Section 7.4 Pathways for Drug Deactivation and Elimination 421 TABLE 7.2Classes of substrates for flavin monooxygenase Functional Group Product R-NR2 R-NR2 R-NHR H R-N R-NR R=NR NHOH R-N-NHR R R-N-NHR R 's0 R-CNH- R-CNH2 RNH RNH 一SH RN+ H R 2RSH RSSR RSSR 2RSO heme iron-oxo species may undergo electrophilic addition to the aromatic ring to give either a tetrahedralintermediate radical(7.15)or cation(7.16)(Scheme 7.4).similar to the mechanism described for the addition of the corresponding heme iron-oxo species to alkenes as was discussed in Chapter 4(Section 4.3.D:Scheme 4.37).Because [1.2]-shifts of hydrogen and alkyl radicals are energetically unfavorable,an electron could be transferred to the Felv of the heme,giving a carbocation species bound to Felll(7.16).A[1.2]-shift of hydride to a cation is energetically favorable,leading to the cyclohexadienone(7.17.pathway c). which would tautomerize to the phenol (7.18).Arene oxide(7.19)formation may arise from either 7.15 or 7.16.Usually,arene oxides can undergo rearrangements to arenols,hydration (catalyzed by epoxide hydrolase)to the corresponding trans-diol,reaction with glutathione (catalyzed by glutathione S-transferase)to the B-hydroxy sulfide,and reactions with various macromolecular nucleophiles(Scheme 7.5). The rearrangement of an arene oxide to an arenol is known as the NIH shift because a research group at the NIH proposed that mechanism based on studies with specifically deuterated substrates(Scheme 7.6).1781 Ring opening occurs in the direction that gives the more stable carbocation (ortho-or para-hydroxylation when R is electron donating).Also
422 Chapter 7 Drug Metabolism P450 H +O2+NADPH+H H NADP'+HO Scheme 7.3Cytochrome P450 oxidation of naphthalene electron transfer 7.19 7.15 7.16 日7.17 7.19 HO- 日7.18 Scheme 7.4Addition-rearrangement mechanism for arene oxide formation because of an isotope effect on cleavage of the C-D bond,the proton is preferentially removed. leaving the migrated dcuterium.Although there is an isotope effect on the cleavage of the C-D versus the C-H bond,this step is not the rate-determining step in the overall reaction: consequently,there is no overall isotope effect on this oxidation pathway when deuterium is incorporated into the substrate. In competition with the [1,2]-hydride(deuteride)shift is deprotonation(dedeuteronation; Scheme 7.7).The percentage of each pathway depends on the degree of stabilization of the intermediate carbocation by R:the more stabilization(i.e.,the greater electron donation ofR). the less need for the higherenergy hydride shift,and the more deprotonation(dedeuteronation) occurs.79)For example,when R=NH2.OH,NHCOCF3 and NHCOCH3,only 0-30%of the product phenols retain deuterium(NIH shift).but when R=Br.CONH2,F.CN.and Cl, 40-54%deuterium retention is observed. Whenever a deuterium is incorporated into a drug for metabolism studies,however.the problem of metabolic switching has to be considered.This is when deuteration at one site in the molecule changes the partition between two metabolic pathways because the deuterium isotope effect on C-H bond cleavage at the deuterated site leads to an increase in metabolism at a different site in the molecule.8
Section 7.4 Pathways for Drug Deactivation and Elimination 423 rearrangement OH R epoxide hydrolase H2O OH OH glutathione S-transferase GSH ✉OH SG macromolecular nucleophiles X OH Scheme 7.5Possible fates of arene oxides P450 B H 02 NADPH 一一 HO Scheme 7.6 Rearrangement of arene oxides to arenols(NIH shift) HO Scheme 7.7 Competing pathway for the NIH shift
424 Chapter 7 Drug Metabollam CH NH. 7.20 CH NH, HO Scheme 7.8 NIH shift of a chloride ion The NIH shift also occurs with substituents as well as with hydrogen.For example. rat liver metabolizes p-chloroamphetamine (7.20)to 3-chloro-4-hydroxyamphetamine8 (Scheme 7.8).However,a common approach to slow down metabolism of aromatic com- pounds is to substitute the ring with a para-fluorinel82)or para-chlorine atom,which deactivates the ring and decreases the rate of oxidative metabolism.831 For example,the anti-inflammatory drug diclofenac(7.21,Voltaren)is metabolized to 4-hydroxydiclofenac with a half-life of I hour.The related analog with a para-chloro substituent,fenclofenac (7.22.Flenac).has a half-life of more than 20 hours.84] COOH COOH NH C diclofenac fenclofenac 7.21 7.22 Arelated substituent NIHshift was observed in the metabolic oxidation of the antiprotozoal agent tinidazolel851(7.23.Scheme 7.9.Fasigyn). As in the case of electrophilic aromatic substitution reactions,it appears that the more electron rich the aromatic ring(R is electron donating).the faster the microsomal hydroxyl- ation will be Aniline (electron rich)for example,undergoes extensiveand para-hydroxylation,whereas the strongly eecro-orrioridrug probenecid(4. Benemid)undergoes no detectable aromatic hydroxylation.(881 In the case of drugs with two rmiringsthe more electron-ichone(r lesselecro poor).generally.is hydroxylated. The antipsychotie drug chlorpromazine(7.25.R=H.Thorazine),for example,undergoes 7-hydroxylation(5.ROH)However,the bindingorenion theactivestefh hydoxylasess well as the activtion of theromtic rings playsnmonboth the rate and position of hydroxylation on the aromatic ring
Section 7.4 Pathways for Drug Deactivation and Elimination 425 CH tinidazole SO,Et 7.23 SO,Er CH SO.E Scheme 7.9 NIH shift of a nitro group SO2N(CH2CH2CHj)2 co. HOOC- CH,CH:CH2NMez probenecid chlorpromazine(R=H) 7.24 7.25 R R H phenytoin(R:R2=R3 H) 7.26 Aromatic hydroxylation,as is the case for all metabolic reactions,is species specific. In humans,para-hydroxylation is a major route of metabolism for many phenyl-containing drugs.The site and stereoselectivity of hydroxylation depend on the animal studied.In humans the antiepilepsy drug phenytoin (7.26:R=R2=R3=H.Dilantin)is para-hydroxylated at the pro-S phenyl ring (7.26.R=OH,R2=R3=H)10 times more often than is the pro-R ring (7.26,R3 =OH.R=R2 =H).In dogs,however,meta-hydroxylation of the pro-R phenyl ring (7.26.R2=OH.R=R3H)is the major pathway.The overall ratio of R:RR hydroxylation in dogs is 18:2:1 Mera-hydroxylation may be catalyzed by