Background: Selpercatinib is a highly selective and potent CNS active RET inhibitor approved for treatment of advanced RET fusion-positive (RET+) NSCLC. Treatment or prevention of CNS disease is critical in RET+ NSCLC patients (pts), who have nearly a 50% lifetime prevalence of brain metastases (mets) (Drilon et al. JTO 2018). LIBRETTO-431 is the first study to compare intracranial (IC) efficacy of a targeted therapy to chemo/IO in pts with oncogene driven NSCLC. Methods: LIBRETTO-431 (NCT04194944) is a randomized, open-label, phase 3 trial comparing 1L selpercatinib vs chemotherapy (cisplatin/carboplatin + pemetrexed) +/- pembrolizumab. As previously reported, the study met its primary endpoint of PFS by blinded independent central review (BICR) at the pre-planned interim analysis. IC analyses included CNS and non-CNS PD, IC PFS and IC responses by BICR per RECIST 1.1 in all pts who had a baseline and one or more post-baseline CNS scans (CNS-evaluable) and were designated to receive pembrolizumab if randomized to the control arm (CNS-pembro population). Adverse events were evaluated in the CNS safety population. Results: A total of 192 of 261 pts enrolled were CNS-evaluable (selpercatinib: 120, control: 72). Baseline characteristics were generally balanced with the selpercatinib arm having a slightly lower proportion of pts with BICR-assessed baseline brain mets (21% vs 25%) and prior CNS radiotherapy (RT; 6% vs 10%) compared to the control arm. Selpercatinib delayed CNS PD as evidenced by a lower 12 mo cumulative incidence rate (CIR) for CNS PD, as well as delaying non-CNS PD compared to control in pts with and without brain mets (Table). In pts with measurable brain mets at baseline (n=29), median time to IC response per RECIST 1.1 was similar between selpercatinib and control (1.4 mo [range: 1.2-2.9] vs 1.6 mo [range: 1.2-2.9]); however, as previously reported the IC response rates were higher (82% vs 58%) and more durable (12 mo DOR rate 76% vs 63%) with selpercatinib vs control (Zhou et al. NEJM 2023). IC responses to selpercatinib were more common in pts without prior CNS RT (14/15, 93%) than with prior CNS RT (3/6, 50%). Conclusions: Selpercatinib delayed IC progression in advanced RET+ NSCLC pts with or without baseline brain mets and achieved higher IC response rates compared to chemotherapy + pembrolizumab. LIBRETTO-431 is the first study to demonstrate IC efficacy improvement of a targeted therapy vs chemo/IO in a biomarker selected NSCLC population. These data further support selpercatinib as the preferred 1L regimen in pts with advanced RET+ NSCLC. Clinical trial information: NCT04194944.