Bcl-xL is a key mediator of apoptosis following KRAS^G12C inhibition in KRAS^G12C-mutant colorectal cancer

Novel covalent inhibitors of KRAS^G12C have shown limited response rates in patients with KRAS^G12C-mutant (MT) colorectal cancer. Thus, novel KRAS^G12C inhibitor combination strategies that can achieve deep and durable responses are needed. Small-molecule KRAS^G12C inhibitors AZ’1569 and AZ’8037 were used. To identify novel candidate combination strategies for AZ’1569, we performed RNA sequencing, siRNA, and high-throughput drug screening. Top hits were validated in a panel of KRAS^G12CMT colorectal cancer cells and in vivo. AZ’1569-resistant colorectal cancer cells were generated and characterized. We found that response to AZ’1569 was heterogeneous across the KRAS^G12CMT models. AZ’1569 was ineffective at inducing apoptosis when used as a single agent or combined with chemotherapy or agents targeting the EGFR/KRAS/AKT axis. Using a systems biology approach, we identified the antiapoptotic BH3-family member BCL2L1/Bcl-xL as a top hit mediating resistance to AZ’1569. Further analyses identified acute increases in the proapoptotic protein BIM following AZ’1569 treatment. ABT-263 (navitoclax), a pharmacologic Bcl-2 family inhibitor that blocks the ability of Bcl-xL to bind and inhibit BIM, led to dramatic and universal apoptosis when combined with AZ’1569. Furthermore, this combination also resulted in dramatically attenuated tumor growth in KRAS^G12CMT xenografts. Finally, AZ’1569-resistant cells showed amplification of KRAS^G12C, EphA2/c-MET activation, increased proinflammatory chemokine profile and cross-resistance to several targeted agents. Importantly, KRAS amplification and AZ’1569 resistance were reversible upon drug withdrawal, arguing strongly for the use of drug holidays in the case of KRAS amplification. Taken together, combinatorial targeting of Bcl-xL and KRAS^G12C is highly effective, suggesting a novel therapeutic strategy for patients with KRAS^G12CMT colorectal cancer.