CONCEPTS

Clin Pharmacokinet 2000 Jun; 38 (6): 493-504 0312-5963/00/0006-0493/$20.00/0 © Adis International Limited. All rights reserved.

Induction of Drug Metabolising Enzymes Pharmacokinetic and Toxicological Consequences in Humans Uwe Fuhr Institute for Pharmacology, Clinical Pharmacology, University of Cologne, Cologne, Germany

Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Induction via the Aryl Hydrocarbon (Ah) Receptor . . . . . . . . . . . . . 2. Ethanol-Type Induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Peroxisome Proliferator Induction . . . . . . . . . . . . . . . . . . . . . . . 4. Constitutive Androstane Receptor (CAR)/Phenobarbital-Type Induction 5. Pregnane X Receptor (PXR)-Type Induction . . . . . . . . . . . . . . . . . 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Currently, 5 different main mechanisms of induction are distinguished for drug-metabolising enzymes. The ethanol type of induction is mediated by ligand stabilisation of the enzyme, but the others appear to be mediated by intracellular ‘receptors’. These are the aryl hydrocarbon (Ah) receptor, the peroxisome proliferator activated receptor (PPAR), the constitutive androstane receptor (CAR, phenobarbital induction) and the pregnane X receptor [PXR, rifampicin (rifampin) induction]. Enzyme induction has the net effect of increasing protein levels. However, many inducers are also inhibitors of the enzymes they induce, and the inductive effects of a single drug may be mediated by more than one mechanism. Therefore, it appears that every inducer has its own pattern of induction; knowledge of the main mechanism is often not sufficient to predict the extent and time course of induction, but may serve to make the clinician aware of potential dangers. The possible pharmacokinetic consequences of enzyme induction depend on the localisation of the enzyme. They include decreased or absent bioavailability for orally administered drugs, increased hepatic clearance or accelerated formation of reactive metabolites, which is usually related to local toxicity. Although some severe drug-drug interactions are caused by enzyme induction, most of the effects of inducers are not detected in the background of nonspecific variation. For any potent inducer, however, its addition to, or withdrawal from, an existing drug regimen may cause pronounced concentration changes and should be done gradually and with appropriate monitoring of therapeutic efficacy and adverse events. The toxicological consequences of enzyme induction in humans are rare, and appear to be mainly limited to hepatoxicity in ethanol-type induction.

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Exposure of animals and humans to a drug or environmental chemical often causes an up-regulation in the amount of enzymes that are capable of metabolising the inducing agent as well as other compounds.[1,2] This is simply an adaptive process to maintain homeostasis upon a signal, which is a general principle in the regulation of protein expression, availability and degradation. A similar situation is present in the regulation of many membrane receptors, for example β-adrenergic receptors, causing a change in pharmacodynamics on prolonged exposure to ligands. For pharmacokinetic processes, such changes are also observed in the case of P-glycoprotein, where induction causes accelerated membrane transport of xenobiotics.[3] From an evolutionary point of view, enzyme induction is thought to be a defence mechanism against varying exposure to toxic components in plants eaten as food. Therefore, it is to be expected that the induction results in decreased concentrations of the active compounds, which is true in most, but not all, cases. For those substances that are inactive but are biotransformed to active metabolites, enzyme induction may in contrast result in higher concentrations of the active components. The pharmacokinetic or toxicokinetic consequences depend on the localisation of the protein with increased activity, and on the pharmacological activity of the parent compounds and their metabolites. If enzymes (or carriers) in the gut wall are the primary targets of induction, bioavailability of orally administered drugs may be decreased considerably. An example is the 25-fold reduction of the area under the concentration-time curve (AUC) of (S)-verapamil by rifampicin (rifampin).[4] If hepatic enzymes are induced, the clearance of the substrate may be increased (e.g. more rapid elimination of caffeine in smokers[5]). In some cases, if local enzyme expression in various tissues is enhanced, the local formation of carcinogens or other highly reactive species may be accelerated. An example is the increased toxicity of paracetamol (acetaminophen) by ethanol.[6] The following clinically relevant consequences © Adis International Limited. All rights reserved.

