Review Article

Clinical Pharmacokinetics 11: 107-132 (1986) 0312-5963/86/0003-0107/$13.00/0 © ADIS Press Limited All rights reserved

Clinical Pharmacokinetics of Cyclosporin Richard J. Ptachcinski, Raman Venkataramanan and Gilbert J. Burckart Departments of Pharmacy Practice and Pharmaceutics, University of Pittsburgh, School of Pharmacy, Pittsburgh

Summary

Cyclosporin (cyclosporin A) is a unique immunosuppressant used to prevent the rejection of transplanted organs and to treat diseases of autoimmune origin. Therapeutic drug monitoring of cyclosporin is essential for several reasons: (a) wide variability in cyclosporin pharmacokinetics has been observed after the oral or intravenous administration of the drug. The variability in the kinetics of cyclosporin is related to a patient's disease state, the type of organ transplant, the age of the patient and therapy with other drugs that interact with cyclosporin; (b) maintaining a blood concentration of cyclosporin required to prevent rejection of the transplanted organ; (c) minimising drug toxicity by maintaining trough concentrations below that which toxicity is most likely to occur; and (d) monitoring for compliance since patient non-compliance with drug regimens is a significant reason for graft loss after 60 days. Clinical monitoring and pharmacokinetic studies ofcyclosporin can be performed using different biological fluids (plasma, serum or whole blood) and different analytical techniques (radioimmunoassay or high pressure liquid chromatography). The available analytical methods provide different results when using blood, plasma. or serum. Comparison of therapeutic ranges and pharmacokinetic parameters should be made with careful attention given to the method of cyclosporin analysis. Following oral administration, the absorption of cyclosporin is slow and incomplete. Peak concentrations in blood or plasma are reached in I to 8 hours after dosing. The bioavailability of cyclosporin ranges from less than 5% to 89% in transplant patients; poor absorption has frequently been observed in liver and kidney transplant patients and in bone marrow recipients. Factors that affect the oral absorption of cyclosporin include the elapsed time after surgery, the dose administered, gastrointestinal dysfunction, external bile drainage, liver disease, and food. Cyclosporin is widely distributed throughout the body. Following intravenous administration, the drug exhibits multicompartmental behaviour. The volume of distribution (whole blood; HPLC) ranges from 0.9 to 4.8 L/kg. Cyclosporin is high~v b~und to erythrocytes and plasma proteins and has a blood to plasma ratio of approximately 2. In plasma, approximately 80% of the drug is bound to lipoproteins. The distribution of cvclosporin in blood can be affected by a patient's haematocrit and lipoprotein profile." . Cyclosporin is extensively metabolised, primarily by mono- and dihydroxylation as well as N-demethylation, and is considered a low-to-intermediate clearance drug. The clearance of cyclosporin (whole blood; HPLC) ranges from 2.0 ml/min/kg in children with congestive heart failure to 11.8 ml/min/kg in paediatric kidney transplant patients. The

Pharmacokinetics of Cyc1osporin

108

terminal elimination half-life is highly variable and ranges from 6.3 hours in healthy volunteers to 20.4 hours in patients with severe liver disease (blood; HPLC). Factors af fecting the metabolism of cyc/osporin include liver disease, age, and concurrent drug therapy. The major route of elimination of cyc/osporin is via the bile, primarily as metabolites of the drug. Renal excretion is a minor elimination pathway. Renal failure and haemodialysis do not alter the pharmacokinetics of cyc/osporin. Several drugs are known to interact with cyc/osporin, including microsomal enzyme inducing and inhibiting agents. Several drugs including amphotericin B, aminoglycoside antibiotics and co-trimoxazole may potentiate the nephrotoxicity of cyclosporin. The dose of cyc/osporin used in a patient should be adjusted after conSidering factors such as the initial response to therapy, the patient's age, transplant type, disease state and concurrent drug therapy. Initial doses are usually in the range of 10 to 20 mg/kg/day orally or 2.5 to 5 mg/kg/day as an intravenous infusion, and should be adjusted based on the clinical status of the patient and cyclosporin blood concentrations. Long term oral maintenance doses of less than 3 mg/kg/day have resulted in adequate immunosuppression in some patients. The therapeutic range for cyclosporin is poorly defined and depends on the biological fluid being analysed, the analytical technique and the time after transplant. Cyc/osporin concentration monitoring should be used in conjunction with other assessment criteria such as serum biochemical parameters, radiological studies, biopsy results and the clinical status of the patient. Even though our understanding of cyc/osporin is incomplete, a thorough knowledge of different factors that affect its kinetics will aid the clinician in optimising immunosuppression with this promising new agent.

Cyclosporin (cyclosporin A) is a unique compound of fungal origin that has demonstrated potent immunosuppressive activity with a lack of myelotoxicity. Wide variability in response to cyclosporin is observed in organ transplant patients; this may be due to significant interpatient variability in its pharmacokinetics. Cyclosporin blood concentration monitoring is an essential part of the management of patients receiving cyclosporin. Cyclosporin monitoring must be performed with an understanding of the pharmacokinetics of the drug and with a knowledge of the clinical management problems of patients receiving this drug.

1. Analytical Methods The quantitation of cyclosporin in biological fluids is essential for the clinical management of transplant patients as well as for pharmacokinetic studies. Clinicians are presently faced with the choice of measuring cyclosporin in plasma or whole blood by either radioimmunoassay (RIA) [Don-

atsch et a1. 19811 or high pressure liquid chromatography (HPLC) [Niederberger et a1. 1980; Sawchuck & Cartier 19811. The analysis of different biological specimens by either of the available methods provides different results. Knowledge of the analytical methodology used for clinical monitoring and pharmacokinetic studies is essential for a meaningful interpretation of the results. A comparison of the available methods for the analysis of cyclosporin is presented in table I. 1.1 Biological Fluids Commonly Analysed

Cyclosporin monitoring and pharmacokinetic studies have been performed using whole blood, serum and plasma. The concentrations of cyclosporin in blood are approximately twice those measured in plasma which reflects the extensive distribution of cyclosporin into erythrocytes (lemaire & Tillment 1982; Niederberger et al. 1983). The relative distribution of cyclosporin between blood cells and plasma depends on several factors

Pharmacokinetics of Cyclosporin

109

including the temperature of separation of plasma from erythrocytes, the patient's haematocrit, the drug concentration and the lipoprotein content and composition in patient plasma (see section 2.2). Plasma samples separated at room temperature have up to 50% lower cyclosporin concentration than samples separated at 37°C (Diepernick 1983; Smith 1983 & Wenk 1983). The temperature dependent concentration of cyclosporin in plasma is due to altered red blood cell and lipoprotein uptake of cyclosporin at different temperatures. The optimal temperature for separation of plasma from blood for obtaining physiologically meaningful pharmacokinetic parameter estimates is 37°C in order to simulate in vivo cyclosporin distribution in blood. Since the temperature dependent cyclosporin distribution is reversible, samples stored at < 37"C can be re-equilibrated at 37°C for plasma separation. This approach can be used for pharmacokinetic studies but may be impractical for routine clinical monitoring due to the lack of availability of temperature controlled centrifuges in many clinical laboratories. When using plasma for

drug monitoring one consistent temperature must be used to separate plasma in order to minimise any errors due to temperature dependent alterations in cyclosporin distribution in blood. Since the concentration of cyclosporin in plasma is lower than that in the blood, there tends to be a greater variability in cyclosporin estimations in plasma as compared with whole blood (Johnston 1986). Whole blood cyclosporin concentrations appear to be more reproducible than plasma concentrations (Wenk 1983). Heparinised blood can be stored for up to 10 days at room temperature without any change in cyclosporin concentration. The red blood cells can be haemolysed by freeze-thaw, sonication, or by solvent extraction so that the total cyclosporin blood concentration is measured. 1.2 Radioimmunoassay (RIA) RIA is readily available to most clinicallaboratories and may be used to ob.tain an estimate of cyclosporin concentrations in blood or plasma. The RIA kit can be obtained from the manufacturer of

Table I. Comparison of HPLC and RIA analysis for cyclosporin Parameter

HPLC

RIA

Biological fluids analysed

Blood, plasma, serum, urine, bile, breast milk, cerebrospinal fluid and different tissues

Blood, plasma, serum, bile, urine, tissues, breast milk, cerebrospinal fluid

Methodology

Requires tedious extraction procedure

Fairly straight-forward method

Specificity

Highly specific for unchanged drug

Nonspecific, metabolites cross-react with antiserum to varying degrees

Sensitivity

25 ",gIL (lml sample)

20 ",gIL (5",1 sample)

Linearity

25-4000 ",gIL

130-600 ",gIL (100-2500 ",gIL by log-Iogit plot)

Applicability to other structural analogues

Easily adaptable to other cyclosporin analogues

Varying degrees of cross-reactivity with cyclosporin analogues

Availability of required instruments

Requires equipment not commonly used in clinical laboratories

Equipment commonly available in most clinical laboratories

Other considerations

Abbreviations: HPLC

Kit stability is limited. Of questionable use in presence of liver impairment

= high pressure liquid chromatography; RIA = radioimmunassay.

Table II. HPLC assays to quantitate cyclosporin in biological fluids

"tI ::r

II>

References

Allwood & Lawrence (1981)

Sample

Serum

Preparation

Column/ conditions

Mode of analysis

1 ml serum + 3 ml methanol; adsorb onto C18 Sep-pak. elute with 1.5ml methanol

",Bondapak C·18 270 x 4mm. 5",m/ 55°C

1ml plasma; extract acidified sample with diethyl ether. wash ether with base. dry ether. inject 100",1

Ultrasphereoctyl 250 x 4.6mm 5",m 72°C

Isocratic

Isocratic

Mobile phase

Methanol/water (95/5)

Internal standard

External

Detection wavelength (nm) 205

Minimum detection limit (ng) 100

Chromatography time (min) 5

3

II>

8

~

::s

g. '"0

....

