiology may be present. An example is provided by the time course of mycophenolic acid AUC after hepatic transplantation. In 11 patients with liver transplants given mycophenolate mofetil orally at doses of 1.75 to 2.25g twice daily, there was a substantial and statistically significant decline of around 33% in the apparent clearance of plasma mycophenolic acid between week 2 and week 3, with a continued slower decline from there onwards to month 3.[26] The mean ± SD AUC∞ of mycophenolic acid after a dose of mycophenolate mofetil 1.75g given intravenous over 1 hours at day 24 post-transplant to 6 patients with liver transplants was 42.85 ± 21.7 © Adis International Limited. All rights reserved.

mg/L • h,[27] lower than the interdosage interval AUC12 seen for the same dose given oral on day 20 after renal transplantation (table III). Most hepatic transplant recipients will have had severe hepatic oxidative impairment prior to transplantation, and, therefore, have induced renal glucuronidation. These results would then be consistent with a continued enhancement of renal mycophenolic acid metabolism into the first few weeks of the post-transplant period, with a transplanted liver which has returned hepatic function to near normal. 8. Enterohepatic Circulation Following Administration Reference has already been made (section 4) to the secondary peaks seen in the plasma mycophenolic acid profile following single dose administration of both oral and intravenous mycophenolate mofetil.[11,15,17] Plasma MPAG also showed secondary peaks which were less pronounced and delayed relative to the mycophenolic acid peaks, similar to the relationship for the primary Cmax peak. This result is most consistent with a secondary mycoClin Pharmacokinet 1998 Jun; 34 (6)

Mycophenolate Mofetil

phenolic acid input. Studies in rats[12] suggested entrohepatic cycling as the source of this secondary material. Following administration of oral [14C]mycophenolate mofetil to bile-cannulated rats, 77% and 21% of the dose was recovered from bile and urine, respectively, 24 hours later. The plasma concentration of mycophenolic acid declined rapidly, suggesting the persistence of mycophenolic acid in the plasma depended on an uninterrupted biliary flow. Estimates of AUC with biliary drainage were less than one-third of those in intact animals. Bile collected in this manner was readministered through an intraduodenal cannula to other rats also having a biliary drainage catheter in situ. Approximately 85% of the administered radioactivity was recovered in a combined collection of the bile and urine of the recipient animals, showing extensive reabsorption of the bile-derived material. Thus, in rats, there is good evidence that entrohepatic cycling has a major role in the pharmacokinetics of mycophenolate mofetil. In a pilot study of mycophenolate mofetil in the prevention of rejection following hepatic transplantation,[26] 17 patients received mycophenolate mofetil 1.75 to 2.5g twice daily. Bile was collected from a T-tube placed in a choledochocholedochostomy, and refed via a nasogastric tube. On 2 separate days at the end of week 2, samples from a 12 hours interdosage interval collection of bile were analysed for mycophenolic acid and MPAG. The mean (range) biliary excretion of MPAG in the 12 hours collection was 18% (16.5 to 26.0%) of the administered dose. In all patients except 1, less than 0.1% of the administered dose was excreted into bile as mycophenolic acid. Although bile collection was likely to be incomplete, and bile secretion diminished, these results show significant amounts of drug substance are excreted into bile, and that essentially all this material is the conjugated metabolite MPAG. In a formal study,[12] entrohepatic cycling was investigated further in healthy individuals by using cholestyramine in a crossover study with single doses of mycophenolate mofetil 1.5g. Cholestyramine was given for 24 hours prior to mycophenol© Adis International Limited. All rights reserved.

