Modeling acquired TKI resistance and effective combination therapeutic strategies in murine RET+ lung adenocarcinoma

Oncogenic RET gene rearrangements drive a subset of lung adenocarcinomas (LUAD) and the tyrosine kinase inhibitors (TKIs) selpercatinib and pralsetinib are approved therapeutics. However, acquired resistance remains a hurdle to durable management. Two RET+ lung cancer cell lines (TR.1, TR.2) were established from a Trim24-Ret mouse model and lung tumors resulting from their orthotopic transplantation initially responded to selpercatinib followed by prompt progression within ∼3 weeks of initiating TKI treatment. Cell lines derived from the selpercatinib-resistant TR.1 and TR.2 tumors exhibited in vitro sensitivity to MET and ERBB-targeted TKIs, indicating acquired bypass signaling through these receptor tyrosine kinases (RTKs). The TKI-resistant cell lines showed no evidence for MET gene amplification, but exhibited transcriptional induction of genes that function within MET and ERBB2:ERBB4 interaction networks including ligands (HGF, NRG1), adaptors (GAB1) and co-receptors (NRP1). Exogenous HGF, but not NRG1 reversed in vitro growth inhibition by selpercatinib in TR.1 and TR.2 cells. Mice bearing orthotopic TR.1 or TR.2 lung tumors progressing on selpercatinib underwent significant re-shrinkage upon co-treatment with the MET inhibitor, crizotinib, although similar to the clinical experience, progression again occurred. By contrast, upfront treatment with selpercatinib and crizotinib in orthotopic tumors yielded complete elimination of 7 of 9 TR.1 tumors and both deepened and prolonged the duration of response in TR.2 tumors. The findings provide new insight into mechanisms of acquired resistance through bypass signaling and highlight the therapeutic benefit of simultaneous upfront blockade of driver oncogenes and dominant resistance mechanisms in LUAD.