Data from The farnesyltransferase inhibitor R115777 (tipifarnib) in combination with tamoxifen acts synergistically to inhibit MCF-7 breast cancer cell proliferation and cell cycle progression <i>in vitro</i> and <i>in vivo</i>

Abstract

<div>Abstract<p>Cross-talk between receptor tyrosine kinases and estrogen receptor is at least partly responsible for the development of acquired resistance to endocrine therapies. Hence, targeting receptor tyrosine kinases and their downstream partners with inhibitors/antagonists may reverse this resistance. Although <i>ras</i> mutations are rare in breast cancer (2%), aberrant function of Ras signal transduction pathways is common. We therefore investigated the efficacy of the farnesyltransferase inhibitor (FTI) R115777 (tipifarnib) in combination with tamoxifen in MCF-7 human breast cancer models both <i>in vitro</i> and <i>in vivo</i>. There was a synergistic antiproliferative interaction between R115777 and 4-hydroxy-tamoxifen <i>in vitro</i> as calculated by median effect analysis. The combination resulted in a significantly greater G<sub>1</sub> arrest than either drug alone and this was associated with marked inhibition of cyclin D1 and induction of the cell cycle inhibitor p27<sup>kip1</sup>. Combining R115777 with either tamoxifen or estrogen withdrawal <i>in vivo</i> produced a significantly greater inhibition of tumor growth and lower xenograft cell proliferation than either therapy alone. These results suggest that the combination of this FTI with endocrine therapy may be of therapeutic benefit in the treatment of breast cancer. Enhanced G<sub>1</sub> arrest due to modulation of cell cycle regulatory proteins may be the underlying mechanism for the positive interaction between FTIs and tamoxifen. [Mol Cancer Ther 2007;6(9):2458–67]</p></div>