Droloxifene, a new antiestrogen: Its role in metastatic breast cancer

Droloxifene, a new antiestrogen, has theoretical advantages over tamoxifen based on preclinical data. These include higher affinity to the estrogen receptor, higher antiestrogenic to estrogenic ratio, and more effective inhibition of cell growth and division in ER positive cell lines, as well as less toxicity, including reduced carcinogenicity in animal models. Droloxifene also exhibits more rapid pharmacokinetics, reaching peak concentrations and being eliminated much more rapidly than tamoxifen. A phase II study compared droloxifene in dosages of 20, 40, and 100 mg daily in postmenopausal women with metastatic, or inoperable recurrent, or primary locoregional breast cancer who had not received prior hormonal therapy. Of 369 patients randomized, 292 were eligible and 268 evaluable for response. Response rates (CR + PR) were 30% in the 20 mg group, 47% in the 40 mg group, and 44% in the 100 mg group (40 mg vs 20 mg, p = 0.02; 100 mg vs 20 mg, p = 0.04; pooled 40 + 100 mg vs 20 mg, p = 0.01). Median response duration also favoured the higher dosages (20 mg group = 12 months; 40 mg group = 15 months; 100 mg group = 18 months). When adjusted for prognostic factors, time to progression was significantly better for the 100 mg (p = 0.01) and the 40 mg (p = 0.02) group compared to the 20 mg group. Droloxifene increased SHBG and suppressed FSH at all dosages and suppressed LH at the 40 and 100 mg dosages. These hormonal effects increased with increasing dosage. Shortterm toxicity was generally mild, and similar to that seen with other antiestrogens. Droloxifene appears active and tolerable. It may have a particular role in situations in which rapid pharmacokinetics, or an increased antiestrogenic to estrogenic ratio, are required.