447

ate mofetil administration, with a dose 1 hour before the mycophenolate mofetil, and continued for the 3 days of sampling post-administration. Cholestyramine had a marked and highly statistically significant effect on the profile of mycophenolic acid. The mean (range) AUC72 of mycophenolic acid was decreased by 37% (10 to 61%), and this was almost entirely attributable to the reduction of mycophenolic acid concentrations from 6 hours onwards. Recovery of urinary MPAG was correspondingly reduced by cholestyramine administration. There was no difference in Cmax with or without cholestyramine, which indicates that the absorption of mycophenolate mofetil was not affected. As an anionic resin, cholestyramine would be expected to bind to mycophenolic acid and the lack of effect on the Cmax supports the idea that mycophenolate mofetil is absorbed intact, without any significant degree of intraluminal de-esterification. These studies support the occurrence of significant entrohepatic cycling in humans. Typical of this process, there was a substantial 6-fold interindividual variation in the contribution of entrohepatic cycling to the AUC, implying a major role for this process in interindividual variability. Since MPAG was the recirculating entity, but the recirculation peak in plasma was mycophenolic acid, deglucuronidation by the gut flora presumably occurred in the colon prior to reabsorption. Entrohepatic cycling represents a basis for potential pharmacokinetic effects in disease states, such as colonic disease or severe diarrhoea; and for drug interactions. 9. Pharmacokinetic Interactions with Mycophenolate Mofetil Since mycophenolate mofetil is solely metabolised by glucuronidation, direct pharmacokinetic interactions with drugs metabolised by cytochrome P450 oxidation are not generally expected. Pharmacokinetic interaction with other drugs metabolised by glucuronidation is a theoretical possibility, although a clinically significant interaction is very unlikely. Potential general mechanisms for interactions with mycophenolate mofetil Clin Pharmacokinet 1998 Jun; 34 (6)

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Bullingham et al.

involve either entrohepatic cycling, or renal tubular competition between MPAG and other drugs undergoing transport mediated renal excretion. As regards the former, competitive interaction could occur between MPAG and other drugs at the biliary transporter, although no example has been reported. Once MPAG is excreted into the gut, cholestyramine and other bile acid sequestrants can clearly reduce the plasma mycophenolic acid AUC and hence clinical effectiveness. Deglucuronidation of MPAG in the colon is mediated by the gut flora, and in particular by the Gram-negative anaerobes which contain most of the glucuronidase activity. Antibiotics with activity against such organisms may reduce entrohepatic cycling and hence the mycophenolic acid AUC. With a renal tubular interaction, plasma mycophenolic acid is not affected. No major clinical sequelae appear to follow from raised plasma MPAG concentrations. The clinical significance of any tubular interaction will thus depend on the magnitude

and consequence of raised plasma concentrations of the other interacting drug. Being a competitive interaction, high plasma MPAG concentrations are likely to increase the magnitude of the interaction. Renal impairment may thus be the clinical situation where such interactions need special consideration. The reported pharmacokinetic interactions of mycophenolate mofetil with concomitant medications are summarised in table V. 9.1 Effect of Food

The effect of a standard meal containing 46.5% fat has been reported in a study of 12 patients with rheumatoid arthritis given a single dose of mycophenolate mofetil 2g in a crossover manner.[17] The plasma mycophenolic acid AUC24 was statistically equivalent in the fed and fasted state. Relative to the fasting state, the median tmax of mycophenolic acid was increased from 1.0 to 2.0 hours and mean Cmax was statistically significantly decreased by

Table V. Effect of concomitant drug medication on mean pharmacokinetic parameters of orally administered mycophenolate mofetil Drug medication

