Bcl-xL and association with apoptosis following KRASG12C inhibition in KRASG12C mutant colorectal cancer.

Abstract

3557 Background: Novel covalent inhibitors of KRASG12C have shown modest response rates in KRASG12C mutant (MT) colorectal cancer (CRC) patients. Thus, novel KRASG12C inhibitor combination strategies that can achieve deep and durable responses are needed. Methods: The small molecule KRASG12C 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 KRASG12CMT CRC cells and in vivo xenograft models. AZ’1569 acquired resistant CRC models were generated and characterised. Results: Response to AZ’1569 was heterogeneous across the KRASG12CMT 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 anti-apoptotic BH3-family member BCL2L1/Bcl-xL as 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 in a panel of KRASG12C MT CRC cells. Furthermore, this combination also resulted in dramatically attenuated tumour growth in KRASG12C MT CRC xenografts. Finally, AZ’1569 acquired resistant KRASG12C MT CRC cells showed amplification of KRASG12C, EphA2/c-MET activation, increased pro-inflammatory chemokine profile and cross-resistance to standard-of-care chemotherapy and several targeted agents. Importantly, the 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 KRASG12C is highly effective, suggesting a novel therapeutic strategy for KRAS G12CMT CRC patients. The cross-resistance to other targeted therapies and importantly conventional chemotherapy in the AZ’1569 acquired resistant cells poses a challenge, with implications for the optimal use of KRASG12C inhibitors as a second or third line option.