PT  - JOURNAL ARTICLE
AU  - Richard C. T. Casabar
AU  - Andrew D. Wallace
AU  - Ernest Hodgson
AU  - Randy L. Rose
TI  - Metabolism of Endosulfan-α by Human Liver Microsomes and Its Utility as a Simultaneous in Vitro Probe for CYP2B6 and CYP3A4
AID  - 10.1124/dmd.106.010199
DP  - 2006 Oct 01
TA  - Drug Metabolism and Disposition
PG  - 1779--1785
VI  - 34
IP  - 10
4099  - http://dmd.aspetjournals.org/content/34/10/1779.short
4100  - http://dmd.aspetjournals.org/content/34/10/1779.full
SO  - Drug Metab Dispos2006 Oct 01; 34
AB  - Endosulfan-α is metabolized to a single metabolite, endosulfan sulfate, in pooled human liver microsomes (Km = 9.8 μM, Vmax = 178.5 pmol/mg/min). With the use of recombinant cytochrome P450 (P450) isoforms, we identified CYP2B6 (Km = 16.2 μM, Vmax = 11.4 nmol/nmol P450/min) and CYP3A4 (Km = 14.4 μM, Vmax = 1.3 nmol/nmol P450/min) as the primary enzymes catalyzing the metabolism of endosulfan-α, although CYP2B6 had an 8-fold higher intrinsic clearance rate (CLint = 0.70 μl/min/pmol P450) than CYP3A4 (CLint = 0.09 μl/min/pmol P450). Using 16 individual human liver microsomes (HLMs), a strong correlation was observed with endosulfan sulfate formation and S-mephenytoin N-demethylase activity of CYP2B6 (r2 = 0.79), whereas a moderate correlation with testosterone 6 β-hydroxylase activity of CYP3A4 (r2 = 0.54) was observed. Ticlopidine (5 μM), a potent CYP2B6 inhibitor, and ketoconazole (10 μM), a selective CYP3A4 inhibitor, together inhibited approximately 90% of endosulfan-α metabolism in HLMs. Using six HLM samples, the percentage total normalized rate (% TNR) was calculated to estimate the contribution of each P450 in the total metabolism of endosulfan-α. In five of the six HLMs used, the percentage inhibition with ticlopidine and ketoconazole in the same incubation correlated with the combined % TNRs for CYP2B6 and CYP3A4. This study shows that endosulfan-α is metabolized by HLMs to a single metabolite, endosulfan sulfate, and that it has potential use, in combination with inhibitors, as an in vitro probe for CYP2B6 and 3A4 catalytic activities. The American Society for Pharmacology and Experimental Therapeutics