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of enzyme induction may be derived from these considerations: • contribution to variability in the pharmacokinetics of many drugs, making appropriate dosage selection more difficult or even impossible • increased toxicity, including increased local toxicity with higher susceptibility to various types of cancer or other local tissue damage • protection against xenobiotics which were not administered intentionally • therapeutic use in order to overcome genetic defects in enzyme expression. It is difficult to assess the significance of enzyme induction in clinical practice. Although clearly there are drug-drug interactions with high clinical relevance caused by induction, such as the phenobarbital-warfarin interaction,[7] clinical studies are sparse in this area. Indeed, most of the data available on enzyme induction originate from studies in laboratory animals, and even for typical examples, such as increased paracetamol toxicity in patients with alcoholism, evidence is circumstantial and not up-to-date. Despite these limitations, several sources of inducers are known for humans. Some of them are present in food, such as cruciferous vegetables (e.g. broccoli). Because of the huge amounts consumed in the population when compared with other chemical inducers, tobacco and alcohol are certainly inducers with major clinical relevance. For the opposite reason, i.e. limited exposure and subsequent uptake into the human body, environmental loads from pollution appear to be less important for humans. Finally, there are many drugs with relevant inducing effects on protein expression. Currently 5 types of enzyme induction have been established (table I).[1,2] These include: induction via the aryl hydrocarbon (Ah) receptor; induction by ethanol or peroxisome proliferators; induction via the constitutive androstane receptor (CAR); and induction via the pregnane X receptor (PXR). Data about the PXR[8] and CAR[9] remain to be confirmed and completed, and there is some overlap in the inducing effects via the different pathways that is not fully understood.[6,10,11] Furthermore, 2 Clin Pharmacokinet 2000 Jun; 38 (6)

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Table I. Important human drug metabolising enzymes subject to induction Type of induction

Main mechanism

Most important induced enzymes

Ah receptor–mediated/polycyclic aromatic hydrocarbons

Increased expression

CYP1A1, CYP1A2, GSTs, UGTs, ALDHs

Ethanol

Enzyme stabilisation

CYP2E1

Peroxisome proliferators

Increased expression

CYP4 enzymes (?), enzymes involved in lipoprotein and fatty acid metabolism

CAR/phenobarbital

Increased expression

Most pronounced effects on CYP2B6, clear effects on CYP2C8, CYP2C9, CYP3A4, CYP1A2 and some UGTs

PXR/glucocorticoids, rifampicin (rifampin) Increased expression

CYP3A enzymes, mainly in the gut

Ah = aryl hydrocarbon; ALDH = aldehyde dehydrogenase; CAR = constitutive androstane receptor; CYP = cytochrome P450; GST = glutathione S-transferase; PXR = pregnane X receptor; UGT = UDP-glucuronosyltransferase.

additional nuclear receptors, designated liver X receptor and farnesoid X receptor, may be involved in enzyme induction.[12] Despite such problems, for the purposes of this review we will adhere to a structure based on these 5 mechanisms. Interestingly, it appears that therapeutic drugs preferentially affect the latter 2 mechanisms, whereas the inductive effects of ‘recreational’ drugs are virtually limited to the first 2 mechanisms. Induction has the net effect that more protein is present. Thus, enzyme induction usually results in an increased turnover of xenobiotics. However, additional mechanisms may be present that make the increased amount of protein partially unavailable for metabolic activity. Possible mechanisms include: • inducers increase expression of the enzyme but are high-affinity substrates for the enzyme • inducers improve the stability of the enzyme by covalent or otherwise virtually irreversible binding to the binding site • inducers improve the stability of the enzyme by reversible but extensive binding to the binding site. It becomes obvious that the effects of enzyme induction on pharmacokinetics may be very complex and a detailed knowledge of the processes involved is required to understand its consequences. 1. Induction via the Aryl Hydrocarbon (Ah) Receptor This mechanism of induction is based on increased protein synthesis, mainly initiated by the binding of the inducer to the intracellular Ah receptor. © Adis International Limited. All rights reserved.

