Fulvestrant, an estrogen receptor downregulator, reduces cell turnover index more effectively than tamoxifen.

This study was performed to determine whether drug action on breast tumours could also be analysed using a cell turnover index (CTI), a composite measurement of both proliferation and apoptosis.Data were obtained from a randomized, placebo-controlled trial comparing three single doses (50, 125 and 250 mg) of intramuscular fulvestrant (Faslodex, previously known as ICI 182,780) with oral tamoxifen 20 mg/day for 14-21 days in women with early, operable, ER-positive breast cancer. CTI was calculated as the ratio of the proliferation index to the apoptotic index.Fulvestrant 250 mg significantly reduced CTI compared with both placebo (p = 0.0003) and tamoxifen (p = 0.026). The effect on CTI with tamoxifen was not significantly different from that with placebo.This study suggests that CTI may be a useful indicator of drug action on breast tumour growth. A significant reduction in growth was found with fulvestrant 250 mg compared with tamoxifen.