Targeted Degradation of SOS1 Exhibits Potent Anticancer Activity and Overcomes Resistance in KRAS-Mutant Tumors and BCR-ABL-Positive Leukemia

SOS1 is an essential guanine nucleotide exchange factor for Ras that also plays a critical role in the activation of the small GTPase RAC mediated by Bcr-Abl in leukemogenesis. Despite this, small molecule inhibitors targeting SOS1 have shown limited efficacy in clinical trials for KRAS mutant cancers, and their potential as a therapeutic approach for chronic myeloid leukemia (CML) remains largely unexplored. In this study, we developed a potent SOS1 PROTAC SIAIS562055, which was designed by connecting a CRBN ligand to an analogue of the SOS1 inhibitor BI-3406. SIAIS562055 exhibited sustained degradation of SOS1 and inhibition of downstream ERK pathways, resulting in superior anti-proliferative activity compared to small molecule inhibitors. SIAIS562055 also potentiated the activity of both KRAS inhibitors in KRAS-mutant cancers and ABL inhibitors in BCR-ABL+ CML. In KRAS-mutant xenografts, SIAIS562055 displayed promising antitumor potency as a monotherapy and enhanced ERK inhibition and tumor regression when combined with KRAS inhibitors, overcoming acquired resistance. In CML cells, SIAIS562055 promoted the active uptake of Bcr-Abl inhibitors by upregulating the carnitine/organic cation transporter SLC22A4. SIAIS562055 and Bcr-Abl inhibitors synergistically enhanced inhibition of ABL phosphorylation and downstream signaling, demonstrating robust antitumor activities in both mouse xenografts and primary CML patient samples. In summary, this study suggests that PROTAC-mediated SOS1 degradation represents an effective therapeutic strategy for treating not only KRAS-mutant cancers but also BCR-ABL-harboring leukemia.