n

Study design

MPA plasma PK

MPAG plasma PK Drug plasma PK

Reference

Aluminium/magnesium hydroxide (Maalox®) 10ml qid orally

10

Crossover study in patients with RA using an oral SD of 2g

Cmax ↓ 37% AUC ↓ 15%

Cmax ↓ 26% AUC ↓ 11%

NA

17

Cotrimoxazole 800mg/160mg SD orally

12

Crossover study in healthy Cmax ↔ individuals using an SD of 1.5g AUC ↓ 7%

Cmax ↔ AUC ↔

ND

12

Aciclovir 800mg SD orally

12

Crossover study in healthy individuals using an SD of 1g

Cmax ↔ AUC ↑ 9%

Cmax ↔ AUC ↑ 10%

Cmax ↑ 18% AUC ↑ 18%

12

Cyclosporinb SS orally

10

Sequential study in patients with RT using 1.5g bid

ND

ND

Cmax ↔ AUC ↔

12

Ganciclovir 5 mg/kg IV infusion

12

Crossover in patients with RT using a SD of 1.5g

Cmax ↓ 10% AUC ↔

Cmax ↔ AUC ↔

Cmax ↔ AUC ↔

20

Oral contraceptivec SD

18

Crossover study in healthy women using a SD of 1g

Cmax ↓ 9% AUC ↔

Cmax ↑ 7% AUC ↔

EE: Cmax ↑ 14% EE: AUC ↔ NET: Cmax ↑ 10% NET: AUC ↑ 8%

12

Cholestyramine 4g tid orally

12

Crossover study in healthy individuals using a SD of 1.5g

Cmax ↓ 6% AUC ↓ 39%

Cmax ↔ AUC ↓ 34%

NA

12

a

Cotrimoxazole (trimethoprim-sulfamethoxazole) twice daily for 7 days.

b

(Sandimmune®) dose at steady state was 275-450 mg/day on a bid schedule; mean trough blood cyclosporin concentration at steady state was ~150 μg/L.

c

Two tablets of Ortho Novum® [norethindrone 1 mg (NET) and ethinyl estradiol 35 mg (EE)].

Abbreviations and symbols: bid = twice daily; IV = intravenous administration; MPA = mycophenolic acid; MPAG = mycophenolic glucuronide; n = number of participants; NA = not applicable; ND = not done; pk = pharmacokinetic; PO = orally; qid = 4 times daily; RA = rheumatoid arthritis; RT = renal transplant; SD = single dose; SS = steady-state; ↔ indicates effect was less than ± 5%; ↑ indicates effect was an increase; ↓ indicates effect was a decrease.

© Adis International Limited. All rights reserved.

Clin Pharmacokinet 1998 Jun; 34 (6)

Mycophenolate Mofetil

24.6%, changes which are consistent with delay in gastric emptying in the fed state. Statistically significant increases in mean plasma MPAG AUC24 and Cmax of 29.8% and 14.1%, respectively, were seen in the fed state relative to fasting, suggesting food may affect glucuronidation. These changes with food are minor, and as the clinical effectiveness appears to correlate with plasma mycophenolic acid AUC, it suggests there is no compelling reason for the drug only to be taken with food. 9.2 Effect of Concomitant Antacids

The effect of co-administration of an antacid (Maalox®, 10ml containing 1200mg of aluminium hydroxide and 600mg of magnesium hydroxide) was studied in another limb of the crossover study in patients with rheumatoid arthritis (described above).[17] Four doses of the antacid were given at 4-hourly intervals on the day before mycophenolate mofetil administration. Mycophenolate mofetil 2g was then given with antacid, and followed by 3 more doses of antacid at 4-hour intervals. The mean plasma AUC24 and Cmax for mycophenolic acid were statistically significantly reduced by 16.8% and 37.7%, respectively, whereas the median tmax was unchanged. The MPAG parameters showed similar decreases with the antacid. The parallel changes in mycophenolic acid and MPAG are consistent with reduced absorption. The plasma mycophenolic acid profile showed reductions in both the initial and secondary peaks, suggesting an effect on both primary and secondary (enterohepatic cycling) absorption. Based on the chemical structure, chelation by the antacid was suggested as a possible mechanism. The reduction in plasma mycophenolic acid with antacid could potentially reduce the clinical effectiveness of orally administered mycophenolate mofetil. Over 80% of the patients received antacids at some time in the large-scale studies,[1-3] suggesting the extent of any effect must be of relatively minor clinical significance. © Adis International Limited. All rights reserved.

