Biotransformation of the antiemetic 5-HT3 antagonist tropisetron in liver and kidney slices of human, rat and dog with a comparison to in vivo

Species differences in the biotransformation of the antiemetic tropisetron, a potent 5-hydroxytryptamine type 3 (5-HT3) receptor antagonist, were evident in liver slice incubates of human, rat and dog, and reflected the species differences observed in vivo with respect to the relative importance of individual pathways. The dominant biotransformation pathway of tropisetron (10 microM) in human liver slices was formation of 6-hydroxy-tropisetron, whereas in rat liver slices it was 5-hydroxy-tropisetron, and in dog liver slices N-oxide formation. Initial rates of tropisetron metabolite formation in the liver slices (8 mm in diameter, 200 +/- 25 microns thickness) of human (83 +/- 61 pmol/h/mg slice protein), rat (413 +/- 98 pmol/h/mg slice protein) and dog (426 +/- 38 pmol/h/mg slice protein) would predict less of a first-pass effect in humans compared to the rat or the dog. For human and rat, the prediction matched well with the species ranking of tropisetron bioavailability; however, for dog the in vitro data overestimated the apparent first-pass effect. The jejunum is not expected to contribute to the first-pass effect in humans, since human jejunum microsomes did not metabolize tropisetron. The major organ of excretion for tropisetron and its metabolites is the kidney, but the contribution of the kidney to the overall metabolism of tropisetron would be small. Species independent N-oxide formation (2-12 pmol/h/mg slice protein) was the major pathway in human, rat and dog kidney slices, and was comparable to N-oxide formation in the rat and human liver slices but was 1/10 the rate in dog liver slices. This study has demonstrated that the liver is the primary site of tropisetron biotransformation, and the usefulness of organ slices to characterize cross species differences in the dominant biotransformation pathways.

Uptake and metabolism of cyclosporin A and SDZ IMM 125 in the human in vitro skin2 dermal and barrier function models

Treatment of psoriasis with the immunosuppressant cyclosporin A (CSA) is beneficial orally but topical treatment is less efficacious. A comparison of CSA to its hydroxyethyl derivative SDZ IMM 125 (IMM) as to bioavailability to epidermal and dermal cells and the potential for inactivation by biotransformation was investigated using a human dermal skin model (skin2 ZK1100) and a barrier function model (skin2 ZK 1300). The dermal ZK1100 model demonstrated that both cyclosporins could be metabolized by human dermis to the known primary hydroxylated metabolite, M17/AM1 for CSA and the hydroxylated analogue of IMM, IMM1. Application of the cyclosporins to the stratum corneum of the barrier function model ZK 1300 demonstrated that both CSA and IMM would be bioavailable to the epidermal and dermal skin cells. Systemic concentrations would be expected to be low due to the slow permeation of the compounds and because mostly metabolites would reach the circulation. The difference between the two cyclosporins was the rate and extent of biotransformation with IMM metabolite formation being about 1/4 that of CSA.

The biotransformation of the ergot derivative CQA 206-291 in human, dog, and rat liver slice cultures and prediction of in vivo plasma clearance

Liver slice cultures from humans, dogs, and rats were used to investigate the biotransformation of the dopaminergic ergot agonist CQA 206-291 and to predict pharmacokinetic values for hepatic intrinsic clearance and plasma clearance. CQA 206-291 was extensively metabolized in the liver slice cultures and in vivo. The HPLC metabolite patterns from the liver slice cultures were similar for all three species, indicating the occurrence of the same metabolic pathways for CQA 206-291 biotransformation. The rate of formation of CQ 32-084, a pharmacologically active N-deethylated metabolite, exceeded that of metabolite d, a primary metabolite, by 1.4 fold in human liver slices, and by 1.7 fold in rat liver slices. In dog liver slice cultures, metabolite d formation exceeded CQ 32-084 formation by 1.3 fold and was formed at a statistically significantly greater rate (3 fold) than in either human or rat liver slices. The metabolism of ergots like CQA 206-291 by human fetal liver was also demonstrated in this study. However, the prominent metabolite from fetal and adult human liver microsomes was metabolite d with minor amounts of CQ 32-089 being formed. A major route of excretion for the metabolites of CQA 206-291 is the kidney, yet the kidney does not contribute to the metabolism of CQA 206-291. Kidney slices derived from humans, rats, and dogs did not metabolize CQA 206-291 within 24 hr. CQA 206-291 intrinsic clearance was derived from the half-life of parent drug disappearance in the liver slice and hepatocyte cultures, and from the ratio of Vmax/Km of human and rat liver microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)