In vitro hydrolysis of oxazepam succinate half-ester by a stereospecific soluble esterase from different animal species

Prodrug strategies to overcome poor water solubility

Drug design in recent years has attempted to explore new chemical spaces resulting in more complex, larger molecular weight molecules, often with limited water solubility. To deliver molecules with these properties, pharmaceutical scientists have explored many different techniques. An older but time-tested strategy is the design of bioreversible, more water-soluble derivatives of the problematic molecule, or prodrugs. This review explores the use of prodrugs to effect improved oral and parenteral delivery of poorly water-soluble problematic drugs, using both marketed as well as investigational prodrugs as examples. Prodrug interventions should be considered early in the drug discovery paradigm rather than as a technique of last resort. Their importance is supported by the increasing percentage of approved new drug entities that are, in fact, prodrugs.

Synthesis, stereoselective enzymatic hydrolysis, and skin permeation of diastereomeric propranolol ester prodrugs

Four diastereomeric propranolol ester prodrugs (1S2S, 1S2R, 1R2S, 1R2R) were synthesized by treating pure R- and S-propranolol hydrochloride with pure enantiomers R- and S-phenylbutyryl chloride. A HPLC technique using alpha-1 acid glycoprotein (chiral AGP) column was developed to study the racemization of propranolol enantiomers during synthesis and hydrolysis studies. A reversed phase HPLC method was also developed to simultaneously analyze propranolol and the ester prodrug. Hydrolysis of these esters was studied in different rat tissue homogenates, i.e., liver, intestine, plasma, skin, brain, and pure plasma cholinesterases, i.e., butyryl cholinesterase (EC 3.1.1.8) and acetyl cholinesterase (EC 3.1.1.7). In vitro percutaneous permeation studies across full thickness shaved rat skin were performed using standard side-by-side diffusion cells at 37 degrees C. The disappearance of the diastereomeric ester prodrugs in rat tissue homogenates followed apparent first-order kinetics and was stereoselective. The ratio of brain to plasma hydrolytic rate constants are 27.8, 5.58, 6.07, and 2.97 for 1S2S, 1R2R, 1R2S, and 1S2R esters, respectively. Hydrolysis of all four diastereomeric ester prodrugs was faster by acetyl cholinesterase than butyryl cholinesterase and is stereoselective. The permeability coefficients [Kp x 10(3) (cm h-1)] are 1.40 +/- 0.30, 1.41 +/- 0.27, 42.20 +/- 1.24, 29.26 +/- 3.41, 16.27 +/- 3.12, 12.99 +/- 2.84 for (R)-propranolol, (S)-propranolol, 1S2S, 1R2S, 1S2R, and 1R2R ester prodrugs, respectively. The results indicate that the 1R2S diastereomeric ester prodrug of propranolol shows greatest stability in liver and intestinal tissues while it exhibits fairly rapid conversion in plasma. The results also suggest the configuration on the second chiral carbon atom to be the determinant in the rate of hydrolysis of all the diastereomeric prodrugs in all biological media examined. The Kp of all four prodrugs markedly increased compared to that of the parent drug, with 1S2S showing a 30-fold increase in skin permeability, the highest among all four prodrugs.

Stereochemical determinants of the nature and consequences of drug metabolism

Enantiomeric discrimination in drug disposition depends on the mechanism of the process under consideration. Absorption, distribution and excretion are generally passive processes which do not differentiate between enantiomers, but enzymic metabolism and protein binding, to plasma or tissue proteins, can show a high degree of stereoselectivity. In terms of metabolism, chiral discrimination occurs at both substrate and product levels, giving rise to five distinct stereochemical courses for drug metabolism, namely (i) prochiral-->chiral, (ii) chiral-->chiral, (iii) chiral-->diastereoisomer, (iv) chiral-->non-chiral and (v) chiral inversion. As a result, the metabolic and pharmacokinetic profiles of enantiomers after administration of racemic drugs can be very variable, so that the exposure to the two enantiomers may be very different. There now an enormous number of examples of each of these possibilities. The net result of the interaction of the stereoselectivities of these various processes can obscure the fact that one (or more) shows a marked stereoselectivity. This is particularly the case for metabolism: while the ratios of the total plasma clearance of the enantiomers of a wide range of drugs never exceed 2, individual metabolic pathways often show much greater stereoselectivity. This is particularly evident for those high-affinity, low-capacity enzyme systems which exhibit genetic polymorphism, namely the human cytochromes P450 2C18 and 2D6. This review provides an introduction to the stereoselectivity of drug metabolism.

