title: 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.
abstract: 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 human liver microsomal P450 1A2 preferentially metabolized M-5 to predominantly M-23OH. Human liver microsomes were capable of activating M-5 to a covalently bound metabolite faster than rat liver microsomes: reduced glutathione prevented the bindings. A cysteine adduct derivative of M-23OH at the C-6-position was structurally confirmed. On the contrary, rat liver microsomal P450 1A2 could metabolize M-5 to equally M-23OH, M-22OH (a C-6-position hydroxyl derivative, 6-hydroxy-5-n-butyl-pyrazolo[1,5-a]pyrimidine), or an unknown metabolite. These results suggest that differences in the regiospecific metabolic function of human and rat P450 1A2 would be responsible for the human-specific metabolic activation of the primary metabolite of OT-7100 to a proximate form. It is presumed that hepatotoxicity associated with OT-7100 could be likely related to the formation of a human-specific reactive metabolite from M-23OH. OT-7100 activation by inducible P450 1A2 may therefore exhibit marked individual differences.
authors.affiliation: Nutrient/Drug Metabolism and Pharmacology, Otsuka Pharmaceutical Factory, Inc., 115 Tateiwa, Muya-cho, Naruto, Tokushima 772-8601, Japan. kuribasy@otsukakj.co.jp
authors.forename: Shunji
authors.initials: S
authors.lastname: Kuribayashi
authors.forename: Kiyoto
authors.initials: K
authors.lastname: Goto
authors.forename: Shinsaku
authors.initials: S
authors.lastname: Naito
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Kamataki
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yamazaki
mesh_terms: D000818:Animals; D056486:Chemical and Drug Induced Liver Injury / Q000209:etiology / Q000378:metabolism*; D003545:Cysteine / Q000737:chemistry; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism*; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D006801:Humans; D006868:Hydrolysis; D009682:Magnetic Resonance Spectroscopy; D008862:Microsomes, Liver / Q000201:enzymology; D011720:Pyrazoles / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D011743:Pyrimidines / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D013045:Species Specificity
chemical_list: D065609:Cytochrome P-450 CYP1A2 Inhibitors; C118618:OT 7100; D011720:Pyrazoles; D011743:Pyrimidines; D019388:Cytochrome P-450 CYP1A2; D003545:Cysteine