(j

'< n

0-

Carruthers et al. (1983)

Plasma

Kahan et al. (1982)

Blood Plasma

1ml blood extracted with diethyl ether; dry. inject 15",1

",Bondapak C-18 300 x 3mm 10",m 25°C

Leyland-Jones et al. (1982)

Plasma

0.6ml plasma adsorbed onto Clin-elute; elute with ethyl acetate. dry. inject 100",1

Niederberger et al. (1980)

Plasma Urine

Nussbaumer et al. (1982)

Sawchuck & Cartier (1981)

Acetonitrile/water/ CyD methanol (47/20/ 33)

210

Gradient

Isopropranolol/ CyD heptane (14/86) to (36/64) in 30 min

220

100

45

Zorbax TMS 250 x 4mm 5",m 55°C

Isocratic

Acetonitrile/water (60/40)

CyD

210

100

20

1ml plasma or urine; extract with diethyl ether. dry. inject 100",1

Lichrosorb RP-8 125 x 3mm 5",m 72°C

Gradient

Acetonitrile/water/ CyD methanol (20/60/ 20) to (75/5/20) in 26 min

210

25

30

Plasma Blood

0.5ml blood or plasma + 1.5ml methanol (65%). precipitate protein. inject lml

Lichrosorb C-18 150 x 4.6mm 5",m 70°C

Column switching

6 Solvent switching scheme with step gradient

CyD

210

20

30

Plasma Blood

2ml blood or plasma; extract with diethyl ether. dry acidify. wash with hexane. alkaline extract into diethyl ether. dry. inject 90",1

Supelcosil LC-18 150 x 4.6mm 5",m 75°C

Isocratic

Acetonitrile/water (68.5/31.5)

CyD

202

25

10

31

30

'" '8::I. ::s

o

'tI

Yee et al. (1982)

Serum

2ml serum absorbed onto 3ml Baker Cyano extraction column, elute with 1ml methanol/water (40/60), dry reconstitute with mobile phase inject 100,,1

Ultrasphere ODS 250 x 4.6mm

Gradient

5"m 70°C

Acetonitrile/water (35/65) to (5/15) at 15 min

CyD

215

50

16

::r II> .... 3II>

n

0

er.

::s

.-.a. n 0

()

Smith & Robinson (1984)

Lensmeyer & Fields (1985)

Plasma Blood

Plasma Serum Blood

0.5ml plasma or blood + 1.2ml acetonitrile/water (97.5/2.5) inject supernatant onto automated sample wash system

Supelcosil LC-8 150 x 4.6mm

Column switching

5"m 75°C

Acetonitrile/water (55/45) switched to (75/25)

202

8 (plasma)15 20 (blood)

Isocratic

Acetonitrile/water (51/49)

CyD

214

CyD

215

10-15

10

5-6"m 58°C

Kates & Latini (1984)

Serum Blood

1ml serum or blood, extracted with diethyl ether and methanol (75%) applied to Baker-10 SPE (Disposable Cyano Extraction column), elute, dissolve residue with mobile phase, inject 165,,1

Brown Lee RP-8 MLPC, 100 x 4.6mm 10"m 70°C

Isocratic

Acetonitrile/water (72/28)

Gfeller et al. (1980)

Plasma

React with derivatizing agent after extraction

Lichrosorb RP-8 250 x 4.6mm

Isocratic

Methanol/water (83/17)

10"m 73°C

.S-

'0 0

::s

5"m 75°C Zorbax Cyanopropyl 250 x 4.6mm

'<

::I.

Supelcosil LC-18 150 x 4.6mm

1ml plasma, serum or blood + 2ml acetonitrile/ water (70/30), applied to disposable solid phase cyanopropyl column

Abbreviation: CyD = cyclosporin D.

External

220 with 2naphthylselenyl chloride derivative

<50

2

20

Pharmacokinetics of Cyclosporin

cyc1osporin. RIA provides a rapid and sensitive means for estimating cyc1osporin concentrations in biological fluids based on the principle of competitive binding. The kit uses tritium-labelled drug and therefore suffers from the problems of low counting efficiency and limited kit stability. The assay is linear in the normal range of trough drug concentrations observed in blood (100 to 2500 J.lgfL by log-logit plot). RIA requires only a limited amount of blood or plasma so the test can be performed after capillary sampling in children. RIA suffers from two major problems. Firstly, concentrations above the linear range of the standard curve are frequently observed in patients with liver impairment or in the course of a pharmacokinetic study. Under these conditions, samples must be diluted with blank blood or plasma prior to analysis. Secondly, the antibody in the RIA analysis cross-reacts with the metabolites of cyc1osporin and therefore is unable to differentiate between the parent compound and its metabolites (Donatsch et al. 1981). A quality control programme for RIA analysis of cyc1osporin has revealed wide between centre variability and overestimates of sensitivity with a significant number of false positive results (J ohnston 1986). In spite of these deficiencies, RIA remains an acceptable means to monitor concentrations of cyc1osporin in select patients (Kahan et al. 1982; Keown et al. 1983). 1.3 High-Pressure Liquid Chromatography (HPLC) Several HPLC procedures are available to measure cyc1osporin in biological fluids and are summarised in table II. HPLC procedures require extensive sample extraction or a column-selective isolation technique. The latter procedure requires specialised instrumentation not commonly available to all laboratories. The extraction of cyc1osporin from biological fluids involves solid-liquid or liquid-liquid extraction methods. In the solidliquid method, cyc1osporin is separated from endogenous substances using columns such as Baker cyano columns (Kates 1984; Vee et al. 1984). In the liquid-liquid extraction method, cyc1osporin is

112

extracted with ether. In some procedures further sample purification is carried out using hexane (Sawchuck & Cartier 1981). The isolated cyc1osporin can be chromatographed using a C-18 or cyano column maintained at a temperature of 55 to 75°C. The mobile phase generally consists of various combinations of acetonitrile, methanol and water. Cyc1osporin and cyc1osporin D (internal standard) usually elute within 12 minutes. After liquid-liquid extraction procedures, late peaks may occasionally lengthen the chromatography time to as long as 25 to 30 minutes. HPLC is specific for unchanged cyc1osporin and is sensitive to concentrations as low as 10 J.lgfL using 1ml of blood. HPLC is linear over the range of 25 to 4000 J.lgfL and is useful for the analysis of blood, bile, plasma, serum, urine, cerebrospinal fluid (CSF) and breast milk. The HPLC methodology is very easily adapted to specifically measure metabolites of cyc1osporin (Freeman 1984), and other immunosuppressive compounds of the cyc1osporin family, and we now use an HPLC method for the determination of cyc1osporin G using cyc1osporin as the internal standard (Burckart et al. 1985b). 1.4 RIA: HPLC Ratio Concentrations obtained by RIA analysis are consistently higher than those obtained by HPLC due to the cross-reactivity of cyc1osporin metabolites with the antiserum used in the RIA kits (Abisch et al. 1982; Donatsch et al. 1981). The percentage cross-reactivity of some of the metabolites tested ranges from 4 to 32%. The RIA : HPLC ratio of cyc1osporin concentrations in blood, plasma and serum is highly variable (Abisch et al. 1982; Carruthers et al. 1983; Vee et al. 1984) and depends on the patient's liver function, the time of blood sampling in reference to the time of drug administration, the absolute cyc1osporin concentration and whether the patient is receiving other drugs which may alter hepatic metabolising enzyme function. RIA : HPLC ratios of I : 1 to 15 : 1 in blood have been observed in some patients following transplantation. The metabolites of cyc1osporin

Pharmacokinetics of Cyciosporin

113

do not possess any significant pharmacological activity in animals and have not been tested in humans (Maurer 1985). Assay results from the RIA should be used with particular caution during periods of impaired hepatic function. In the first 2 weeks following liver transplantation, during liver rejection or in the event of a technical failure (i.e. hepatic artery thrombosis), the RIA: HPLC ratio is dramatically increased (Burckart et al. 1985a); we have observed ratios of 15 : I in liver transplant patients during periods of hepatic dysfunction, indicating that RIA results may be occasionally misleading. The accumulation of cyclosporin metabolites that crossreact with the RIA may be responsible for this discrepancy and since the metabolites do not have any significant immunosuppressive activity (Maurer 1985) they have little effect on the drug's therapeutic effect. The toxicity of the metabolites of cyclosporin has not been determined. Cyclosporin blood concentration versus time data in a patient with liver disease as analysed by RIA and HPLC are presented in figure I. The RIA: HPLC ratio changes over the dosing interval studied (from I : I to 4 : I) and the pharmacoki-

2000 e

f

::J'

j

i ~

.8.c:

1000

~ ·00

500

~

••

200

o

•

0

0

0 0

0

•

100

• • •

•

'"0

~ ()

~

tXI

0

14 21 Time (hours)

7

26

35

42

49

56

Fig. 1. Whole blood cyclosporin concentrations in a patient with liver disease as measured by RIA (0) and HPLC (e) following a dose of 2.0 mg/kg administered as an intravenous infusion over 2 hours (Venkataramanan 1965d).

netic parameters calculated are different (Yee et al. 1984, unpublished observation) depending on the analytical procedure used. Since HPLC measures only the unchanged drug it is the preferred method of analysis of cyclosporin for pharmacokinetic studies. In conclusion, any combination of biological fluids (blood, serum, plasma) or assay techniques (RIA, HPLC) may be used for clinical monitoring of cyclosporin concentrations following transplantation. Centres routinely using RIA analysis must be able to provide HPLC analysis when indicated. HPLC analysis of cyclosporin should be available for patients following liver transplantation, patients with liver disease and patients receiving other drugs which may induce or inhibit microsomal enzyme functions (see section 2.3.1).

2. Pharmacokinetic Properties 2.1 Absorption 2.1.1 Oral Administration Following oral administration, the absorption of cyclosporin is slow and incomplete (Newberger & Kahan 1983; Ptachcinski et al. 1985d). A summary of reported cyclosporin pharmacokinetic parameters following oral administration is presented in table III. Peak concentrations in blood or plasma are observed 1 to 8 hours aftter oral dosing and the absorption half-life ranges from 0.6 to 2.3 hours (Beveridge et al. 1981; Newberger & Kahan 1983). Considerable variation has been observed in the peak concentrations of cyclosporin in blood or plasma in recipients of kidneys, hearts and allogenic bone marrow grafts following oral therapy (Beveridge 1982; Keown et al. 1982; Ptachcinski et al. 1985d). Following the ingestion of cyclosporin 600mg the peak serum (HPLC) concentration varied from 240 to 1250 p,gfL in 6 medical patients (Beveridge et al. 1981). In renal transplant patients receiving 17.5 mgfkg once a day, the peak (RIA) concentrations in serum ranged from 1800 to 3300 p,g/L (Keown et al. 1981). Renal transplant patients receiving 17.5 mgfkg with breakfast had peak whole blood (HPLC) concentrations of 862 to 3431 p,gfL (Ptachcinski et al. 1985e).