449

9.3 Drug Interactions

In addition to cholestyramine, cross-over mycophenolate mofetil-drug pharmacokinetic interaction studies have been conducted with cyclosporin, a combination oral contraceptive (Ortho Novum®, containing norethindrone 1mg and ethinyl estradiol 35μg), cotrimoxazole (trimethoprim-sulfamethoxazole), aciclovir[12] and ganciclovir.[20] The studies with cyclosporin and ganciclovir were done in patients with renal transplants; all other studies were done in healthy individuals. Single doses of both mycophenolate mofetil (1 or 1.5g) and the concomitant drug were given, except for the cyclosporin study where a 2 week course of 1.5g twice daily of mycophenolate mofetil was given on top of a maintenance dosage of cyclosporin. Ganciclovir was given intravenously; all other drug administrations were oral. The plasma concentrations of mycophenolic acid and MPAG were measured in all the studies. Except for cotrimoxazole, where the purpose of the study was to investigate the effect on the pharmacokinetics of mycophenolate mofetil, the plasma concentrations of the concomitant drug or drugs were measured in each case. No statistically significant changes in the plasma pharmacokinetic parameters for mycophenolic acid or MPAG were seen with any of these drugs. None of the concomitant medications showed any statistically significant changes in plasma concentrations with mycophenolate mofetil. With aciclovir, there were small increases in the plasma AUC24 of both MPAG (mean about 10%) and aciclovir (mean about 18%), consistent with a small degree of competition between the 2 drugs for renal tubular secretion. With ganciclovir,[20] there was a small (mean of around 12%) but statistically significant decrease in ganciclovir renal clearance. None of these studies provided a pharmacokinetic basis for the need to adjust either the dose of mycophenolate mofetil or of the concomitant medication. The studies do not exclude the possibility of pharmacodynamic interactions and all have limitations as single dose studies in healthy individuals (excepting ganciclovir single dose in patients and Clin Pharmacokinet 1998 Jun; 34 (6)

450

cyclosporin multiple administration in patients). These factors are of particular significance for the oral contraceptive, as mycophenolate mofetil is a teratogen in animals. Whilst the results exclude a gross effect of mycophenolate mofetil on norethindrone and ethinyl estradiol, a multiple dose study including confirmation of the suppression of ovulation is needed so that it can be certain that it does not affect the clinical effectiveness of the oral contraceptive. 10. Clinical PharmacokineticPharmacodynamic Correlations 10.1 Correlations of Plasma Mycophenolic Acid Concentration with Adverse Events

In the double-blind large-scale clinical studies,[1-3] a blood sample was taken when a severe adverse event occurred. The elapsed time between the last dose of the investigational drug and the time of sampling was obtained from their respective clock-times. At trial end, the treatment assignment code was broken and the stored samples from those patients who had received mycophenolate mofetil were analysed for mycophenolic acid and MPAG.[12] These plasma concentrations were then compared with the plasma concentrations of mycophenolic acid or MPAG in renal transplant patients without severe adverse events, taking into account the elapsed time from the last dose of mycophenolate mofetil, as follows. Several pharmacokinetic studies had been conducted in stable renal transplant patients receiving mycophenolate mofetil, so that the plasma concentrations profile data on mycophenolic acid and MPAG were available over the 0 to 12 hours interdosage interval. Data from a study in patients with DGF were excluded. The combined individual plasma concentrations for mycophenolic acid or MPAG from patients in these studies was then used to construct percentile distributions of plasma concentrations of mycophenolic acid or MPAG over the 12 hours interdosage interval. Plasma concentrations of mycophenolic acid and MPAG collected at the time of an adverse event © Adis International Limited. All rights reserved.

Bullingham et al.

were compared with the percentile distribution appropriate for the elapsed time between administration and the taking of the adverse event sample. The severe adverse events were categorised as diarrhoea, nausea/vomiting, leucopenia, tissue invasive cytomegalovirus (CMV) infection, or all others (not separately classified). None of these categories of adverse event appeared to be related to mycophenolic acid or MPAG concentrations which were extreme relative to the percentile distribution in the patients without severe adverse events (fig. 7). 10.2 Correlations of Mycophenolic Acid Concentration with Immunosuppressive Effect

In vitro, mycophenolic acid concentration shows a classical sigmoidal relation to inhibition of IMPDH, and to the suppression of lymphocyte proliferation. [6] For both the T and B cell mitogeninduced proliferation of human peripheral blood lymphocytes, and the mixed lymphocyte reaction (MLR), the free mycophenolic acid concentration for 50% inhibition of response (IC50) was in the range 20 to 100 nmol/L (approximately 0.3 to 1.4 mg/L). Complete inhibition was produced at free mycophenolic acid concentrations around 1 mmol/L. By varying albumin concentration it was shown that it was the free concentration of mycophenolic acid, rather than the total (free plus protein-bound) concentration, which determined the immunosuppressive response in lymphocyte transformation studies.[13] For a given plasma protein concentration, the plasma mycophenolic acid free fraction is constant across the clinical range of plasma concentrations of mycophenolic acid.[13] Total plasma concentrations of mycophenolic acid can be thus used as a surrogate for free mycophenolic acid concentration. In an early dose-ranging study,[19] plasma samples from some of the patients receiving oral mycophenolate mofetil were taken during the interdosage interval and used in vitro in similar mitogen lymphocyte transformation assays.[6] The percentage of inhibition from plasma samples taken preClin Pharmacokinet 1998 Jun; 34 (6)

