The Ras inhibitor farnesylthiosalicylic acid (Salirasib) disrupts the spatiotemporal localization of active Ras: a potential treatment for cancer.

Chronic activation of Ras proteins by mutational activation or by growth factor stimulation is a common occurrence in many human cancers and was shown to induce and be required for tumor growth. Even if additional genetic defects are present, "correction" of the Ras defect has been shown to reverse Ras-dependent tumorigenesis. One way to block Ras protein activity is by interfering with their spatiotemporal localization in cellular membranes or in membrane microdomains, a prerequisite for Ras signaling and biological activity. Detailed reports describe the use of this method in studies employing farnesylthiosalicylic acid (FTS, Salirasib), a Ras farnesylcysteine mimetic, which selectively disrupts the association of chronically active Ras proteins with the plasma membrane. FTS competes with Ras for binding to Ras-escort proteins, which possess putative farnesyl-binding domains and interact only with the activated form of Ras proteins, thereby promoting Ras nanoclusterization in the plasma membrane and robust signals. This chapter presents three-dimensional time-lapse images that track the FTS-induced inhibition of membrane-activated Ras in live cells on a real-time scale. It also describes a mechanistic model that explains FTS selectivity toward activated Ras. Selective blocking of activated Ras proteins results in the inhibition of Ras transformation in vitro and in animal models, with no accompanying toxicity. Phase I clinical trials have demonstrated a safe profile for oral FTS, with minimal side effects and promising activity in hematological malignancies. Salirasib is currently undergoing trials in patients with pancreatic cancer and with nonsmall cell lung cancer, with or without identified K-Ras mutations. The findings might indicate whether with the disruption of the spatiotemporal localization of oncogenic Ras proteins and the targeting of prenyl-binding domains by anticancer drugs is worth developing as a means of cancer treatment.