Enantioselectivity of esterases in human brain

Subcellular fractions of three human brain specimens were found to contain esterase activities which hydrolyzed racemic oxazepam 3-acetate (rac-OXA). All three human brain preparations were highly selective toward the S-enantiomer of rac-OXA.

Stereoselective enzymatic hydrolysis of various ester prodrugs of ibuprofen and flurbiprofen in human plasma

The hydrolysis kinetics of various alkyl, glycolamide, aminoethyl, and 2-(1-imidazolyl)ethyl esters of ibuprofen and flurbiprofen in 80% human plasma were investigated using a direct high-performance liquid chromatographic assay for the enantiomers of these acids. In each case, the R-isomer ester was found to undergo faster plasma-catalyzed hydrolysis than the corresponding S-isomer. The difference in the hydrolysis rates between the enantiomeric forms ranged from a factor of 1.4 for the N,N-diethylglycolamide ester of ibuprofen to a factor of 50 and 25 for the 2-(1-imidazolyl)ethyl ester of ibuprofen and flurbiprofen, respectively. Therefore, enantioselective differences in plasma-catalyzed ester prodrug hydrolysis must be taken into account when evaluating prodrugs of racemic mixtures of chiral drugs.

Stereoselective hydrolysis of O-acetyl propranolol as prodrug in rat tissue homogenates

The stereochemical characteristics of the hydrolysis of O-acetyl propranolol were studied using phosphate buffer (pH 7.4), rat plasma, and rat tissue homogenates. In the phosphate buffer, no difference was observed in the hydrolysis rate between the esters of (R)- and (S)-propranolol. In rat plasma and tissue homogenates, hydrolysis of the ester was both accelerated and stereoselective. Hydrolysis of O-acetyl (R)-propranolol was five times faster than that of the (S)-isomer in rat plasma. However, in the liver and intestine homogenates, the (S)-isomer was hydrolyzed faster than the (R)-isomer. Interconversion between the (R)- and (S)-isomers was not observed under the experimental conditions. The same stereoselective hydrolysis was also observed with racemic O-acetyl propranolol. However, observed rate constants for the hydrolysis were lower than those for the pure isomers. These results indicate that enzymatic hydrolysis of O-acetyl propranolol occurred stereoselectively and the selectivity of the plasma enzyme was different from those of liver and intestine enzymes.

In vitro hydrolysis of RR,SS-threo-methylphenidate by blood esterases--differential and enantioselective interspecies variability

Enantioselective in vitro hydrolysis of methylphenidate (MPH) by the blood esterases of seven mammalian species is reported. The species included rats, rabbits, dogs, cattle, horses, monkeys, and humans. In vitro incubations up to 8 h were carried out in plasma, red blood cells, and whole blood of the various species. Enantioselective differences were evident among the different species on comparison of the data obtained from the three biological fluids. The esterases present in plasma appeared to show greater activity in the hydrolysis of MPH in all species where comparison with the other two biofluids was possible. Only in the case of humans did esterases present in plasma and red blood cells demonstrate opposite enantioselectivity in the hydrolysis of MPH. Thus after 8 h incubation, the RR-MPH/SS-MPH ratios in plasma and red blood cells were 0.31 and 1.16, respectively.

