CTNI-43. MULTI-CENTER PHASE I STUDY OF CONCURRENT PAXALISIB AND RADIATION THERAPY (RT) IN PATIENTS WITH SOLID TUMOR BRAIN METASTASES (BM) OR LEPTOMENINGEAL METASTASES (LM) HARBORING PI3K PATHWAY ALTERATIONS

Abstract INTRODUCTION Partial or whole brain RT is a standard treatment for solid tumor BM and LM. However, tumors harboring PI3K pathway mutations have poorer CNS control after RT alone. Combining RT with paxalisib, a CNS-penetrant PI3K/mTOR inhibitor, may overcome radioresistance. METHODS This multi-center, phase I trial (NCT04192981) studied concurrent paxalisib with cranial RT (30Gy in 10 fractions) for PI3K pathway-altered BM/LM. Part A was a 3 + 3 dose escalation cohort of paxalisib at 45, 60 or 75mg to establish maximum tolerated dose (MTD). Part B was an expansion at MTD to further validate safety and early efficacy. CNS response was assessed using Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) criteria. RESULTS From 3/2020-2/2024, 21 patients (median age 58y, n=18 with BM, n=10 with breast cancer) were enrolled. PIK3CA mutations were the most common pathway alteration (n=15, 71%) followed by PTEN deletion (n=3, 14%). Part A enrolled 12 patients of which 9 were evaluable (3 did not complete protocol therapy). There were no dose-limiting toxicities (DLTs) at 45mg, and 2 DLTs at 60mg: grade 3 nausea and grade 4 neutropenic enterocolitis. Subsequently, 9 additional patients were enrolled during Part B at MTD of 45mg, of which 1 withdrew consent prior to completion of study interventions. No additional DLTs were observed during Part B. In total, there were 14 evaluable patients who received 45mg/day paxalisib concurrently with RT, two of whom died of intercurrent extracranial progression prior to CNS restaging. Of the remaining 12, there was robust response signal. At 1-month post-treatment, 6/12 (50%) had partial response (PR) and the remaining had stable disease (SD). To date, best CNS response is 67% PR (8/12) and 33% SD (4/12). CONCLUSIONS A MTD of 45 mg/day has been confirmed for paxalisib with concurrent brain RT for PI3K pathway-altered BM/LM. Given significant unmet need, this novel therapeutic combination warrants further investigation given promising activity signal.