Identification of a Highly Cooperative PROTAC Degrader Targeting GTP-loaded KRAS(on) Alleles

Kirsten rat sarcoma viral oncogene homolog (KRAS) is a frequently mutated oncogene in multiple types of cancer and is a high priority target for oncology drug development. There are many different KRAS mutations, including mutations which favour the GTP-loaded ‘active’ state of KRAS, (KRAS(on)), that can lead to tumorigenesis. However, small molecule interventions thus far have predominantly targeted single mutations of ‘inactive’ GDP-loaded KRAS, (KRAS(off)), such as KRASG12C. Here, we address this gap through the development of heterobifunctional VHL-based PROTACs capable of engaging and degrading KRAS(on), thus addressing a wider range of KRAS mutations. By studying ternary complex affinity, stability and binding modes using SPR binding assays and X-ray cocrystal structures we identified PROTACs that exhibit the ability to cooperatively and stably bind GCP-loaded KRAS as representative of KRAS(on) variants. Degrader activity profiling in relevant cancer cells supported the discovery of ACBI4, a PROTAC which forms a highly stable ternary complex between VHL and GTP-bound KRAS and which potently degrades KRASG12R, leading to anti-proliferative effect. ACBI4 provides a new chemical tool for studying the impact of degrading KRAS(on) mutants which is not possible with current pan-KRAS inhibitors or degraders.