Racemization kinetics of enantiomeric oxazepams and stereoselective hydrolysis of enantiomeric oxazepam 3-acetates in rat liver microsomes and brain homogenate

Enantiomers of oxazepam and of 3-O-acyl, 1-N-acyl-3-O-acyl, and 3-O-methyl ether derivatives of oxazepam were resolved on HPLC columns packed with Pirkle's chiral stationary phases [CSP; (R)-N-(3,5-dinitrobenzoyl)phenylglycine or (S)-N-(3,5-dinitrobenzoyl)leucine] bonded either ionically or covalently to spherical particles of gamma-aminopropylsilanized silica, and on a column packed with poly-N-acryloyl-(S)-phenylalanine ethyl ester bonded covalently to silica gel (Chiraspher). Resolution was achieved, with several mobile phases of different solvent compositions and with varying chromatographic resolutions, on all of the chiral stationary phases tested. Resolved enantiomers of oxazepam undergo racemization, whereas enantiomers of 3-O-acyl and 3-O-methyl derivatives are stable. Racemization half-lives of oxazepam enantiomers were determined by monitoring changes in ellipticity as a function of time on a spectropolarimeter immediately (within 30 s) following resolution of enantiomers and were found to substantially vary, depending on the solvents used. Rates of hydrolysis of racemic and enantiomeric 3-O-acyl-oxazepams by esterases in liver microsomes and brain homogenate of rats were determined by a simple and sensitive CSP-HPLC method. The relative rate of hydrolysis was 3R greater than racemate much greater than 3S by rat liver microsomes and 3S greater than racemate much greater than 3R by rat brain homogenate.

Identification and characterization of hepatic carboxylesterases hydrolyzing hydrocortisone esters

The present study has provided evidence for the existence of three distinct carboxylesterases involved in the hydrolysis of steroid esters, where two enzymes are possibly responsible for the metabolism of hydrocortisone hemisuccinate (HCHS) at pH 5.5 and 8.0, and a third enzyme for the metabolism of hydrocortisone acetate (HCAC) at pH 8.0, in isolated rat liver microsomes. The activity of all three enzymes in rat liver was induced significantly by the administration of phenobarbital while no such function in enzyme activity was observed in animals receiving 3-methylcholanthrene or benzo[a] pyrene under similar experimental conditions. The increase in the activity of HCHS esterase I (HCHS-E1) active at pH 5.5, HCHS esterase II (HCHS-E2) active at pH 8.0, and HCAC esterase (HCAC-E) was approximately 7 to 8, 3- and 3-fold respectively. On the other hand, the degree of induction of nonspecific microsomal carboxylesterase acting on p-nitrophenylacetate (PNPA) was significantly less. The Km values for the hydrolysis of HCHS at pH 5.5 and 8.0 and HCAC by rat liver microsomes obtained from control rats were 2.45, 2.02 and 1.6 mM, respectively, and these Km values were not changed significantly in preparations obtained from rats treated with phenobarbital. The distinct in vitro responses displayed by hepatic microsomal steroid esterases to various inhibitors were able to distinguish three different enzymes which also differed from nonspecific carboxylesterases. The activity of HCAC-E was inhibited by NaAsO2 and AgNO3 while that of HCHS-E1 and HCHS-E2 remained unaffected. Selective inhibition of HCHS-E1 by NaF, HgCl2 and p-chloromercuribenzoate and that of HCHS-E2 by NiSO4 indicated the possible existence of different enzymes or isozymes of a carboxylesterase catalyzing HCHS hydrolysis. The effects elicited by the inhibitors on the activity of PNPA esterase were different from those observed with steroid esterases. Furthermore, the present study has also indicated species variations in the distribution of steroid esterases in the livers of rat, mouse, dog and cat.

