Hugh R. Sullivan

Disposition of methadone in methadone maintenance

Six detoxified opiate addicts housed in a closed metabolic ward received methadone in stepwise increasing doses of 10, 20, 40, and 80 mg/day during 1 month. Four were given 14C‐methadone at the lowest dose and again at the highest dose. Of the subjects receiving radiomethadone, 2 excreted the major part of the radioactivity in urine and 2 about equally in urine and feces. In addition to methadone, 7 metabolites were isolated and identified in urine and 3 metabolites in feces. About 75% of the urinary and fecal radioactive metabolites were unconjugated. Urinary excretion of methadone and its major N‐monomethylated metabolite accounted for 17% to 57% of the given dose. The ratio of metabolite to parent drug increased in 5 of 6 subjects, and the urinary recovery of unchanged methadone decreased during the period. The results indicate that enhanced demethylation of methadone may occur during oral administration to man.

A particulate arachidonate lipoxygenase in human blood platelets.

Abstract The microsomal fraction of human platelets contains a lipoxygenase activity in addition to the thromboxane-synthesizing activity. The enzymatic activity was stimulated by tryptophan, but inhibited by catecholamine, methemoglobin, and hydroquinones.

Biosynthesis of thromboxane B2: Assay, isolation, and properties of the enzyme system in human platelets

The microsomal fraction of human platelets catalyzed the conversion of arachidonic acid to an unstable platelet-aggregating factor and a hydrolyzed product on the thin-layer chromatography (TLC). This product was isolated on TLC, purified by silica gel column chromatography and identified by combined gas chromatography-mass spectrometry as the hemiacetal derivative of 8-(1-hydroxy-3-oxopropyl)-9, 12L-dihydroxy-5, 10-heptadecatrienoic acid (thromboxane B2). The enzymatic activity was dependent upon methemoglobin and tryptophan as cofactors. Reduced glutathione had no effect either alone or in combination with other cofactors. Methemoglobin could be replaced by hematin or hemin; and tryptophan by 3-indolacetic acid or catecholamines. The apparent requirement for methemoglobin is due to the reductive activity of ferriprotoporphyrin IX. The reaction, however, catalyzed by the ferriprotoporphyrin IX in the thromboxane synthesizing system is different from that described for the decomposition of lipid peroxides. Certain transition metals and hydrogen donors, such as hydroquinone and ascorbate, which have been shown to stimulate the catalytic activity of ferriproroporphyrin IX in the decomposition of 15-hydroperoxy-prostaglandin E1 are inhibitors of thromboxane B2 formation. This enzyme preparation also transformed eicosa-8. 11, 14-trienoic acid to an unknown product on TLC. The enzyme system was rapidly inactivated upon incubation in the reaction mixture.

The metabolite pattern of d-propoxyphene in man. The use of heavy isotopes in drug disposition studies.

Abstract Through the combined use of deuterium labeling and GC-MS analysis, eight urinary metabolites of d-propoxyphene have been identified in man following a single oral dose of 130 mg of d-propoxyphene. The metabolites present were: norpropoxyphene, dinorpropoxyphene, cyclic dinorpropoxyphene, propoxyphene carbinol, norpropoxyphene carbinol, dinorpropoxyphene carbinol, p-hydroxypropoxyphene and p-hydroxynorpropoxyphene. The latter two metabolites occur as conjugates. The persistent metabolite, previously identified as norpropoxyphene, was found to be a mixture of norpropoxyphene and dinorpropoxyphene.

Microsomal hydroxylation of ethylbenzene. Stereospecificity and the effect of phenobarbital induction

Abstract The microsomal monooxygenase system responsible for the oxidative metabolism of drugs has been studied intensively for the past decade. Considerable progress has now been made toward the understanding of the oxygen activating pathways of this syste, (cf. Omura et. al.). However the nature of the final step, the interaction of an activated oxygen species and substrate, is little understood. One probe of this reaction would be a study of the stereochemical nature of the reaction. The experiments presented here represent a reinvestigation of the hydroxylation of ethylbenzene to methylphenylcarbinol. In earlier work in intact rabbits, Smith, Smithies and Williams working with glucuronides found that both the D and L carbinols were formed although the ratio of D to L was not determined. We have now found that intact rats convert ethylbenzene to methylphenylcarbinol which is a mixture of 90.3% of the D(+) isomer and 9.7% of the L(-) isomer. In vitro hydroxylation in a microsomal preparation proved to be less stereospecific; the product contained 80.9% of the D(+) isomer. Studies with phenobarbital induced rats were of particular interest. In this case a reduction in stereospecificity was observed in both the in vivo and in vitro experiments. The role of the microsomal membranes in drug hydroxylations is discussed in light of these findings.

