Inhibition of zaleplon metabolism by cimetidine in the human liver: in vitro studies with subcellular fractions and precision-cut liver slices.
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.
Renwick
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TNO BIBRA International Ltd, Woodmansterne Road, Carshalton, Surrey SM5 4DS, UK.
Ball
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Tredger
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Price
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Walters
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Kao
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Scatina
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JA
Lake
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D000081:Acetamides / Q000037:antagonists & inhibitors* / Q000493:pharmacokinetics*; D002927:Cimetidine / Q000493:pharmacokinetics*; D003600:Cytosol / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000493:pharmacokinetics*; D006801:Humans; D006993:Hypnotics and Sedatives / Q000037:antagonists & inhibitors* / Q000493:pharmacokinetics*; D020128:Inhibitory Concentration 50; D007700:Kinetics; D008099:Liver / Q000187:drug effects* / Q000378:metabolism; D008956:Models, Chemical; D011743:Pyrimidines / Q000037:antagonists & inhibitors* / Q000493:pharmacokinetics*; D013347:Subcellular Fractions / Q000378:metabolism
D000081:Acetamides; D004791:Enzyme Inhibitors; D006993:Hypnotics and Sedatives; D011743:Pyrimidines; D002927:Cimetidine; C085665:zaleplon