I4123

{RCQ- represents palmitate or oleate residues and ArCHiCH^X^O- a 2-arylpropionyl residue )

benoxaprofen undergo significant inversion in man, whereas the reaction either does not occur or is relatively minor for indoprofen, flurbiprofen, ketoprofen and carprofen. In addition to chiral inversion a number of these agents show stereoselectivity in plasma protein binding (e.g. fraction unbound ibuprofen enantiomers S>R) and in other routes of metabolism, e.g. glucuronidation and oxidation. In the majority of cases following administration of the racemic drug the plasma concentrations and areas under the plasma concentration versus time curves (AUC), of the active S-enantiomers exceed those of their R-antipodes (e.g. benoxaprofen, carprofen, fenoprofen, flurbiprofen, ibuprofen, indoprofen), but others, e.g. ketoprofen and tiaprofenic acid, show similar plasma concentrations and AUCs. The dispositional properties of these agents are also complicated by enantiomer-enantiomer interactions. For example both enantiomers of ibuprofen show concentration-dependent plasma protein binding and compete for binding sites, which may explain the differences observed in their pharmacokinetic parameters when administered as single enantiomers or as the racemic mixture.

For those 2-arylpropionates marketed as racemic mixtures and for which chiral inversion is a significant route of metabolism, the effective dose of the active agent is unknown. In the case of these agents the R-enantiomers act essentially as pro-drugs for their active S-antipodes. The extent of the inversion reaction would also be expected to vary within the population, possibly with disease state and thus any attempt to relate plasma concentrations to clinical effect must take the stereochemistry of the circulating drug into account. The use of the single S-enantiomers of these agents offers a number of advantages: accurate dosing, simplification of pharmacokinetics and concentration-effect relationships and avoidance of the potential problems due to hybrid triglyceride formation and inhibition of fatty acid metabolism.

Sulindac

Sulindac (4.7), a benzylidine analogue of indomethacin, is a pro-drug (see Chapter 7) the anti-inflammatory activity of which resides in the sulphide metabolite (4.124), the other major metabolite is the sulphone derivative (4.125). Sulindac is used as the racemic

(4.134) (4.125)

sulphoxide and little is known concerning the stereoselectivity of the reduction to the active sulphide or the oxidation to the sulphone. The stereochemistry of the sulphide (4.124) oxidation back to sulindac (4.7) has been investigated in vitro and the reaction appears to show stereoselectivity for the formation of the (+)-enantiomer. The stereochemistry of the material in plasma will therefore depend on potential stereoselectivity in sulphoxide reduction, sulphide oxidation and oxidation of the sulphoxide to the sulphone (4.125). It is therefore likely that the enantiomeric composition of sulindac will vary with time but the clinical significance of this is unknown.

4.6.6 Antimicrobial agents

P-Lactam antibiotics

The majority of the P-lactam antibiotics are semisynthetic agents, the stereochemistry of the 6-aminopenicillanic (6-APA; 4.126) and 7-aminocephalosporanic (7-ACA; 4.127) nucleii being determined as 35, 5R, 6Rand 6R, 7R respectively. The introduction of an a-substituted acyl side chain on the 6-APA and 7-ACA nucleii results in the introduction of an additional chiral centre and the formation of two epimers, e.g. ampicillin (4.35), carbenicillin (4.128) and cephalexin (4.36). In the case of ampicillin (4.35) and cephalexin (4.36) the official preparations are those containing the D-configuration in the side chains which correspond to the R-designation using the sequence rule system. The differential absorption of the epimers of cephalexin have been referred to previously (Section 4.4.1). The two epimers of ampicillin differ in terms of their aqueous solubility and activity, the epimer of the D-configuration in the side chain having two to five fold greater activity, depending on the test system used, than that of the L-epimer. Unlike the above examples carbenicillin is used as an epimeric mixture. The individual epimers of carbenicillin show only slight differences in activity, but more importantly are stereochemically unstable undergoing rapid epimerisation in solution. The contribution of the stereochemical instability to the observed lack of difference in activity is by no means clear, but in the case of carbenicillin separation of the individual epimers would appear to be a futile exercise.

