Chapter 7 Drug Metaboliam 416 15 12 direct renal clearance 9 metabolic clearance 6 OO 0 -1.0 -0.5 0 0.5 1.0 1.5 2.0 Log D7.4 Figure 7.1Effect of lipophilicity on direct renal clearance and on metabolism drug(see Chapter 8).The pharmacological response of a drug may be altered if a metabolite has a new activity:in some cases,the metabolite has the same activity and a similar or different potency as the drug.A change in drug absorption and drug distribution(that is,the tissues or organs in which it is concentrated)also can result when it is converted into a much more polar species. The majority of drug metabolizing enzymes also catalyze reactions on endogenous compounds.Consequently,the function of these enzymes may be metabolic disposition of endogenous cellular modulators,and it may be fortuitous that they also catalyze the metabolism of drugs and other xenobiotics.The greater affinity of the endogenous substrate over drugs in many cases seems to support this notion;however,many of these enzymes are very broad in specificity or are induced only on the addition of a particular chemical.so it is not clear that at least some of these enzymes have not evolved to protect the organism from undesirable substances. Aswas discussed in Chapter3(Section 3.2.E.2.p.143)and Chapter4(Section 4.1.B.1.a. p.176).the interaction of a chiral molecule with a receptor or enzyme produces a diastere- omeric complex.Therefore,it is not surprising that the processes in drug metabolism also are stereoselective,if not stereospecific.511 Stereoselectiviry can occur with enantiomers of drugs,in which case one enantiomer may be metabolized to a greater extent by one pathway and the other enantiomer predominantly by another pathway to give two different metabolites. Another type of stereoselectivity is the conversion of an achiral drug into a chiral metabolite. In both of these cases a difference in rates leads to unequal amounts of metabolites rather than exclusively to one metabolite.In many cases,however,a racemic drug is metabolized as if it were twodifferent xenobiotics,each enantiomer displaying itsown pharmacokinetic and phar macodynamic profile In fact,it was concluded by Higniter2 that"warfarin enntomer should be treated astwodrugs"Silbereral.53 concluded that"S-(-)-and R-()-propranolol
Secon7.4 Pathways for Drug Deactivation and Elimination are ssentiay two distinct entities pharmacologically"These are just two of many examples 417 of drugs whose enantiomers are metabolized by different routes.In some cases the inactive enantiomer can produce toxic metabolites which could be avoided by the administration of o theactivenothercses,the inctivesmrmaythmetabo of the active isomer. The metabolism of enantiomers may depend on the route of administration.For example, the racemic antiarrhythmic drug verapamil hydrochloride (7.9.Isoptin)is 16 times more potent when administered intravenously than when taken orally because ofeeive hepatic presystemic elimination that occurs with oral administration(the first-pass effect:see Section 7.1).(55]The(-)-isomer,which is 10 times more potent than the(+)-isomer,is preferentially metabolized during hepatic metabolism and,therefore,there is much more of the less potent (+)-isomer available by the oral route than by the intravenous route. HC OCHs CH;O OCH CH3 verapamil hydrochloride 7.9 In some cases one enantiomer of a drug can be metabolized to the other enantiomer. The therapeutically inactive R-isomer of the analgesic ibuprofen(7.10,Advil)is converted enzymaticallys61 in the body to the active S-isomer.571 The racemization occurs by initial conversion of the carboxylic acid group of ibuprofen to the corresponding S-CoA thioester, followed by racemization,then hydrolysis to the twoenantiomers of ibuprofen.58)If aracemic mixture is administered,a 70:30 mixture of S:R is excreted;if a 6:94 mixture of S:R is administered,an 80:20 mixture of S:R is excreted. COOH ibuprofen 7.10 In the next two sections the various types of metabolic biotransformations are described. I do not intend to imply that those reactions are