DDDR-26. Preclinical evaluation of first-in-class PROTAC survivin degrader HMAK101 for radiosensitization of CNS malignancies

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.