Enhancement of the antitumor activity of tamoxifen and anastrozole by the farnesyltransferase inhibitor lonafarnib (SCH66336).

Lonafarnib is an orally bioavailable farnesyltransferase inhibitor. Originally developed to block the membrane localization of Ras, subsequent work suggested that farnesyltransferase inhibitors mediate their antitumor activities by altering the biological activities of additional farnesylated proteins. Breast tumor models that express wild-type Ras have been shown to be sensitive to farnesyltransferase inhibitors. We have determined the effects of combining lonafarnib with the antiestrogen 4-hydroxy tamoxifen on hormone-dependent breast cancer cell lines in vitro. The effects of combining lonafarnib with tamoxifen or the aromatase inhibitor anastrozole on the growth of two different MCF-7 breast tumor xenograft models were also evaluated. In four of five human breast cancer cell lines, lonafarnib enhanced the antiproliferative effects of 4-hydroxy tamoxifen. The combination prevented MCF-7 cells from transitioning through the G1 to S phase of the cell cycle and augmented apoptosis. This was associated with reduced expression of E2F-1 and a reduction in hyperphosphorylated retinoblastoma protein. Lonafarnib plus 4-hydroxy tamoxifen also inhibited the mammalian target of rapamycin signal transduction pathway. In nude mice bearing parental MCF-7 or aromatase-transfected MCF-7Ca breast tumor xenografts, lonafarnib enhanced the antitumor activity of both tamoxifen and anastrozole. These studies indicate that lonafarnib enhances the efficacy of endocrine agents clinically used for treating hormone-dependent breast cancer.