The Farnesyl Transferase Inhibitor Darlifarnib (KO-2806) Resensitizes Relapsing Tumors to RAS Inhibition

Resistance remains a key issue limiting the clinical benefit from RAS-targeting therapeutic agents and necessitates combination approaches. In this study, we identified persistent mTORC1 activity in preclinical KRAS-mutant non-small cell lung cancer (NSCLC) and colorectal cancer models as a frequent, nongenetic driver of inherent and adaptive resistance to RAS inhibition. This vulnerability was targetable with the farnesyl transferase inhibitor darlifarnib (KO-2806), which blocks mTORC1 activation via RHEB while sparing mTORC2 to limit associated toxicities. The addition of KO-2806 to NSCLC or colorectal cancer tumors progressing on mutant-selective RAS inhibitors led to rapid and durable tumor regression. In contrast, switching from mutant-selective to pan-RAS inhibitor monotherapy resulted in only stasis of NSCLC tumors and had no effect on colorectal cancer tumor progression. Furthermore, the addition of KO-2806 rescued sensitivity of progressing tumors to the pan-RAS inhibitor RMC-6236. These results establish mTORC1 as an important mediator of escape from RAS inhibition and highlight KO-2806 as a promising RAS companion inhibitor in patients with prior RAS inhibitor exposure. SIGNIFICANCE: KO-2806 salvages RAS inhibitor activity by controlling parallel mTORC1 in RAS inhibitor-resistant tumors in which vertical inhibition of MAPK is insufficient to restore sensitivity, providing a combination strategy for resistant patients.