Received for publication October 5, 2006.
Revised March 7, 2007.
Accepted for publication March 8, 2007.

METABOLISM OF PRAZOSIN IN RAT, DOG AND HUMAN LIVER MICROSOMES AND CRYOPRESERVED RAT AND HUMAN HEPATOCYTES AND CHARACTERIZATION OF METABOLITES BY LC/MS/MS

Abstract

Prazosin, (2-[4-(2-furanoyl)-piperazin-1-yl]-4-amino-6,7-dimethoxyquinazoline), is an antihypertensive agent that was introduced to the market in 1976. It has since established an excellent safety record. However, in vitro metabolism of prazosin has not been investigated. This study describes the in vitro biotransformation of prazosin in liver microsomes from rats, dogs, and humans as well as rat and human cryopreserved hepatocytes and characterization of metabolites using liquid chromatography tandem mass spectrometry. The major in vivo biotransformation pathways reported previously in rats and dogs include demethylation, amide hydrolysis and O-glucuronidation. These metabolic pathways were also confirmed in our study. In addition, several new metabolites were characterized including a stable carbinolamine, an iminium species and an enamine all formed via oxidation of the piperazine ring. Two ring-opened metabolites generated following oxidative cleavage of the furan ring were also identified. Using semicarbazide hydrochloride as a trapping agent, an intermediate arising from opening of the furan ring was captured as a pyridazine product. In the presence of glutathione, three glutathione conjugates were detected in microsomal incubations, although they were not detected in cryopreserved hepatocytes. These data support ring opening of the furan via a reactive -keto-, -unsaturated aldehyde intermediate. In the presence of UDPGA, prazosin underwent conjugation to form an N-glucuronide not reported previously. Our in vitro investigations have revealed additional metabolic transformations of prazosin and have demonstrated the potential of prazosin to undergo bioactivation through metabolism of the furan ring to a reactive intermediate.

Key words: glucuronidation, glutathione conjugates, hepatocytes, in vitro-in vivo prediction, mass spectrometry, metabolite identification, reactive intermediate