Stereochemical aspects of carbonyl reduction of the original anticancer drug oracin by mouse liver microsomes and purified 11β-hydroxysteroid dehydrogenase type 1

Abstract

Oracin, 6-[2-(2-hydroxyethyl)aminoethyl]-5,11-dioxo-5,6-dihydro-11 H-indeno[1,2- c] isoquinoline, is a potential cytostatic drug for oral use and presently in phase II of clinical trials. Major advantages of this novel chemotherapeutic are the possibility of oral administration, its negative results in the Ames test on mutagenicity, and the lack of cardiotoxicity. Metabolic studies on oracin have revealed that the principal metabolite in all laboratory animals is 11-dihydrooracin (DHO), which is produced by carbonyl reduction of the parent compound. Since the carbonyl moiety of oracin is a pro-chiral centre, reduction may lead to the two stereoisomer forms (+)-DHO and (−)-DHO. The aim of the present study was to infer if 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD 1) is responsible for carbonyl reduction of oracin in mouse liver and if this enzyme exhibits stereospecificity in DHO formation. 11β-HSD 1 was purified from mouse liver microsomes, and the kinetics and stereospecificity regarding DHO formation were determined and compared to values obtained from the whole microsomal fraction. We could show that purified mouse liver 11β-HSD 1 catalyzes the stereospecific carbonyl reduction of oracin, thereby following a sigmoidal dose–response kinetics. Due to a different ratio of (+)-DHO and (−)-DHO (93:7) formed by purified 11β-HSD 1 compared to that produced in whole microsomes (70:30), the existence of at least one other oracin carbonyl reducing enzyme can be expected in mouse liver microsomes. This suggestion is further supported by the fact that the Hill coefficient of 2 for purified 11β-HSD 1 (which is supporting earlier data on the cooperativity of this dimeric enzyme) changes to a Hill coefficient of 3 in whole microsomes (which is indicative for another enzyme participating in oracin carbonyl reduction).