Phase I trial of the cyclin-dependent kinase inhibitor flavopiridol in combination with docetaxel in patients with metastatic breast cancer

Purpose: The purpose of this study was to determine the toxcities and characterize the pharmacokinetics of docetaxel and flavopiridol in patients with metastatic breast cancer. Experimental Design: Docetaxel was administered at an initial dose of 60 mg/m² followed in 24 hours by a 72-hour infusion of flavopiridol at 50 mg/m²/d every 3 weeks. Because dose-limiting myelosuppression occurred, the schedule was amended to docetaxel, 50 mg/m ², followed by escalating doses of flavopiridol (starting dose, 26 mg/m²/d) as a 1-hour infusion daily for 3 days. Pharmacokinetic studies were performed. Ki67, p53, and phosphorylated retinoblastoma protein (phospho-Rb) in paired tumor and buccal mucosa biopsies (obtained pre- and posttreatment) were examined by immunohistochemistry. Results: Eleven patients were enrolled. Five patients received docetaxel and 72-hour flavopiridol. Dose-limiting toxicity was grade 4 neutropenia. Six patients received docetaxel and 1-hour flavopiridol, and the dose-limiting toxicity was grade 3 hypotension. Pharmacokinetics of flavopiridol and docetaxel were consistent with historical data. Nuclear staining with p53 increased and phospho-Rb decreased in 10 pairs of buccal mucosa biopsies posttreatment (P = 0.002 and P = 0.04, respectively). No significant changes in Ki67, p53, or phospho-Rb were detected in six paired tumors. Two patients sustained stable disease for >3 months (72-hour flavopiridol), and one partial response was observed (1-hour flavopiridol). Conclusions: Docetaxel combined with 72-hour flavopiridol was not feasible because of dose-limiting neutropenia. Dose escalation of a 1-hour infusion of flavopiridol with docetaxel was also not possible. The changes in p53 and phospho-Rb in buccal mucosa suggest that a biological effect with flavopiridol was achieved.