Background: GBM is the most common malignant primary brain tumor in adults. Pts with rGBM have a poor prognosis. EGFR is amplified (amp) in ~50% of GBMs and is a compelling therapeutic target. ABT-414 is an antibody-drug conjugate composed of an EGFR-directed antibody conjugated to a microtubule toxin, MMAF. ABT-414 binds a unique epitope exposed during EGFR activation, either through ligand stimulation or mutation such as EGFR variant III (EGFRvIII), releasing MMAF into the cancer cell. Here, we report a pooled safety and efficacy analysis of ABT-414 +/- TMZ in EGFR amp, rGBM. Methods: M12-356 is a Phase 1, open-label, multi-arm study. Results from the 2 arms accruing rGBM pts were pooled for analysis. Eligible adults had rGBM, centrally confirmed EGFR amp, and KPS ≥ 70. Pts received 0.5-1.25 mg/kg ABT-414 on days 1 and 15 +/- 150-200 mg/m² TMZ on days 1-5 of 28-day cycles until progression (per RANO). Results: As of 11 January 2017, 126 pts were treated. The most common adverse events (AEs, ≥ 25% pts) were ocular (90%) and included blurred vision (64%) and photophobia (31%), which were mainly reversible. Common non-ocular AEs were fatigue (36%) and headache (30%). Grade 3/4 AEs (≥ 5% pts) included ocular toxicities (29%) and decreased platelets/thrombocytopenia (10%). Serious AEs included seizure and keratitis (2% each). Of 125 pts evaluable by RANO, 52% had improvement or stabilization as best response (2 CR, 9 PR, 54 SD), and the remaining 60 (48%) had PD. Of 115 pts with measurable disease at baseline, the objective response rate (ORR) was 10% (2 CR + 9 PR). For 5 pts, re-resection for radiographic PD revealed mostly necrotic tissue and pts were classified as SD, suggesting the ORR may be an underestimate. Of all 126 pts, the 6-month PFS rate (PFS6) was 26%; median OS was 8.5 months. Conclusions: In this Phase 1 trial of EGFR amp, rGBM, we observed encouraging disease control (52%, CR + PR + SD) and PFS6 (26%) rates. Toxicity was mainly ocular, and reversible. A global, randomized trial of ABT-414 vs. ABT-414 + TMZ vs. TMZ/lomustine in EGFR amp, rGBM has completed accrual with results expected later this year (NCT02343406). Clinical trial information: NCT01800695