Forty-two patients with advanced solid tumors were entered into a dose-finding study of the combination of doxorubicin with the cyclosporin analogue SDZ PSC 833 (PSC), given by oral route. Patients received PSC at escalating doses, ranging from 2.5 to 25 mg/kg/day, for 5 days, in doses given every 12 h. Doxorubicin was given by i.v. push on day 3 of PSC administration, 4 h after the morning dose of PSC. Pharmacokinetic analyses of PSC and doxorubicin were performed. A total of 38 patients received a combination of PSC and doxorubicin, and 27 received doxorubicin alone in the first course. The major toxicity of the combination was hematological and was significantly more severe than that with doxorubicin alone; severe myelosuppression was already observed at the first PSC dose level, which required doxorubicin dose reduction from 50 to 35 mg/m2. At all dose levels of PSC, up to 17.5 mg/kg/day, there were at least two patients with grade 3 or 4 hematological toxicity, which was manageable in less heavily pretreated patients. A further PSC dose escalation was performed to 25 mg/kg/day, together with doxorubicin, at a further reduced dose of 20 mg/m2. At this dose, central nervous system toxicity became the most relevant side effect. The increase of toxicity in the combined treatment was supported by a significant increase of the area under the plasma concentration-time curve to infinity of doxorubicin (54%) and a 10-fold increase of the area under the plasma concentration-time curve to infinity of doxorubicinol. The pharmacological interaction was not dependent on the plasma concentration of PSC. The total body clearance of doxorubicin decreased by 30%. PSC plasma concentrations of >1 microM at the time of doxorubicin administration were, in general, found at a dose of 7.5 mg/kg/day or higher. One patient had a partial response. In conclusion, PSC plasma concentrations that can revert multidrug resistance in experimental models could be achieved in patients who have solid tumors and who are treated with doxorubicin. However, a marked pharmacological interaction was found between doxorubicin and PSC, which led to substantial increase in hematological toxicity and required marked reduction of the doxorubicin dose. Further study of PSC may be warranted, in association with the investigation of P-glycoprotein expression and concentration of drugs in the tumor tissues.