CYTOCHROME P450 AND DRUG INTERACTIONS

D.K. BADYAL AND A.P. DADHICH

Indian Journal of Pharmacology 2001; 33: 248-259 EDUCATIONAL FORUM

Correspondence: D.K. Badyale-mail:[email protected]

CYTOCHROME P450 AND DRUG INTERACTIONS

D.K. BADYAL, A.P. DADHICH

Department of Pharmacology, Christian Medical College and Hospital, Ludhiana-141008.

Manuscript Received: 31.8.2000 Revised: 7.11.2000 Accepted: 10.3.2001

The cytochrome P450(CYP) enzyme family plays a dominant role in the biotransformation of a vastnumber of structurally diverse drugs. There are several factors that influence CYP activity directly or atenzyme regulation level. Many drug interactions are a result of inhibition or induction of CYP enzymes.Inhibition based drug interactions form a major part of clinically significant drug interactions. Six isoenzymesof the CYP enzyme system are mainly involved in metabolism of most of the drugs. CYP3A4 isoenzymeis the most predominant isoenzyme in the liver and is involved in the metabolism of approximately 30-40% of drugs.

The advances in the identification, isolation and characterisation of different isoenzymes of CYP enzymesystem made it possible to correlate a specific isoenzyme activity with the metabolism of a specific drug.This will help in prediction of in vivo drug interactions of new drugs based on their in vitro interactiondata. Based on knowledge of CYP isoenzymes involved in the metabolism of drugs, physicians maybetter anticipate drug interactions. This will enhance the use of rational drug therapy and better drugcombinations.

Cytochrome P450 isoenzymes drug interactionsKEY WORDS

SUMMARY

INTRODUCTION

The cytochrome P450(CYP) enzyme system con-sists of a superfamily of hemoproteins that catalysethe oxidative metabolism of a wide variety ofexogenous chemicals including drugs, carcinogens,toxins and endogenous compounds such as steroids,fatty acids and prostaglandins1. The CYP enzymefamily plays an important role in phase-I metabolismof many drugs. The broad range of drugs that un-dergo CYP mediated oxidative biotransformation isresponsible for the large number of clinically signifi-cant drug interactions during multiple drug therapy.

Clinical case reports or studies usually provide thefirst evidence of interaction between drugs. Severalrecent studies discuss such drug interactions at themolecular or enzyme level and therefore constitutean important link between clinical and experimentalresearch. Central to this approach is an understand-ing of the catalytic importance of individual CYPisoenzymes in particular metabolic pathways.

NOMENCLATURE OF CYP

Nomenclature of CYP enzyme system has been es-tablished by CYP nomenclature committee2. The namecytochrome P450 is derived from the fact that theseproteins have a heme group and an unusual spec-trum. These enzymes are characterised by a maxi-mum absorption wavelength of 450 nm in the reducedstate in the presence of carbon monoxide. Naming acytochrome P450 gene include root symbol “CYP” forhumans(“Cyp” for mouse and Drosophila), an Arabicnumeral denoting the CYP family (e.g. CYP1, CYP2),letters A, B, C indicating subfamily (e.g. CYP3A,CYP3C) and another Arabic numeral representing theindividual gene/isoenzyme/isozyme/isoform (e.g.CYP3A4, CYP3A5)2. Of the 74 gene families so fardescribed, 14 exist in all mammals. These 14 familiescomprise of 26 mammalian subfamilies2.

Each isoenzyme of CYP is a specific gene productwith characteristic substrate specificity. Isoenzymesin the same family must have >40% homology in their

CYP450-DRUG INTERACTIONS

amino acid sequence and members of the same sub-family must have >55% homology3. In the human liverthere are at least 12 distinct CYP enzymes2. At presentit appears that from about 30 isozymes, only six isoen-zymes from the families CYP1, 2 and 3 are involvedin the hepatic metabolism of most of the drugs. Theseinclude CYP1A2, 3A4, 2C9, 2C19, 2D6 and 2E13.