Moxalactam (4.22), a l-oxacephem derivative, is also used as a mixture of two epimeric forms, designated R and S with respect to the acyl side chain chiral centre. The antimicrobial activity of the compound resides predominantly in the R-epimer which is approximately twice as active as (S^-moxalactam depending on the test system used. The two epimers are stereochemically unstable and undergo epimerisation to yield equilibrium mixtures in the ratio R:S of 50:50 and 45:55 in buffer and serum respectively. The rates of epimerisation vary depending on the environment and epimeric form but at 37°, in serum, the half-life of epimerisation is the same for both compounds (1.5 h). Following intravenous infusion of the epimeric mixture to man the serum elimination half-life of the "total" drug is about 2.3 hours, and the serum concentrations of the less active S-epimer are approximately twice those of the R-epimer within four hours with a ratio R/S in renal clearance of 1.5.

In terms of the relative merits of single isomers versus stereoisomeric mixtures compounds such as moxalactam present considerable problems as the half-life of epimerisation under physiological conditions is only slightly shorter than the serum elimination half-life. It would be difficult under such circumstances to recommend the use of a single stereoisomer.

fi-Lactam Pro-drugs

The poor oral availability of a number of the penicillins and cephalosporins has resulted in the synthesis of lipophilic ester pro-drugs (see Chapter 7). The majority of these are not simple esters but involve the introduction of an acyloxymethyl or acyloxyethyl function into the molecule. These groups undergo rapid enzymatic hydrolysis in vivo to yield the corresponding hydroxymethyl or hydroxyethyl esters which, being hemiacetal derivatives, spontaneously cleave with liberation of the active P-lactam and the corresponding aldehyde. The introduction of an hydroxyethyl function into the promoiety results in an additional chiral centre and therefore a pair of diastereoisomers, e.g. cefuroxime axetil (4.129) and cefdaloxime pentexil (4.130), which may differ in terms of their physicochemical properties and also their susceptibility to enzymatic hydrolysis.

Cefuroxime axetil (4.129) undergoes hydrolysis in vivo to yield cefuroxime, acetaldehyde and acetic acid and is used as an equal parts mixture of the two epimers. Following oral administration to man the pro-drug can not be detected in the systemic circulation and shows a bioavailability based on urinary recovery of cefuroxime of between 30 to 50%.

Stereoselectivity in the hydrolysis of the pro-drug occurs by both serum and intestinal mucosal esterases isolated from both rat and dog tissue. In all cases the S-epimer is hydrolysed selectivity but the selectivity varies between 2.5 to 14 fold with both tissue and species. Such Stereoselectivity in hydrolysis in the gut may contribute to the observed bioavailability of the liberated cefuroxime in man.

Cefdaloxime is poorly absorbed from the gastrointestinal tract and has been esterified to yield the pivaloylethyl pro-drug (4.130). The bioavailability and pharmacokinetics of cefdaloxime have been investigated following the administration of the individual and an equal parts mixture of the pro-drug diastereoisomers to experimental animals. Following administration to the dog the bioavailability of cefdaloxime was three times greater after dosing with the S-epimer than the R. It has been reported that a similar situation occurs in man and the 5-epimer has been selected for further development.

Stereoselectivity in the absorption of diastereoisomeric pro-drugs and therefore the subsequent availability of the drug, may arise as a result of differential solubility at the absorption site, rates of diffusion through the gut wall and enzymatic activity in the intestinal mucosa, liver and blood, and as such the potential problems associated with the introduction of a chiral promoiety into a molecule need to be taken into consideration at the compound design stage.

Quinolones

The quinolones are synthetic antibacterial agents based on the 1,4-dihydro-4-oxopyridine-3-carboxylic acid ring system. An important subgroup of these agents possess a tricyclic fused ring structure with a chiral centre in the saturated ring, e.g. ofloxacin (4.131), flumequine (4.132) and methylflumequine (4.133).

