Cyclosporin A reverses multidrug resistance (MDR) and increases the in vivo cytostatic activity and toxicity of the anticancer agent etoposide (VP 16-213). SDZ PSC 833 (PSC 833), a non-immunosuppressive, non-toxic cyclosporin and very active modifier of P-gp 170-mediated MDR, elicits similar effects when administered with adriamycin. The underlying mechanisms, however, are not yet understood. The present pharmacological interaction study with PSC 833 and VP 16-213 was carried out to reveal the nature of this enhancement of cytostatic activity and toxicity. Rats pre-treated with either PSC-833 or solvent received a single dose of VP 16-213. Plasma levels of VP 16-213 were measured by high-performance liquid chromatography (HPLC). The resulting increase in cytostatic activity and toxicity of VP 16-213 mediated by PSC 833 was paralleled by marked changes in the pharmacokinetic parameters of VP 16-213 in vivo. Bioavailability and blood levels of VP 16-213 were significantly increased 30 min after administration if PSC 833 had been given before. The disappearance rate of VP 16-213 from the intravascular compartment was considerably slowed down by PSC 833. In drug-sensitive xenografts of human colon carcinoma, the PSC-833-induced pharmacologic changes in vivo could be counteracted by dose reduction of VP 16-213 while a full therapeutic potential was maintained. Doses of VP 16-213, 1.5 to 2 times smaller, combined with PSC 833, were as effective in terms of tumor-growth inhibition as the maximum tolerated dose of VP 16-213 alone. Thus, pharmacologic interactions between PSC 833 or other resistance modifiers and VP 16-213 and other cytostatic agents require careful attention if they are to be used in humans to overcome MDR.