Clinical pharmacology of tamoxifen in patients with breast cancer: correlation with clinical data.

Blood tamoxifen levels were determined for patients with metastatic breast cancer following initial and chronic dosing at twice daily 10 mg/m2 or a 20 mg/m2 single dose. Median time to response was six weeks. Blood tamoxifen levels at that time were ten-fold greater than those obtained after an initial single dose; however, steady-state values were not achieved until 16 weeks of chronic dosing. On a loading dose schedule of 40 mg/m2 twice daily for seven days and 20 mg/m2 daily thereafter, blood levels greater than or equal to 10 mg/m2 twice daily steady-state values were reached in one week. Levels drawn at peak and trough times suggest that tamoxifen may be given on a once-daily basis. Tamoxifen half-life was 9-12 hours after the initial dose and seven days after chronic dosing.