the only ones that occur with those par- ticular drugs:rather,they are just examples of the particular metabolic biotransformation under discussion.Some examples are included to show the effect of stereochemistry on drug metabolism.Keep in mind,however,that when a racemic mixture is administered,the metabo- lites observed may be different from those detected when a pure isomer is used.Many of the metabolites that are described are derived from in vitro studies (metabolites produced using either purified enzymes,subcellular fractions,whole cells,or tissue preparations).However, after analysis of a large amount of data on in vitro metabolism and a comparison with in vivo metabolism.it appears that it is valid to look at in virro data as a reasonable guide for the prediction of invivo metabolism.9 It should be noted that some drugs are excreted without being metabolized at all
Chapter 7 Drug Metabollsm 418 7.4.B Phase I Transformations Mueller andMll showed that thenmetabolism mthyminazobe B.1 Oxidative Reactions obestudedrat liver homogenates.It was demonstrated that then systemwas functionifnctinmideadenne dinucltie phosphate(NADP+)m euyand both the microsmal and solubefractionsfom the liver homogenates erenedLater Brodiefound that the oxidative activitywasnh microsomal and that the soluble fraction coud be replaced byither NADPHr NADPH-generaing system.This system was active toward a broad spectrum of structurally diverse compounds.Because it required both and a reducing system,it was classified s amixefn2that is.one atom of the is transferred to the substrate.and the other undergoes a two-electron reduction and is converted to water.This classification was confirmed when it was shown63]that aromatic hydroxylation of acetanilide by liver microsomes in the presence of resulted in incorporation of one atom ofinto the productand that a heme protein was an essential compoent for this reaction.When this heme protein was reduced and exposed to carbon monoxide,a strong absorption in the visible spectrum at 450 nm resulted.Because of this observation,these microsomal oxidases were named cytochrome P450. Cytochrome P450 now represents a superfamily of enzymes containing a heme cofac- tor but with structurally variable active sites that catalyze the same reaction on different substrates,namely,the oxidation of steroids,fatty acids.and xenobiotics.651 These related cytochrome P450 enzymes are referred to as isozymes.Although about 500 genes encode different cytochrome P450 isozymes,only about 15 of these are very important in drug metabolism.The primary site for these enzymes is the liver,but they also are present in lung.kidney.gut,adrenal cortex,skin,brain,aorta,and other epithelial tissues.The heme is noncovalently bound to the apoprotein.Cytochrome P450 is associated with another enzyme. NADPH-cytochrome P450reductase,a flavoenzyme that contains one molecule each of flavin adenine dinucleotide(FAD)and flavin mononucleotide(FMN).661 Heme-dependent oxida- ion reations were discussed in Chapter4.Section 4.3.D.p.212.As shown in Scheme 4.35. the NADPH-cytochrome P450 reductase reduces the flavin.which,in tu,transfers an elec ton tothe heme-oxygen complex of cytochrome P450.Actually,the FAD accepts electrons from the NADPH.the FADH-then transfers electrons to the FMN.and the FMNH-donates the electron to the heme or heme-oxygen complex of cytochrome P450.1671 In general,cytochrome P450 catalyzes either hydroxylation or epoxidation of various sus(Table 7.1)and is believed to operate via radicatermediates (see Chapter. Scheme 4.35).When the concenrations of cytochrome P450and other drug metabolizng dug metabois becmesltered.Many rugsnd hemichr theirown metabolismor the metabolism ofotherdrugsin of theirnuc of cytochrome P450and NADPH-cytochrome P450rductas Thedmtboioftherugs when multiperug osOne om mechanism leadingtou hnof ychrome P450ymby ther ro(perc)an herbal remedy for therea ofephhsomm reereNaturePTheof herbhery hyperforn ()ouno athe pregnane Xrecepr which is thekey reuaor of cytochrome 53sym
