MECHANISMS FOR HIGH METHOXYMORPHOLINO DOXORUBICIN CYTOTOXICITY IN DOXORUBICIN-RESISTANT TUMOR-CELL LINES

Methoxymorpholino doxorubicin (MMRDX) is an anthracycline analogue tha t is able to overcome tumor cell resistance to classical anthracycline s. Mechanisms for increased MMRDX cytotoxicity were analyzed in a smal l cell lung carcinoma cell line (GLC(4)), its 300-fold doxorubicin-res istant and multidrug resistance-associated protein (MRP)-over-expressi ng subline (GLC(4)/ADR), an ovarian carcinoma cell line (A2780) and it s 100-fold doxorubicin resistant and P-glycoprotein (P-gp)-over-expres sing subline A2780AD. Cross-resistance, measured with the MTT assay at MMRDX concentration resulting in 50% growth inhibition, was 1.8-fold in GLC(4)/ADR and 4.5-fold in A2780AD compared to their respective par ental cell lines. Cellular MM RDX accumulation was equal in GLC(4) and GLC(4)/ADR and 2-fold lower in A2780AD compared to A2780. Doxorubicin fluorescence was analyzed with confocal laser scan microscopy. Fluore scence was nuclear in sensitive, and cytoplasmic in resistant, cell li nes, while MMRDX fluorescence was found in the nucleus in all cell lin es. Pre-incubation with the MRP blocker MK571 restored in GLC(4)/ADR c ells the nuclear doxorubicin fluorescence pattern, as observed in GLC( 4) cells. MMRDX, thus, can largely overcome cross-resistance in these P-gp- and MRP-overexpressing doxorubicin-resistant cell lines. Our res ults suggest that MMRDX is not a substrate for MRP-mediated resistance . (C) 1997 Wiley-Liss, Inc.