Background: BAL101553 (prodrug of BAL27862) is a novel TCC that promotes tumor cell death by modulating the spindle assembly checkpoint. BAL27862 is a lipophilic, small molecule (MW 387) shown in rats to penetrate the brain (1:1 plasma ratio) and has shown promising antitumor activity in orthotopic preclinical models of GBM as monotherapy or in combination with radiotherapy (RT) with/without chemotherapy. In a completed Phase 1 study with 2-h IV infusions (Days 1, 8, 15, q28d, NCT01397929, CDI-CS-001, Lopez et al. J Clin Oncol 34, 2016 suppl; 2525), dose-limiting vascular effects were observed and appeared C_max related. Preclinical data suggest that antiproliferative effects of BAL101553/27862 are driven by exposure (AUC); thus vascular toxicity and antitumor activity are mediated by different PK drivers. In this ongoing study (NCT02490800, CDI-CS-002), daily oral BAL101553 was initially examined in solid-tumor patients; no vascular toxicities were observed at doses up to the MAD of 30 mg QD. Given this absence, the study was amended to enroll separate cohorts of patients with progressive or recurrent GBM or high-grade glioma. Methods: This is an ongoing multicenter, open-label, Phase 1 dose-escalation study using a 3+3 design to determine the MTD, characterize dose-limiting toxicities and assess the PK, PD and antitumor activities of daily oral administration of BAL101553 in consecutive 28-day cycles at a starting dose of 8 mg QD. Patients with histologically-confirmed GBM or high-grade glioma, with progressive or recurrent disease after prior RT with/without chemotherapy, are eligible for enrollment. This includes patients with histologically-confirmed low-grade glioma with unequivocal evidence by imaging of transformation to high-grade glioma. Adverse events are assessed using CTCAEv4; tumor response by RANO (every 2 cycles). The dose escalation allows for doubling of dose levels depending on observed toxicities. PD assessments include circulating tumor cells. PK profiles are assessed throughout the first two treatment cycles. Clinical trial information: NCT02490800