Secon 7.4 Pathways for Drug Deactivation and Elimination 419 TABLE 7.1 Classes of substrates for cytochrome P450 Functional Group Product R-人 OH ArCH,R ArCHR OH CHR OH RCH-R R CHR OH RCHR RCHR' OH RCHz-X-R' RCH-XR RCHO+RXH (X =N.O.S.halogen) OH R-X-R' R-X-R (X=NR.S) transcription.71 Because of the increased expression of this P450 enzyme by hyperforin, drugs that are metabolized by this isozyme,which may be more than half of all drugs,are rapidly degraded when taken with St.John's wort. hyperforin 7.11 The other most common mechanism leading to drug-drug interactions arises when mul- tiple drugs are administered and one of the drugs inhibits drug metabolism of the others asa result of its inhibition of cytochrome P450 isozymes or other enzymes.Adverse effects and toxicity of drugs from drug-drug interactions have been one of the major concems in clinical practices and in new drug approval.A better characterization of the metabolic pathways and the enzymes involved in its metabolism is useful in understanding the underlying mechanisms
420 Chapter 7 Drug Metabolism ofdrudrugineractionOn the bright side.as discussed in Chapter5.Section.4E.6 rukown to inhibit cytochrome P450(such asinir)odbe used in mbnio with her drug(suchas lopinavir)to block metabolism of the other druginteniolly Some properties of molecules lead to preferred metabolism by cytochrome P50. Lipophilicity is important for the binding of a molecule tocytochrome P450.whichxn why increasing lipophilicity leads toan increase in metabolic clearance (see Figure7.1. p.416).The next most important physicochemical property of a molecule for cytochrome P450 metabolism is the presence of an ionizable group.This group may have a keyroe binding o the active site and determine regioselectivity of the metabolic reaction.f th ionizable group isa secondary or tertiary amine.it may direct the reaction toocuathat part of the molecule(see below).The reaction and site of reaction catalyzed by cytochrome P450s are determined by(1)the topography of the active site of the particular isozyme,(2) the degree of steric hindrance of the heme iron-oxo species to the site of reaction,and(3)the ease of hydrogen atom abstraction or electron transfer from the compound that is metabolized by the isozyme. Another important family of enzymes involved in drug oxidation is the microsomal flavin monooxygenases731(see Chapter 4.Section 4.3.C.p.205);mechanism of oxidation was described in Scheme 4.34,p.212.According to this scheme,the flavin peroxide intermediate is an electrophilic species,indicating that the substrates for this enzyme are nucleophiles such as amines,thiols,and related compounds(Table 7.2).The enzyme contains one FAD molecule per subunit and,as in the case of cytochrome P450.requires NADPH to reduce the flavin.It has been found that nucleophilic compounds containing an anionic group are excluded from the active site of this enzyme.Because most endogenous nucleophiles contain negatively charged groups,this may be how Nature prevents normal cellular components from being oxidized by this enzyme.Oxygenation of some of these compounds leads to reactive intermediates,74 for example,mercaptopyrimidines and thiocarbamides.Metabolism by flavin monooxygenases often can be differentiated from other oxygenases because of its unique stereoselectivity.For example,cytochrome P450s oxidize(S)-nicotine(7.12)to a mixture of cis-(7.13)and trans- N-1'-oxides(7.14),but flavin monooxygenase oxidizes it exclusively to the trans-N-1'-oxide. CH CH (S)-nicotine CH 7.12 7.13 7.14 Other enzymes involved in oxidative drug metabolism include prostaglandin H syn- thase (see Section 7.4.B.1.e.p.430).alcohol dehydrogenase,aldehyde dehydrogenase(see Section 7.4.B.1.i,p.447),xanthine oxidase,monoamine oxidase,and aromatase.These enzymes,however,are involved,for the most part,in the metabolism of endogenous compounds. a. Aromatic Hydroxylation Boyadhypoheed hatromaicomounds ere metabolizednil ng().This postulate wascomedn9byagrou of HeaNH)hosdptaxromthemicrm ofthan(Scheme 73).Kinetic isotope effectstudiohowever indc thatrectnehihly unliely proces nsed ncd