Farnesyl transferase inhibitors: the next targeted therapies for breast cancer?

The ras family of proto-oncogenes are upstream mediators of several essential cellular signal transduction pathways involved in cell proliferation and survival. Point mutations of ras oncogenes result in constitutively active Ras and have been shown to be oncogenic. However, ras activation can occur in the absence of ras mutations secondary to upstream receptor activation. The first important step in Ras activation is farnesylation by farnesyl transferase, and inhibitors of this enzyme have been demonstrated to inhibit Ras signaling, and have anti-tumor effects. However, it is now clear that farnesyl transferase inhibitors (FTIs) have activity independent of Ras, most likely due to effects on prenylated proteins downstream of Ras, which explains their activity in several malignancies, including breast cancer, where ras mutations are rare. Several FTIs are in clinical development for the treatment of solid tumors. Preclinical evidence suggests that FTIs can inhibit breast cancers in vitro and in vivo, and a phase II trial of the FTI, R115777, in patients with advanced breast cancer produced encouraging results. Based on prior successful outcomes with agents targeting the estrogen and epidermal growth factor receptor pathways in breast cancer, the FTIs, used alone or more likely with other agents, may be the next exciting targeted therapy in breast cancer.