In in vitro cell proliferation assays, VT3989 showed strong to very strong synergy in combination with osimertinib in several EGFR mutant NSCLC cell lines tested. In vivo, we observed strong VT3989 and osimertinib combination effect in NCI-H1975 and HCC827 cell line-derived xenograft models. We also tested the combination in several EGFR mutant NSCLC patient-derived xenograft models and demonstrated that the addition of VT3989 enhanced the efficacy of osimertinib and delayed tumor regrowth compared to osimertinib alone.