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2024 Background: Patients with RET fusion-positive NSCLC have an estimated 25% incidence of CNS metastasis at diagnosis, and up to 40% during disease progression. Effective anti-RET therapy that penetrates the blood-brain barrier is essential to extending survival. TAS0953/HM06 is a structurally distinct RET-specific inhibitor that exhibits a distinct binding mode to RET and is effective against RET solvent front (G810) and gatekeeper (V804) mutations. TAS0953/HM06 also inhibits growth of xenograft tumors established from RET fusion-driven tumors of multiple histologies. TAS0953/HM06, therefore, represents a potentially effective strategy to overcome the emergence of acquired resistance to first generation RET-selective inhibitors. Here, we compared the brain penetration and efficacy of TAS0953/HM06 to selpercatinib (FDA-approved RET inhibitor) in models of intracranial RET fusion-positive cancers, specifically NSCLC and sarcoma. Methods: We compared the brain: plasma ratio of unbound TAS0953/HM06 and selpercatinib in mice to determine the unbound partition coefficient, Kpuu, brain. We injected ECLC5 (NSCLC cell line, TRIM33-RET) and HMSC-RET (immortalized human mesenchymal stem cells in which SPECCL1-RET was introduced by CRISPR-Cas9 genomic engineering, sarcoma model) cells expressing luciferase into the cerebellum of mice. Tumor-bearing mice were treated with TAS0953/HM06 (50 mg/kg BID), selpercatinib (10 mg/kg BID) or vandetanib (multi-kinase RET inhibitor, 50 mg/kg QD), and assessed weekly for tumor growth via bioluminescence imaging. Results: Kpuu, brain, of TAS0953/HM06 and selpercatinib were 1.3 and 0.20, respectively. Substances with brain Kpuu > 0.3 in mice are regarded as brain-penetrable. TAS0953/HM06 was superior to selpercatinib at inhibiting growth of ECLC5 (p < 0.0001) and HMSC-RET (p = 0.0005) brain xenograft tumors, and increasing survival of tumor-bearing animals (ECLC5: TAS0953/HM06 139±0.5 days, selpercatinib 95+2.3 days, p = 0.002; HMSC-RET: TAS0953/HM06 41± 2.2 days, selpercatinib 20±3 days, p = 0.0001). Vandetanib, which is highly brain-penetrant, did not cause a significant decrease in growth of either brain tumor xenograft models. At the doses used, the 3 RET inhibitors induced similar regression in several peripheral subcutaneous xenograft tumor models. Conclusions: Our data in animal models suggest that TAS0953/HM06 penetrates the CNS more effectively than selpercatinib, and is superior at decreasing CNS disease and extending survival. TAS0953/HM06 represents a promising new therapeutic option for patients with RET fusions with acquired resistance mutations, including those with brain metastasis and those resistant to first-generation selective RET inhibitors. TAS0953/HM06 is currently undergoing a biomarker-driven phase 1/ 2 clinical trial for patients with solid tumors driven by RET alterations (NCT04683250).

12122Background: While multiple genetic mechanisms have been identified in EGFR mutant lung cancers as mediators of acquired resistance (AR) to EGFR tyrosine kinase inhibitors (TKI), either first g...