Human carboxylesterase (CES) 1A and CES2, two major forms of human CES, dominate the pharmacokinetics of most prodrugs such as imidapril and irinotecan (CPT-11). Antihypertensive drugs are often prescribed for clinical therapy concurrently with others. Moreover, two or more antihypertensive drugs are ubiquitously combined. The influences of antihypertensive drugs on the activity of CES remain undefined. In the present study, the inhibitory effects of 17 antihypertensive drugs on the CES1A1 and CES2 activities were evaluated. Imidapril and CPT-11 were used as substrates and cultured with liver microsomes in vitro. The imidapril hydrolase activities by recombinant CES1A1 and human liver microsomes (HLM) were intensely inhibited by telmisartan and nitrendipine (K-i = 0.49 +/- 0.09 and 1.12 +/- 0.39 mu M for CES1A1, 1.69 +/- 0.17 mu M and 1.24 +/- 0.27 mu M for HLM, respectively). However, other drugs did not exert strong inhibition. The enzyme hydrolase activity of recombinant CES2 was substantially inhibited by diltiazem and verapamil (K-i = 0.25 +/- 0.02 and 3.84 +/- 0.99 mu M, respectively). Hence, diltiazem, verapamil, nitrendipine and telmisartan may attenuate the drug efficacy of catalyzed prodrugs by changing the activities of CES1A1 and CES2.