DRUG INTERACTIONS

Metabolic drug interactions between drugs representa major concern for the pharmaceutical industry, forregulatory agencies and clinically for health care pro-fessionals and their patients. It has been estimatedthat drug interactions may be fourth to sixth leadingcause of death in hospitalised patients in UnitedStates (U.S.)4. During the past few years a revolutionhas taken place in our understanding of drug inter-actions, mostly as a result of advances in the mo-lecular biology of the CYP enzyme system. Sev-eral factors directly or indirectly influence the CYPactivity. Many drug interactions are a result of induc-tion or inhibition of CYP enzymes.

Enzyme inhibition: Inhibition based drug interac-tions constitute the major proportion of clinically im-portant drug interactions. A drug may inhibit the CYPisoenzyme whether or not it is a substrate for thatisoenzyme. If the two drugs are substrate for thesame CYP isoenzyme then metabolism of one or boththe drugs may be delayed. Erythromycin and mida-zolam both are substrates for 3A4 isoenzyme so,there is competition for enzyme sites and metabo-lism of midazolam is inhibited5. Fluoroquinolonesantimicrobials and azole antifungals, although notmetabolised by CYP3A4 isoenzyme, cause rapidreversible inhibition of CYP3A4 isoenzyme6,7.Macrolide antimicrobials, fluoxetine, lidocaine andamiodarone cause slowly reversible inhibition of CYPisoenzymes7,8. These drugs are converted throughmultiple CYP dependent steps to nitroso derivativesthat bind with high affinity to the reduced form of CYPenzymes. Thus CYP enzymes are unavailable forfurther oxidation and synthesis of new enzymes istherefore the only means by which activity can berestored and this may take several days9. Cimetidineand macrolide antimicrobials directly form complexwith heme moiety of CYP isoenzyme9,10. Cimetidine,amiodarone and stiripentol are non-specific inhibi-tors of CYP450 enzyme system9,11. Reversible inhi-bition can be competitive or non-competitive.

The most selective CYP inhibitors generally fall intothe category known as mechanism based inhibitors.These drugs are substrates for the target CYP andare converted to reactive species that covalently bindto CYP isoenzymes leading to their inactivation. Thistype of inhibition is known as irreversible or mecha-nism based or suicide inhibition9,12. Several 17α -ethinyl substituted steroids e.g. ethinylestradiol,gestodene and levonorgesterol are reported to causemechanism based inhibition9,12.

The extent of competitive or non-competitive inhibi-tion for reversible inhibition is determined by the rela-tive binding constants of substrate and inhibitor forthe enzyme and by the inhibitor’s concentration. Thecritical factor for the “inhibition index” (the ratio of theintrinsic clearance in presence of inhibitor to that inthe absence of inhibitor), is the inhibitor’s concentra-tion relative to its Ki value (Ki -inhibition constantdetermined in vitro). Thus, the most potent revers-ible 3A inhibitors including azole antifungals and firstgeneration HIV protease inhibitors have Ki value be-low 1 µM. Inhibition is uncommon for compounds withKi value >75-100 µM8. For irreversible inhibition thecritical factor for inhibition is the total amount ratherthan the concentration of the inhibitor to which CYPisozyme is exposed. Lipophilic and large molecularsize drugs are more likely to cause inhibition8. Twocharacteristics make a drug susceptible to inhibitoryinteractions: one metabolite must account for >30-40% metabolism of a drug and that metabolic path-way is catalysed by a single isoenzyme3. Inhibitorwill decrease the metabolism of substrate and gen-erally lead to increased drug effect or toxicity of thesubstrate. If the drug is a prodrug then the effect isdecreased. The process of inhibition usually startswithin the first dose of the inhibitor and onset andoffset of inhibition correlates with the half-life of thedrug involved13.

Enzyme induction: Drug interactions involving en-zyme induction are not as common as inhibitionbased drug interactions but equally profound andclinically important. Exposure to environmental pol-lutants as well as large number of lipophilic drugscan result in induction of CYP enzymes. The mostcommon mechanism is transcriptional activation lead-ing to increased synthesis of more CYP enzyme pro-teins13. If a drug induces its own metabolism, it iscalled autoinduction as is the case with carbama-zepine3. If induction is by other compounds it is called

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foreign induction. Metabolism of the affected drug isincreased leading to decreased intensity and dura-tion of drug effects. If the drug is a prodrug or it ismetabolised to an active or toxic metabolite then theeffect or toxicity is increased. It is somewhat difficultto predict the time course of enzyme induction be-cause several factors including the half-life of drugand enzyme turnover determine the time course ofinduction13,14. Understanding these mechanisms ofenzyme induction and inhibition is extremely impor-tant in order to give appropriate multiple drug therapy.