Table III. Reported pharmacokinetic parameters [arithmetic mean (± SO) or harmonic mean (range)] for cyclosporin following oral administration References

Patient population Number Biolog- Dose Study Assay C..... of ical fluid (mg/kg) dosing ("gIL) interval patients (h)

t....,.

C_

k.

(h)

("gIL)

(h- 1)

t,.. (h)

"(h-1)

t"", (h)

"0

::r

i3

t~

F

(h- 1)

(h)

(%)

3'" '" (")

0

~

::s

"=-. '" ..., (")

Medical

Robson et al. (1984)

Primary biliary cirrhosis

10

Robson et al. (1984)

Primary biliary cirrhosis

10

Blood

5.0

24

RIA

Venkataramanan et al. (1985a)

Liver disease cirrhosis

9

Blood

10.0

48

HPLC

435

Beveridge et al. (1981)

Bone marrow transplant

8

Serum

12.5

48

HPLC

480 (±435)

Lokeic et al. (1983)

Bone marrow transplant

38

Plasma

15

24

RIA

Atkinson et al. (1983b)

Bone marrow transplant, no GI dysfunction

11

Serum

12.5

12

RIA

910 (±503)

3.0

380 (±327)

Atkinson et al. (1983b)

Bone marrow transplant with chemoradiation enteritis

22

Serum

12.5

12

RIA

354 (±313)

4.0

181 (±167)

Atkinson et al. (1983b)

Bone marrow transplant with acute GVHD of intestine

6

Serum

12.5

12

RIA

153 (±59)

6.0

93 (±61)

Ptachcinski et al. (1985d)

Adult transplant

Blood

14

24

HPLC

1103 (±570)

4.0 (±1.8)

27.6 0.069 10.0 (± 0.043) (3.8-49.6)( ± 21.2)

Kahan et al. (1985)

Renal transplant < 170 14 days after surgery

Serum

12-14

24

RIA

1004 (±737)

4.3 (±3.2)

9.4 (±5.6)

Ptachcinski et al. (1985a)

Paediatric renal transplant

7

Blood

13.3 (±6.9)

12-24

HPLC

6

Serum

Blood

10.oa

5.0

48

24

HPLC

0.698 0.591 1.0 (±0.261) (0.6-1.6) (±0.14)

Beveridge et al. (1981)

HPLC

1.2 0.028 (0.9-1.6) (±0.01)

24.8 (17.640.1)

538 (±359)

3.3 (±0.5)

850 (±500)

3.1 (±6.2)

(±57)

12 (±5.4)

1243 (±780)

3.5 (±7.0)

227 (±183)

13.3 (±6.3)

77

0

(j '<

!2.

0

'"0

'0

::J.

::s

11.9

4.7

5.0 (±2.1)

185()b

1.6 (±0.9)

(±1270)

41

4.0

11.3 (±6.8)

24.2 (± 18.1)

30.8 (±10.2)

~

Pharmacokinetics of Cyclosporin

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5 2E

~

Bioavailability Marked variability in the extent of cyclosporin absorption has been observed in patients after organ transplantation. The absolute bioavailability of cyclosporin in 5 bone marrow transplant patients varied from 20 to 50%, with a mean of 34% (Wood et at al. 1983). In adult kidney transplant patients the absolute bioavailability ranged from < 5% to 89% with a mean of 27.6% (Ptachcinski et al. 1985d). The cyclosporin blood concentration versus time profile of a kidney transplant patient during a bioavailability study is presented in figure 2. The bioavailability of cyclosporin in children following renal transplantation is 31 % (Ptachcinski et al. 1985a) and following cardiac transplantation in adults is 35% (Venkataramanan et al. 1985b). Adult liver transplant recipients have a mean bioavailability of 27% with a range of 8 to 60% (Burckart 1986). Paediatric liver transplant patients absorb < 5 to 19% of an orally administered dose of cyclosporin in the immediate postoperative period (Burckart et al. 1985a). Frequent incomplete absorption of cyclosporin « 110% 0% bioavailability) is observed in paediatric liver (Burckart et al. 1985a), adult kidney (Ptachcinski et al. 1985d) and bone marrow transplant patients (Atkinson et al. 1983, 1984) as well as in patients with liver disease (Venkataramanan et al. 1985a).

II .S

8

~ J~ £

2.1.2 Effect of Food on Absorption Coadministration of cyclosporin with food may alter cyclosporin bioavailability. A preliminary report (Keown et al. 1982) suggested that food delayed and impaired the absorption of cyclosporin, but a later report by the same group stated that food had no consistent effect (Keown et aL al. 1983) 1983).. We recently studied the effect of food on the absorption of cyclosporin in renal transplant patients (Ptachcinski et al. 1985e). A significant increase in the Cmax CmiR, max,, C min , and AVC following cyclosporin administration with food was observed compared with the fasted state (table IV). The effect of food on the absorption of cyclosporin may be related to the nature of the diet and the time of drug administration in relation to food intake, and the fre-

Pharmacokinetics of Cyc1osporin

Time (hours)

Fig.2. Cyclosporin blood concentration (HPLC) in a renal transplant patient following 3.2 mg/kg intravenously (0) and 17.3 mgt kg orally (0). Cyclosporin clearance = 5.3 ml/min/kg; Vd" = 3.7 L/kg; tv, = 15.2 hours; F = 17.9% (Ptachcinski 1985d).

quency of blood sampling during the study. Additional studies are required to confIrm this increase in the absorption of cyclosporin with food following other transplant procedures (i.e. liver transplantation, where no gallbladder is present) and to determine the effect of dietary composition on the absorption of cyclosporin. 2.1.3 Intramuscular Administration Cyclosporin is very poorly absorbed following intramuscular administration (Beveridge et al. 1981; Keown et al. 1981). Studies by Beveridge (1981) [n = 9] and Keown et al. (1981) [n = 6] were conducted to investigate the absorption of cyclosporin dissolved in miglyol (100 g/L) following administration by the intramuscular route. Even after the administration of 20 mg/kg, the maximum concentration of cyclosporin in plasma (HPLC) was less than 100 ~g/L in most patients; large interpatient variation in plasma cyclosporin concentrations was also noted (Beveridge 1981). Hence, this route of administration does not appear to be useful for treating patients with cyclosporin.

116

2.1.4 Time- and Dose-Dependent Changes in Absorption Time-dependent improvement in cyclosporin absorption has been observed in liver transplant recipients (Burckart et al. 1985a). Time-dependent improvement in bile flow with the stabilisation of liver function in these patients may be responsible for such observations. In kidney transplant patients, 3- to 5-fold increases in oral bioavailability over time have also been noted (Kahan et al. 1983a). In one study, the mean bioavailability was 24.2 (± 18.1)% during the fIrst 2 weeks after kidney transplantation and increased to 50.2 (± 7.9)% at 6 to 12 months after transplantation (Kahan et al. 1985). The exact nature of the underlying mechanism for this improvement is not known. The pharmacokinetics of cyclosporin appears to be dependent on the dose administered in some patients. We compared the average cyclosporin concentration (AUC/r) following the administration of 17, 8 and 6 mg/kg in 8 renal transplant patients (Ptachcinski et al. 1985c). No difference in the average cyclosporin concentration (CSS) was observed between the 3 doses in half of the patients studied. The mean CSs in these patients was 596 (± 205), 551 (± 58) and 551 (± 286) ng/ml for cyclosporin administered once, twice or three times daily, respectively. However, signifIcantly higher average cyclosporin concentrations were observed with lower doses in the remaining patients with respective mean CSs of 497 (± 268), 800 (± 274) and 1241 (± 354) ~g/L after dosing of cyclosporin once, twice, or three times daily. Dose-dependent absorption and/or distribution of cyclosporin may contribute to the observed variability in the average cyclosporin concentration. Time dependent improvement in the absorption of cyclosporin has been observed with chronic dosing of the oral dosage form of the drug. Dose-dependent kinetics may partially explain these observations if a greater percentage of the lower maintenance dose is absorbed compared with the higher doses used early in therapy. The actual mechanism(s} responsible for such an observation have not been completely characterised.

Pharmacokinetics of Cyclosporin

117

Table IV. Cyclosporin pharmacokinetic parameters (arithmetic mean ± SO) in 18 renal transplant patients following oral administration with or without food a Parameter

Food

Fasting

Statistical significance

Cma• ("gIL)

1465 ± 565

1120 ± 548

p

< 0.01

Cm," ("gIL)

267 ± 131

228 ± 121

p

< 0.05

tma• (h)

4.05 ± 1.49

3.81 ± 1.58

11430 ± 5883

7881 ± 4508

AUC o.... ("gIL· h) t'l,. (h)

11.4 ± 8.1

11.5 ± 8.8

p> 0.05 p

< 0.001

p> 0.10

a

Based on whole blood concentrations as measured by HPLC following a dose of 15 mg/kg (reprinted with permission from Ptachcinski et al. 1985e). Abbreviations: see table III.

2.1.5 Effect of Liver Disease on Absorption The absorption of fat and fat-soluble material is impaired in patients with liver disease. Because cyclosporin is fat soluble, its bioavailability is very low (12%) in patients with severe liver disease (Venkataramanan et al. 1985a) compared with other patient populations (,., 30%). In the study by Venkataramanan et al. (1985) patients with a total serum bilirubin of < 10 mg/l00ml (n = 5) absorbed more than 5% of the administered drug, while the patients with a total serum bilirubin of > 10 mg/l00ml (n = 4) absorbed less than 5%. Surgically induced cholestasis in dogs (n = 7) resulted in a decrease in cyclosporin bioavailability from 23.5 (± 9.7)% before surgery to 7.4 (± 3.7)% one week after surgery (Zagloul, unpublished observation). Bile and bile salts are essential for the absorption of lipids and lipid-soluble compounds. Therefore, the poor bioavailability of cyclosporin in patients with liver disease and in liver transplant patients during the immediate postoperative period may be related to the lack of sufficient bile and bile salts necessary for cyclosporin absorption.

