Synergistic effect of a retinoid X receptor-selective ligand bexarotene (LGD1069, Targretin) and paclitaxel (Taxol) in mammary carcinoma.

We have previously shown that the retinoid X receptor (RXR) ligand bexarotene (LGD1069, Targretin) is efficacious as a chemopreventive and chemotherapeutic agent in rat N-nitroso-N-methylurea (NMU)-induced mammary carcinomas (Cancer Res 58: 479-484, 1998). To determine additional role for bexarotene in breast cancer treatment, we evaluated the effect of bexarotene on the efficacy of paclitaxel (Taxol) treatment in a rat NMU-derived mammary tumor cell line, NMU-417, in vitro and in rat NMU-induced mammary tumors in vivo. Our growth inhibition results showed that the bexarotene/paclitaxel combination produced a concentration-dependent synergy in NMU-417 tumor cell line. Synergistic growth inhibition by the combination was associated with an increase in cell death induced by both agents. In rat NMU-induced mammary tumor model in vivo, the benefit of combination therapy was observed as early as 1 week after treatment and increased as treatment continued. At the end of 6 weeks of treatment, the bexarotene/paclitaxel combination produced an overall objective response rate of 94% compared with a rate of 12% in paclitaxel-treated and 58% in bexarotene-treated animals, an effect that was more than the additive effects produced by single agents. Although both bexarotene alone and the bexarotene/paclitaxel combination reduced tumor multiplicity to similar extent, the combination regimen produced a statistically significant decrease in total tumor burden compared to single agents and untreated controls (two-tailed, p < 0.05). Combination therapy did not further alter body weight nor increase toxicity when compared to single agents. In summary, our results demonstrated the potential of using a RXR selective ligand in combination with chemotherapy for the treatment of breast cancer.