Isoenzymes and drug interactions: The advancesin CYP enzyme system has made it possible to as-sociate specific enzyme activity with the formationof a particular metabolite and in some cases to iden-tify the major isoenzyme responsible for the totalclearance of a drug. Presently we know the individualisoenzymes involved in the metabolism of largenumber of important drugs. Induction or inhibition ofthese isoenzymes leads to clinically significant druginteractions. Individual isoenzymes and their druginteractions are as follows:

3A Isoenzymes: Members of the 3A subfamily arethe most abundant CYP enzymes in the liver and ac-count for about 30% of CYP proteins in the liver. Highlevels are also present in the small intestinal epithe-lium and thus makes it a major contributor to presys-temic elimination of orally administered drugs14. Thereis considerable interindividual variability in hepatic andintestinal CYP3A activity(about 5-10 fold)1. Since 40-50% of drugs used in humans involve 3A mediatedoxidation to some extent, the members of this sub-family are involved in many clinically important druginteractions8. 3A4 is the major isoenzyme in the liver.3A5 is present in the kidneys. Inducers of 3A4 usuallydo not upregulate 3A5 activity5,8.

Important drug interactions of 3A4 are listed inTable 1. Noteworthy is high concentration ofterfenadine, astemizole and cisapride when thesedrugs are taken concomitantly with azole antifungalsor macrolide antibiotics or fluoxetine or fluvoxamine.High concentration of these drugs lead to life threat-ening cardiac arrhythmias like torsades de pointesand ventricular fibrillation16,18,27. Terfenadine has beenwithdrawn in USA and the European Union32.Fexofenadine, the active metabolite of terfenadine,is now marketed as a noncardiotoxic alternative toterfenadine. U.S. Food and Drug Administration(FDA)

is currently considering to contraindicate the use ofselective serotonin reuptake inhibitors(SSRI) with thenon-sedating antihistamines41. Mibefradil, a newercalcium channel blocker, that preferentially blocks T-type calcium channels, has already been voluntarilywithdrawn from the market because of large numberof drug interactions42.

Grapefruit juice(GFJ) is often taken at breakfast inthe western countries when drugs are also oftentaken. GFJ contains bioflavonoids mainly naringinand furacoumarin which cause mechanism basedinhibition of presystemic elimination of a number ofdrugs and increase their bioavailability43-45.

An important interaction involving induction of 3A isthe reduction in efficacy of oral contraceptives byrifampicin and rifabutin, because of an induction ofthe 3A mediated metabolism of estradiol andnorethisterone, the components of oral contracep-tives50.

2D6: This isoenzyme represents <5% of total CYPproteins57. It has aroused great interest because ofits large number of substrates(30-50 drugs) and itsgenetic polymorphism3. It is also called debrisoquinehydroxylase after the drug that led to the discovery ofi ts genetic deficiency. Many psychotropic,antiarrhythmic and β-adrenergic receptor blocker drugsare substrates as well as inhibitors of 2D69,59. Impor-tant drug interactions of 2D6 are listed in Table 2.

IA2: This is the only isoenzyme affected by tobacco.Cigarette smoking may lead to threefold increase in1A2 activity. Theophylline is metabolised in part by1A2, which explains why smokers require higherdoses of theophylline than non-smokers70. Alcoholinhibits metabolism of caffeine, a substrate of 1A2.Alcohol has been reported to mask the 1A2 inducingpotential of smoking71. Exposure to polyaromatichydrocarbons found in charbroiled food can also in-duce this isoenzyme. This isoenzyme also causesmetabolic activation of procarcinogens to carcino-gens e.g. aromatic and heterocyclic amines72. Inter-actions involving 1A2 are shown in Table 2.

