- Recent prior therapy, defined as 1. Any non-monoclonal anti-cancer therapy within 14 days or 5 half-lives, whichever is longer, prior to the first dose of GSK3326595. Any nitrosoureas or mitomycin C within 42 days prior to the first dose of GSK3326595. Prior therapy with biologic agents (including monoclonal antibodies) is permitted so long as 28 days have elapsed since therapy and all therapy-related AEs have resolved to =< Grade 1, 2. Any radiotherapy within 14 days or major surgery within 28 days prior to the first dose of GSK3326595. For subjects in the GBM cohort, subjects must have completed radiation therapy at least 28 days prior to the first dose of GSK3326595. 3. Anti-androgen therapies for prostate cancer, such as bicalutamide, must be stopped 4 weeks prior to enrollment. Second-line hormone therapies such as enzalutamide or abiraterone should be stopped 2 weeks prior to enrolment. Subjects with prostate cancer should remain on luteinizing hormone releasing hormone (LHRH) agonists or antagonists. Subjects with prostate cancer may also remain on low-dose prednisone or prednisolone [up to 10 milligram (mg)/day] and still be eligible for this study.