Human Cytochrome P450 1A2 Involvement in the Formation of Reactive Metabolites from a Species-Specific Hepatotoxic Pyrazolopyrimidine Derivative, 5-n-Butyl-7-(3,4,5-trimethoxybenzoylamino)pyrazolo[1,5-a]pyrimidine

5-n-Butyl-7-(3,4,5-trimethoxybenzoylamino)pyrazolo[1,5-a]pyrimidine) (OT-7100) is a pyrazolopyrimidine derivative with potential analgesic effects. Exclusively limited elevations in serum levels of aspirate- and alanine-aminotransferase were abnormally observed in a clinical study, in contrast to no toxicological potential to experimental animals. The aim of this study was to clarify the mechanism responsible for species-specific hepatotoxicity of this model compound. OT-7100 was primarily metabolized to a carboxylic acid derivative and an amino derivative (5-n-butyl-pyrazolo[1,5-a]pyrimidine, M-5) by hydrolysis in humans and rats. In human liver, pyrazolo[1,5-a]pyrimidine derivative M-5 was further metabolized to mainly M-23OH (a C-3-position hydroxyl derivative, 3-hydroxy-5-n-butyl-pyrazolo[1,5-a]pyrimidine). Studies with recombinant cytochrome P450s (P450s), correlation analysis using a panel of human liver microsomes as well as immunoinhibition with anti-P450 antibodies collectively suggested that hum...