Background: Preclinical GBM data targeting the checkpoint molecule Lag-3 have shown promising anti-tumor immune response with resultant improved survival when combined with anti-PD-1. Here we report our experience from a multi-arm safety study in patients with recurrent GBM treated with anti-Lag-3 and in combination with anti-PD-1. Methods: A phase I, open label, multicenter, multi-arm dose-finding/safety study of anti-LAG-3 (BMS-986016) alone or in combination with anti-PD-1 in patients at first recurrence of GBM was carried out in The Adult Brain Tumor Consortium (ABTC) (1501). The primary objectives were safety and to define MTD (DLT rate < 33%) for both the mono and combination arms. The major secondary objective was efficacy. The key inclusion criteria were: adults with first recurrence of GBM following RT+TMZ, TLC≥1000/ul, KPS≥ 60%, on a stable corticosteroid regimen, measurable disease, and written informed consent. Three pre specified dose levels of anti-Lag-3 at 80mg, 160mg, and 800mg were tested. Anti-PD-1was given at a flat dose of 240 mg in combination with anti-LAG-3 at 80 mg and 160 mg. Results: To date, the phase I portion of study completed its accrual and 33 patients were enrolled into the anti-LAG-3 alone or in combination with anti-PD-1 arms. The median age and KPS was 56 and 90 respectively. 39% tumors were MGMT methylated and the median treatment cycle was 3. The highest safe dose for Anti-LAG-3 alone is 800 mg without a DLT. Two DLT were observed in combination arms of Anti-LAG-3 +anti-PD-1 (80 mg/240mg), a grade 3 muscle weakness and a grade 4 edema. Three DLTs were observed in the higher Anti-LAG-3 + anti-PD-1 group (160 mg/240mg): grade 3 hypertension, syncope, and edema. 80% of the DLTs occurred after cycle 2 of the treatment. The estimated overall mOS was 8 months. Seven (44%) patients in the combination arm are still alive and 3 out of the 7 are living beyond 20 months suggesting a subset benefit. Conclusions: The phase I part of trial has completed enrollment. The MTD is 800mg for anti-LAG-3 as a monotherapy. For the combination arms, 160 mg of Anti-LAG-3 and 240 mg of anti-PD-1 was the MTD. DLTs were late onset events. Clinical trial information: NCT02658981.