Selective Time- and NADPH-Dependent Inhibition of Human CYP2E1 by Clomethiazole.

The sedative clomethiazole (CMZ) has been used in Europe since the mid-1960s to treat insomnia and alcoholism. It has been previously demonstrated in clinical studies to reversibly inhibit human CYP2E1 in vitro and decrease CYP2E1-mediated elimination of chlorzoxazone. We have investigated the selectivity of CMZ inhibition of CYP2E1 in pooled human liver microsomes (HLMs). In a reversible inhibition assay of the major drug-metabolizing cytochrome P450 (P450) isoforms, CYP2A6 and CYP2E1 exhibited IC50 values of 24 µM and 42 µM, respectively with all other isoforms exhibiting values >300 µM. When CMZ was preincubated with NADPH and liver microsomal protein for 30 minutes before being combined with probe substrates, however, more potent inhibition was observed for CYP2E1 and CYP2B6 but not CYP2A6 or other P450 isoforms. The substantial increase in potency of CYP2E1 inhibition upon preincubation enables the use of CMZ to investigate the role of human CYP2E1 in xenobiotic metabolism and provides advantages over other chemical inhibitors of CYP2E1. The KI and kinact values obtained with HLM-catalyzed 6-hydroxylation of chlorzoxazone were 40 µM and 0.35 minute(-1), respectively, and similar to values obtained with recombinant CYP2E1 (41 µM, 0.32 minute(-1)). The KI and kinact values, along with other parameters, were used in a mechanistic static model to explain earlier observations of a profound decrease in the rate of chlorzoxazone elimination in volunteers despite the absence of detectable CMZ in blood.

Human cytochrome p450 induction and inhibition potential of clevidipine and its primary metabolite h152/81.

Clevidipine is a short-acting dihydropyridine calcium channel antagonist under development for treatment of perioperative hypertension. Patients treated with clevidipine are likely to be comedicated. Therefore, the potential for clevidipine and its major metabolite H152/81 to elicit drug interactions by induction or inhibition of cytochrome P450 was investigated. Induction of CYP1A2, CYP2C9, and CYP3A4 was examined in primary human hepatocytes treated with clevidipine at 1, 10, and 100 microM. Clevidipine was found to be an inducer of CYP3A4, but not of CYP1A2 or CYP2C9, at the 10 microM and 100 microM concentrations of clevidipine tested. Induction response for CYP3A4 to 100 microM clevidipine was approximately 20% of that of the positive control inducer rifampicin. The response of H152/81 was similar. Using cDNA-expressed enzymes, clevidipine inhibited CYP2C9, CYP2C19, and CYP3A4 activities with IC(50) values below 10 microM, whereas CYP1A2, CYP2D6, and CYP2E1 activities were not substantially inhibited (IC(50) values >70 microM). The K(i) values for CYP2C9 and CYP2C19 were 1.7 and 3.3 microM, respectively, and those for CYP3A4 were 8.3 and 2.9 microM, using two substrates, testosterone and midazolam, respectively. These values are at least 10 times higher than the highest clevidipine concentration typically seen in the clinic. Little or no inhibition by H152/81 was found for the enzyme activities mentioned above (IC(50) values >or= 69 microM). The present study demonstrates that it is highly unlikely for clevidipine or its major metabolite to cause cytochrome P450-related drug interactions when used in the dose range required to manage hypertension in humans.

Highly selective inhibition of human CYP3Aa in vitro by azamulin and evidence that inhibition is irreversible.

Azamulin [14-O-(5-(2-amino-1,3,4-triazolyl)thioacetyl)-dihydromutilin] is an azole derivative of the pleuromutilin class of antiinfectives. We tested the inhibition potency of azamulin toward 18 cytochromes P450 using human liver microsomes or microsomes from insect cells expressing single isoforms. In a competitive inhibition model, IC(50) values for CYP3A (0.03-0.24 microM) were at least 100-fold lower than all other non-CYP3A enzymes except CYP2J2 ( approximately 50-fold lower). The IC(50) value with heterologously expressed CYP3A4 was 15-fold and 13-fold less than those of CYP3A5 and CYP3A7, respectively. The reference inhibitor ketoconazole was less selective and exhibited potent inhibition (IC(50) values <10 microM) for CYP1A1, CYP1B1, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP4F2, and CYP4F12. Inhibition of CYP3A by azamulin appeared sigmoidal and well behaved with the substrates 7-benzyloxy-4-trifluoromethylcoumarin, testosterone, and midazolam. Preincubation of 4.8 microM azamulin in the presence of NADPH for 10 min inhibited approximately 95% of testosterone 6beta-hydroxylase activity compared with preincubation in the absence of NADPH. Catalytic activities of CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1 were unaffected by similar experiments. Incubation of azamulin with heterologously expressed CYP3A4 yielded a type I binding spectrum with a spectral dissociation constant of 3.5 microM, whereas no interaction was found with CYP2D6. Azamulin exhibited good chemical stability when stored in acetonitrile for up to 12 days. Aqueous solubility was found to be >300 microM. Azamulin represents an important new chemical tool for use in characterizing the contribution of CYP3A to the metabolism of xenobiotics.

Cytochrome P450 fluorometric substrates: identification of isoform-selective probes for rat CYP2D2 and human CYP3A4.

We have tested a panel of 29 cDNA-expressed rat and human enzymes with 9 fluorometric substrates to determine the P450 isoform selectivity in the catalysis of the substrates to fluorescent products. The substrates examined were dibenzyl fluorescein, 7-benzyloxyquinoline (BQ), 3-cyano-7-ethoxycoumarin, 3-cyano-7-methoxycoumarin, 7-methoxy-4-trifluoromethylcoumarin, 3-[2-(N,N-diethyl-N-methylamino)ethyl]-7-methoxy-4-methylcoumarin (AMMC), 3-[2-(N,N-diethyl-N-methylamino)ethyl]-7-methoxy-4-trifluoromethylcoumarin, 7-benzyloxyresorufin, and 7-benzyloxy-4-trifluoromethylcoumarin (BFC). For most substrates, multiple cDNA-expressed cytochrome P450 isoforms were found to catalyze the formation of the fluorescent product. However, among the combinations tested, rat CYP2D2 displayed high selectivity for AMMC demethylation (a substrate selective for CYP2D6 in human liver microsomes). AMMC demethylation activity was 15-fold lower in microsomes isolated from female Dark Agouti rats, a model known to have a low abundance of CYP2D2, and apparent K(M) values were similar for cDNA-expressed CYP2D2 and male Sprague-Dawley liver microsomes. BFC dealkylation and BQ dealkylation were selective but not exclusive for human CYP3A4. A small role for CYP1A2 could be demonstrated. The CYP3A4 selectivity in hepatic microsomes was supported by studies using chemical and antibody inhibitors and a correlation analysis within a panel of liver microsomes from individual donors. BQ demonstrated a higher degree of selectivity for and higher rates of metabolism by CYP3A than BFC. However, per unit enzyme the fluorescent signal is lower for BQ than BFC. AMMC, BQ, and BFC should find uses as enzyme-selective probe substrates.