This complex, together with another protein, Ah receptor nuclear translocator (Arnt), increases enzyme expression by binding to an enhancer/promoter region.[1] The main targets of induction are cytochrome P450 (CYP) 1A1 and 1A2, but the concentrations of additional enzymes, including glutathione S-transferases (GSTs) and UDP-glucuronosyltransferases (UGTs), are also increased.[13-15] CYP1A2 appears to be the most important of these enzymes in terms of mediating rate-limiting metabolic steps for systemic elimination of drugs in humans. It is mainly expressed in the liver. There are only a few currently used drugs that are primarily metabolised by CYP1A2: these are theophylline, caffeine, tacrine, clozapine and flutamide.[5,16] The greatest exposure to inducers acting via the Ah receptor comes from tobacco smoke. Several thousand components have been found in tobacco smoke, and many of them belong to the polycyclic aromatic hydrocarbons, which are the classical inducers of the Ah receptor pathway. In a recent population study using caffeine elimination as a phenotyping reaction for CYP1A2 activity, the quantitative effect of smoking on the enzyme was examined.[5] Smoking showed a dose-dependent effect: caffeine clearance increased 1.22-, 1.47-, 1.66- and 1.72-fold for between 1 and 5, 6 and 10, 11 and 20, and >20 cigarettes smoked per day, respectively. Accordingly, smoking accelerates systemic elimination of other substrates depending on CYP1A2,[17] including theophylline (28% increase in clearance[18]), tacrine and clozapine (some 20% higher doses were required to achieve target conClin Pharmacokinet 2000 Jun; 38 (6)

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Log [Caffeine clearance (ml/min/kg bodyweight)]

centrations in smokers[19]). Additionally, smoking also had an effect on the metabolism of drugs for which CYP1A2 is only one metabolising enzyme among others, including verapamil, flecainide, propoxyphene, propranolol, diazepam and chlordiazepoxide.[17,20] Data on 3 Ah receptor inducers originating from food are available. Interestingly, coffee caused induction of CYP1A2, which may reflect autoinduction by caffeine or originate from roasted coffee bean constituents, resulting in a 1.45-fold increase of caffeine clearance per litre of coffee drunk daily (fig. 1).[5] Grilled meat has been shown to induce CYP1A2 (and CYP1A1).[21] Finally, cruciferous vegetables cause significant, but small, increases in the turnover of CYP1A2 substrates[22] reaching less than 20% in the case of broccoli even when extensive amounts are consumed.[23] Studies have tried to use this inductive effect (as well as phenobarbital induction) for the experimental treatment of Crigler-Najjar syndrome, which is characterised

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by unconjugated hyperbilirubinaemia, in order to accelerate bilirubin elimination.[24] Therapeutic drugs acting as inducers via the Ah receptor are rare. In 1990, reports of induction by omeprazole[25] produced considerable upheaval. While the first in vitro results suggested that the effect might be very pronounced, in vivo studies in humans showed that the effect depended on omeprazole concentration and did not exceed a 1.5-fold mean increase in CYP1A2 activity.[26] With drugs as inducers of this receptor type, the main concern was not that a dosage adjustment for CYP1A2 substrates would be required, but that this type of induction mediates an increased activation of procarcinogens. Indeed, it has been speculated that 90% of all known procarcinogens are activated by CYP1A1 and 1A2.[27] The expression of CYP1A1 in humans may be increased considerably in liver, placenta and lung tissue upon exposure to inducers such as tobacco smoke. In some studies, a relationship between CYP1A2 activity as a marker of Ah receptor pathway induction and susceptibility to bladder, colon and lung cancer has been proposed.[28-30] However, it is not clear whether this relationship is causal as, for example, smoking causes both increased CYP1A2 expression and cancer. During discussion of the inductive properties of omeprazole, it became obvious that it is not clear whether activation of the Ah receptor pathway confers a risk by increased activation of procarcinogens or a benefit by a more rapid elimination of potentially dangerous chemicals. A possible genetic polymorphism in Ah receptor–mediated inducibility may contribute further to the complexity of the situation.[31]

