LAUSR

selected variants of the H1650 and H1975 cell lines for resistance to osimertinib to create models in which mechanisms of resistance can be characterized and tested for potential methods to overcome that resistance. Method: Cells were exposed continuously to a single concentration of osimertinib, in some cases with the inclusion of verapamil to avoid possible selection of multidrug-resistant cells. After 4 weeks, with weekly changes of drug-medium, clonal cell lines were selected from H1650 cultured in the presence of 25 or 50 mM osimertinib plus 10 mM verapamil, and selected from H1975 cultured in 6 or 10 mM osimertinib alone or 5 mM osimertinib plus 10 mM verapamil. Result: The H1650 osimertinib-resistant cell lines (H1650/osi-25a/VPL and H1650/osi-50a/VPL) are 1.6to 1.8-fold resistant to osimertinib. The H1975 osimertinib-resistant cell lines (H1975/osi-6b, osi-10c, and osi5b/VPL) were, respectively, 90-, 95-, and 38-fold resistant to osimertinib. None of the cell lines was cross-resistant to imatinib. The compound 2-(benzylsulfonyl)-1-(1H-indol-3-yl)-1,2-dihydroisoquinoline (IBR2) is an inhibitor of the DNA repair protein RAD51 and enhances cytotoxicity of EGFR inhibitors against numerous cell lines (Ferguson et al., JPET 2018, doi.org/10.1124/jpet.117.241661). IBR2 decreased osimertinib-resistance by up to 80% in the H1650and H1975-derived osimertinib-resistant cell lines. Further analyses of the resistant cell lines are being undertaken. Conclusion: The H1650 and H1975 osimertinib-resistant cell lines are a valuable resource in which to test methods to circumvent resistance to third generation EGFR TKIs.