Background: Recurrent GBM (rGBM) has dismal prognosis. Almost 50% GBM tumors harbor amplified (amp) EGFR. ABT-414 is a tumor specific ADC combining an antibody targeting a unique conformation of EGFR (ABT-806) to a microtubule cytotoxin, monomethyl auristatin F (MMAF). Here we report the safety and efficacy of ABT-414 monotherapy at recommended phase 2 dose (RPTD) in EGFR amp, rGBM. Methods: M12-356 (NCT01800695) is an open-label, phase 1, 3-arm study: Arm A (ABT-414+radiation/temozolomide (TMZ) in newly diagnosed GBM (nGBM)), Arm B (ABT-414+TMZ in nGBM as adjuvant therapy, or in rGBM) and Arm C (ABT-414 monotherapy in rGBM). Each arm had an escalation cohort to determine the RPTD and an expansion cohort to establish the safety and preliminary efficacy at RPTD. Results of Arm C expansion cohort at 1.25 mg/kg RPTD (IV infusion) are shown here. Eligible patients (pts) were adults with KPS score ≥ 70, EGFR amp (confirmed centrally), rGBM, normal end-organ function and no prior bevacizumab. Results: As of January 7, 2016, 48 EGFR amp, rGBM pts were treated in this cohort. The median age was 59 years (range, 35-80). Most pts had prior therapies: 40% had 1, 48% had 2, 10% had ≥ 3 prior therapies. Most common treatment emergent adverse events (TEAEs) ( ≥ 25% pts) were blurred vision (60%), headache, photophobia (29% each), dry eye, eye pain, fatigue (27% each). The most common serious AE ( > 1 pt) was seizure (8%). Grade 3/4 TEAEs ( > 1 pt) were keratitis (15%), corneal epithelial microcysts (8%), hemiparesis, hyperglycemia, muscular weakness, seizure (6% each), blurred vision, ulcerative keratitis (4% each). No dose-limiting toxicities were reported. Best RANO responses of 44 pts with complete data were: 2 partial responses, 18 stable disease, 24 progressive disease. The 6-month progression-free survival (PFS6) estimate was 30% [95% CI = 17, 44]. Conclusions: ABT-414 monotherapy, at 1.25 mg/kg RPTD, displayed frequent yet reversible ocular toxicities. An encouraging tumor stability/response and PFS6 were observed in this highly refractory EGFR amp, rGBM. A global randomized trial of ABT-414, alone or with TMZ, vs. TMZ or lomustine, is underway in EGFR amp, rGBM (NCT02343406). Clinical trial information: NCT01800695