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451

Adverse events

MPA (mg/L)

50

Diarrhoea

40

Nausea and vomiting

Leucopenia

CMV

Other

30 20 10

MPAG (mg/L)

0

350 300 250 200 150 100 50 0 0 2 4 6 8 10 12 U

0 2 4 6 8 10 12 U

0 2 4 6 8 10 12 U

0 2 4 6 8 10 12 U

0 2 4 6 8 10 12 U

Time after administration (h)

Fig. 7. Relationship of plasma concentrations of mycophenolic acid (MPA) [upper series of graphs] and mycophenolic glucuronide (MPAG) [lower series of graphs] to severe adverse events in patients with renal transplants receiving twice daily mycophenolate mofetil. The headings diarrhoea, nausea and vomiting, leucopenia, CMV (tissue-invasive cytomegalovirus disease), or other (which included all adverse events not falling into any of the previous categories) show the terms to which the reported adverse events were mapped by a standard thesaurus. In each graph, the continuous lines show (in ascending order) the 50th, 90th and 95th percentile of the plasma concentration-time profile determined for a reference group of patients without delayed graft function (DGF) and not experiencing adverse events. Plasma concentration results from individual patients with adverse events are marked, at the time the sample was taken after the last mycophenolate mofetil dose; results from patients where the time of the last dose was not recorded are placed together at the extreme right of the time axis as a column labelled U.

dose, and 2 and 6 hours post-dose are shown for 5 patients in figure 8. The actual numerical values obtained for the extent of inhibition are dependent on assay conditions, particularly sample dilution and so cannot be directly related to inhibitory effect in vivo. The figure does show that the inhibitory effect increases after mycophenolate mofetil administration in line with the expected increase in plasma concentrations of mycophenolic acid and is maintained through the 6 hours time-point. Using appropriate dilutions of plasma, suppression of transformation followed the plasma concentration profile of mycophenolic acid at low mycophenolate mofetil doses, but as the mycophenolate mofetil dose increased, mycophenolic acid concentrations in plasma were reached that were associated with maximal inhibition of lymphocyte transformation. At this stage, the Cmax of mycophenolic acid had © Adis International Limited. All rights reserved.

no direct relation to in vitro inhibition since it was well above the maximum inhibitory concentration, although it could presumably indirectly reflect how long in vivo the plasma concentration would remain above the maximal inhibitory concentration. As the mycophenolate mofetil dose increased further, an increasing proportion of the interdosage interval achieved a plasma mycophenolic acid concentration which was above this maximal inhibitory concentration. These data provide insight into the nature of the immunosuppressive action of the drug in vivo, and in particular to the ease of reversing the immunosuppressive effect. Simple washing of lymphocytes inhibited by exposure to mycophenolic acid fully restores their ability to respond to mitogens in vitro, even when the cells have been harvested from animals treated for several months with mycophenolate mofetil. Clin Pharmacokinet 1998 Jun; 34 (6)

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Inhibition of lymphocyte proliferation (%)

MMF dose

MMF dose

100

50

0 Pre-dose 2h

6h

Pre-dose

Time to lymphocyte assay in relation to dose

Fig. 8. Percentage of inhibition of phytohaemagglutinin (PHA)-

induced lymphocyte proliferation by plasma samples taken on day 20 after renal transplant from 5 patients receiving cyclosporin, corticosteroids and mycophenolate mofetil (MMF) twice daily.[19] Sampling was pre-mycophenolate mofetil dose, and 2 and 6 hours post-dose. Assay used the same dilution and a standard protocol.

