9032 Background: Selpercatinib, a first-in-class highly selective, potent, CNS-active RET kinase inhibitor, is approved in multiple countries for treatment of RET fusion+ lung or thyroid cancers. Selpercatinib demonstrated durable antitumor activity in previously treated pts with RET fusion+ NSCLC in an ongoing Phase 1/2 trial, LIBRETTO-001 (Besse et al., ASCO 2021). Methods: Pts with RET fusion+ NSCLC enrolled in the global, multicenter, LIBRETTO-001 trial (NCT03157128; 16 countries, 89 sites). Primary endpoint was objective response rate (ORR). Secondary endpoints included progression-free survival, duration of response, and safety. This post-hoc intrapatient analysis was based on a 30 March 2020 data cutoff date. Historical physician-reported best overall response (BOR) from last systemic therapy received prior to enrollment was compared with selpercatinib BOR by independent review committee per RECIST v1.1, with each patient serving as his/her own control. Results: In efficacy-evaluable pts (N = 218) who previously received platinum-based chemotherapy (chemo), median pt age was 61 years, the majority with ECOG of 0/1 (37%/61%), with a median of 2 (range: 1-15) prior systemic therapies. Overall, 57% of patients responded to selpercatinib while 16% responded to the immediate prior therapy. ORR improvements with selpercatinib were observed regardless of prior therapy: chemotherapy + immune checkpoint inhibitor (ICI) (57% vs 14%), single-agent ICI (48% vs 3%), or chemotherapy (58% vs 15%). A total of 108 patients (49%) did not respond to immediate prior therapy but responded to selpercatinib. Fewer patients had progressive disease as their BOR with selpercatinib (2%) compared to the immediate prior therapy (28%). The median duration of therapy for selpercatinib was notably extended compared with that of the immediate prior therapy (11.8 vs. 3.4 months, respectively). Conclusions: In pts with RET fusion+ NSCLC treated on LIBRETTO-001, systemic therapies administered prior to enrollment achieved less meaningful clinical benefit than selpercatinib. Selpercatinib demonstrated consistent efficacy regardless of the type of prior therapy. Clinical trial information: NCT03157128.