Combination of Selpercatinib and Trametinib Overcomes Resistance to RET Inhibitors in RET-Mutant Medullary Thyroid Carcinoma

PURPOSE: Medullary thyroid carcinoma (MTC) is often driven by activating mutations in the RET receptor tyrosine kinase. Multikinase and selective inhibitors targeting RET are highly effective for RET-mutant MTC, but acquired resistance is commonly observed, limiting clinical efficacy. MATERIALS AND METHODS: We performed a comprehensive genomic and pharmacological analysis of acquired resistance in a previously described in vitro model of RET-mutant MTC generated from long-term treatment with the RET inhibitor vandetanib. Molecular studies using spectral karyotyping, multiplex interphase fluorescence in situ hybridization, whole-exome sequencing, and RNA sequencing revealed several mechanisms of acquired resistance. Whole-genome CRISPR knockout screening was performed to identify potential genes mediating intrinsic resistance. High-throughput drug screening was used to identify additional therapeutic targets. The combination of RET and MEK inhibitors was evaluated in preclinical animal models. RESULTS: Genomic profiling revealed that resistant MTC cells acquired RET copy-number gain and the clinically observed secondary RET mutation p.G810S. Whole-genome CRISPR knockout screening on MTC cells treated with two different RET inhibitors highlighted that NF1 deletion and subsequent RAS/MAPK activation were sufficient to establish resistance to RET inhibition. High-throughput drug screening also indicated that MTC cells are sensitive to RAS/MAPK inhibition, particularly in combination with RET inhibitors. The combination of RET and MEK inhibitors was synergistic in both RET-inhibitor-naïve and resistant MTC in mouse xenograft models. CONCLUSION: Resistance to RET inhibitors can be acquired through RET copy-number gain and secondary mutations as well as NF1 loss-mediated MAPK pathway activation. This mechanism of resistance can be overcome with dual inhibition of RET and downstream RAS/MAPK signaling, demonstrating clinical potential in RET-mutant MTC.