Inhibition of zaleplon metabolism by cimetidine in the human liver:in vitrostudies with subcellular fractions and precision-cut liver slices

Abstract

1. The effect of cimetidine on the metabolism of zaleplon (ZAL) in human liver subcellular fractions and precision-cut liver slices was investigated. 2. ZAL was metabolized to a number of products including 5-oxo-ZAL (M2), which is known to be formed by aldehyde oxidase, N-desethyl-ZAL (DZAL), which is known to be formed by CYP3A forms, and N-desethyl-5-oxo-ZAL (M1). 3. Human liver microsomes catalysed the NADPH-dependent metabolism of ZAL to DZAL. Kinetic analysis of three microsomal preparations revealed mean (+/-SEM) S(50) and V(max) of 310 +/- 24 micro M and 920 +/- 274 pmol/min/mg protein, respectively. 4. Human liver cytosol preparations catalysed the metabolism of ZAL to M2. Kinetic analysis of three cytosol preparations revealed mean (+/-SEM), K(m) and V(max) of 124 +/- 14 micro M and 564 +/- 143 pmol/min/mg protein, respectively. 5. Cimetidine inhibited ZAL metabolism to DZAL in liver microsomes and to M2 in the liver cytosol. With a ZAL substrate concentration of 62 micro M, the calculated mean (+/-SEM, n = 3) IC50 were 596 +/- 103 and 231 +/- 23 micro M for DZAL and M2 formation, respectively. Kinetic analysis revealed that cimetidine was a competitive inhibitor of M2 formation in liver cytosol with a mean (+/-SEM, n = 3) K(i) of 155 +/- 16 micro M. 6. Freshly cut human liver slices metabolized ZAL to a number of products including 1, M2 and DZAL. 7. Cimetidine inhibited ZAL metabolism in liver slices to M1 and M2, but not to DZAL. Kinetic analysis revealed that cimetidine was a competitive inhibitor of M2 formation in liver slices with an average (n = 2 preparations) K(i) of 506 micro M. 8. The results demonstrate that cimetidine can inhibit both the CYP3A and aldehyde oxidase pathways of ZAL metabolism in the human liver. Cimetidine appears to be a more potent inhibitor of aldehyde oxidase than of CYP3A forms and hence in vivo is likely to have a more marked effect on ZAL metabolism to M2 than on DZAL formation. 9. The results also demonstrate that precision-cut liver slices may be a useful model system for in vitro drug-interaction studies.

Potent Inhibition of Human Liver Aldehyde Oxidase by Raloxifene

“…This is due to the fact that not many drugs have been shown to be primarily cleared by aldehyde oxidase-mediated metabolism, nor are there many drugs that are potent inhibitors. Zaleplon oxidation has been recently established as an aldehyde oxidase-mediated major clearance mechanism for this drug Renwick et al, 2002). The activation of famciclovir to the active antiviral agent penciclovir requires aldehyde oxidase-mediated oxidation (Clarke et al, 1995;Rashidi et al, 1997).…”

Hydralazine As a Selective Probe Inactivator of Aldehyde Oxidase in Human Hepatocytes: Estimation of the Contribution of Aldehyde Oxidase to Metabolic Clearance

“…Notable substrates used in in vitro work include N-methylnicotinamide and phthalazine. Drugs known to have an important contribution of aldehyde oxidase in human include zaleplon Renwick et al, 2002) and famciclovir in which AO is involved in metabolism of a prodrug to the active antiviral agent penciclovir (Clarke et al, 1995;Rashidi et al, 1997).…”

Prediction of Human Drug Clearance by Multiple Metabolic Pathways: Integration of Hepatic and Intestinal Microsomal and Cytosolic Data

“…Hepatic and intestinal cytosolic scaling factors were derived from literature sources quoting cytosolic protein abundance or activity data in both the cytosolic fraction and tissue homogenate (Wynne et al, 1992;Boogaard et al, 1996;Gibbs et al, 1998;Renwick et al, 2002;Mutch et al, 2007). Hepatic CL int, SULT values obtained in the current study were scaled using 80.7 mg/g liver, whereas intestinal CL int, SULT values were scaled using 18 mg/g intestine, a value derived from only one literature source available and based on data from 12 donors (Gibbs et al, 1998).…”