Abstract only 9057 Background: Osi (3 rd gen EGFR TKI) has robust activity in 1 st line EGFR mutant NSCLC and TKI resistant T790M pos NSCLC but progressive disease (PD) occurs and outcomes with Osi alone are poor in T790M neg , C797X pos and EGFR exon 20 insertion (ins20) disease. This study examined the EGFR monoclonal antibody Neci with Osi in select settings of EGFR TKI resistance. Methods: Using a 3+3 design, Neci was examined in advanced EGFR mutant NSCLC at dose levels (DL) of 600 mg (DL1) & 800 mg (DL2) D1, D8 IV q21 days + Osi 80 mg qd. Four expansion cohorts (ExC; 18 each) included: A) T790M neg PD on 1 st /2 nd gen TKI as last therapy, B) T790M neg PD on 3 rd gen TKI, C) T790M pos PD on 3 rd gen TKI, D) EGFR ex20ins PD on chemotherapy. Central T790M testing by ddPCR in ExC A-C. Additional studies performed include: NGS panel of > 400 genes, EGFR FISH, plasma for PK and serial EGFR ctDNA by ddPCR as exploratory analyses. Adverse events (AEs) graded (Gr) by CTCAEv5 with ORR and PFS by RECIST 1.1. Results: Dose escalation and ExC B completed accrual. In total 55 pts were evaluable (Table). 1 pt had DLT at DL2, Gr 3 sinus bradycardia. Drug related Gr 3 AEs were seen in 27% (15) of pts, mainly rash (7;13%). ctDNA showed decreased mutant allele frequency with therapy in all pts studied with detected EGFR at baseline, with complete plasma clearance in a pt with detectable C797S. Conclusions: The RP2D (Osi 80 mg qd and Neci 800 mg D1, D8 IV on q21d cycle) is feasible and tolerable. In ExC A,T790M neg pts with 1 st /2 nd gen EGFR TKI as last treatment, using curtailed sampling, the pre-specified efficacy signal was reached for Osi + Neci comparing favorably to Osi alone in analogous pts from the AURA trial. Clinical activity was also seen in EGFR-dependent resistance (T790M pos C797S pos ) after PD on 3 rd gen TKI and in EGFR ins 20. Clinical trial information: NCT02496663. Table: see text