Combined treatment of Bcl-2 antisense oligodeoxynucleotides (G3139), p-glycoprotein inhibitor (PSC833), and sterically stabilized liposomal doxorubicin suppresses growth of drug-resistant growth of drug-resistant breast cancer in severely combined immunodeficient mice.

We studied the possibility of increasing sensitization of drug-resistant MDA435/LCC6 multidrug-resistant (MDR) human breast cancer cells to doxorubicin (DOX) by increasing cellular drug retention with P-glycoprotein (P-gp) inhibitor PSC833 in combination with induction of cell death through down-regulation of Bcl-2 protein using Bcl-2 antisense (G3139). In in vitro cytotoxicity assays, the combination of G3139 with DOX exhibited 40% increased cytotoxicity in both wild-type (WT) and MDR cells. PSC833 increased the cytotoxicity of DOX and Taxol with complete and partial reversal of the resistance of MDR cells to DOX and Taxol, respectively. The presence of G3139 did not increase the cytotoxicity of PSC833 combined with DOX or Taxol in both cell lines. In vivo studies with WT and MDR cell lines transplanted into severely combined immunodeficient mice demonstrated that G3139 (5 mg/kg) was able to suppress the growth of both WT and MDR tumors to an equivalent extent. PSC833 (100 mg/kg) partially restored the sensitivity of resistant tumors to DOX, and the combination of G3139 and PSC833 with liposomal DOX showed maximum growth suppression of MDR tumors compared with individual treatments. The improved efficacy of this treatment was attributed to Bcl-2 antisense-induced apoptosis, combined with cellular retention of DOX in tumor cells via P-gp blockade.