Metabolism of Prazosin in Rat, Dog, and Human Liver Microsomes and Cryopreserved Rat and Human Hepatocytes and Characterization of Metabolites by Liquid Chromatography/Tandem Mass Spectrometry

Abstract

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 UDP-glucuronic acid, prazosin underwent conjugation to form an N-glucuronide not reported previously. Our in vitro investigations have revealed additional metabolic transformations of prazosin and have shown the potential of prazosin to undergo bioactivation through metabolism of the furan ring to a reactive intermediate.Prazosin (Fig. 1), a short-acting vasodilator discovered in the mid1970s, has been widely used in treating hypertension and congestive heart failure (Constantine, 1974;Althuis and Hess, 1977;Stanaszek et al., 1983). It was the first of a new class of direct-acting vasodilators acting by ␣-adrenoreceptor blockade. Prazosin was introduced to the U.S. market as MINIPRESS by Pfizer (New York, NY) in 1976. Prazosin is well tolerated, with the most common side effect associated with treatment being postural hypotension. Although no longer a major drug among the antihypertensive agents, based on prazosin's ability to antagonize centrally located ␣ 1 -adrenergic receptors, a new indication for treatment of post-traumatic stress disorders (PTSD) encountered during civilian life is being explored in clinical studies (Taylor and Raskind, 2002). In addition, prazosin is also being investigated in treating combat-related nightmares characteristic of PTSD among soldiers recently returned from Operation Iraqi Freedom (Daly et al., 2005).In vitro metabolism of prazosin in animals or humans has not been reported. Early metabolism studies with 14 C-labeled prazosin (label in quinazoline ring) in rats and dogs revealed that it undergoes extensive hepatic metabolism with biliary excret...

Metabolite Profiling of Justicia gendarussa Burm. f. Leaves Using UPLC-UHR-QTOF-MS

“…Metabolites (1), (5), and (6) were previously reported for J. gendarussa leaves [15,16]. Metabolite (2) was reported as a demethylated product of prazosin in liver microsomes for rats, dogs, and humans [29].…”

Production of a reactive metabolite of troglitazone by electrochemical oxidation performed in nonaqueous medium

“…Research into reactive metabolites has been performed to reduce the risk of drug-induced toxicity by liver microsome oxidation or electrochemical oxidation of drug candidates using various trapping agents, such as thiol compounds (glutathione, cystein and its derivatives), cyanide or semicarbazide [9][10][11][12]; however, it is difficult to detect all reactive metabolites as their trapped conjugate, much less in their intact form. Current researches of reactive drug metabolites using an on-line electrochemical cell-mass spectrometer (EC-MS) have been performed in aqueous medium, and reactive metabolites were detected as their conjugates using glutathione [1,13,14]; however, electrochemical oxidation of a drug in nonaqueous medium has rarely been reported in research into drug metabolism.…”

Human UDP-glucuronosyltransferase UGT1A4 forms tertiary N-glucuronides predominately with the energetically less favored tautomer of substituted 1H-indazole (benzpyrazole)

“…This neutral loss has been described mainly for N-glucuronides (Amore et al 1997;Erve et al 2007;Kassahun et al 1997), a thiocarbamate N-glucuronide Fig. 1 CID product ion spectra of m/z 439.1 of glucuronides 2 (upper spectrum) and 3 (lower spectrum) magnified by a factor of two.…”