10.3 Correlations of Plasma Mycophenolic Acid Pharmacokinetic Parameters with Clinical Effectiveness

Connecting the plasma pharmacokinetics-pharmacodynamic correlations for mitogen-stimulated antiproliferative effect on lymphocytes in vitro to the clinical end-point of acute rejection is not straightforward. Lymphocytes may be pre-incubated with mitogen without exposure to mycophenolic acid for 12 hours, and suppression of proliferation still achieved by subsequent addition of mycophenolic acid.[6] A significant proportion of patients (20 to 45%) will not experience acute rejection when just given cyclosporin and corticosteroids, without mycophenolate mofetil. Acute rejection is an all-or-none end-point, partly dependent on the criteria and techniques used to diagnose it. Nonetheless, the key role of lymphocytes in acute rejection, their likely exposure to mycophenolic acid concentrations similar to those in the central plasma compartment, and the above ex vivo results, suggest some correlation between the plasma pharmacokinetics and clinical effectiveness might be anticipated. © Adis International Limited. All rights reserved.

Because of the binary response, logistic regression was used to relate plasma interdosage interval AUC12 of mycophenolic acid to the probability of rejection using data from an early dose-ranging study.[19] A highly statistically significant correlation was found,[22] such that as the AUC12 of mycophenolic acid increased, the probability of acute rejection decreased in a sigmoidal fashion. C max also showed a similar relationship, but AUC12 and Cmax were highly correlated. Bivariate logistic regression where AUC and Cmax are simultaneously used has indicated that Cmax makes a negligible additional contribution to the fit of the model relative to that based on AUC12 only.[12] The plasma AUC12 of mycophenolic acid estimated from the fitted curve which was required to achieve a 50% reduction in rejection was around 30 mg/L • h, which is in the region of the mean value obtained from the 1g twice daily administration schedule in the early post-transplant period (table III). The AUC12 required to achieve a 90% reduction in rejection was around 55 mg/L • h, which is in the region of mean values obtained from mycophenolate mofetil 1.5 to 1.75g twice daily (table III). It should be noted that these are estimates of dose administration for suppression of rejection only. The intent-to-treat analyses of the large clinical studies,[1-3] which incorporate withdrawals from adverse events, indicate that the 1g twice daily dose is optimal. The principal dose-related adverse events responsible for this result were diarrhoea, leucopenia and tissue-invasive CMV disease. Profiles were obtained from a subset of patients in one of the large clinical studies,[2] some of whom experienced biopsy-proven rejection in the first 6 months post-transplant. Standard normal quantile plots (which plot a value on the Y-axis as a function of the same value on the X-axis expressed in units of standard deviation around the mean) were used for the corresponding calculated AUC12 of mycophenolic acid (fig. 9), using logarithmically transformed AUC12 values for the Y-axis. Separate plots are shown for the 1 and 1.5g twice daily mycophenolate mofetil doses in figure 9 (top and bottom, respectively) with the AUC12 values of patients who Clin Pharmacokinet 1998 Jun; 34 (6)

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453

came largely from the lower end of the distribution of the AUCs. For the 1.5g twice daily group, the AUC12 values of patients who experienced rejection were much more evenly distributed around the mean; AUCs for 2 of the 4 patients with rejection were above the mean and 2 were below. Thus mycophenolate mofetil appears to have a pharmacokinetic-pharmacodynamic correlation, such that an increase in the plasma concentration of mycophenolic acid reduces the probability of the clinical outcome of acute rejection. The results of the logistic regression approach and that of the trial profiles were self-consistent, and imply that a low AUC12 of mycophenolic acid, caused by interindividual pharmacokinetics variation or noncompliance, is an important cause of a rejection event for the 1g twice daily dose.

Patients experiencing rejection Without rejection

4.0 3.8 3.6 3.4 3.2 3.0

Ln MPA AUC

2.8 2.6

4.0 3.8

10.4 Therapeutic Drug Monitoring

3.6 3.4 3.2 3.0 2.8 2.6 –3

–2

–1 0 1 Standard normal quantile

2

3

Fig. 9. (top) Standard normal quantile plot of mycophenolic acid (MPA) area under the concentration-time curve (AUC) for patients (n = 26) receiving mycophenolate mofetil 1g twice daily.[2] (bottom) Standard normal quantile plot of mycophenolic acid AUC for patients (n = 23) receiving mycophenolate mofetil 1.5g twice daily.[2]

rejected identified in the plots. The plots were linear for both the 1 and 1.5g data sets, indicating log-normal distribution of the AUC12 values. For the mycophenolate mofetil 1g twice daily group, AUCs in 5 of the 6 patients with rejection are below the mean, AUCs in 3 of the 6 patients with rejection are in the lowest quantile. Clearly in this subset, the patients who experienced rejection © Adis International Limited. All rights reserved.