2C9: S-warfarin and phenytoin, both involved in largenumber of drug interactions, are metabolised mainlyby 2C93,73. St John’s wart a herbal antidepressant hasbeen reported to decrease levels of warfarin by induc-tion of 2C974. Interactions of 2C9 are shown in Table 2.

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Table 1. Drug interactions involving CYP3A4 isoenzymes.

Drugs affected (substrates)

INHIBITORS

Azole antifungals: Midazolam8, tacrolimus15, terfenadine16, triazolam17

Itraconazole Cisapride18, quinidine19, astemizole20, methylprednisolone21, buspirone22,felodipine23, vincristine24, atorvastatin25

Ketoconazole Terfenadine16, astemizole20, cyclosporine9, triazolam, alprazolam26

Fluconazole Terfenadine16, triazolam17

Macrolide antimicrobials: Tacrolimus15, astemizole20

Erythromycin Carbamazepine3, triazolam17, buspirone22, terfenadine27, simvastatin28

Clarithromycin Pimozide29, cyclosporin30, midazolam31

Selective serotonin re-uptakeinhibitors(SSRIs): Midazolam4, cisapride32

Fluoxetine Diazepam, alprazolam33, midazolam8, terfenadine34,35

Paroxetine Alprazolam33

Calcium channel blockers: Tacrolimus15

Verapamil Simvastatin28, carbamazepine, cyclosporine36

Diltiazem Triazolam17, carbamazepine, cyclosporine36, quinidine, simvastatin37,midazolam, alfentanil38

Nifedipine Midazolam4

Protease inhibitors Terfenadine, astemizole, cisapride39, midazolam40

Grapefruit juice Felodipine, nifedipine, nimodipine, nitrendipine, terfenadine, cyclosporin, midazolam,carbamazepine, simvastatin43, verapamil, prednisolone, ethinylestradiol44, artemether45

Ciprofloxacin Tacrolimus46

Cimetidine Carbamazepine3, quinidine19, cyclosporine, calcium channel blockers, benzodiazepines47

Propofol Midazolam48

Nafimidone, omeprazole Carbamazepine3,49

INDUCERS

Rifampicin Protease inhibitors9, diazepam, triazolam, midazolam17, estradiol, norgesterol50, lidocaine51,zopiclone, zolpidem52, ondansetron53

Rifabutin Protease inhibitors9, estradiol, norgesterol50

Carbamazepine Protease inhibitors9, midazolam17, itraconazole54, vincristine55

Phenytoin, Phenobarbitone Midazolam17, vincristine55, carbamazepine56

2C19: This isoenzyme also exhibits genetic polymor-phism. It is involved in metabolism of a number ofclinically important drugs e.g. omeprazole, diazepam,antidepressants and antimalarials76. Important inter-actions are listed in Table 2.

2E1: This isoenzyme is involved in metabolism oflow molecular weight toxins, fluorinated ether vola-

ti le anesthetics and procarcinogens79. Thisisoenzyme is inducible by ethanol and is responsi-ble in part for metabolism of acetaminophen. Themetabolite of acetaminophen formed is highly reac-tive and hepatotoxic. Alcohol dependant patients maybe at increased risk of acetaminophen hepatotoxicitybecause of induction of 2E1 by alcohol79. Isoniazidhas biphasic effect on 2E1 activity, a direct inhibitory

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Table 2. Drug interactions involving CYP2D6 isoenzymes.

Drugs affected (substrates)

CYP2D6

INHIBITORS

SSRIs: Metoprolol58, tramodol, codeine, ondansetron, tamoxifen59

Fluoxetine Imipramine, desipramine, nortriptyline, haloperidol33,34

Propafenone Propranolol, metoprolol, debrisoquine60

Cimetidine Propranolol, quinidine47

Quinidine Sparteine, debrisoquine61

Terbinafine Nortriptyline62

Amiodarone Flecainide63

CYP1A2

INHIBITORS

Fluvoxamine Melatonon64, tacrine, imipramine, clomipramine, theophylline, caffeine, clozapine65