2.1.6 Absorption in Patients with External Biliary Drainage The absorption of cyclosporin is slow and erratic in liver transplant patients with external bile drainage via aT-tube. There is also a significant

increase in the trough blood concentration following T-tube clamping (Andrews et al. 1985). Following clamping or removal of the T-tube, the rate and extent of absorption appears to increase as indicated by faster and higher peak blood concentrations (fig. 3). Comparative bioavailability studies in the same liver transplant patients with and without external bile drainage indicate a mean increase of 487% in the dose-normalised AUC following Ttube clamping (unpublished observation). This observation cannot be totally attributed to enterohepatic recycling of cyclosporin since < 1% of a dose of cyclosporin is excreted in the bile as unchanged drug over 1 dosing interval (Venkataramanan et al. 1985c) [see section 2.4]. Increased bile flow into the gut following T-tube clamping is most likely responsible for the improved cyclosporin absorption. Whenever external bile diversion is instituted or discontinued, adjustments in cyclosporin dosage must be made.

2.1.7 Effect of Gastrointestinal Disease on Absorption Orally administered cyclosporin is poorly absorbed in the presence of intestinal disease (Atkinson et al. 1983b, 1984). In bone marrow transplant patients, intestinal dysfunction may be mediated by chemoradiation enteritis secondary to the conditioning regimen for marrow transplantation, acute

Pharmacokinetics of Cyclosporin

118

2000

)1000 ~

1

I . . . ,- .-. . 500

"e- __ ---e\

I

I

'
100 L..----::-----!"_--=-_-:-_-: ..:---_---:,--e 2 4 6 Time (hours)

8

10

12

Fig. 3. Cyclosporin blood concentrations (HPLC) after oral administration in a liver transplant patient prior to (e) [dose = 8oomg) and following (0) [dose = 900mg) clamping of the Ttube (Venkataramanan 1985d).

graft-versus-host disease of the intestine, or candida enteritis. Patients with greater than 500ml of diarrhoea per 72 hours have a significantly lower area under the cyclosporin serum concentration versus time curve indicating impaired drug absorption (Atkinson et a1. 1984). Impaired cyclosporin absorption has also been noted in paediatric liver transplant patients with diarrhoea (Burckart et al. 1985a). In these patients, intravenous cyclosporin should be administered to provide adequate immunosuppression. 2.2 Distribution Following intravenous administration, cyclosporin exhibits multicompartmental behaviour (Follath et a1. 1983; Vee et a1. 1984). The initial rapid distribution half-life is 0.10 hours and the second slower distribution half-life is 1.1 hours (Follath et a1. 1983). Such behaviour is attributed to the high lipid solubility of cyclosporin and its ability to easily diffuse through biological membranes.

Distribution in Blood In blood, cyclosporin is highly bound to erythrocytes and plasma proteins. The relative distribution of cyclosporin in blood is a function of drug concentration, haematocrit, temperature and lipoprotein concentration. At a blood concentration of 500 J.l,g/L, 58% of the drug is associated with erythrocytes, 4% with granulocytes, 5% with lymphocytes and the remaining 33% is distributed within the plasma (leMaire & Tillement 1982). In normal subjects, the blood to plasma ratio (B : P) is approximately 2.0, indicating a greater affinity of cyclosporin for red blood cells than for plasma proteins. In liver and renal transplant patients the B : Pis 1.32 and 1.36 respectively. The lower ratio may be the result oflower haematocrits in these patients. The plasma concentration of cyclosporin increases linearly with whole blood concentrations up to 1000 J.l,g/L. Above this concentration the distribution of cyclosporin between blood and plasma becomes non-linear. Blood cells appear to be saturated by cyclosporin at concentrations above 5 mg/L (Niederberger et a1. 1983). Temperature is another factor that will affect the distribution of cyclosporine between red blood cells and plasma. With a temperature decrease from 37·e to 21·e about 50% of cyclosporin diffuses from plasma to red blood cells where it binds to haemoglobin; this process is reversible upon re-equilibration at 37"e for 2 hours (Niederberger et a1. 1983; Smith et a1. 1983). The concentration-dependent red blood cell uptake of cyclosporin is more pronounced at lower temperatures than at higher temperatures due to greater partitioning of the drug into erythrocytes. In plasma, cyclosporin is primarily bound to lipoproteins (about 80%) while very little is bound to other plasma proteins such as albumin and globulin (leMaire & Tillement 1982; Niederberger et a1. 1983). The binding of cyclosporin to plasma proteins is independent of concentration between 0.02 and 20.0 mg/L (Niederberger 1983). However, the binding is markedly influenced by temperature; about 70% of the drug is bound at 4·e, 93% at 20·e and 98% at 37·e (Mraz et al. 1983; Niederberger

Pharmacokinetics of Cyc1osporin

et al. 1983). This temperature-dependent plasma protein binding alone does not explain the influence of temperature on the distribution of cyclosporin in blood (Niederberger et al. 1983). Of the different lipoproteins in plasma, the highdensity lipoprotein (HDL) binds 57% of the drug compared with 25% by low-density lipoproteins (LDL) and approximately 2% by the very-low-density lipoproteins (VLDL). The amount of cyclosporin bound to chylomicrons is negligible (Niederberger 1983). The plasma protein binding of cyclosporin appears to be highly variable among different patients and among different animal species. The unbound fraction of cyclosporin in plasma from transplant patients at 37"C as measured by ultracentrifugation methods range from 0.05-0.17 (Venkataramanan, unpublished observation). The unbound fraction of cyclosporin in dogs, humans, rats, and rabbits are 0.07, 0.15, 0.19, and 0.33, respectively (Zagloul, unpublished observation). These values appear to be related to the quantity oflipoproteins in the different species tested. Tissue Distribution In accordance with its lipophilic nature, body fat contains a high concentration of cyclosporin in humans. Other tissues such as the liver, pancreas, lungs, kidneys, adrenal glands, spleen and lymph nodes contain higher cyclosporin concentrations than serum (Atkinson et al. 1983a; Kahan et al. 1983b; Niederberger & Wiscott 1982; Ried et al. 1983). The effect of various disease states on the tissue distribution of cyclosporin requires further investigation. The drug remains in body tissues for a considerable duration after the discontinuation of therapy. Low levels of cyclosporin are found in brain tissues (Niederberger 1983). Cyclosporin was not detectable by HPLC in the CSF of four transplant patients receiving the drug (Ptachcinski, unpublished observation) which confirms the results of a study conducted in mice (Fazakorley 1985). These data suggest that cyclosporin does not cross the blood-brain barrier. This observation is inconsistent with clinical evidence of CNS toxicity (i.e. seizures) in patients receiving cyclosporin therapy

119

(Beaman et al. 1985; Boogaerts et al. 1982; Shah et al. 1984). Volume of Distribution In 1 study the volume of distribution of cyclosporin (HPLC; whole blood) in kidney transplant patients was 4.5 (± 3.6) L/kg (Ptachcinski et al. 1985d). Kahan et al. (1985) reported a volume of distribution of 8.6 (± 5.9) L/kg in another group of kidney transplant patients (RIA, serum). The results offurther studies show that patients with liver disease have a volume of distribution of 3.9 (± 1.8) L/kg (HPLC, whole blood) whereas normal volunteers, children with heart failure and patients following heart transplantation reportedly have a smaller volume of distribution of 1.2, 0.9 and 1.3 L/kg (HPLC, whole blood), respectively (Venkataramanan et al. 1985b; unpublished observation). The smaller volume of distribution may be due to a higher haematocrit in normals and heart transplant recipients compared with other transplant recipients. Yee et al. (1984) reported a volume of distribution of 3.1 to 4.3 L/kg in bone marrow transplant patients based on HPLC-serum data. Transplacental Transfer and Excretion in Milk During pregnancy, cyclosporin crosses the placenta and is found in the amniotic fluid (Flechner et al. 1985). Cyclosporin is detectable in a newborn's peripheral blood up to 48 hours after birth and is also detectable in breast milk of mothers receiving the drug. Although the concentration of cyclosporin in milk is low, breastfeeding is not recommended for mothers receiving this drug (Flechner et al. 1985).

2.2.1 Factors Affecting Distribution Lipoprotein profiles are significantly altered in patients with renal or hepatic impairment and furthermore the functional status of a transplanted kidney or liver may influence the concentrations of the various lipoproteins in these patients. Moreover, hyperlipidaemia has been observed in patients following renal transplantation and one would therefore anticipate changes in unbound cyclo-

Pharmacokinetics of Cyclosporin

120

drug in the blood is associated with erythrocytes at the lower temperature (Niederberger et al. 1983).

sporin associated with changes in the lipoprotein profile. However, changes in the unbound cyclosporin concentration are probably minimal due to its large volume of distribution.

2.3 Metabolism Haematocrit The distribution of cyclosporin between red blood cells and plasma is dependent upon a patient's haematocrit (Niederberger 1983). Transplant recipients often have low haematocrit levels due to chronic disease or intraoperative blood loss, resulting in altered drug distribution in these patients. In blood with a low haematocrit, a greater portion of the drug resides in plasma. The large variation in the haematocrit of patients results in a marked variability in the blood/plasma ratio of cyclosporin. The effect of changes in haematocrit on the blood/plasma ratio of cyclosporin is more marked when plasma is separated at 6°C compared with separation at 37°C since a large portion of the

Hydroxylation

Cyclosporin undergoes extensive hepatic metabolism in humans and animals (Maurer et al. 1984); metabolites and unchanged drug are excreted into bile (Wood et al. 1983). The biotransformation pathway of cyclosporin is similar in all the animal species studied (Beveridge 1982). Of the 17 suspected metabolites of cyclosporin, 9 have been isolated and identified (Maurer et al. 1984). All the identified metabolites have the intact cyclic oligopeptide structure of the parent drug. Structural modifications during metabolism include mono- and dihydroxylation as well as N-demethylation (fig. 4). Based on blood clearance estimates obtained by

--+

CH3

H , 1

~

CH3

C I' H'\ CH2 ~)I

CH3

~

,I CH

I

~H2

/

Non-enzymatic intramolecular formation of tetrahydrofuran derivative

CH

~3

CH 3,,.3

~

~

~ ~

~

I

~3

tH

I2

CH 3

I

CH3 - N - CH - CO - N - - CH - C - N - - CH - CO - N - CH - C - N - CH2 ILL II L I L II I CH oc 10 II 0 1 H Z 0 3 CO I

3

I

•

••

H - C - CH2 - CHL9 /f I I Hydroxylation CH 3 CH 3 - N 8 7 H 6 10 L I L oc - CH - - N - CO - CH - N - C - CH - - N I I I II I I CH3 CH3 ~ ~H2 CH3 H .. ................. C ,;

CH 3

I"CH 3

!H

Hydroxylation

Fig. 4. Structure of cyclosporin with locations (Sites) of metabolism.