−2 0

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2. Ethanol-Type Induction

Coffee drunk per day (L)

Fig. 1. Inductive effect of coffee drinking on the activity of the

human cytochrome P450 (CYP) enzyme CYP1A2. The data originate from a published epidemiological study conducted in 863 healthy volunteers[5] where CYP1A2 activity was derived from caffeine clearance. When taking other significant covariates into account (body mass index, smoking, oral contraceptives, country of residence and gender), coffee drinking significantly increased enzyme activity in a dose-dependent manner, as shown by an 1.45-fold increase in caffeine clearance per litre of daily coffee intake.

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This type of induction is different. Ethanol-type induction appears to be limited to a single main target, CYP2E1. Although transcriptional or posttranscriptional activation and message stabilisation may be involved in an increased de novo synthesis of CYP2E1, the most important mechanism providing more protein is stabilisation of the enzyme.[32] This is mediated by the binding of Clin Pharmacokinet 2000 Jun; 38 (6)

Clinical Consequences of Enzyme Induction

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Treatment just added

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Concentration

Baseline

No apparent change

Metabolism slowed down

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Enzyme

Enzyme

Enzyme

Enzyme

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Enzyme

Enzyme

S

Subtrate (drug)

I

Inhibitor

Baseline

Accelerated metabolism Concentration

Long term treatment

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Fig. 2. Pharmacokinetic consequences of cytochrome P450 (CYP) 2E1 stabilisation by active site occupation. (a) Beginning of treatment with inducer/inhibitor. The diagrams show changes in the concentration vs time profile of a single dose of a drug where

induction affects hepatic clearance as the only route of elimination. In this example, for simplicity it is assumed that the inhibitory effect of the inducer/inhibitor is a 2-fold reduction in clearance of the drug, that induction results in a 2-fold increase in clearance and that the inhibitor is either absent or present at a constant concentration. The baseline pharmacokinetics of the drug are included in all diagrams for a comparison. The closed circles represent drug concentrations during administration of the inducer/inhibitor. Inhibition occurs as soon as the ligand is available, but induction by decreased enzyme degradation to (partially) compensate for the inhibitory effect takes many hours to fully develop in this case. (b) End of treatment with inducer/inhibitor. Immediately after elimination of the inducer/inhibitor, enzyme becomes temporarily available for more rapid metabolism of the substrate.

inducers to the active site of the enzyme. It appears that 2 mechanisms of enzyme degradation exist, slow and fast.[33] In rats, CYP2E1 elimination half-lives of 7 and 37 hours were estimated for these 2 mechanisms of enzyme degradation; in humans conclusive data © Adis International Limited. All rights reserved.

are not available, but it appears that these half-lives are longer but retain the roughly 5-fold difference.[33] Rapid destabilisation of the enzyme presumably occurs during catalytic cycling and is not the result of intrinsic instability of the protein.[34] EthanolClin Pharmacokinet 2000 Jun; 38 (6)

