Background: GBM remains almost universally fatal and new therapies are needed. The epidermal growth factor receptor (EGFR) is a key oncogenic target. ABT-414, an antibody-drug conjugate with a toxic payload (monomethylauristatin F) targeted to activated EGFR, showed efficacy in preclinical models. We report the data of patients (pts) with recurrent GBM (Arms B + C) from an ongoing phase 1, open-label, 3-arm study (NCT01800695). Methods: Eligible pts ( ≥ 18 years) had recurrent supratentorial GBM and KPS ≥ 70. ABT-414 was given i.v. every 2 weeks combined with TMZ re-challenge (150–200 mg/m² days 1–5 q28; Arm B) or as monotherapy (Arm C). ABT-414 doses were escalated using a modified continual reassessment method. Primary objectives were safety, maximum tolerated dose (MTD), and recommended phase 2 dose (RP2D) of ABT-414; secondary objectives included antitumor activity. Results: As of Jan 2015, accrual included 18 pts in Arm B treated in 4 dose groups (0.5, 1.0, 1.25, 1.5 mg/kg) and 28 pts in Arm C treated at 1.25 mg/kg. Median age was 51/57 years (Arms B/C), 50%/50% were male, 56%/75% had KPS ≥ 80, and 11/7 pts had amplified EGFR. Treatment-emergent adverse events (AEs) occurring in ≥ 25% of pts either in Arm B or C were blurred vision (n = 11/15), fatigue (n = 6/10), foreign body sensation in the eyes (n = 8/5), photophobia (n = 5/7), nausea (n = 9/2), constipation (n = 5/1), and GGT increase (n = 5/1). Grade 3/4 AEs in ≥ 10% of Arm B or C pts were keratitis (n = 1/3), GGT increase (n = 3/0), fatigue (n = 2/0), and thrombocytopenia (n = 2/0). Dose-limiting toxicities in Arm B were corneal deposits and GGT increase (n = 1 each); none occurred in Arm C during dose escalation. ABT-414 MTD was 1.5 mg/kg in Arm B and 1.25 mg/kg in Arm C; RP2D is 1.25 mg/kg for both arms. Best objective responses were complete response (CR) in 1 pt and partial responses (PR) in 4 pts in Arm B, and 1 CR and 1 PR in Arm C. EGFR amplification was found in all pts with confirmed responses. Confirmed responses were durable, ranging from 5–16 months. Conclusions: The ABT-414 RP2D is 1.25 mg/kg for monotherapy or combined with TMZ. The unique ABT-414 safety profile included ocular AEs. ABT-414 appears to have antitumor activity in recurrent GBM pts, particularly those with amplified EGFR. Clinical trial information: NCT01800695