The antibacterial activity of these agents has been shown to reside in the enantiomers of the 5-absolute configuration, the R-enantiomers being considerably less active than the corresponding racemates, the 5-enantiomers having approximately twice the activity of the racemates. In the case of ofloxacin (4.131) the difference in in vitro enantiomeric activity ranges from 8 to 128 fold against both Gram-positive and Gram-negative bacteria. In addition the corresponding non-chiral analogues of methylflumequine (4.133) and ofloxacin (4.131), i.e. structures (4.133) and (4.131) where R1=R2=H, are more active

COOH

COOH

5l

flumequine (4.132)

CHj

H

H

(R)-

flumequine (4.132)

H

CHj

H

(S)-

methylflumequine (4.133)

CH3

H

CH

methylfluineqiiine (4.133)

H

CH,

CH;

ofloxacin (4.131)

Cit,

H

-

(R) ■

ofloxacin (4.131)

H

CHj

-

than the R-enantiomers but less active than the racemates. Such data implies steric constraints at the site of action with the orientation of the methyl group attached to the chiral centre hindering the interaction in the case of the R-enantiomers and enhancing the interaction of the S-enantiomers.

The target enzyme of the quinolones is believed to be DNA gyrase (bacterial topoisomerase II) and good correlations between the IC50 concentrations for enzyme inhibition and antimicrobial activity, as determined by MIC concentrations, have been obtained for this series of compounds. In the case of ofloxacin the rank order of potencies for enzyme inhibition is identical to that observed for MIC activity, with the S-enantiomer being 9.3 and 1.3 fold more active in terms of enzyme inhibition, than the R-enantiomer and the racemate respectively. As there are similarities between DNA gyrase and mammalian topoisomerase II it is useful to evaluate the activity of the quinolones on the enzyme and hence their effects on mammalian cells. The rank order of potency of ofloxacin isomers against mammalian topoisomerase II is the same as that obtained with DNA gyrase, i.e. S>R,S>R. However, the relative activity of the two enantiomers decreases from 12.4 with DNA gyrase to 1.8 against topoisomerase II. More importantly the S-enantiomer is 6.7 fold more selective than the R-isomer with respect to the DNA gyrase. The non chiral analogue (4.131, R1=R2 =H) of ofloxacin is the least selective of the compounds examined. Thus, the presence and orientation of the methyl group at the chiral centre not only determines the potency of these compounds but also increases their selectivity of action.

A number of quinolone derivatives have also been developed which are substituted at carbon-7 of the bicyclic ring system and contain a chiral centre in the substituent, e.g. temafloxacin (4.134). In comparison to ofloxacin and related derivatives, differences in

the enantiomeric activities of the 7-substituted compounds are of relatively minor significance. For example the enantiomers of temafloxacin show only small differences in activity in in vitro test systems and possess similar activities against DNA gyrase. This difference in stereoselectivity of action between the two series of quinolones is presumably due to the centre of chirality in the 7-substituted compounds being in a position remote from the critical binding region of these molecules.

4.7 TOXICOLOGY

The process of drug safety evaluation is complex, expensive and time consuming involving acute and chronic toxicity testing, mutagenicity and genetic toxicology, reproductive toxicology, carcinogenicity and clinical safety evaluation both pre- and post-marketing. There is also a need to carry out mechanistic and toxicokinetic studies in order to determine the animal exposure to both the drug and metabolites and to aid in the extrapolation of animal data to man. At present there is relatively little published data on the comparative toxicity of single enantiomers versus racemic drugs and even less information arising from clinical studies. However, examples may be cited which are illustrative of aspects of stereochemical considerations in safety evaluation.

Fenvalerate (4.135), is a synthetic pyrethroid insecticide which contains two chiral centres, and thus four stereoisomers are possible. Administration of the compound in the diet to a range of animal species resulted in granulomatous changes in the liver, lymph nodes and spleen. Separation and toxicological evaluation of the individual stereoisomers indicated that the toxicity was associated with only one of the four isomers. Subsequent metabolic studies indicated that the toxicity was associated with the formation and disposition of a cholesteryl ester, (Rj-2-(4-chlorophenyl)isovaleric acid cholesterylester (4.136), formed by transesterification of the single toxic stereoisomer of fenvalerate. Fortunately the active isomer of fenvalerate may be synthesised stereospecifically. While not a drug this example does indicate that stereochemical considerations may prevent a compound being discarded following an adverse toxicological evaluation.

A similar situation in terms of stereoselective toxicity appears to occur with the potassium channel activator cromakalim the activity of which resides in the (-)-(3S,4R)-enantiomer (4.137). Administration of high doses of the racemate to the monkey resulted in the development of heart lesions which appear to be associated with the (+)-enantiomer. This compound is now under development as the single (-)-enantiomer.

CHCOOCH

CH(CH3)2

0 0

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