Background: Tumor mutational burden (TMB) may predict immunotherapy response in solid tumors. Response to immune checkpoint inhibitor (ICI) treatment in the rare gliomas with biallelic mismatch repair deficiencies have been attributed to their hypermutation phenotype (HMP). This has not been tested in other gliomas. We developed a phase II clinical trial using nivolumab in recurrent IDH-mutant gliomas evaluating response in tumors with HMP (approximately 10% of cases) and in tumors with non-HMP (NHMP). Here we report the results of the interim analysis of the NHMP cohort. Methods: Adults with recurrent IDH-mutant glioma, KPS ≥ 60, normal organ function, with known somatic TMB (analyzed at NIH) were enrolled to a phase II trial. Nivolumab is given at 480mg IV every 28-day cycle with a maximum of 16 cycles. The primary endpoint is PFS rate at 6 months (PFS6) in both HMP and NHMP cohorts. Responses to treatment are evaluated by MRI every 2 cycles using iRANO criteria. A Simon’s two-stage design was used for conducting the HMP/NHMP cohort independently. For NHMP cohort, the null and alternative hypotheses for PFS6 are 0.3 and 0.5, respectively. Fifteen patients were planned to accrue in stage I. If ≥10 have disease progression at 6 months, the cohort will be terminated, otherwise continue to stage II and accrue additional 31 patients, resulting a total cohort size of 46. The design controls the type I error at 0.05 and yields a power of 0.8. Tumor samples and peripheral blood were collected for correlative studies. Patient-reported outcomes (PRO) were evaluated by longitudinal symptom burden analysis using Brain Tumor Module of the MD Anderson Symptom Inventory. Results: As of May 2022, 17 patients were enrolled to NHMP cohort and 15 were treated (TMB˂5 mut/Mb). Among 15 evaluable patients, 10 were male, median age 39.5 and KPS 90. Histological diagnosis included 12 astrocytoma (grade 3, n = 4; grade 4, n = 8) and oligodendroglioma (grade 2, n = 1; grade 3, n = 2). Median number of prior recurrences is 3. At the time of interim analysis, four patients were receiving ongoing treatment, but all have received more than 6 cycles, with 100% PRO completion. Eleven patients were off study treatment, 2 completed all 16 cycles, 9 had disease progression. Six out of 15 (40%) patients were progression free at 6 months. Per design, the NHMP cohort proceeded to stage II. One treatment related grade 3 immune-related colitis was reported. Conclusions: Nivolumab is well tolerated and has demonstrated efficacy in a cohort of patients with recurrent IDH-mutant gliomas with low TMB. Accrual to this NHMP cohort continues along with continued enrollment to the HMP cohort. Clinical benefit measured by objective response, PFS, OS and longitudinal PRO measure along with longitudinal immune monitoring will help address whether TMB correlates with tumor, immunologic and clinical response, and benefit of ICI therapy in IDH-mutant gliomas. Clinical trial information: NCT03718767.