PURPOSE: Adavosertib is an oral small-molecule inhibitor of Wee1. The Adult Brain Tumor Consortium conducted a phase I study evaluating adavosertib in combination with radiation (RT) and temozolomide (TMZ) in patients with newly diagnosed glioblastoma (GBM), as well as a surgical window-of-opportunity study in recurrent GBM. PATIENTS AND METHODS: The MTD of adavosertib was determined in adult patients with newly diagnosed GBM using a standard 3+3 design in two separate cohorts: with concurrent RT/TMZ or with adjuvant TMZ. A combination cohort with both concurrent and adjuvant adavosertib at MTD followed. We also performed intratumoral drug distribution studies in patients with recurrent GBM undergoing surgery. RESULTS: As separate cohorts, the MTD for concurrent adavosertib with RT/TMZ was 200 mg daily Monday through Friday × 6 weeks during RT, and the MTD for adjuvant adavosertib with TMZ was 425 mg daily for 5 days of each 28-day cycle. However, six of 12 patients experienced dose-limiting toxicities (DLT) in the combination cohort. The mean ratios of the intratumoral to plasma concentration of adavosertib were 4.18 ± 3.36 for contrast-enhancing tissue and 0.74 ± 0.63 in nonenhancing tissue. CONCLUSIONS: Adavosertib at 200 mg daily Monday through Friday × 6 weeks with RT/TMZ and at 425 mg daily on a 5-day/28-day cycle with TMZ had an unacceptable DLT rate. Additional dose levels in combination cohorts resulted in DLT, and we deemed concurrent adavosertib too toxic for further examination. Adavosertib 425 mg daily on a 5-day/28-day cycle with adjuvant TMZ is the recommended phase II dosage. Tissue pharmacokinetics in tissue homogenates and by microdialysis provided complementary information about drug penetration.