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

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Author: Renwick, A.B. · Ball, S.E. · Tredger, J.M. · Price, R.J. · Walters, D.G. · Kao, J. · Scatina, J.A. · Lake, B.G.
Type:article
Date:2002
Institution: TNO Voeding
Source:Xenobiotica, 10, 32, 849-862
Identifier: 42186
doi: doi:10.1080/00498250210158221
Keywords: Nutrition · Aldehyde oxidase · Cimetidine · Cytochrome P450 3A · Reduced nicotinamide adenine dinucleotide phosphate · Zaleplon · Cell fractionation · Drug inhibition · Drug metabolism · Human · Human cell · Human tissue · In vitro study · Liver metabolism · Liver microsome · Liver slice · Acetamides · Cimetidine · Cytosol · Dose-Response Relationship, Drug · Drug Interactions · Enzyme Inhibitors · Humans · Hypnotics and Sedatives · Inhibitory Concentration 50 · Kinetics · Liver · Models, Chemical · Pyrimidines · Subcellular Fractions · Staphylococcus phage 3A

Abstract

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<sub>50</sub> and V<sub>max</sub> of 310±24 μM 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<sub>m</sub> and V<sub>max</sub> of 124±14 μM 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 μM, the calculated mean (±SEM, n=3) IC<sub>50</sub> were 596±103 and 231±23 μM 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<sub>i</sub> of 155±16 μM. 6. Freshly cut human liver slices metabolized ZAL to a number of products including M1, 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<sub>i</sub> of 506 μM. 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. Chemicals/CAS: Acetamides; Cimetidine, 51481-61-9; Enzyme Inhibitors; Hypnotics and Sedatives; Pyrimidines; zaleplon, 151319-34-5