Abstract 4007: Efficacy of vepafestinib in preclinical models of RET fusion-driven sarcoma models

Background: Vepafestinib (TAS0953/HM06, Vepa) is a 2nd generation RET-selective inhibitor that effectively penetrates the brain, and inhibits the wildtype RET kinase domain (KD) and RET KD mutants (G810, V804, Y806, L730) (presented at AACR-NCI-EROTC 2021 meeting). RET rearrangements are found in an increasing number of soft tissue sarcomas, including infantile fibrosarcoma (IFS). Here we investigated the efficacy of Vepa in comparison to other RET-selective inhibitors in preclinical models of pediatric sarcomas harboring RET fusions. Methods: Multiple preclinical models of SPECC1L::RET-driven sarcomas were established: 1) Paired patient-derived xenograft (PDX) and cell line models from a brain metastasis (BM) of an IFS tumor (SR-Sarc-0001); 2) A human mesenchymal stem cell line with RET fusion introduced with CRISPR-Cas9 (HMSC-RET); 3) A murine BM model produced by injection of luciferase-expressing HMSC-RET into the cerebellum. CNS penetration of Vepa was assessed by pharmacokinetic profiling in the prefrontal cortex (PFC), cerebrospinal fluid (CSF), and plasma in freely-moving male Han Wistar rats after oral administration of 3, 10, or 50 mg/kg single doses. Results: Exposure of SR-Sarc-0001 and HMSC-RET cells to Vepa resulted in dose- and time-dependent decreases in phosphorylation of RET, ERK1/2, AKT, STAT3 and S6, expression changes in cell cycle regulators (p27 up, cyclin D1 down), induction of pro-apoptosis proteins (c-PARP, BIM), and loss of MYC expression. Growth of SR-Sarc-0001 (IC50: 0.09 µM, 95% CI: 0.03-0.2) and HMSC-RET cells (IC50: 0.2 µM, 95% CI: 0.09-0.5), but not parental HMSC cells (IC50 > 1 µM), was suppressed by Vepa, with concomitant elevation of caspase 3/7 activity. Vepa was more effective than vandetanib and similar to the FDA-approved RET inhibitors, selpercatinib (Selp) and pralsetinib (Pral), in all in vitro assays. Significant regression of SR-Sarc-0001 PDX tumors was seen after Vepa treatment (64.8 ± 0.5%). Notably, no regrowth was observed up to 46 days after cessation of Vepa treatment, whereas 25 days after stopping Selp (10 mg/kg BID) and Pral (15 mg/kg BID) treatment, 1/5 and 3/5 tumors started to regrow, respectively. Similar efficacy was observed in the HMSC-RET xenograft model. Vepa was more effective than Selp at blocking HMSC-RET brain xenograft tumor growth (p=0.001) and increasing survival (p=0.0001). CNS penetration of Vepa was excellent, with near-equivalent concentrations detected in the PFC, CSF, and plasma-fr