The above results would suggest that the therapeutic drug monitoring (TDM) of plasma concentrations of mycophenolic acid might be advantageous. However, TDM is usually most advantageous for drugs with large interindividual pharmacokinetic variability, in which the range of doses to achieve a given effect is wide. Mycophenolate mofetil has low interindividual variability, and the choice of well-tolerated doses for mycophenolate mofetil in uncomplicated renal transplantation with concomitant cyclosporin and steroids is restricted to 2 to 3 g/day. Patients who had a low AUC12 of mycophenolic acid on 1g twice daily mycophenolate mofetil administration might benefit from increased dose, although there is reduced tolerability of doses above 2 g/day. The pharmacokinetic-pharmacodynamic correlations have been obtained with mycophenolic acid AUC; the trough (pre-administration) plasma concentration of mycophenolic acid is much easier to measure clinically but shows considerable intraindividual variability, presumably because of entrohepatic cycling. TDM for mycophenolic acid is presently of unproven clinical utility for the patients with uncomplicated renal transplants but merits further investigation.[28,29] In non-routine Clin Pharmacokinet 1998 Jun; 34 (6)

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circumstances, such as patients with short bowel syndrome or ileostomy, measurement of the plasma concentrations of mycophenolic acid may be of considerable value for patient management. IMPDH activity in plasma has been shown to vary phasically with mycophenolic acid concentration.[30] This activity has been suggested for use in TDM.

Bullingham et al.

pharmacokinetics-pharmacodynamic relationship makes the pharmacokinetics of mycophenolate mofetil directly relevant to the clinical outcome of transplantation. Areas where further investigation of mycophenolate mofetil is desirable include the mechanisms of the early nonstationary pharmacokinetics, the extent of renal metabolism of mycophenolic acid, and the effect of different types of liver disease.

11. Conclusions As an oral prodrug of mycophenolic acid, mycophenolate mofetil meets its intended role well. It undergoes rapid and essentially complete absorption, complete presystemic de-esterification to mycophenolic acid, and delivers mycophenolic acid with close to 100% systemic availability. Mycophenolic acid is itself well-behaved kinetically. It is almost completely metabolised to the pharmacologically inactive and stable phenolic glucuronide MPAG, which is excreted in urine and in amounts which eventually account for almost all of the administered drug. Entrohepatic cycling, involving biliary excretion of MPAG, is an important feature of the pharmacokinetics of mycophenolic acid. There is a relationship between the plasma pharmacokinetics (specifically the interdosage interval AUC12) and clinical effectiveness (suppression of acute rejection) in renal transplantation of mycophenolic acid, although no similar relationship has, so far, been observed for adverse events. From its conjugative metabolism and the clinical interaction studies described, the potential of mycophenolic acid to cause interactions with other drugs seems small. These properties of mycophenolate mofetil lead to a small intra- and interindividual variability for plasma mycophenolic acid and predictable pharmacokinetics changes in pathophysiological situations, except perhaps in the case of the liver with its multiple roles in mycophenolic acid metabolism and excretion. Mycophenolate mofetil offers some interesting opportunities for further understanding of the clinical pharmacology of entrohepatic cycling and drug glucuronidation. Moreover, the mycophenolic acid © Adis International Limited. All rights reserved.

Acknowledgements The statistical analysis and expertise of Dr Michael Hale is gratefully acknowledged.

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Correspondence: Dr R.E.S. Bullingham, 270 Ventura Avenue, Palo Alto, CA 94306, USA. E-mail: [email protected] Reprints: Dr A.J. Nicholls, Roche Global Development – Palo Alto, 3401 Hillview Avenue, Palo Alto, CA 94303, USA. E-mail: [email protected]

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