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type inducers stabilise the enzyme from degradation by the fast mechanism, resulting in an accumulation of the enzyme.[33] However, in this case the increased amount of protein is not generally available for drug metabolism. The inducers bound to the binding site may act as competitive inhibitors. With this mechanism, immediately after the start of exposure towards an ethanol-type inducer, inhibition of the enzyme will usually be predominant (fig. 2a). After induction has fully developed, effective enzyme activity may return to pre-exposure levels, with higher amounts of the enzyme but with a lower specific activity. It is only after the end of exposure that a more rapid turnover of substrates is observed, thought to be typical for drug induction. This effect, however, would be limited to a short period of time as the accumulated enzyme is degraded and reaches pre-exposure amounts (fig. 2b).[6,33] Only a few drugs are metabolised by CYP2E1. For a single drug, the muscle relaxant chlorzoxazone, CYP2E1 is the relevant enzyme for overall elimination. The compound is therefore used as a probe drug for CYP2E1 phenotyping. However, CYP2E1 mediates a minor metabolic pathway for several other drugs, which often results in the formation of toxic metabolites. In ethanol metabolism, CYP2E1 accounts for less than 30% of formaldehyde formation.[6] The formation of reactive metabolites and inorganic fluoride by CYP2E1 has been described for several fluorinated volatile anaesthetics, including enflurane, halothane, methoxyflurane and sevoflurane. Fluoride formation is associated with nephrotoxicity, and reactive metabolites are thought to be involved in the formation of neoantigens causing halothane hepatitis.[35] A reaction mediated by CYP2E1 of considerable practical importance is the formation of N-acetyl-p-benzochinonimine from paracetamol.[36] Finally, CYP2E1 mediates the activation of many carcinogens. From the mechanism described above, it is not surprising that CYP2E1 substrates are often also inducers. The list of inducers includes ethanol and many organic solvents (such as acetone, benzene and carbon tetrachloride) and the antituberculosis © Adis International Limited. All rights reserved.

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drug isoniazid.[6,33] Furthermore, in patients with obesity and diabetes mellitus, increased CYP2E1 activity is observed, possibly mediated by ketone utilisation[37] and brought about by processes other than enzyme stabilisation. The pharmacokinetic consequences of ethanoltype induction of CYP2E1 are minor. As the contribution of CYP2E1-dependent metabolic pathways to overall metabolism appears to be negligible for nearly all drugs, chlorzoxazone is the only drug requiring a higher dosage in the presence of higher CYP2E1 activity. In contrast, toxicological consequences are more important, of possible relevance and highly complex. Ethanol itself is known to cause hepatotoxicity. The distribution of toxic lesions in the hepatic centrilobular regions coincidences with the expression of CYP2E1. This is also the area where cytotoxicity upon paracetamol overdose is predominant. Although inductive ethanol effects on CYP2E1 are mediated by ligand stabilisation, which is associated with enzyme inhibition, chronic ethanol intake appears to increase the net CYP2E1 availability for other substrates. This may be caused by fluctuations in ethanol concentrations, or by a relatively low affinity of ethanol for the CYP2E1 binding site compared with other ligands.[33] Indeed, regular ethanol consumption probably increases the risk of paracetamol-induced hepatotoxicity in humans,[6] although conclusive data are not available. The likely mechanism for increased toxicity is that the first step of paracetamol activation mediated in humans by CYP2E1 is accelerated by long term ethanol exposure, whereas inactivating enzymes are unchanged or even expressed in decreased amounts if alcoholic liver disease is already present (fig. 3). Similarly, it appears that the hepatotoxicity of inhaled anaesthetics is higher in individuals with a history of chronic ethanol use. This is supported by the observation that liver damage by halothane was blocked efficiently in non-alcoholic patients by pretreatment with disulfiram, a specific CYP2E1 inhibitor in humans.[35] The second most important inducer/inhibitor of CYP2E1 is isoniazid. In several studies[33] conducted Clin Pharmacokinet 2000 Jun; 38 (6)

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in humans, the immediate effect of standard doses of isoniazid was an inhibition of CYP2E1 to approximately 40% of the pre-dose value. This was determined both by formation of toxic paracetamol metabolites and/or by chlorzoxazone elimination. When this inhibitory action disappeared after elimination of the drug, in contrast a higher CYP2E1 activity was observed, reaching approximately 160% of the initial activity 24 hours after isoniazid intake. Depending on individual elimination rates for isoniazid, the inductive effect of isoniazid usually disappeared after 1 additional day.[33] In conclusion, the pharmacokinetic consequences of exposure to an ethanol-type inducer acting by ligand stabilisation differ considerably from those to be expected from increased enzyme expression. 3. Peroxisome Proliferator Induction Two classes of drugs, glitazones and fibrates, are known to bind to the 2 peroxisome proliferator activated receptors (PPARs), PPAR α and PPARγ.[38] However, the term ‘peroxisome proliferator’ may not be appropriate in humans, as peroxisome proliferation has been observed in laboratory animals but not in humans.[38] PPARα is involved in the transcription of genes encoding for proteins that control lipoprotein and fatty acid metabolism, whereas PPARγ controls adipocyte differentiation

