EFFECTS OF AMIODARONE, CYCLOSPORINE-A, AND PSC-833 ON THE CYTOTOXICITY OF MITOXANTRONE, DOXORUBICIN, AND VINCRISTINE IN NON-P-GLYCOPROTEIN HUMAN SMALL-CELL LUNG-CANCER CELL-LINES

The multidrug resistance (MDR) modulators amiodarone (AM), cyclosporin A (CsA), and PSC 833 were tested for their potential to modulate cyto toxicity of doxorubicin (DOX), vincristine (VCR), and mitoxantrone (MX ) in a sensitive human small cell lung carcinoma cell line GLC4 in its DOX-resistant non-P-glycoprotein subline GLC4-Adr, and in its cisplat in-resistant subline GLC4-CDDP. GLC4-Adr, in which overexpression of t he so-called multidrug resistance-associated protein has been demonstr ated, is 91-fold resistant for DOX, 22-fold for VCR, and 7.5-fold for MX, compared with its sensitive cell line. AM previously modulated DOX and VCR resistance in the P-glycoprotein-positive human colon cancer cell line COLO 320. Cytotoxicity was studied in the microtiter well te trazolium assay. In the small cell lung carcinoma cell lines described above, AM did not increase cytotoxicity of DOX, but increased VCR cyt otoxicity; moreover, AM was shown to be a potent modulator of MX cytot oxicity. CsA did not potentiate DOX cytotoxicity, but, at a concentrat ion of 4 mu M, it modestly increased VCR cytotoxicity in GLC4. However , 0.8 and 4.0 mu M CsA protected against MX cytotoxicity in GLC4 and G LC4-CDDP, but no effect was observed in GLC4-Adr. At the much higher I D10 concentration CsA modulated MX cytotoxicity 1.6-fold in GLC4-Adr a nd slightly in GLC4 and GLC4-CDDP. PSC 833, a nonimmunosuppressive CsA analogue, did not alter the cytotoxicity of DOX or MX in these cell l ines, but potentiated VCR cytotoxicity in GLC4-Adr at a concentration of 0.4 mu M. The modulation of MX cytotoxicity by AM and the protectio n by CsA was confirmed in a clonogenic assay. In the colony-forming un it granulocytemonocyte assay, no additional MX toxicity on normal bone marrow by AM was observed. Flow cytometry of cellular MX fluorescence was performed in order to elucidate the mechanism behind the AM-induc ed increased MX cytotoxicity. This revealed an increase in cellular MX after 1-h incubation of MX combined with AM and an inhibition of effl ux from GLC4 and GLC4-Adr; CsA and PSC 833 had no effect on MX efflux. An increase in MX-induced cleavable complexes by AM in GLC4 was obser ved using the K+/sodium dodecyl sulfate coprecipitation assay, but no effect of CsA was found. In conclusion, AM enhances, MX and VCR cytoto xicity in these sensitive, non-P-glycoprotein DOX and cisplatin-resist ant small cell lung carcinoma cell lines. It also inhibits efflux of M X and causes more MX-induced cleavable complexes.