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Department of Molecular Oncology, Ligand Pharmaceuticals, Inc., San Diego, California

Requests for reprints: William W. Lamph, Department of Molecular Oncology, Ligand Pharmaceuticals, Inc., 10275 Science Center Drive, San Diego, CA 92121. Phone: 858-550-7500; Fax: 858-550-7730. E-mail: wlamph{at}ligand.com

Acquired drug resistance represents a major challenge in the therapeutic management of breast cancer patients. We reported previously that the retinoid X receptor–selective agonist bexarotene (LGD1069, Targretin) was efficacious in treating animal models of tamoxifen-resistant breast cancer. The goal of this study was to evaluate the effect of bexarotene on development of acquired drug resistance and its role in overcoming acquired drug resistance in advanced breast cancer. Paclitaxel, doxorubicin, and cisplatin were chosen as model compounds to determine the effect of bexarotene on the development of acquired drug resistance. Human breast cancer cells MDA-MB-231 were repeatedly treated in culture with a given therapeutic agent with or without bexarotene for 3 months. Thereafter, cells were isolated and characterized for their drug sensitivity. Compared with parental cells, cells treated with a single therapeutic agent became resistant to the therapeutic agent, whereas cells treated with the bexarotene combination remained chemosensitive. Cells with acquired drug resistance, when treated with the combination, showed increased sensitivity to the cytotoxic agent. Furthermore, cells treated with the combination regimen had reduced invasiveness and angiogenic potential than their resistant counterparts. These in vitro findings were further confirmed in an in vivo MDA-MB-231 xenograft model. Our results suggest a role for bexarotene in combination with chemotherapeutic agents in prevention and overcoming acquired drug resistance in advanced breast carcinoma.

Key Words: bexarotene • mammary carcinoma • multidrug resistance • retinoid X receptor

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¹ Hermann and Lamph, Unpublished results.

Received 1/13/05; revised 2/23/05; accepted 3/ 9/05.