and adipogenesis. PPARα ligands are used to treat patients with hypertriglyceridaemia and hypercholesterolaemia, and PPARγ is a possible target for the treatment of patients with liposarcoma.[39] By administering therapeutic doses of fibrates such as gemfibrozil or bezafibrate, triglyceride plasma concentrations in patients are usually lowered by between 30 and 40% and LDL-cholesterol concentrations by between 10 and 15%, whereas HDL cholesterol concentrations are increased by 5 to 10%.[40,41] Although the mechanism of action of peroxisome proliferators in mouse and rabbit has been investigated extensively, there are many gaps in our knowledge of the molecular mechanisms mediating the effects of fibrates and glitazones in humans. It appears that drug metabolising enzymes are not involved to a relevant extent in this type of induction. As the pharmacokinetic and toxicological consequences of peroxisome proliferators are currently difficult to define, this mechanism will not be discussed further. 4. Constitutive Androstane Receptor (CAR)/Phenobarbital-Type Induction Phenobarbital was among the first agents to be recognised as increasing the expression of CYP enzymes and reducing the effect of drugs metabolised Protein adducts

O

HN

O

N

CH3

CYP2E1

OH STs UGTs

Hydroxyl radicals

Lipid peroxyl radicals

H2 O 2

Lipid hydroperoxides

CH3

O GSTs

Nontoxic conjugates excreted renally

GSTs

Water

GSTs

Lipid hydroxides

Fig. 3. Pathways for formation of reactive and inactive substances of paracetamol (acetaminophen). The human cytochrome P450 (CYP) enzyme CYP2E1 appears to mediate the rate-limiting step in formation of the reactive metabolite N-acetyl-p-benzochinonimine

from paracetamol. Thereafter, the toxic effects depend on the balance between detoxification and covalent binding to proteins and other macromolecules. GST = glutathione S-transferase; ST = sulfotransferase; UGT = UDP-glucuronosyltransferase.

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by such enzymes in humans.[42,43] Soon thereafter, the clinical relevance of interactions of this type was described.[7] Typically, high inducer concentrations are required to achieve phenobarbital-type induction compared with other types of induction.[8] The expression of some 50 genes appears to be affected by phenobarbital.[44] Although dramatic increases in the expression of several drug-metabolising enzymes were observed, down-regulation also occurred in a major fraction of these genes. Until recently no receptor for phenobarbital or other chemicals causing the same pattern of change in protein expression had been found,[45] but new findings suggest that the orphan receptor CAR is the molecular target and mediator of phenobarbitaltype induction.[9,46] It is not entirely clear which drug-metabolising enzymes may be induced by phenobarbital-type induction and to what extent, as in vitro studies in human hepatocytes and/or mechanistic human in vivo studies are sparse. Therefore, current data are mainly based on drug interaction studies with phenobarbital and phenobarbital-type inducers. For the CYP enzymes, it appears that the most pronounced inductive effect is on CYP2B6, and that clear effects also exist on CYP2C8, 2C9, 3A4, 1A2 and some UGTs, whereas no induction of human CYP2C19 or 2D6 was seen.[10,47-49] Thus, the drugs metabolised by enzymes subject to phenobarbital-type induction include a major fraction of all drugs undergoing biotransformation. It is beyond the scope of this article to provide listings of substrates for human CYPs (such a list is available, for example, in Rendic and Di Carlo[27]). Many chemicals are known to have an induction pattern similar to that of phenobarbital, at least in animals.[2,27] The most important drugs used in humans showing this type of induction are anticonvulsants, mainly phenobarbital, phenytoin, carbamazepine and primidone, and the 2 oxazaphosphorines, cyclophosphamide and ifosfamide.[10,49,50] The following remarks are therefore focused on these compounds. As reported for isoniazid, with these anticonvulsants both induction and inhibition occur in paral© Adis International Limited. All rights reserved.

