SDZ PSC 833, a non-immunosuppressive cyclosporin analogue reverses multidrug resistance (MDR) in vitro by inhibiting P-glycoprotein (P-gp) mediated drug efflux. We performed a dose escalation study of SDZ PSC 833 combined with VAD chemotherapy in refractory multiple myeloma (MM). Twenty-two MM patients who were refractory to doxorubicin/vincristine/dexamethasone (VADr, n=11) or had failed multiple regimens (n=6) or were melphalan-refractory (MELr, n=5), were treated with one to three cycles of VAD combined with oral SDZ PSC 833, which was administered at escalating dosages starting at 5 mg/kg/day to 15 mg/kg/day for 7 days. The median trough and peak blood levels of SDZ PSC 833 ranged from 461/1134 ng/ml at 5 mg/kg/day to 821/2663 ng/ml at 15 mg/kg, respectively. With addition of SDZ PSC 833 (5 mg/kg) the mean plasma AUC 0-->96 h of doxorubicin as compared with control patients treated with VAD increased from 779 to 1510 ng/ml/h (P=0.0071), while the doxorubicin clearance was reduced from 47.6 to 27.8 l/h/m2 (P=0.0002). The clearance of doxorubicinol was reduced accordingly. Because of the increased plasma AUC, the dose of doxorubicin and vincristine had to be reduced in 13 patients to 50% (n=1) or 75% (n=12). A further dose-escalation of SDZ PSC 833 did not lead to a proportional increase of doxorubicin AUC. Toxicity WHO CTC grade 2 or 3 included hypoplasia (18/22), constipation (10/22), hyponatremia (3/22) and infections (6/22). A partial response or stable disease was achieved in eight and six patients, respectively. In 17 evaluable patients the mean percentage of pretreatment bone marrow plasma cells which expressed P-glycoprotein was 40%. The pretreatment in vitro rhodamin retention in CD38++ myeloma cells was reversible by 2 microM SDZ PSC 833 with 15-98% in 7/9 tested patients. In 4/5 responding patients analyzed before and after treatment with VAD + SDZ PSC 833, a reduction of P-gp + plasma cells was observed. It is concluded, that the blood concentrations of SDZ PSC 833 attained in MM patients increase with dose after oral administration. It can be safely combined with VAD chemotherapy. SDZ PSC 833 diminishes the clearance of doxorubicin, leading to an increase of the plasma AUC of doxorubicin. In addition, it is an effective inhibitor of P-gp mediated efflux of doxorubicin in myeloma tumor cells in vitro. Therefore, a proportional dose-reduction of doxorubicin and vincristine is warranted. Phase II/III studies in refractory MM are in progress to evaluate the therapeutic efficacy of SDZ PSC 833 with VAD.