I

N - CH

I

5 H 4 ilL I L C - CH - N - CO - CH I I .... CH, C CH 3 CH3

0

3.... Demethylation

?1.

,;

I"CH3

r

CH 3

Hydroxylation

Pharmacokinetics of Cyclosporin

HPLC, cyclosporin can be classified as a low to intermediate clearance drug. Both its clearance and elimination half-life are highly variable among patients and seem to be influenced by the type of transplant, age, disease state and concurrent drug therapy. A summary of reported and unreported pharmacokinetic parameters following the intravenous administration of cyclosporin is presented in table V. The half-life of cyclosporin as measured by RIA appears to be slightly longer than the half-life as measured by HPLC. In 5 healthy volunteers mean half-life of cyclosporin based on RIA was 8.4 hours compared with 6.3 hours (HPLC) for the parent drug (unpublished observation). In another study the harmonic mean half-life of cyclosporin based on RIA was 16.1 hours versus 12.2 hours (HPLC) for the parent drug in patients with renal failure (Follath et a1. 1983). Similar observations were made in 8 patients with severe liver disease where the mean half-life of cyclosporin based on RIA was 23.9 hours compared witth 20.4 hours (HPLC) for the parent drug (unpublished observation).

121

bodyweight basis. The harmonic mean clearance of cyclosporin in a study involving 26 paediatric liver transplant patients was 8.4 mljmin/kg (range 1.9 to 13.9 mljmin/kg) which is higher than the clearance of cyclosporin in adult liver transplant patients (Burckart et a1. 1985a). Paediatric kidney transplant recipients reportedly have a significantly higher harmonic mean clearance of cyclosporin (11.8 mljmin/kg; range 9.8 to 15.5 mljmin/kg) compared with adult renal transplant patients (5.7 mljmin/kg) [Ptachcinski et a1. 1985a]. Kahan et a1. (1985) reported a higher cyclosporin clearance from serum (RIA) in patients less than 45 years of age (13.3 ± 9.0 ml/min/kg) than patients older than 45 years (9.5 ± 5.2 ml/min/kg). The higher clearance in the paediatric population appears to be the result of more rapid removal of cyclosporin from the body. Therefore, children may require more frequent and larger doses of cyclosporin per kilogram bodyweight to achieve blood concentrations of the drug similar to those observed in adults. The kinetics of cyclosporin have not been studied in the geriatric population. Impairment in renal function with age is not expected to contribute to any changes in cyclosporin elimination.

2.3.1 Factors Affecting Metabolism Liver Disease Since cyclosporin is primarily eliminated by hepatic metabolism, its clearance is impaired in patients with liver disease. In 8 patients with biopsy-proven cirrhosis the harmonic mean clearance of cyclosporin was 2.8 mljmin/kg (unpublished observation) which is approximately half the clearance value observed in kidney and liver transplant patients. The harmonic mean half-life of cyclosporin in these patients was prolonged to 20.4 (range 10.8 to 48.0) hours. Because of the marked influence of liver disease on cyclosporin kinetics, blood concentration monitoring is essential In patients with severe liver dysfunction. Age The kinetics of cyclosporin are influenced by the age of the patient. Paediatric patients appear to clear the drug more rapidly compared to adults, on a

Other Drugs The metabolism of cyclosporin is significantly influenced by changes in the activity of the hepatic drug-metabolising systems. There have been several reports of pharmacokinetic interactions between cyclosporin and other drugs (Kerr 1984; Wood 1983). However, the mechanisms of many of these interactions have not been completely characterised. Phenytoin reduces the area under the cyclosporin plasma concentration-time curve (AUC) by 50% when administered in maintenance anticonvulsant doses (Freeman et al. 1984; Keown et al. 1982). Phenobarbitone is also known to increase the clearance of cyclosporin; clearance estimates as high as 15 mljmin/kg have been observed in a patient receiving phenobarbitone (Burckart et al. 1984). Antitubercular treatment with rifampicin and isoniazid has been shown to cause significant

Table V. Reported and unreported pharmacokinetic parameters [arithmetic mean (±SO) or harmonic mean (range)] for cyclosporin following intravenous administration References

Patient population

Number Biological of fluid subjects

Dose (mg/kg)

Study dosing interval

Assay

"(h-')

t.... (h)

p (h-')

t.... (h)

CL (ml/min/kg)

Vc (L/kg)

Vd.. (L/kg)

'"0

::r

..,I»

3

I»

(")

0

i'I"

S·

(h)

"c. (")

Ptachcinski (unpublished observations)

Healthy volunteers

Ptachcinski (unpublished observations)

Healthy volunteers

Follath et al. (1983)

Patients with renal failure

4

Blood

2.1-3.5

48

HPLC

0.7 (±.2)

1.1 (±0.3) 1.0 (0.7-1.3)

0.057 (±0.04)

15.8 (±8.4) 12.2 (6.4-26.9)

5.9 (±1.4)

0.14 (±0.05)

3.5 (±2.7)

Follath et al. (1983)

Patients with renal failure

4

Blood

2.1-3.5

48

RIA

0.4 (±.1)

1.7 (±0.3) 1.6 (1.3-2.0)

0.043 (±0.01)

16.5 (±3.1) 16.1 (14.1-20.9)

4.8 (±1.3)

0.16 (±0.1)

3.2 (±1.3)

Newberger & Kahan (1983)

Renal transplant

3

Serum

2.1"

24

RIA

6.8

0.1 (0.07-0.14)

0.223

2.9 (2.6-5.0)

Newberger & Kahan (1983)

Renal transplant (post-transplant)

2

Serum

2.1"

24

RIA

2.7

0.3 (0.25-0.27)

0.164

4.2 (3.0-7.1)

Venkataramanan (unpublished observations)

Patients with liver failure

8

Blood

2.0

48

HPLC

0.034 (±0.015)

20.4 (10.8-48)

2.8" (1.9-4.8)

3.9 (±1.8)

Venkataramanan (unpublished observations)

Patients with liver failure

8

Blood

2.0

48

RIA

0.029 (±0.017)

23.9 (10.8-47.6)

1.2 (0.8-3.6)

2.9 (±0.5)

Stewart et al. (1985)

Patients with liver disease

9

Serum

5.0

20

HPLC

0.5 (±0.2)

8.7 (±3.3)

30.4 (± 15.5)

2.7 (±2.3)

13.8 (±11.2)

Yee et al. (1984)

Bone marrow transplant serum bilirubin < 1.2 mg/dl

10

Serum

1.4-2.5

12

HPLC

0.3 (±0.1)

6.7 (±1.6)

12.8 (±1.6)

0.8 (±0.2)

4.3 (±0.9)

Yee et al. (1984)

Bone marrow transplant serum bilirubin < 1.2 mg/dl

10

Serum

1.4-2.5

12

RIA

0.4 (±0.1)

7.4 (±2.0)

8.3 (±0.9)

0.7 (±0.2)

3.1 (±0.8)

5

Blood

2.2 (±0.2)

48

HPLC

1.5 (±1.0)

0.45 (0.2-1.5)

0.112 (±0.04)

6.2 (4.7-9.5)

3.9 (2.9-5.5)

0.36 (0.3)

1.3 (±0.3)

'"

0-, ()

'< (")

5

Blood

2.2 (±0.2)

48

RIA

1.5 (± 1.1)

0.47 (0.24-2.0)

0.089 (±0.42)

7.8 (4.9-19.1)

2.8 (2.2-3.4)

0.48 (0.43)

1.5 (±0.6)

0"

'"0

'0

;:l.

::s

N N

'"..,

::r

I»

Lokiec et al. (1983)

Bone marrow transplant patients

38

Plasma

4.0

12

RIA

1.0 (±0.8)

6.8 (±2.5)

5.2· (1.5-10)

Yee et al. (1984)

Bone marrow transplant serum bilirubin > 2.0 mg/dl

11

Serum

1.4-2.5

12

HPLC

0.5 (±0.1)

12.7 (±6.1)

9.8 (±2.1)

0.7 (±0.2)

3.5 (± 1.1)

Bone marrow transplant serum bilirubin> 2.0 mg/dl

11

0.4 (±0.1)

10.3 (±2.3)

4.8 (±0.9)

0.6 (±0.2)

3.1 (±0.9)

Ptachcinski (1985d)

Adult renal transplant patients

41

Blood

10.7 (4.3-53.4)

5.7 (0.6-23.9)

4.5 (±3.6)

Kahan et al. (1985)

Renal transplant patients < 14 days after surgery

95

Serum

8.0 (±3.8)

13.1 (±8.8) 9.3

8.6 (±5.9)

Ptachcinski (1985a)

Paediatric renal transplant patients

7

Blood

7.3 (6.1-16.6)

11.8 (9.8-15.5)

4.7 (±1.5)

Burckart (1986)

Adult liver transplant patients

6

Blood

Burckart et al. (1985a)

Paediatric liver transplant patients

26

Blood

Venkataramanan (1985b)

Heart transplant patients

4b

Blood

Yee et al. (1984)

Serum

1.4-2.5

12

RIA

24

HPLC

24

RIA

4.5 (± 1.3)

12-24

HPLC

1.8 (±0.5)

12

HPLC

5.5 (4.9-7.6)

8

HPLC

8.4 (1.9-13.9)

12

HPLC

4.7

0.065 (±0.04)

3 ,..,

I»

0

~

0

..g". 0 .....,

() '< ,..,

.. '8

0"

::l. 0

a

0.8 (±0.4)

0.095 (±0.02)

0.105 (±0.029)

6.4 (5.2-9.3)

4.0 (2.1-5.3)

1.3 (±0.2)

Assuming 70kg.

b

Patients not receiving enzyme-inducing drugs. Abbreviations: see table III.