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lel.[50] This makes it extremely difficult to predict the concentrations of phenobarbital, phenytoin, carbamazepine and/or primidone when administered concomitantly. It is conceivable that the dynamic equilibrium between mutual induction and/or inhibition achieved after prolonged administration at constant dosages may easily be disturbed by external influences such as infectious diseases, food effects on pharmacokinetics, and physical exercise. Taking into account the long half-life of some anticonvulsants and the several days needed for enzyme induction to fully develop, it may take 3 weeks to achieve a new dynamic equilibrium upon any change in drug administration or metabolism.[50] This problem, together with the narrow therapeutic range of anticonvulsants and the existence of active metabolites, is a strong argument for therapeutic drug monitoring of these agents. From epidemiological studies, it appears that among the 4 inducing anticonvulsants mentioned, phenobarbital and phenytoin are not affected by mutual interactions to a relevant extent.[51] The effect of comedication on carbamazepine is verifiable but minor. Despite the potent inductive power of these anticonvulsants when given at high doses to laboratory animals, in a study by Yukawa and Aoyama[52] the mean increase in carbamazepine clearance was only 7% for valproic acid (valproate sodium) comedication, 16% for phenobarbital and 27% for the combination of 2 or more anticonvulsant drugs. Accordingly, the proportion of underdosed patients increased significantly from 3% to approximately 30%, because no appropriate dosage adjustments were carried out.[51] However, the situation is even more complex since carbamazepine metabolites are affected to a varying extent.[53] Inducing effects were more pronounced for valproic acid, which has no relevant inductive properties of its own. For this drug, comedication with phenobarbital, carbamazepine or phenytoin increased the proportion of patients with drug concentrations below the therapeutic threshold from 30% to between 60 and 90%.[51] Of course, as well as the mutual interaction between anticonvulsant drugs, there are many imporClin Pharmacokinet 2000 Jun; 38 (6)

Clinical Consequences of Enzyme Induction

tant interactions of anticonvulsants with other drugs (comprehensive reviews are available[50,54-56]). Induction may cause a several-fold decrease in plasma concentrations. This appears to be most important for oral administration of drugs undergoing a pronounced first-pass metabolism. For instance, pretreatment with phenobarbital 100 mg/day reduced the AUC of oral verapamil more than 4-fold, with only a minor effect on the AUC of intravenous verapamil.[57] Oxazaphosphorines are substrates and inducers of enzymes subject to phenobarbital-type induction.[10] Therefore, oxazaphosphorines are expected to participate in drug-drug interactions during antineoplastic polychemotherapy.[58] The impact on therapeutic outcome, however, remains to be examined. Part of the problem is that these drugs are able to induce their own metabolism. This has been shown in human primary hepatocytes[10] and patients, where clearance of high dose cyclophosphamide increased from one day to the next by 50%.[59] On the other hand, the pharmacokinetics of oxazaphosphorines are sensitive to enzyme induction by other drugs. Indeed, concentrations of active cyclophosphamide metabolites in blood were increased after pretreatment with phenobarbital.[60] As for drugs of the Ah receptor induction type, there is much debate as to whether enzyme induction of the phenobarbital type confers a cancer risk. Currently, it appears that there is no evidence for an increased risk in patients undergoing anticonvulsant therapy.[61] It should be pointed out that the molecular mechanism of phenobarbital-type induction often shows partial overlap with that of the PXR,[11] which was recently recognised to mediate CYP3A4 induction, for example by rifampicin.[8] As an aside, it may be more appropriate to call these receptors ‘mediators’, as their ligands do not have a high affinity for these molecules and de-suppression rather activation may be the more correct characterisation. Besides diverse affinities for the 2 ‘receptors’, differences in the impact of inducers on the expression of various proteins, including drugmetabolising enzymes, may be brought about by © Adis International Limited. All rights reserved.