Fluoxetine Clozapine66, desipramine33,34

Ciprofloxacin Caffeine, theophylline, antipyrine6, tacrolimus46, clozapine66

Grapefruit juice Caffeine44

Cimetidine Theophylline, warfarin47

Verapamil, diltiazem Antipyrine67, theophylline68

Estradiol, levonorgestrel Tacrine69

Omeprazole Theophylline49

INDUCER

Tobacco Theophylline70

CYP2C9INHIBITORS

Ketoconazole, metronidazole S-warfarin70,73

Fluconazole Phenytoin9, S-warfarin73

Amiodarone, benzbromarone, S-warfarin63,75

Cimetidine, stiripentol Phenytoin3,47

INDUCER

Rifampicin Phenytoin53

CYP2C19

INHIBITORS

Fluvoxamine Imipramine, clomipramine, amitriptyline, diazepam, chloroguanide65

Fluoxetine Imipramine, diazepam3

Omeprazole Diazepam49

Ticlopidine Phenytoin3

Cimetidine Imipramine, benzodiazepines47

Ketoconazole Omeprazole4

INDUCER

Artemisinin Omeprazole77

CYP2E1

INHIBITORS

Disulfiram, isoniazid Chloroxazone78

INDUCER

Ethanol Acetaminophen79

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effect immediately after its administration followed byan inducing effect because of CYP proteinstabilisation1. Important interactions of 2E1 are listedin Table 2.

Knowledge of substrates, inhibitors and inducers ofCYP isoenzymes assists in predicting clinically sig-nificant drug interactions.

Other factors affecting the expression of CYPproteins: Apart from drugs, the following factors mayaffect expression of CYP proteins and lead to signifi-cant alterations in drug interactions.

Age: Activity of CYP enzymes decreases with ad-vancing age in both the sexes. Metabolism ofantipyrine (metabolised by at least 10 CYP isoen-zymes, CYP1A2, 2A6, 3A4, 2B6, 2C8, 2C9, 2C18,2C19, 2D6 and 2E1), lidocaine, diazepam andtheophylline decreases in the elderly80. In vivo activi-ties of CYP1A2, 3A4, 2C9 and 2D6 have been re-ported to be low at birth, but maximally increased atthe young adult stage and decreased in old age81.

Gender: Gender based differences in metabolic ac-tivity of hepatic CYP isoenzymes have been identi-fied in humans. Women exhibit higher baseline 3A4activity than men and therefore a greater extent ofinteractions on average81. Clearance of diazepam andprednisolone is more in women, but clearance ofsome drugs like propranolol are more in men81. Malesubjects typically have been used in most drug stud-ies. US FDA now encourages inclusion of women inclinical trials and one of the reasons for this inclu-sion is variation in drug metabolising enzymes82.

Hormones: Testosterone deficiency decreases CYPactivity. This may the reason for decreased activity ofCYP enzymes in the elderly81. Estrogen has been foundto decrease oxidation of some drugs e.g. imipramine.Oral contraceptives(estrogen/progesterone combina-tion preparations) were reported to decrease clearanceof diazepam and chlordiazepoxide83. Menstrual cyclephases have variable effect on CYP activity.Theophylline clearance was found to be decreased inluteal phase81. Increased metabolism of antipyrine wasreported near the time of ovulation83. No effect of men-strual cycle phases was found on the clearance of pa-racetamol, nitrazepam, salicylates and propranolol83.Pituitary-liver axis is thought to be an important regula-tor of CYP expression. Growth hormone deficiency maylead to down regulation of CYP enzymes84.

Genetic polymorphism: As each isoenzyme is aspecific gene product, genetic variations influencethe expression of isoenzymes in different individualsand hence the capacity of the individual to metabo-lise drugs. Genetic polymorphism with clinical impli-cations has been described for 2D6, 2C19, 2C9 and1A29,85,86. These isoenzymes exhibit polymorphismwith number of allelic variants, the frequency of whichoften varies between different populations.