N

w

Pharmacokinetics of Cyclosporin

decreases in trough cyclosporin blood concentrations (Allen et al. 1985; Langhoff & Madsen 1983; Van Buren et al. 1984). Phenytoin, phenobarbitone and rifampicin presumably induce the enzymes responsible for the metabolism of cyclosporin. Cyclosporin blood concentrations should be closely monitored in patients concurrently receiving drugs known to induce microsomal enzymes. Several reports have described the interaction between cyclosporin and ketoconazole (Ferguson et al. 1982; Morgenstern et al. 1982). The combination of cyclosporin and ketoconazole results in elevated cyclosporin concentrations in transplant patients as well as in animals (Morgenstern et al. 1982; White et al. 1984). The presumed mechanIsm for this interaction is an inhibition of cyclosporin metabolism by ketoconazole. A similar interaction between cyclosporin and erythromycin has recently been reported (Ptachcinski 19850. Due to the potential nephrotoxicity associated with high cyclosporin blood concentrations, these drug combinations should be used only with extreme caution. Cyclosporin blood concentrations and renal function studies must be frequently monitored in such patients. Other drugs which inhibit microsomal enzyme function - including cimetidine, ranitidine and clotrimazole - may interact with cyclosporin; however, no controlled studies documenting such interactions have been conducted. The administration of high dose methylprednisolone for acute graft rejection may increase cyclosporin concentrations as measured by RIA (Klintmalm & Sawe 1984; Ost et al. 1985). We have studied the potential interaction between cyclosporin and high dose steroids in 7 renal transplant recipients (Ptachcinski, unpublished observation). A higher cyclosporin clearance (6.7 vs 5.7 ml/min/ kg) and a shorter cyclosporin half-life (9.9 vs 12.2 hours) were observed in patients receiving high dose steroids compared to when they were receiving maintenance doses of steroids. Contrary to observations using RIA, HPLC blood concentrations of cyclosporin were generally lower in patients on high dose steroids. One possible explanation for this observation is that microsomal enzyme induction secondary to steroid dosing may result in the for-

124

mation of large quantities of cyclosporin metabolites that are eliminated relatively slowly (measured by RIA) with low concentrations of the parent compound (measured by HPLC). Long term steroid therapy may also affect the pharmacokinetics of cyclosporin. Steroids can induce microsomal enzyme systems (Shukla et al. 1984), which would result in a higher clearance of cyclosporin in patients receiving steroids compared with those treated with cyclosporin alone. The results of our pharmacokinetic studies in normals and transplant patients support this theory since the clearance of cyclosporin in transplant patients (receiving long term steroids) was higher than the clearance observed in normals (not receiving steroids). There is some evidence to suggest that cyclosporin may decrease prednisolone metabolism in renal transplant patients (Ost 1984; Ost et al. 1985). Such an interaction may theoretically lead to the potentiation of the effect of prednisolone in patients simultaneously receiving cyclosporin. Additional studies are required to completely characterise the interaction between cyclosporin and steroids. 2.4 Excretion Renal excretion is a minor pathway of elimination for cyclosporin in humans and animals (Beveridge 1982). Less than I % of an administered dose is excreted unchanged in the urine. Approximately 6% of an administered dose of radioactive cyclosporin is excreted in the urine in 96 hours (Beveridge 1982). The major route of elimination of cyclosporin appears to be via the biliary system. In rats, biliary excretion accounts for nearly 50% of the intravenously administered cyclosporin (Beveridge 1982). In dogs, most of the orally administered drug (76% of the dose) is excreted in the faeces (Beveridge 1982). Studies in liver transplant patients with Ttubes indicate that biliary excretion is the major pathway for cyclosporin elimination in humans as well (Venkataramanan et al. 1985b). Biliary concentrations of cyclosporin are much higher than blood concentrations obtained at the same time.

Pharmacokinetics of Cyclosporin

The concentration of cyclosporin in bile by RIA was 18 to 36 times greater than when analysed by HPLC. Cyclosporin metabolites are therefore concentrated in the bile to a greater extent than in blood. The total amount of cyclosporin excreted in the bile over 1 dosing interval is directly related to the volume of bile output (Venkataramanan 1985c). Patients with normal liver function have higher bile output than patients with impaired hepatic function and therefore excrete larger amounts of cyclos"porin in the bile. However, less than 1% of an administered dose of cyclosporin is excreted in the bile as parent drug. More than 44% of a dose of cyclosporin appears in the bile as metabolites when measured by RIA. Kahan et a1. (1983a) reported that secondary peaks following a dose of cyclosporin occurred in 36% of their patients. These observations led the authors to conclude that enterohepatic recycling was the cause of the secondary peaks. The presence of small amounts of unchanged cyclosporin in the bile as measured by HPLC suggest that enterohepatic recycling is not a major factor influencing the kinetics of cyclosporin in liver transplant patients (Venkataramanan et a1. 1985b). The discrepancy between the conclusions of these two reports may be explained by methodological differences in assay. Kahan and co-workers used RIA to analyse plasma and ileostomy drainage, and cyclosporin metabolites may indeed be enterohepatically recycled. Other factors contributing to the presence of secondary peaks in the blood or serum cyclosporin concentration versus time profile (HPLC or RIA) in some patients are not completely understood at present.

2.4.1 Factors Affecting Excretion The pharmacokinetics of cyclosporin are not significantly altered in the presence of renal failure (Follath et a1. 1983). Therefore, dosage adjustments are not necessary in patients with renal impairment. Since less than 6% of the dose appears in the urine as measured by RIA, accumulation of metabolites, if any, will be only minimal.

125

Haemodialysis Transplant recipients may require haemodialysis or peritoneal dialysis to treat drug-induced toxicity, surgical complications or acute tubular necrosis. In 1 study the clearance of cyclosporin in liver transplant patients was similar whether they were on or off haemodialysis; less than 1% of a dose of cyclosporin was removed during 4 hours of dialysis (Venkataramanan et a1. 1984). This finding is due to the fact that very little cyclosporin is excreted unchanged in the urine, and that it has a high molecular weight (1202), is highly lipid soluble, is significantly bound to plasma proteins, and is a drug with a large volume of distribution. Since haemodialysis does not significantly alter the total body clearance of cyclosporin, dosage alterations are not necessary for patients undergoing haemodialysis. The effect of peritoneal dialysis on the pharmacokinetics of cyclosporin has not been studied, but no significant change would be expected.

3. Drug Interactions The interactions between cyclosporin and other drugs include those that influence the metabolism of cyclosporin and interactions which potentiate the drug's nephrotoxicity. Table VI summarises the drug interactions involving cyclosporin. Interactions altering cyclosporin's metabolism are discussed in section 2.3.1. Several drugs have been reported to potentiate the nephrotoxicity of cyclosporin. These include the aminoglycoside antibiotics (Whiting et a1. 1983), amphotericin B (Kennedy et a1. 1983), co-trimoxazole (trimethoprim-sulphamethoxazole) [Ringden et a1. 1984; Thompson et a1. 1983], melphalan (Morgenstern et a1. 1982), frusemide (furosemide) [Whiting et a1. 1984] and cephalosporin antibiotics (cefotaxime and cefuroxime) [Whiting et a1. 1983]. The concurrent use of cyclosporin and indomethacin has resulted in a rapid deterioration of renal function in 2 renal transplant recipients (Ptachcinski, unpublished observation) which may be related to changes in renal blood flow secondary to prostaglandin inhibition (Petrie 1986). Renal func-

Pharmacokinetics of Cyclosporin

126

Table VI. Reported and unreported drug interactions with cyclosporin

Drug

References

Mechanism

Effect observed

Management

Increased blood concentrations of cyclosporin leading to nephrotoxicity

Avoid use of ketoconazole if possible

Pharmacokinetic interactions

Ketoconazole

Morgenstern et al. Impairment of (1982); Ferguson et al. cyclosporin (1982); White et al. (1984) metabolism

Erythromycin

Ptachcinski (1985f)

Impairment of cyclosporin metabolism

Increased blood concentrations of cyclosporin

Monitor blood concentrations of cyclosporin

Methylprednisolone (high dose)

Klintmalm and Sawe (1984); Ost et al. (1985)

Not known

Increased plasma concentrations of cyclosporin as measured by RIA. Decreased blood concentrations of cyclosporin as measured by HPLC. Significance unknown

Monitor blood concentrations of cyclosporin closely during periods of rejection. Verify cyclosporin concentration with HPLC

Phenytoin

Freeman et al. (1984); Induces CYCIOSPorin{ Lowering of cyclosporin {Increase cyclosporin metabolism blood concentrations with dose as necessary to Keown et al. (1982) { associated rejection of maintain blood Allen et al. (1985); transplanted organs concentrations and organ Van Buren et al. (1984) function Burckart et al. (1984)

Rifampicin Phenobarbitone Sulphadimidinetrimethoprim (intravenous)

Wallwork (1983)

Not known

Decrease in serum cyclosporin levels but significance unknown

Continue cyclosporin blood concentration monitoring programme 2 to 3 times per week

Pharmacological interactions

Aminoglycosides Amphotericin B Melphalan Trimethoprim Co-trimoxazole

Whiting et al. (1983) Kennedy et al. (1983) Morgenstern et al. (1982) Thompson (1983) Ringden et al. (1984); Thompson et al. (1983)

Not known

Nephrotoxicity Nephrotoxicity Nephrotoxicity Nephrotoxicity Nephrotoxicity

Nephrotoxicity - adjust doses of drug as indicated by renal function. If possible, reduce cyclosporin dosage

Methylprednisolone

Boogaerts (1982)

Not known

Convulsions

Avoid other precipitating states such as severe hypertension or electrolyte imbalance

Non-steroidal antiinflammatory drugs (NSAIDs)

Unpublished observation

Not known

Nephrotoxicity

Avoid the use of NSAIDs if possible

Pharmacokinetics of CycIosporin

tion usually returns to baseline upon discontinuation of the NSAID. The combination of any of these agents and cyclosporin may be required in some patients. Close monitoring of renal function is essential in patients receiving cyclosporin and other nephrotoxic agents.