Characterization of aspirin hydrolase of guinea-pig liver cytoplasm

Previous subcellular fractionation studies of guinea-pig liver had shown aspirin hydrolysing activity to be located mainly in the microsomal fraction, and was due to a single carboxylesterase (EC 3.1.1.1) (White, K.N. and Hope, D.B. (1981) Biochem. J. 197, 771-773). However, activity had been found in all cell fractions, and in this study they were analysed simultaneously by slab gel electrophoresis for aspirin hydrolysing activity. Two enzymes were identified, one of which was associated only with the particulate cell fractions and was the microsomal carboxylesterase described previously. The other activity was located exclusively in the cytoplasmic fraction, and could be inhibited by bis(4-nitrophenyl)phosphate, so identifying it as a carboxylesterase. It has an unusually low molecular weight of 35 000, compared with values of 60 000, or multiples thereof, normally found for liver carboxylesterases. It contributes about 14% of the total aspirin hydrolysing activity of liver homogenates, and could be distinguished from its particulate counterpart by differences in molecular weight and in sensitivity to inhibition by bis(4-nitrophenyl)phosphate (see White, K.N. and Hope, D.B. (1984) Biochim. Biophys. Acta 785, 138-147).

Esterase activity of rat muscle

The esterasic capacity of a series of skeletal muscles in response to three hemisuccinate ester drugs was investigated in rats and compared to that on alpha-naphthylacetate as a reference esterase substrate. Marked variations between different muscles and between given muscles of animals of different sex were observed, indicative of a complex heterogeneity in muscular expression of esterase activity.

Biochemical and pharmacological properties of oxazepam

Oxazepam is the final metabolic product in vitro and in vivo of a large number of pharmacologically active benzodiazepines. Oxazepam shows antimetrazol activity varying in intensity and duration according to the animal species considered. This difference is in part related to different "sensitivity" and in part due to different disposition of oxazepam. Particularly relevant is the difference in biliary excretion by various animal species. Oxazepam is currently available as a racemate but two optical isomers can be separated as succinate half esters. The (+) form appears to be more active than the (-) form, probably because more oxazepam is released from the (+) than the (-) isomer in vivo. In vitro studies confirm that the liver hydrolyzes the (+) oxazepam succinate half ester more than the (-) form. Other work has aimed at analyzing the effects of oxazepam on brain chemical mediators, with particular reference to the cholinergic system. Finally it is shown that oxazepam, similarly to other benzodiazepines, increased aggressiveness in male mice during chronic treatment.

Stereospecificity of esterases hydrolyzing oxazepam acetate

Esterases hydrolyzing the racemic acetate ester of the centrally acting drug oxazepam in mice were examined. Radiolabeled ester administered intravenously was hydrolyzed rapidly in the liver, kidneys, and brain. The distribution of the enzyme activity of liver and brain subcellular fractions was measured. Kinetic data and structure investigation of partially hydrolyzed racemic ester pointed to the stereoselectivity of liver and brain esterases. The preferred hydrolysis of the (R)-(-)-isomer in liver homogenates was attributed mainly to microsomal enzymes, while that of the (S)-(+)-isomer in brain was considered to be due to the mitochondrial fraction. This phenomenon was a common property of all species tested.

Relationship of in vitro hydrolysis of 17-chloroacetylajmaline and 17-acetylajmaline in different animal species

17-Chloroacetylajmaline and 17-acetylajmaline are reported to have in vivo antiarrhythmic activity and are metabolized by hydrolysis. Since the hydrolysis product, ajmaline, may be the actual antiarrhythmic agent, the hydrolysis of these derivatives by various tissues of the guinea pig, rat, and mouse was determined in vitro by a titrimetric method and compared to hydrolysis by alpha-naphthylacetate. The heart is the most active tissue in the guinea pig for hydrolyzing 17-chloroacetylajmaline. The hydrolyzing activity is greater in the guinea pig than in rat or mouse heart, corresponding with the more significant pharmacological activity in the guinea pig. 17-Chloroacetylajmaline has a significantly lower Km value than 17-acetylajmaline, which is in agreement with the in vivo activity.