About 5-10% of Caucasians and 1% of Asians arepoor metabolisers of drugs metabolised by 2D69. Thefrequency of poor metabolisers in Indian populationis about 2-4.8%87. Poor metabolisers are more proneto adverse drug reactions because metabolism isdecreased and blood levels are high. CYP2D6 is in-volved in O-demethylation of codeine to morphine.Hence poor metabolisers exhibit impaired or no an-algesia after codeine administration8,87. Polymor-phism of 2D6 contributes to the pronouncedinterindividual variability observed in the eliminationof important drugs including tricyclic antidepressants,antipsychotics, mexiletine and several β-adrenergicreceptor blockers59. About 5% of whites and 20% ofAsians are poor metabolisers of drugs metabolisedby 2C199. The frequency of poor metabolisers in In-dian population is about 11-20%76. The Chinese arepoor metabolisers of 2C19 and are more prone tosedative effect of diazepam and they are prescribedlower doses of diazepam. In case of 1A2 only 3-4%of white population is poor metaboliser80.

An area of growing interest is that relating to conse-quences of inhibitory interactions of drugs subjectedto isoenzyme polymorphism. Dual drug therapy foreradication of helicobacter pylori is more beneficialin poor metabolisers of 2C19 than extensivemetabolisers. This is because of decreasedmetabolisn of omeprazole in poor metabolisers andinhibition of omeprazole metabolism byclarithromycin89. Poor metabolisers of phenytoin (less2C19 activity) will require lower doses of phenytoinand chances of interaction between phenytoin andfelbamate will be less3. Species variation in expres-sion of these isoenzymes may create difficulty in ex-trapolating results of drug interactions from animalsto humans. For example, in humans there is only oneactive 2DCYP isoenzyme, the 2D6 isoenzyme. In ratsthere are 5 or 6 different 2D isoenzymes. So, study-ing the effect of a drug interactions in rats may notbe relevant to humans90.

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Hepatic disease: Many studies have shown effectof liver disease on CYP enzymes. In cirrhotic pa-tients expression of 1A2, 2E1 and 3A isoenzymeswas decreased. There are reports of alteration ofclearance of drugs metabolised by 3A4 in patients ofcirrhosis9. Activity of 2C19 was reported to be de-creased in patients of liver disease, but activity of2D6 was unaltered91. Levels of 2C subfamily havebeen reported to be upregulated in patients of he-patic carcinoma9. Detailed knowledge of these isoen-zymes affected in disease states would be used toenhance the design of rational drug therapy.

Inflammation: Acute phase response inflammatorymediators have been reported to suppress CYP ac-tivity in humans. This inhibition can lead to abnor-mally high plasma levels and toxicity of drugs thatare metabolised by CYP dependent enzymes andhave low therapeutic ratio. This phenomenon hasbeen observed with oral anticoagulants in herpeszoster, theophylline in acute respiratory viral infec-tion, nifedipine in acute febrile infection and quininein malaria92. Tumour necrosis factor and interleukin-1, two major inflammatory cytokines probably play arole. These factors have been reported to inhibit CYPenzymes in rats and mice92.

Nutrition: Starvation and obesity are known to in-duce CYP enzymes in rodents, but in humans theseconditions inhibit CYP enzymes. Obesity has beenreported to increase metabolism of enflurane andsevoflurane in humans93.

Environmental factors: Cigarette smoking is knownto induce CYP enzymes. Smoking has been foundto increase clearance of phenacetin andtheophylline70. Charbroiled food can induce CYP en-zymes72.

Pregnancy: Pregnancy may induce CYP enzymes.Increased metabolism of metoprolol was found inpregnant women due to induction of 2D6isoenzyme94.

Keeping in view the above factors the dose of thesubstrate for affected isoenzymes must be altered.

PREDICTION OF INTERACTIONS

The prediction of inhibition based interactions hasbeen possible for new drug candidates as a result ofidentification of CYP isoenzymes and an increased

awareness of their in vitro and in vivo behaviour. Forany new drug the spectrum of drug interactions canbe predicted even before the drug reaches the clini-cal phase of development96,99. These predictions arebased on the following principles:

If the drug of interest is substrate: In vivo and invitro inhibition is isoenzymes specific and substrateindependent. Metabolism of all drugs that are me-tabolised by the same isoenzyme is inhibited by in-hibitors of that isoenzyme. Therefore, these drugsexhibit the same spectrum of interactions. For ex-ample phenytoin, warfarin and tolbutamide are me-tabolised by the same isoenzyme and exhibit a simi-lar spectrum of interactions, even though these drugsare unrelated chemically, pharmacologically andtherapeutically3,73.