4. Individualising Dosage Regimens 4.1 Initial Therapy Cyclosporin is frequently administered 4 to 12 hours preoperatively, when used for cadaveric organ transplantation. Patients receiving a kidney from a living donor often receive cyclosporin for 48 hours prior to transplantation. However, some transplant centres do not administer the drug until a postoperative diuresis has occurred. Almost every transplant centre uses a somewhat different regimen for the administration of cyclosporin. Factors such as age, type of transplant, disease state, and concurrent drug therapy must all be considered when initiating therapy with cy~lo­ sporin. Initial doses are usually in the range of 10 to 20 mg/kg/day orally or 2.5 to 5 mg/kg/day as a 2- to 6-hour intravenous infusion. Any schedule of cyclosporin dosage should be considered a starting point from which therapy is adjusted based on clinical, biochemical and immunological parameters. Initial doses of cyclosporin are often lower in heart or heart-lung recipients because of the high incidence of oliguria and renal failure in the postoperative period (Griffith et al. 1982). Immunosuppressive regimens in these patients often include other drugs such as anti thymocyte globulin or azathioprine. 4.2 Maintenance Therapy Cyclosporin dosage and concentrations in blood should be high in the first 2 weeks following transplantation, and should be gradually reduced thereafter. The risk of graft rejection is greatest in the early postoperative period and aggressive therapy is required during this time. Long term maintenance doses ofless than 3 mgjkg/day of oral cyclo-

127

sporin have accomplished adequate immunosuppression in some patients. Dosage reduction should be performed slowly and the smallest dose that maintains organ function should be sought. Long term maintenance with intravenous cyclosporin (4 to 6 weeks) is necessary in some patients who do not absorb the drug well orally. Cyclosporin is poorly absorbed in patients following liver transplantation, in patients with graft-versus-host disease involving the gastrointestinal tract and in other patients with diarrhoea (Atkinson et al. 1984; Burckart et al. 1985a). Following liver transplantation, patients at the University of Pittsburgh initially receive intravenous therapy alone. When oral intake is possible, oral cyclosporin is added to the intravenous regimen. The intravenous dose of cyclosporin is gradually tapered and finally discontinued when adequate blood concentrations are obtained. Dosage adjustments with cyclosporin are usually made without the aid of extensive pharmacokinetic calculations. The reasons for this include the rapid physiological changes in patients requiring dosage adjustments and the long elimination half-life of the drug which prevents the accurate calculation of pharmacokinetic parameters in the clinical setting. Rational dosage adjustments should be made after considering the patient's liver function, the concentration of cyclosporin in the blood, the time after transplant as well as the clinical and biochemical results relating to drug efficacy and toxicity.

5. Therapeutic Drug Monitoring The variability in response to a given dose of cyclosporin was recognised early in the clinical trials with the drug. The monitoring of cyclosporin blood concentrations is currently recommended by the manufacturer of cyclosporin. The quantitation of cyclosporin in biological fluids is essential for routine monitoring for several reasons: (a) wide variability in cyclosporin pharmacokinetics is observed after oral or intravenous administration of the drug; (b) blood concentration monitoring allows the maintenance of drug concentrations suf-

Pharmacokinetics of Cyclosporin

ficient to prevent the rejection of the transplanted organ; (c) drug toxicity can be minimised through blood concentration monitoring; and (d) blood concentration monitoring provides a means to assess compliance since patient non-compliance with drug regimens is a significant reason for graft loss after 60 days. However, cyclosporin blood concentrations should be used in concert with biochemical parameters, radiological studies, biopsy results, and the clinical examination of the patient. A programme of cyclosporin blood concentration monitoring and biochemical monitoring should start immediately following the initiation of therapy. A biochemical profile (electrolytes, kidney and liver function), should be performed daily. A cyclasporin blood concentration determination should be obtained on day 2 or 3 following transplantation and then twice weekly until the patient's clinical condition and cyclosporin dosage are stable. The cyclosporin blood concentration should also be checked after any dosage adjustment. Two to 3 days are required to achieve a new steady-state cyclosporin concentration after a dosage adjustment in patients with normal hepatic function (Ptachcinski et al. 1985d), which would therefore be the optimal time to determine cyc1osporin blood concentration. A longer period is required to achieve a new steady-state cyclosporin concentration in patients with impaired liver function. The period of intensive monitoring may last only a few weeks in renal transplant patients but may continue for several months in liver transplant patients. After the early period of intensive monitoring, the cyclosporin blood concentration and biochemical monitoring can be gradually reduced to every 1 to 2 months. A high degree of patient awareness of factors that may influence cyclosporin therapy, such as the development of diarrhoea, is constantly necessary. A periodic monitoring programme should continue indefinitely while the patient is on cyclosporin therapy. Preliminary observations in 2 patients suggest the presence of diurnal variations in the blood clearance of cyclosporin in liver transplant patients. The clearance was higher in the late evening and early mornings (7.1 and 6.2 ml/min/kg) than in the day time hours (5.1 and 4.3 ml/min/

128

kg) [Venkataramanan 1986]. Therefore, trough drug

concentrations should be monitored at a specific time of the day. 5.1 Cyclosporin Therapeutic Ranges The desired therapeutic trough cyclosporin concentrations vary significantly between centres (table VII). Much of this variability is because of different biological fluids monitored and the different analytical techniques used in various centres (see section 1). The therapeutic range for cyclosporin also appears to change with time following transplantation. In the early course of therapy, patients may require cyclosporin blood concentrations in the upper end of the therapeutic range, while several months after transplantation clinically stable patients may have a trough cyclosporin blood concentration at or below the lower end of the therapeutic range (Klintmalm et al. 1985). The usual range of trough blood concentrations in patients doing well on cyclosporin therapy is from 100 to 300 IJ.g/L (HPLC) [Ptachcinski et al. 1985b] or 800 to 1000 IJ.g/L (RIA) [Iwatsuki et al. 1985]. The therapeutic range for cyclosporin in serum (RIA) ranges from 100 to 500 IJ.g/L (Klintmalm et al. 1985). Klintmalm et al. (1985) reported that kidney transplant patients with good graft function had a mean cyclosporin plasma concentration (RIA) of 393 (± 38) IJ.g/L during the first postoperative month, while patients with good graft function 12 months following transplantation had a mean plasma cyclosporin concentration of 111 (± 14) IJ.g/L. The results of studies in bone marrow recipients suggest that trough serum cyclosporin concentrations (RIA) correlate with the risk of developing renal dysfunction; renal dysfunction occurred in 73%, 95% and 100% of patients (n = 63) with mean trough cyclosporin concentrations of <150, 150 to 250, and >250 IJ.g/L, respectively (Yee et al. 1985). Kidney transplant patients with good functioning grafts usually have cyclosporin blood concentrations (HPLC) of 150 to 300 IJ.g/L in the early postoperative period. Several months after transplantation, attempts are made to maintain cyclosporin concentrations of approximately 100

Pharmacokinetics of Cyclosporin

129

Table VII. Desired trough cyclosporin concentrations reported from major transplant centres References

Patient population

Biological fluid

Assay

Desired cyclosporin concentrations v.g/ L)

Najarian et a!. (198S)

Renal transplant

Whole blood

HPLC

100-200

Klintmalm et a!. (198S)

Renal transplant first month

Plasma

RIA

< SOO

Klintmalm et al. (198S)

Renal transplant long term

Plasma

RIA

< SO-lS0

Canadian Trial (1983)

Renal transplant

Serum

RIA

100-400

Merion et a!. (1984)

Renal transplant

Serum

RIA

l00-S00

Zitelli et al. (1983)

Uver transplant

Whole blood

HPLC

100-400

Cooley et a!. (1983)

Heart transplant

Plasma

RIA

100-200

Keown et a!. (1983)

Renal transplant

Serum

RIA

lS0-300

Kahan et al. (1984)

Renal transplant

Serum

RIA

100-200

Yee et al. (198S)

Bone marrow transplant

Serum

RIA

100-2S0

Painvin et a!. (198S)

Heart transplant

Serum

RIA

200-S00

Starzl et a!. (1984)

Uver transplant

Whole blood

RIA

800-1000

Ptachcinski et al. (198Sb)

Renal transplant

Whole blood

HPLC

lS0-300

Abbreviations: HPLC = high-pressure liquid chromatography; RIA = radioimmunoassay.

jJ.gJL. Patients with higher cyclosporin concentrations for prolonged periods often have an elevated serum creatinine which may be due to some chronic cyclosporin nephrotoxicity. Some newer regimens incorporate a third immunosuppressant in therapy following organ transplantation. Antilymphocyte globulin may be administered soon after the transplant procedure and azathioprine added as a third long term immunosuppressant along with prednisone and low dose cyclosporin (Canafax et al. 1984). Adequate trough cyclosporin concentrations in blood with these triple drug regimens may be as low as 50 jJ.g/L (HPLC).

6. Conclusions The pharmacokinetics of cyclosporin are highly variable and blood concentrations of the drug are influenced by factors such as transplant type, patient

age, disease state and concurrent drug therapy. Cyclosporin blood concentration monitoring is essential because of variability in the drug's kinetics, to provide adequate blood concentrations to prevent rejection with the avoidance of drug concentration associated toxicity. Special precautions are necessary when managing children and patients with problems such as malabsorption or those receiving any drug known to interact either pharmacokinetically or pharmacologically with cyclosporin. Dosage adjustments in patients should be made based on the clinical status of the patient, the patient's biochemical profile, blood cyclosporin concentrations, and with the knowledge of the factors which will affect cyclosporin blood concentrations. The success of organ transplantation will continue to improve as our ability to use cyclosporin improves and as new cyclosporin derivatives with fewer side effects become available for clinical use.

Pharmacokinetics of Cyclosporin

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Pharmacokinetics of CycIosporin

