Cyclosporin (CsA) is a potent modulator of multidrug resistance (MDR) and has been combined with
etoposide (VP-16) to purge MDR leukemic cells from human bone marrow (BM) in vitro. We studied the feasibility of this approach in an in vivo model for autologous BM
transplantation using the murine
leukemia cell line P388 and its MDR variant P388/ADR. Colony-forming assays with 2-h
drug exposure revealed a
tumor selectivity of
VP-16 for P388 cells compared to normal murine marrow granulocyte-macrophage colony-forming units (CFU-GM), whereas P388/ADR cells were resistant to
VP-16. Simultaneous incubation with CsA restored sensitivity in these cells. Almost 4 logs of cell kill were achieved by treating P388/ADR cells with 60 microM
VP-16 plus 2.5 microM CsA (combination A) or 40 microM
VP-16 plus 10 microM CsA (combination B), whereas there was a 2.5-log reduction of CFU-GM at these doses. Even though the myelotoxicity of
VP-16 was increased by the addition of CsA, this effect was nonspecific as shown by a similar chemosensitization in sensitive P388 as well as in P388/VP 2.5 cells, an atypical MDR variant lacking
P-glycoprotein. In vivo experiments addressed the ability of BM treated with
VP-16 and CsA to rescue lethally irradiated mice and to purge leukemic cells. In total, 1/14 lethally irradiated mice died due to
sepsis within 10 days after receiving 15 x 10(6) BM cells treated ex vivo with combination A in contrast to 1/4 for combination B. All 16 surviving animals demonstrated long-term engraftment. When simulated remission marrow contaminated with 0.1% P388/ADR was purged with
VP-16 (60 microM) or CsA (2.5 microM) alone, all mice died from
leukemia before day 16 after
transplantation (median 14.3 and 12.2 days). In contrast, nine of ten animals receiving similar marrow purged with combination A survived > 60 days without any evidence of disease (p < 0.01). We conclude that combining
VP-16 and CsA was effective in purging MDR
leukemia cells from transplanted BM in this murine model.