Modulation of Multidrug Resistance by SDZ PSC 833 in Leukemic and Solid‐tumor‐bearing Mouse Models

P‐Glycoprotein inhibitors, including the nonimmunosuppressive cyclosporin D analog SDZ PSC 833 (PSC 833), have been developed to circumvent multidrug resistance. In the present study, the potential of PSC 833 in reversing multidrug resistance was evaluated in various systemic treatment models with leukemic and solid‐tumor‐bearing mice. Having a relatively wide therapeutic window of daily p.o. doses from 12.5 to 75 mg/kg, PSC 833 significantly improved the antileukemic activity of the anticancer drugs adriamycin (ADM), vincristine (VCR) and etoposide (VP‐16) given i.p. or i.v. against i.p.‐inoculated vincristine‐resistant P388 tumor (P388/VCR). PSC 833 in combination with i.p.‐injected anticancer drugs in optimal schedule and dosage induced apparent cures in some leukemic mice, whereas no cures were obtained with the cyclosporin A/anticancer drug combinations. PSC 833 combined with i.v.‐injected anticancer drugs was highly active, but not curative, against P388/VCR and parental P388 tumors (maximum T/C>175%). PSC 833 in combination with intravenous treatment with ADM showed prominent anti‐solid‐tumor activity against s.c.‐inoculated colon adenocarcinoma 26 and human colorectal adenocarcinoma HCT‐15. Against colon adenocarcinoma 26, the PSC 833/ADM combinations induced cure in two or three of six mice. PSC 833/ADM combinations significantly inhibited the growth of the tumor with maximum percent inhibitions of 83 and 73% in the early and advanced stages of the HCT‐15 tumor models, respectively. The present study demonstrated that PSC 833 is highly active in potentiating the antitumor activity of systemically administered ADM, VCR and VP‐16 against four murine and human tumors with a relatively wide therapeutic window of daily p.o. dose range of 12.5–100 mg/kg.