Keown PA, Stiller CR, Ulan RA, Sinclair NR, Wall WJ, et al. Immunological and pharmacological monitoring in the clinical use of cyclosporin A. Lancet I: 686-689, 1981 Kerman RH, Flechner SM, Van Buren CT, Payne W, Kahan BD. Immunologic monitoring of renal allograft recipients treated with cyclosporine. Transplantation Proceedings 15: 2302-2305, 1983 Kerr LE. Drug interactions with cyclosporine. Clinical Pharmacy 3: 348-349, 1984 Klintmalm G, Sawe J. Prednisolone increases cyclosporine levels in renal transplant recipients. Lancet I: 731, 1984 Klintmalm G, Sawe J, Ringden 0, von Bahr C. Magnusson A. Cyclosporine plasma levels in renal transplant patients. Transplantation 39: 132-137, 1985 Langhoff E, Madsen S. Rapid metabolism of cyclosporin and prednisone in kidney transplant patients receiving tuberculostatic treatment. Lancet 2: 1031, 1983 LeMaire M, Tillement JP. Role of lipoproteins and erythrocytes in the in vitro binding and distribution of cyclosporin A in the blood. Journal of Pharmacy and Pharmacology 34: 715-718, 1982 Lensmeyer GL, Fields BL. Improved liquid-chromatographic determination of cyclosporine, with concomitant detection of a cell bound metabolite. Clinical Chemistry 31: 196-201, 1985 Leyland-Jones B, Clark A, Kreis W, Dinsmore R, O'Reilly R, et al. High pressure liquid chromatographic determination of cyclosporin-A in human plasma. Research Communications in Chemical Pathology and Pharmacology 37: 431-444, 1982 Lokiec F, Devergie A, Poirier 0, Gluckman E. Pharmacologic monitoring and the clinical use of cyclosporine. Transplantation Proceedings 15: 2442-2445, 1983 Maurer G. Metabolism of cyclosporine. Transplantation Proceedings 17: 19-26, 1985 Maurer G, Loosli HR, Schreier E, Keller B. Disposition of cyclosporine in several animal species and man. Structural elucidation of its metabolites. Drug Metabolism and Disposition 12: 120-126. 1984 Merion RM, White DJG, Thiru S, Evans DB, Caine RY. Cyclosporine: five years experience in cadaveric renal transplantation. New England Journal of Medicine 310: 148-154, 1984 Morgenstern GR, Powles R, Robinson R, McElwain TJ. Cyc1osporine interaction with ketoconazole and melphalan. Lancet 2: 1342, 1982 Mraz W, Zink RA, Graf A, Preis D, IIIner WD, et aL Distribution and transfer of cyc1osporine among the various human lipoprotein classes. Transplantation Proceedings 15: 2426-2429, 1983 Najarian JS, Fryd DS, Strand M, Canafax DM, Ascher NL, et al. A single institution randomized prospective trial of cyc1osporine versus azathioprine-antilymphocyte globulin for immunosuppression in renal allograft recipients. Annals of Surgery 201: 142-157, 1985 Newberger J, Kahan BD. Cyclosporine pharmacokinetic in man. Transplantation Proceedings 15: 2413-2415, 1983 Niederberger W, leMaire M, Maurer G, Nussbaumer K, Wagner O. Distribution and binding of cyc1osporine in blood and tissues. Transplantation Proceedings 15: 2419-2421, 1983 Niederberger W. Schaub P, Beveridge T. High-performance liquid chromatographic determination of cyc1osporin A in human plasma and urine. Journal of Chromatography 182: 454-458, 1980 Niederberger W, Wiscott E. Circular letter to users of RIA or HPLC to measure levels of cyclosporin A in blood, 1982 Nussbaumer K, Niederberger W, Keller HP. Determination of cyclosporin A in blood and plasma by column switching-HPLC after rapid sample preparation. Journal of High Resolution Chromatography and Chromatography Communications 5: 424428, 1982 Ost L. Effects of cyclosporine on prednisolone metabolism. Lancet

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I: 451 1984 Ost L, Klintmalm G, Ringden O. Mutual interaction between prednisolone and cyc1osporine in renal transplant patients. Transplantation Proceedings 17: 1252-1255, 1985 Painvin GA, Okereke OUJ, Frazier OH, Kahan BD, Van Buren CT, et al. Cardiac transplantation: current results of cyc1osporine-treated patients at the Texas Heart Institute. Transplantation Proceedings 17: 223-224, 1985 Petric R, Keown PA, Freeman OJ, Wallace AC, MacDonald JWD, et al. The role of prostaglandins in the development of cyclosporine nephrotoxicity. Transplantation Proceedings, in press, 1986 Ptachcinski RJ, Burckart GJ, Rosenthal JT, Venkataramanan R, Howrie DL, et al. Cyc1osporine pharmacokinetics in children following cadaveric renal transplantation. Transplantation Proceedings, in press, 1985a Ptachcinski RJ, Hakala TR, Schleicher C, Rosenthal JT, Taylor RJ. et al. Cyclosporine in cadaveric renal transplantation: analysis of three administration regimens. Transplantation Proceedings, in press, 1985b Ptachcinski RJ, Venkataramanan R, Burckart GJ, Rosenthal JT, Taylor RJ, et al. Dose-dependent absorption of cyc1osporine. Drug Intelligence and Clinical Pharmacy 19: 450, 1985c Ptachcinski RJ, Venkataramanan R, Rosenthal JT, Burckart GJ, Taylor RJ, et al. Cyc1osporine kinetics in renal transplantation. Clinical Pharmacology and Therapeutics 38: 296-300, 1985d Ptachcinski RJ, Venkataramanan R, Rosenthal JT, Burckart GJ, Taylor RJ, et al. The effect offood on cyc1osporine absorption. Transplantation 40: 174-176, 1985e Ptachcinski RJ, Carpenter BJ, Burckart GJ, Venkataramanan R, Rosenthal JT. Effect of erythromycin on cyclosporine levels. New England Journal of Medicine 313: 1416-1417, 1985f Ried M, Gibbons S, Kwok D, Van Buren CT, Flechner S, et al. Cyclosporine levels in human tissues of patients treated for one week to one year. Transplantation Proceedings 15: 24342437, 1983 Ringden 0, Myrenfors P, Klintmalm G, Tyden G, Ost L. Nephrotoxicity by cotrimoxazole and cyc1osporin in transplanted patients. Lancet 2: 1016-1017, 1984 Robson S. Neuberger J, Keller HP, Abisch E, Niederberger W, et al. Pharmacokinetic study of cyc1osporin A (Sandimmune) in patients with primary biliary cirrhosis. British Journal of Clinical Pharmacology 18: 627-631, 1984 Sawchuck RJ. Cartier LL. Liquid-chromatographic determination of cyclosporin A in blood and plasma. Clinical Chemistry 27: 1368-1371, 1981 Shah D, Rylance PB, Rogerson ME, Bewick M, Parsons A. Generalized epileptic fits in renal transplant recipients given cyc1osporine A. British Medical Journal 289: 1347-1348, 1984 Shukla VK, Garg SK, Matheu VS. Influence of prednisolone on antipyrine and chloramphenicol disposition in rabbits. Pharmacology 29: 117-120, 1984 Smith HT. Robinson WT. Semi-automated high-performance liquid chromatographic method for the determination of cyclosporine in plasma using column switching. Journal of Chromatography 305: 353-362, 1984 Smith J. Hows J, Gordon-Smith EC. In vitro stability and storage of cyc1osporine in human serum and plasma. Transplantation Proceedings 15: 2422-2425, 1983 Stanl TE, Iwatsuki S, Shaw BW, Gordon RD. Orthotopic liver transplantation in 1984. Transplantation Proceedings 17: 250258, 1984 Stewart CF, Cochran E, Williams J. Cyc1osporine disposition in patients with liver disease. Drug Intelligence and Clinical Pharmacy 19: 451, 1985 Thompson JF, Chalmers DHK, Hunnisett AGW, Wood RFM, Morris RJ. Nephrotoxicity oftrimethoprim and cotrimoxazole in renal allograft recipients treated with cyc1osporine. Transplantation 36: 204-206, 1983

Pharmacokinetics of CycJosporin

Van Buren 0, Wideman CA, Ried M, Gibbons S, Van Buren CT, et al. The antagonistic effect of rifampin upon cyclosporine bioavailability. Transplantation Proceedings 16: 1642-1645, 1984 Venkataramanan R, Ptachcinski RJ, Burckart GJ, Gray J, Van Thiel DH, et al. Cyclosporine bioavailability in liver disease. Drug Intelligence and Clinical Pharmacy 19: 451, 1985a Venkataramanan R, Ptachcinski RJ, Burckart GJ, Hardesty RE, Griffith BT, et al. Cyclosporine pharmacokinetics in heart transplant patients. Transplantation Proceedings, in press, 1985b Venkataramanan R, Ptachcinski RJ, Burckart GJ, Yang SL, Starzl TE et al. The clearance of cyclosporine by hemodialysis. Journal of Clinical Pharmacology 24: 528-531, 1984 Venkataramanan R, Starzl TE, Yang S, Burckart GJ, Ptachcinski RJ, et al. Biliary excretion of cyclosporine in liver transplant patients. Transplantation Proceedings 17: 286-289, 1985c Venkataramanan R, Burckart GJ, Ptachcinski RJ. Pharmacokinetics and monitoring of cyclosporine following orthotopic liver transplantation. Seminars in Liver Disease 5: 357-368, 1985d Venkataramanan R, Yang SL, Burckart GJ, Ptachcinski RJ, Van Thiel DH, et al. Diurnal variations in cyclosporine kinetics. Therapeutic Drug Monitoring, in press, 1986 Wallwork J, McGregor OGA, Wells FC, Cory-Pearch R, English TAH. Cyclosporin and intravenous sulphadimidine and trimethoprim therapy. Lancet I: 366-367, 1983 Wenk M, Follath F. Temperature dependency of apparent cyclosporin A concentrations in plasma. Clinical Chemistry 29: 1865, 1983 White DJG, Blatchford NR, Cauwenberg G. Cyclosporine and ketoconazole. Transplantation 37: 214-215, 1984 Whiting PH, Cunningham C, Thompson AW, Simpson JG. Enhancement of high dose cyclosporin A toxicity by furosemide. Biochemical Pharmacology 33: 1070-1075, 1984 Whiting PH, Simpson JG, Thompson AW. Nephrotoxicity of cyclosporine in combination with aminoglycoside and cepha-

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losporin antibiotics. Transplantation Proceedings 15: 2702-2705, 1983 Wood AJ, Maurer G, Niederberger W, Beveridge T. Cyclosporine: pharmacokinetics, metabolism, and drug interactions. Transplantation Proceedings 15: 2409-2412, 1983 Yee GC, Gmur OJ, Kennedy MS. Liquid-chromatographic determination of cyclosporine in serum with use of a rapid extraction procedure. Clinical Chemistry 28: 2269-2271, 1982 Yee GC, Kennedy MS, McGuire BS, Deeg HJ. Correlation of serum cyclosporine concentration to renal dysfunction in marrow transplant recipients. Drug Intelligence and Clinical Pharmacy 19: 445-446, 1985 Yee GC, Kennedy MS, Storb R, Thomas ED. Pharmacokinetics of intravenous cyclosporine in bone marrow transplant patients. Transplantation 38, 511-513, 1984 Zitelli BJ, Gartner JC, Malatack JJ, Shaw BW, Iwatsuki S, et al. Hepatic homograft survival in pediatric orthotopic liver transplantation with cyclosporine and steroids. Transplantation Proceedings 15: 2592-2596, 1983

Acknowledgements: Research data reported here are supported in part by USPHS grant no. AM3347S-0IAI, and by Research Development grants from the University of Pittsburg and Sandoz Inc.

Address for correspondence and reprints: Dr Richard J. Plach·

cinski. Clinical Pharmacokinetics Laboratory, 807 Salk Hall, University of Pittsburgh, Pittsburgh. PA 15261 (USA).