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additional mechanisms, such as direct effects of drugs at the enzymes, or be attributable to the pharmacokinetic properties of the inducer. For instance, low-affinity inducers undergoing extensive first-pass metabolism in the gut wall would be expected to have no effect on hepatic enzyme expression, but an extensive induction of gut wall enzymes with a pronounced decrease in oral bioavailability of their substrates may occur. 5. Pregnane X Receptor (PXR)-Type Induction The induction formerly termed rifampicin/ glucocorticoid-type appears to be mediated by an orphan receptor. Lehmann and colleagues[8] recently showed that the so-called human pregnane X receptor (hPXR) binds to the rifampicin/dexamethasone response element in the CYP3A4 promoter region as a heterodimer with the 9-cis-retinoic acid receptor RXR and is activated by many CYP3A4 inducers, including several steroids, lovastatin, clotrimazole, rifampicin and phenobarbital. Target drugs of this type of induction are all CYP3A4 substrates, which account for roughly half of all drugs undergoing biotransformation in humans. Although the inductive effects of glucocorticoids have been clearly demonstrated in laboratory animals, albeit at very high doses,[62] in humans these steroids are the victims of drug interactions. To our knowledge, there are no clinical studies showing that glucocorticoids enhance the metabolism of CYP3A4 substrates in humans. In contrast, there are a number of very potent CYP3A4 inducers that are also active in humans when given at therapeutic doses, such as rifampicin and rifabutin. The loss of effect of oral contraceptives when taken with rifampicin was an early observation recently investigated in more detail.[63] In this study, daily doses of 300mg reduced both maximum plasma concentration and terminal elimination half-life of ethinylestradiol and norethindrone by roughly 60% for rifampicin and 30% for rifabutin. Increased spotting confirmed that prevention of inadvertent pregnancy may be compromised. Clin Pharmacokinet 2000 Jun; 38 (6)

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An even more pronounced induction with rifampicin was seen with verapamil. Long term treatment with rifampicin reduced the oral bioavailability of (S)-verapamil by 96%.[4] Accordingly, the effect of verapamil on atrioventricular conduction disappeared. A decrease in verapamil trough concentrations was seen in this study after 4 days of rifampicin treatment and had fully developed after 8 days. In contrast, the AUC after intravenous administration was reduced by only 20%. The difference may be explained by the high concentration of the inducers reached in the gut wall, the site where first-pass metabolism of verapamil is mediated. 6. Conclusions

• Clinical effects of enzyme induction are often deduced from animal data in the absence of valid investigations in humans. It appears that this fact needs to be emphasised as in many publications it is not taken into account. Additional studies in humans are needed. • Enzyme induction is highly variable and often accompanied by other complex mechanisms affecting pharmacokinetic processes. • Every drug has its own pattern of induction of drug metabolising enzymes, and several mechanisms of induction are often activated by a single drug, but to a different extent. • Knowledge of the mechanism of induction is not sufficient to predict the extent of induction in most cases, but may alert the clinician to potential dangers. • For any potent inducer, addition to or withdrawal from an existing drug regimen should be done gradually and with appropriate monitoring of therapeutic efficacy and adverse events. • Pronounced induction may render a drug therapy completely ineffective. Drugs undergoing CYP3A4-mediated first-pass metabolism in the gut are at high risk not only for grapefruit juice interactions[64] but also for induction effects. • The toxicological consequences of enzyme induction are rare and appear to be limited mainly to hepatoxicity in ethanol-type induction. © Adis International Limited. All rights reserved.

Fuhr

Acknowledgements The data were presented in part at the 7th European ISSX Meeting in Budapest, August 1999. The ISSX also printed a manuscript of this presentation as a ‘Short Course Manual’ (ISBN 9-634-20610-7) which was made available at the meeting.

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Correspondence and offprints: Prof. Dr med. Uwe Fuhr, Institut für Pharmakologie, Klinische Pharmakologie, Universität zu Köln, Gleueler Strasse 24, 50931 Köln, Germany. E-mail: [email protected]

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