INTRODUCTION: Novel rearranged in transfection (RET)-specific tyrosine kinase 
inhibitors (TKIs) such as selpercatinib (LOXO-292) have shown unprecedented 
efficacy in tumors positive for RET fusions or mutations, notably RET 
fusion-positive NSCLC and RET-mutated medullary thyroid cancer (MTC). However, 
the mechanisms of resistance to these agents have not yet been described.
METHODS: Analysis was performed of circulating tumor DNA and tissue in patients 
with RET fusion-positive NSCLC and RET-mutation positive MTC who developed 
disease progression after an initial response to selpercatinib. Acquired 
resistance was modeled preclinically using a CCDC6-RET fusion-positive NSCLC 
patient-derived xenograft. The inhibitory activity of anti-RET multikinase 
inhibitors and selective RET TKIs was evaluated in enzyme and cell-based assays.
RESULTS: After a dramatic initial response to selpercatinib in a patient with 
KIF5B-RET NSCLC, analysis of circulating tumor DNA revealed emergence of RET 
G810R, G810S, and G810C mutations in the RET solvent front before the emergence 
of clinical resistance. Postmortem biopsy studies reported intratumor and 
intertumor heterogeneity with distinct disease subclones containing G810S, 
G810R, and G810C mutations in multiple disease sites indicative of convergent 
evolution on the G810 residue resulting in a common mechanism of resistance. 
Acquired mutations in RET G810 were identified in tumor tissue from a second 
patient with CCDC6-RET fusion-positive NSCLC and in plasma from patients with 
additional RET fusion-positive NSCLC and RET-mutant MTC progressing on an 
ongoing phase 1 and 2 trial of selpercatinib. Preclinical studies reported the 
presence of RET G810R mutations in a CCDC6-RET patient-derived xenograft (from a 
patient with NSCLC) model of acquired resistance to selpercatinib. Structural 
modeling predicted that these mutations sterically hinder the binding of 
selpercatinib, and in vitro assays confirmed loss of activity for both anti-RET 
multikinase inhibitors and selective RET TKIs.
CONCLUSIONS: RET G810 solvent front mutations represent the first described 
recurrent mechanism of resistance to selective RET inhibition with 
selpercatinib. Development of potent inhibitor of these mutations and 
maintaining activity against RET gatekeeper mutations could be an effective 
strategy to target resistance to selective RET inhibitors.