MEK1 as a Synthetic Lethal Target with Cabozantinib in Renal Cell Carcinoma: Insights from CRISPR/Cas9 Screening.

Background/Objectives: Cabozantinib is a tyrosine kinase inhibitor that primarily targets MET. It has become an important drug in the treatment of renal cell carcinoma (RCC); however, many patients do not respond to cabozantinib treatment and there is no effective next-line therapy. In this study, we identified molecular-targeted drugs that exhibit synergistic effects with cabozantinib using CRISPR/Cas9 screening. Methods: A kinome-wide synthetic lethal CRISPR/Cas9 screen was used to identify target molecules using 786-o RCC cells. A library was generated, and treatment with vehicle or cabozantinib was carried out, followed by next-generation sequencing to identify candidate genes. A combination index based on the Chou-Talalay method was used to evaluate the synergistic effect of cabozantinib through cell viability assays. Xenograft assays were conducted to determine the effect in vivo. Results: CRISPR/Cas9-based screening revealed four genes (MEK1, DCLK1, DYRK3, and FGFR1) that were candidates for synthetic lethality by cabozantinib in RCC cells. We focused on MEK1 because the MEK1 inhibitor cobimetinib has been approved for melanoma treatment. In a cell proliferation assay using 786-o and A498 RCC cells, the combination of cobimetinib and cabozantinib exhibited a synergistic effect. A xenograft assay also revealed a significant synergistic effect of cobimetinib and cabozantinib. Conclusions: CRISPR/Cas9 screening identified MEK1 as a candidate for a synthetic lethal target with cabozantinib in RCC. The combined inhibition of MET/VEGFR and MEK1 suppressed compensatory MAPK reactivation and downregulated the PI3K-Akt pathway, including the survival-associated genes PPP2R3B and ATF6B, and produced significant tumor growth suppression in vivo. These findings highlight the potential of cabozantinib plus cobimetinib, an already-FDA-approved MEK inhibitor, as a readily translatable combination strategy to overcome cabozantinib resistance in RCC.