Reaction Pathways of in vivo Stereoselective Conversion of Ethylbenzene to (-)-Mandelic Acid

1. Mandelic acid formed in vivo from ethylbenzene as well as from various oxidation intermediates was laevo mandelic acid and was of surprisingly high optical purity. 2. Reaction sequences are proposed for the stepwise oxidation of ethylbenzene to mandelic acid. 3. Although the initial hydroxylation of ethylbenzene to methylphenyl-carbinol is stereoselective, the optical activity of mandelic acid is not established at this point since the optical centre is destroyed in the second step, dehydrogenation to acetopheneone. 4. Acetophenone appears to be a precursor of not only mandelic acid and benzoylformic acid but benzoic acid as well. 5. The route from acetophenone involves conversion to omega-hydroxyacetophenone and subsequent reduction to glycol and/or oxidation to phenylglyoxal. 6. The configuration of mandelic acid is determined either during reduction of hydroxyacetophenone or reduction of phenylglyoxal.

The metabolism of the herbicide diphenamid in rats.

Abstract The in vivo metabolism of the herbicide diphenamid (N.N-dimethyldiphenyl-acetamide) was studied in rats. The compound appeared to be well absorbed and relatively easily metabolized to excretable metabolites. The main route of metabolism was found to be N-dealkylation to nordiphenamid which in turn was excreted as an N-glucuronide. The most interesting metabolite found in urine was the O-glucuronide of N-methyl-N-hydroxymethyl diphenylacetamide. The N-hydroxymethyl compound, often suggested as an unstable intermediate in the N-demethylation reaction, was in this case stabilized by glucuronide formation and excreted in urine. A minor pathway of metabolism of diphenamid was found to be p-hydroxylation.

Metabolism of [14C]Cefaclor, a Cephalosporin Antibiotic, in Three Species of Laboratory Animals

The metabolic fate of the orally effective cephalosporin antibiotic cefaclor (Lilly 99638) has been studied in rats, mice, and dogs. Cefaclor is efficiently absorbed from the gastrointestinal tract as the intact antibiotic. In rats and mice, cefaclor, for the most part, escapes metabolism in the body and is eliminated unchanged as unaltered antibiotic, primarily by renal excretion. In dogs, however, cefaclor is more labile to metabolism and only a portion of the administered antibiotic is eliminated unchanged via the kidney.

The difference in activity between (+)- and (-)-methadone is intrinsic and not due to a difference in metabolism.

The disposition and metabolism of (+)‐ and (‐)‐methadone has been compared in rats. At equal molecular doses, somewhat higher plasma levels of (‐)‐isomer were observed. At equal analgesic doses, brain and plasma concentrations of (+)‐methadone were at least 25 times greater than those of (‐)‐methadone. No qualitative differences were observed between isomers with respect to in vivo metabolic pattern or in vitro N‐demethylation rates. The results strongly support the conclusion that the large differences in analgesic potency between the isomers is due to an intrinsic difference in pharmacologic properties and is not related to a difference in disposition or metabolism.

Species specificity in the metabolism of nabilone. Relationship between toxicity and metabolic routes

The disposition of 14C-nabilone has been examined in dogs and monkeys; it is rapidly absorbed and extensively metabolized in both species. A metabolic pathway initiated by the stereoselective enzymic reduction of the 9-keto moiety of nabilone was of major importance in the biotransformation of nabilone in the dog but was only a minor pathway in the monkey. The resulting long half-lived carbinol metabolites accounted for 77% of the circulating 14C in dog but for only 19% in monkey, 48 h after drug administration. Accumulation of carbinol metabolites in brain tissue was observed in dogs, with 24 h brain concentrations being five to six times higher than the plasma concentrations. No accumulation of carbinol metabolites, nabilone or of 14C occurred in the brain of monkeys. Accumulation of the carbinol metabolites in brain tissue has been implicated as the causative factor for the CNS toxicity observed in dogs treated chronically with nabilone. Comparison of nabilone pharmacokinetics in dog and humans showed the dog to be unique in producing high levels of carbinol metabolites, so that monkey appeared to be a more appropriate model than dog for pre-clinical toxicological and safety studies. Studies with liver 15,000 g supernatants indicated that the enzymic reduction of nabilone to its carbinol metabolite was a viable metabolic pathway in rat, dog and monkey, so that a distinct species difference exists between dog and monkey in the subsequent metabolism and/or elimination of these carbinol metabolites.