Abstract
PURPOSE: Glioblastoma (GBM) and diffuse intrinsic pontine gliomas (DIPG) inevitably develop local recurrence, despite the delivery of maximal tolerated radiation doses. Survivin, an “undruggable” radiation-resistance protein, is highly expressed in these tumors. We report the preclinical development of HMAK101, a first-in-class, CNS-penetrant PROTAC survivin degrader that radiosensitizes GBM and DIPG. METHODS: Pharmacodynamic (PK) study was performed in mouse, rat and cynomolgus monkey. DNA damage was evaluated with Comet assay, quantification of gamma H2AX foci, TUNEL assay and RNA-seq analysis. Toxicity study was performed in preclinical phase 1 dose escalation study. The in vivo efficacy of HMAK101 in combination with radiation (RT+HMAK101) was evaluated in various orthotopic models. RESULTS: HMAK101 downregulates survivin in GBM and DIPG PDCs in a time and dose-dependent manner with a half-maximal degradation concentrations (DC50s) within nanomolar range. HMAK101 induced DNA damage response in GBM and DIPG cells with significant gamma H2AX foci accumulation, DNA fragmentation and G2/M cell cycle arrest. HMAK101 enhanced RT-induced DNA damage, caspase-3/-7 activation and ROS accumulation. In rats, 2 mg/kg I.V. resulted in brain parenchyma concentrations of 15.4 ng/g (2 h) and 5.85 ng/g (6 h), with brain/plasma ratios of 6.1% and 18.9%. In cynomolgus monkey, HMAK101 was detectable in CSF at 2 h post-injection. In orthotopic GBM models, HMAK101 remained detectable in tumor tissue 16 h post-dose, suggesting daily dose regimen. A 14-day toxicity study (0–8 mg/kg daily, I.V.) showed minimal toxicity. Orthotopic syngeneic SB28 model and PDX GBM models demonstrated that RT+HMAK101 significantly prolonged survival and reduced tumor burden as compared RT alone. Xenograft models for DIPG with SF8628 also demonstrated tumor burden reduction with the combination treatment. CONCLUSION: HMAK101 is a novel, brain-penetrant PROTAC that targets survivin with low toxicity. It accumulates in brain tumors and enhances radiation efficacy in preclinical GBM and DIPG models.