Response to estramustine phosphate and paclitaxel in patients with advanced breast cancer: a phase I study

Estramustine phosphate (EMP) is thought to form a chemical link between estradiol and non-nitrogen mustard. An estramustine-binding protein has been isolated in prostate, breast, and brain cancers as well as in malignant melanoma cells. Estramustine phosphate's ability to bind to microtubular-associated proteins and to interfere with mdr-mediated drug efflux are thought to result in its enhancement of paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) activity in cell lines and in its ability to affect hormone-resistant prostate cancer. This phase I study administered combined paclitaxel and EMP to 25 women with ovarian, breast, and other tumors and assessed efficacy and toxicity. Estramustine phosphate was administered at two dose levels, 900 or 1,200 mg/m2 daily on days 1, 2, and 3 in 3-week cycles. On day 3, paclitaxel (150, 180, 210, or 225 mg/m2) was given concomitantly by 3-hour infusion. Therapeutic effects were noted in all patients. Partial responses were noted in three of eight patients with breast cancer who had failed to improve on paclitaxel alone. Three other patients experienced prolonged stable disease. Only moderate toxicities were noted until EMP levels of 1,200 mg/m2 were reached. At these dose levels, gastrointestinal toxicities became more prominent. The addition of EMP to paclitaxel allowed patients to receive paclitaxel for longer periods, and may have enhanced the therapeutic effects of paclitaxel. If so, the mechanisms of such enhancement warrant investigation. The two drugs may work on different aspects of microtubular function, for example, or may reduce efflux of paclitaxel in P-glycoprotein overexpressed tumors.