Ras as a therapeutic target in hematologic malignancies.

The RAS gene product is normally a membrane-localized G protein (N-Ras, K-Ras and H-Ras) of 21 kDa classically described as a molecular off/on switch. It is inactive when bound to guanosine diphosphate and active when bound to GTP. When mutated, the gene produces an abnormal protein resistant to GTP hydrolysis by GTPase, resulting in a constitutively active GTP-bound protein that stimulates a critical network of signal transduction pathways that lead to cellular proliferation, survival and differentiation. At least three downstream effector pathways have been described, including Raf/MEK/ERK, PI3K/AKT and RalGDS, but they are not completely understood. Ras pathways are also important downstream effectors of several receptor tyrosine kinases localized in the cell membrane, most notably the BCR-ABL fusion protein seen in patients with Philadelphia chromosome positive chronic myelogenous leukemia. An important consideration in designing strategies to block Ras stimulatory effect is that Ras proteins are synthesized in the cytosol, but require post-translational modifications and attachment to anchor proteins or membrane binding sites in the cell membrane to be biologically active. Farnesyl transferase inhibitors (FTIs) are probably the best-studied class of Ras inhibitors in hematologic malignancies. They block the enzyme farnesyl-transferase (FTase), which is essential for post-translational modification. However, it has been observed that the Ras proteins also can be geranylgeranylated in the presence of FTIs, thus allowing membrane localization and activation, which limits their effectiveness. It is now hypothesized that their mechanism of action may be through FTase inhibition involving other signal transduction pathways. S-trans, trans-farnesylthiosalicylic acid, which was first designed as a prenylated protein methyltransferase inhibitor, has shown in vitro activity against all activated Ras proteins by dislodging them from their membrane-anchoring sites. Here, Ras biology, its signaling pathways and its implications as a therapeutic target in hematologic malignancies are reviewed.