If the drug of interest is inhibitor: The potential forany new drug to inhibit the various isoenzymes ofCYP can be assessed in vitro using probes. If thenew drug inhibits one isoenzyme at therapeutic con-centration, we can predict that it will interact with anysubstrate of that isoenzyme. It is important to notethat a drug may inhibit an isoenzyme whether or notit is a substrate of that isoenzyme. For example,fluconazole inhibits the major isoenzyme (2C9) me-tabolising phenytoin, but it is not a substrate for thatisoenzyme. In fact, its major route of elimination isrenal8. In principle, the situation for rapidly reversibleinhibition is relatively straightforward i.e. the degreeof inhibition depends only on the dose and elimina-tion kinetics of the inhibitor and its affinity for theisoenzyme. However, for slowly reversible or mecha-nism based inhibition the situation is more complex,since not only are these factors important but in ad-dition the rate of enzyme complexation/inactivationand synthesis as well as the degradation of theholoenzyme are determinants8. Accordingly in vivoprediction of these types of inhibition is difficult. Acritical aspect of any a priori prediction concerns theaccuracy of the Ki determined in vitro. Experimentalconditions such as incubation time, concentration ofdrug at the enzyme site and non-hepatic metabo-lism limit the ability of in vitro system to predict invivo interactions8,99. Although most of the predictionsare still at the qualitative level, quantitative predic-tions have been achieved in some situations to esti-mate the extent of in vivo interactions from in vitrodata. For example, midazolam is almost completely

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metabolised by 3A4 isoenzyme. In vitro interactionsby ketoconazole reveal that Ki is <0.1 µM/L. Know-ing this Ki value and achievable plasma concentra-tion of ketoconazole(>1 µg/L), one could predict fromin vitro model developed by Rowland and Martin thatclearance of midazolam after oral administration ofketoconazole is decreased by 95%. This predictionwas confirmed by a clinical study in which midazolamclearance was decreased by 94% after ketoconazoleadministration4.

CONCLUSION

Drug interactions involving inhibition and inductionof CYP enzymes will undoubtedly continue to be ofscientific interest and clinical importance simply be-cause of enzyme’s role in metabolism of currentlyavailable and future drugs. Mibefradil, terfenadine andcisapride provide classical examples indicating thecritical importance of CYP enzyme mediated druginteractions in the development, regulation and ulti-mate economic success of drugs33,38. Our knowledgeof the isoenzymes involved in metabolism of drugswill allow a prediction of interactions of these drugswith new drugs, to be developed in the future. In-deed an editorial from US FDA suggests that thistype of information will be required for future newdrugs100.

By understanding the unique functions and charac-teristics of CYP enzymes, physicians may better an-ticipate and manage drug interactions. This practicewill increase in future and will result in the formationof a rational information base that would indicate drugcombinations to be avoided. For example inhibitorsor inducers of 3A4 isoenzyme would not be givenalong with substrates of 3A4 and instead will receivealternative drugs that are not inhibitors or inducersof 3A4. This will improve rational drug use and facili-tate better selection of drug combinations.

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REGIONAL RESEARCH LABORATORY, JAMMUDr. I.C. CHOPRA MEMORIAL AWARD

FOR THE YEAR - 2001

To commemorate the contributions of Dr. I.C. Chopra, Ex Head (1957-1964), RegionalResearch Laboratory, Jammu, an annual "Dr. I.C. Chopra Memorial Award" in the area of phar-macology was instituted last year through the kind generosity of Mrs. M. Chopra W/o LateDr. I.C. Chopra.

Applications on the prescribed format are invited for Dr. I.C. Chopra Memorial Award for the year-2001 (Cash award of Rs.10,000 along with a medal and citation) from the Indian Nationals withproven contribution in the field of Drug development & General / Biochemical Pharmacology.

Please contact for further information:The Director,Regional Research Laboratory,Canal Road, Jammu Tawi-180 001.

Last date for receiving filled in applications in all respects is September 30, 2001.

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CYTOCHROME P450 AND DRUG INTERACTIONS