Austin M. Moore

Approximately 30% of all human cancers are driven by mutations in RAS genes, KRAS, HRAS, and NRAS, resulting in the constitutive activation of RAS proteins and stimulation of MAPK/AKT signaling. Non-mutant, i.e., wild-type (WT) RAS can also become activated through mechanisms such as gene amplification or excessive stimulation by mutated or overexpressed receptor tyrosine kinases (e.g., EGFR), thereby promoting cancer progression. Mutant or activated RAS contributes to multiple hallmarks of cancer, including unchecked cellular proliferation, reprogrammed cellular metabolism, immunosuppression, and metastasis. Hence, RAS is of immense clinical importance, with hundreds of laboratories studying various aspects of RAS biology or developing RAS inhibitors. There is perhaps no greater unmet medical need in oncology than the need for a broadly efficacious but safe inhibitor of mutant and activated RAS. Mutant-specific KRAS G12C inhibitors have shown promising therapeutic efficacy, leading to FDA approval of sotorasib and adagrasib, although their use is limited to patients with the relatively rare G12C KRAS mutation. Mutant-specific KRAS inhibitors are also susceptible to adaptive resistance, in part, due to secondary RAS mutations, and compensatory signaling from WT RAS isozymes. A pan-RAS inhibitor capable of blocking all RAS isozymes, regardless of the underlying mutation, offers the potential for broader efficacy and capacity to avert resistance. While just a few years ago, pan-RAS inhibitors were predicted to be severely toxic or even fatal, the apparent safety profile of RMC-6236 (daraxonrasib), a pan-RAS inhibitor currently in clinical trials, suggests otherwise. Indeed, pan-RAS inhibitors are now considered by many in the RAS field to be the most promising class in development. In this review, we summarize the evolution and current status of pan-RAS and pan-KRAS inhibitors in preclinical and clinical development and highlight emerging human-relevant tumor models that are advancing preclinical evaluation.

Approximately 28 million individuals in the United States face the risk of developing precancerous colonic adenomas (polyps) and potentially progressing to colorectal cancer (CRC). While a promising strategy for CRC prevention involves pharmacological intervention, such as cancer chemoprevention or interception, currently, there are no FDA-approved drugs capable of preventing the formation or progression of adenomas to adenocarcinoma. Numerous clinical, epidemiological, and preclinical studies have offered compelling evidence supporting the efficacy of nonsteroidal anti-inflammatory drugs (NSAIDs) in CRC chemoprevention. However, the prolonged use of NSAIDs is not FDA-approved due to potential life-threatening toxicities resulting from cyclooxygenase (COX) inhibition and the depletion of physiological prostaglandins. Despite indications that the COX inhibitory activity of NSAIDs may not be essential for their antineoplastic effects, the absence of a well-defined target impeded the development of derivatives that do not inhibit COX. Earlier research suggests that the inhibition of cyclic guanosine monophosphate phosphodiesterase (cGMP PDE) may be responsible, at least in part, for the antineoplastic activity of the NSAID sulindac. This could potentially offer a novel target for CRC chemoprevention. To identify the cGMP PDE isozyme(s) contributing to the antineoplastic activity of sulindac, we synthesized a chemically diverse library of over 1500 compounds, all sharing the indene scaffold of sulindac. Subsequently, we screened these compounds for their impact on cancer cell growth and PDE inhibitory activity. From this screening, a series of lead compounds emerged. These compounds lacked COX-1 and COX-2 inhibitory activity, surpassing sulindac in potency to inhibit CRC cell growth. Importantly, they demonstrated greater selectivity by not affecting normal cell growth. Through chemical optimization, we identified several development candidates that selectively inhibit PDE5 and/or PDE10. These compounds activate cGMP/PKG signaling, suppressing Wnt/β-catenin transcription. This action counters the growth advantages resulting from APC or CTNNB1 mutations, which are responsible for most human CRCs. This review delves into the scientific literature supporting PDE5 and/or PDE10 as potential targets for CRC chemoprevention or interception. Our findings suggest a promising avenue for developing drugs that may effectively intervene in the progression of colorectal cancer, offering hope for improved preventive strategies in the future.