Use of etoposide in combination with cyclosporin for purging multidrug-resistant leukemic cells from bone marrow in a mouse model.
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.
Kühl, JS; Sikic, BI; Blume, KG; Chao, NJ
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