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

Purpose Novel covalent inhibitors of KRAS^G12C have shown limited response rates in KRAS^G12C mutant (MT) colorectal cancer (CRC) patients. Thus, novel KRAS^G12C inhibitor combination strategies that can achieve deep and durable responses are needed.

Experimental design Small molecule KRAS^G12C inhibitors AZ’1569 and AZ’8037 were employed. 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 CRC cells and in vivo. AZ’1569-resistant CRC cells were generated and characterised.

Results Response to AZ’1569 was heterogeneous across the KRAS^G12CMT models. AZ’1569 was ineffective at inducing apoptosis when used as single-agent or combined with chemotherapy or agents targeting the EGFR/KRAS/AKT axis. Using a systems biology approach, we identified the anti-apoptotic BH3-family member BCL2L1/Bcl-xL as a top hit mediating resistance to AZ’1569. Further analyses identified acute increases in the pro-apoptotic protein BIM following AZ’1569 treatment. ABT-263 (Navitoclax), a pharmacological 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 tumour growth in KRAS^G12CMT xenografts. Finally, AZ’1569-resistant cells showed amplification of KRAS^G12C, EphA2/c-MET activation, increased pro-inflammatory 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.

Conclusions Combinatorial targeting of Bcl-xL and KRAS^G12C is highly effective, suggesting a novel therapeutic strategy for KRAS ^G12CMT CRC patients.