CYP3A2 is one of the most abundantly expressed cytochrome P450s (CYPs) in the rat liver and metabolizes numerous clinically important drugs. Studies were designed to examine efficacy, potency and specificity of antisense inhibition of CYP3A2 in vivo. Three phosphorothioate ODNs were used: 3A2-ATG, antisense to the CYP3A2 mRNA translational start site; 3A2-REV, 5' to 3' reverse sequence of 3A2-ATG; and C-MYC, antisense to the C-MYC mRNA translational start site. Midazolam (MZ) sleep times were used as CYP3A2-specific in vivo marker in male Sprague-Dawley rats. Administration of 1 mg/day 3A2-ATG for 2 days i.p. significantly increased MZ sleep times from 22.4 +/- 0.4 (saline) to 35.3 +/- 1.5 min. Administration of equivalent doses of noncomplementary 3A2-REV or C-MYC produced no significant changes in MZ sleep times (22.4 +/- 0.6 and 22.8 +/- 1.3, respectively). Liver microsomal erythromycin demethylase activity, a specific CYP3A2 assay, was significantly decreased from 124 +/- 13 mumol/mg per min in saline controls to 63.8 +/- 8 in 3A2-ATG-treated rats. Enzyme activities for CYPs 2E1, 1A1/2 and 2B1/2 were not significantly different between saline controls and 3A2-ATG-treated animals. The control ODNs 3A2-REV and C-MYC had no significant changes in enzymatic activities compared to saline. Western blot analysis revealed decreases in CYP3A2 protein but not CYP2B1 protein in 3A2-ATG rat microsomes compared to controls. These studies demonstrate for the first time that antisense ODNs can effectively, potently, and specifically inhibit CYP3A2 in vivo.