Inclusion Criteria:

          1. Be ≥18 years of age.

          2. Have histologically or cytologically confirmed recurrent Grade 2 or 3 LGG
             (oligodendroglioma or astrocytoma according to World Health Organization 2016
             classification).

          3. Have documented IDH1 R132H gene mutation by local testing and known 1p19q or ATRX
             mutation status by local testing.

          4. Have central confirmation of primarily non-enhancing disease by MRI with less than or
             equal to 5 mm slice thickness and up to 1 mm interslice gap on either 2D T2 weighted
             image, 3D T2 weighted image, or FLAIR, with at least 1 non-enhancing tumor measuring
             1×1×1 cm.

          5. Be candidates for clinical resection but for whom surgery is not urgently indicated
             (eg, for whom surgery within the next 2-4 months is appropriate).

          6. Have KPS of ≥60%

          7. Have expected survival of ≥12 months.

        Exclusion Criteria:

          1. Have received prior systemic anti-cancer therapy within 1 month of the first dose of
             AG-120 or AG-881 or have received an investigational agent <14 days prior to their
             first dose of AG-120 or AG-881. In addition, the first dose of AG-120 or AG-881 should
             not occur before a period of ≥5 half-lives of the investigational agent has elapsed.

          2. Have had radiation within 6 months of the first dose of AG-120 or AG-881. (Note: Prior
             biopsy or surgery is allowed.)

          3. Have received any prior treatment with an IDH inhibitor.

          4. Have received any prior treatment with bevacizumab (Avastin).