We have previously reported the reduction in oral bioavailability of cyclosporine A (CsA) by probucol, a lipid-lowering drug. To evaluate the mechanism, we examined the effect of probucol on the transport of CsA across Caco-2 cell monolayers, on the influence of CsA pharmacokinetics in rats, and on the change of ultraviolet-absorption spectrum of the drug. Pretreatment with probucol (50 microM) inhibited (P < 0.001) both the apical-to-basal (-73.1%) and basal-to-apical (-77.8%) fluxes of [3H]-CsA. In rats, probucol orally given 6 h after, but not simultaneous with CsA did not decrease peak CsA concentration or area under the blood CsA concentration-time curve following a single oral dosing of CsA after the pretreatment with probucol for 7 days. These data indicate that P-glycoprotein-mediated active transport from intracellular to apical is not involved in the mechanism of probucol-CsA interaction, and absorption of CsA decreases in the presence of probucol in the gastrointestinal tract. In difference spectral analysis, probucol reduced the absorption peak of CsA in a concentration-dependent manner, indicating that probucol could form a complex with CsA. These results suggest that probucol interferes with CsA absorption probably by physicochemical mechanism such as complex formation, but not the enhancement of P-glycoprotein function.