A Study of DS-1001b in Patients With Chemotherapy- and Radiotherapy-Naive IDH1 Mutated WHO Grade II Glioma

Inclusion Criteria: * Has a histopathologically documented IDH1 mutated WHO grade II glioma according to the 2016 WHO classification. * Has confirmed IDH1 mutation at the R132 locus by testing at the central laboratory conducted during the screening period. * Has no prior anticancer treatment (including chemotherapy and radiotherapy) for glioma except craniotomy or biopsy. * Has at least 1 measurable and non-enhancing lesion. * Has an interval of at least 90 days from the latest surgery. * Has no sign of malignant transformation including the appearance of enhancing lesions and/or rapid growth of non-enhancing lesions. * Has an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 to 1. Exclusion Criteria: * Has had a histopathological diagnosis of WHO grade III or IV glioma. * Has had a contrast enhancing lesion on brain MRI. * Has received a prior treatment with any mutant IDH1 inhibitor. * Has received other investigational products within 28 days before the start of the study drug treatment. * Has an active infection requiring systemic treatment. * Has multiple primary malignancies. * Has a history of clinically significant cardiac disease. * Is a pregnant or lactating woman.