LAUSR

Significance Mutants of RAS are major oncogenes and occur in many human cancers, but efforts to develop drugs that directly inhibit the corresponding constitutively active RAS proteins have failed so far. We therefore focused on SOS1, the guanine nucleotide exchange factor (GEF) and activator of RAS. A combination of high-throughput and fragment screening resulted in the identification of nanomolar SOS1 inhibitors, which effectively down-regulate active RAS in tumor cells. In cells with wild-type KRAS, we observed complete inhibition of the RAS-RAF-MEK-ERK pathway. In a mutant KRAS cell line, SOS1 inhibition resulted in a reduction of phospho-ERK activity by 50%. Together, the data indicate that inhibition of GEFs may represent a viable approach for targeting RAS-driven tumors. Since the late 1980s, mutations in the RAS genes have been recognized as major oncogenes with a high occurrence rate in human cancers. Such mutations reduce the ability of the small GTPase RAS to hydrolyze GTP, keeping this molecular switch in a constitutively active GTP-bound form that drives, unchecked, oncogenic downstream signaling. One strategy to reduce the levels of active RAS is to target guanine nucleotide exchange factors, which allow RAS to cycle from the inactive GDP-bound state to the active GTP-bound form. Here, we describe the identification of potent and cell-active small-molecule inhibitors which efficiently disrupt the interaction between KRAS and its exchange factor SOS1, a mode of action confirmed by a series of biophysical techniques. The binding sites, mode of action, and selectivity were elucidated using crystal structures of KRASG12C–SOS1, SOS1, and SOS2. By preventing formation of the KRAS–SOS1 complex, these inhibitors block reloading of KRAS with GTP, leading to antiproliferative activity. The final compound 23 (BAY-293) selectively inhibits the KRAS–SOS1 interaction with an IC50 of 21 nM and is a valuable chemical probe for future investigations.