title: Biotransformation of tirilazad in human: 3. tirilazad A-ring reduction by human liver microsomal 5alpha-reductase type 1 and type 2.
abstract: Tirilazad mesylate (FREEDOX), a potent inhibitor of membrane lipid peroxidation in vitro, is under clinical development for the treatment of subarachnoid hemorrhage. In humans, tirilazad is cleared almost exclusively via hepatic elimination with a medium-to-high extraction ratio. In human liver microsomal preparations, tirilazad is biotransformed to multiple oxidative products and one reduced, pharmacologically active metabolite, U-89678. Characterization of the reduced metabolite by mass spectrometry and cochromatography with an authentic standard demonstrated that U-89678 was formed via stereoselective reduction of the Delta4 bond in the steroid A-ring. Kinetic analysis of tirilazad reduction in human liver microsomes revealed that kinetically distinct type 1 and type 2 5alpha-reductase enzymes were responsible for U-89678 formation; the apparent KM values for type 2 and type 1 were approximately 15 and approximately 0.5 microM, respectively. Based on pH dependence and finasteride inhibition studies, it was inferred that 5alpha-reductase type 1 was the high affinity/low capacity microsomal reductase that contributed to tirilazad clearance in vivo. In addition, a role for CYP3A4 in the metabolism of U-89678 was established using cDNA expressed CYP3A4 and correlation studies comparing U-89678 consumption with cytochrome P450 activities across a population of human liver microsomes. Collectively, these data suggest that formation of U-89678, a circulating pharmacologically active metabolite, contributes to the total metabolic elimination of tirilazad in humans and that clearance of U-89678 is mediated primarily via CYP3A4 metabolism. Therefore, concurrent administration of therapeutic agents that modulate 5alpha-reductase type 1 or CYP3A activity are anticipated to affect the pharmacokinetics of PNU-89678.
authors.affiliation: Drug Metabolism and Disposition Research, Pharmacia & Upjohn, Kalamazoo, Michigan, USA.
authors.forename: L C
authors.initials: LC
authors.lastname: Wienkers
authors.forename: R C
authors.initials: RC
authors.lastname: Steenwyk
authors.forename: M J
authors.initials: MJ
authors.lastname: Hauer
authors.forename: J C
authors.initials: JC
authors.lastname: Fleishaker
authors.forename: P G
authors.initials: PG
authors.lastname: Pearson
mesh_terms: D001711:Biotransformation; D042944:Cholestenone 5 alpha-Reductase; D002851:Chromatography, High Pressure Liquid; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016166:Free Radical Scavengers / Q000493:pharmacokinetics; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007527:Isoenzymes / Q000378:metabolism*; D013058:Mass Spectrometry; D008862:Microsomes, Liver / Q000201:enzymology*; D018696:Neuroprotective Agents / Q000493:pharmacokinetics; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011281:Pregnatrienes / Q000493:pharmacokinetics*
chemical_list: D004791:Enzyme Inhibitors; D016166:Free Radical Scavengers; D007527:Isoenzymes; D018696:Neuroprotective Agents; D011281:Pregnatrienes; C087053:U 89678; D010088:Oxidoreductases; D042944:Cholestenone 5 alpha-Reductase; C053355:tirilazad