Inhibition of Ras for cancer treatment: the search continues

Abstract

The RAS oncogenes (HRAS, NRAS and KRAS) comprise the most frequently mutated class of oncogenes in human cancers (33%), thus stimulating intensive effort in developing anti-Ras inhibitors for cancer treatment. Despite intensive effort, to date, no effective anti-Ras strategies have successfully made it to the clinic. We present an overview of past and ongoing strategies to inhibit oncogenic Ras in cancer. Since approaches to directly target mutant Ras have not been successful, most efforts have focused on indirect approaches to block Ras membrane association or downstream effector signaling. While inhibitors of effector signaling are currently under clinical evaluation, genome-wide unbiased genetic screens have identified novel directions for future anti-Ras drug discovery.

Figure 1. RAS mutation in human cancers

A. Human Ras proteins. RAS genes encode 188 or 189 amino acid proteins that share the indicated amino acid identity. KRAS encodes K-Ras4A or K-Ras4B due to alternative exon four utilization, with KRAS4B the predominant transcript. B. Frequency of specific RAS mutations. KRAS mutations (17,342 unique samples with mutations in a total of 80,140 unique samples) comprise 86% of all RAS mutations documented in human tumor cells. Next most frequent are NRAS mutations (2,279 mutations in 28,521 samples) and HRAS is the least frequent (652 mutations in 19,589 samples). Data are compiled from COSMIC (http://www.sanger.ac.uk/genetics/CGP/cosmic/0. C. Genetic progression of pancreatic ductal adenocarcinoma. D. Genetic progression of colorectal carcinoma.

Figure 2. Regulation of the Ras GDP-GTP cycle in normal and neoplastic cells

A. Normal Ras. Wild type Ras proteins cycle between inactive GDP-bound and active GTP-bound states. Growth factors stimulate transient activation of Ras through activation of RasGEFs (e.g., Sos). Ras-GTP binds preferentially to downstream effectors (E). RasGAPs (e.g., neurofibromin) accelerate the intrinsic GTP hydrolysis activity, returning Ras to the inactive state. B. Tumor-associated Ras. Missense mutations primarily at glycine-12, glycine-13 or glutamine-61 impair intrinsic and GAP-stimulated GTP hydrolysis activity, rendering Ras persistently active and GTP-bound.

Figure 3. Targeting Ras membrane association for anti-Ras drug discovery

Ras proteins are synthesized initially as cytosolic and inactive proteins. The C-terminal CAAX motif signals for three posttranslational modifications, beginning with cytosolic FTase-catalyzed addition of a C15 farnesyl group and Golgi-associated Rce1 and Icmt catalyzed carboxymethylation of the now terminal farnesylated cysteine residue. Inhibitors of FTase (e.g., tipifarnib and lonafarnib) block all CAAX-signaled modifications. Farnesyl group-containing small molecules (salirasib and TLN-4601) have been evaluated in clinical trials as possible inhibitors of Ras membrane association and oncogenesis.

Figure 4. Effectors of Ras-mediated oncogenesis

Ras-GTP binds preferentially to a spectrum of functionally diverse downstream effectors. Of these, five have been validated in cell culture and/or mouse models for their requirement for mutant Ras-induced oncogenesis. In addition to direct mutational activation of Ras, Ras can also be activated indirectly, for example, by mutational inactivation of the neurofibromin RasGAP or by mutational activation of the epidermal growth factor receptor (EGFR).

Figure 5. Inhibitors of Raf and PI3K effector signaling under clinical evaluation

Small molecule inhibitors of Raf and MEK, and PI3K, AKT and mTOR are currently being evaluated in Phase I-III clinical trials. Also see Tables 1 and 2. Compiled from http://clinicaltrials.gov