Background: Tebentafusp (tebe) is a bispecific consisting of an affinity-enhanced T cell receptor fused to an anti-CD3 effector that can redirect T cells to target gp100+ cells. Tebe significantly improved OS compared to investigator’s choice (IC) in first line (1L) mUM [NCT03070392]. In a phase (ph) 2 study of tebe in 2L+ mUM (NCT02570308), several checkpoint inhibitor (CPI) refractory pts who were retreated with CPI after tebe achieved durable clinical benefit [1]. We therefore evaluated clinical outcomes of post-tebe CPI in patients treated on the ph3 trial of tebe versus investigator’s choice (IC) [NCT03070392]. Methods: In the ph3 trial, 378 HLA-A*02:01+ 1L mUM pts were randomized 2:1 to tebe (n=252) or IC (n=126) [pembrolizumab (82%), ipilimumab (12%) or dacarbazine (6%)]. No crossover to tebe was permitted, investigators were free to choose subsequent therapy, and there was no re-randomization at time of subsequent therapy. This analysis was conducted on the first interim analysis (data extracted Nov-2020). When pts received more than one subsequent therapy, the first was used in these analyses. Medians and 1-yr OS from the start of post-study therapy are obtained from standard Kaplan-Meier analyses; hazard ratios (HR) are from Cox regression models adjusted for age and gender. Results: 106/252 (42%) tebe pts received ≥ 1 subsequent therapy: 35% CPI, 9% chemo, 6% liver directed therapy (LDT), 6% other. 55/126 (44%) of IC pts received ≥ 1 subsequent therapy: 21% CPI, 10% chemo, 12% LDT, 10% other. Median time to first subsequent therapy was longer for tebe pts at 5.2 mo vs. IC pts at 3.8 mo. The median duration from start of first subsequent CPI to end date was longer in the prior tebe pts at 4 mo vs prior IC pts at 2.8 mo. From the start of any first subsequent therapy, prior tebe pts had longer OS compared to prior IC pts, HR 0.67 (95% CI 0.42, 1.07). Most of the subsequent therapy was CPI, and the OS benefit was also seen in this subset, HR 0.62 (95% CI 0.34, 1.14). For prior tebe pts, the median and 1-yr OS rates from start of any first subsequent therapy were 13 mo and 53% and from start of first subsequent CPI were 16 mo and 63%. Both were higher than the sequence of IC followed by any therapy (11 mo and 44%), IC followed by CPI (9 mo and 47%) and a recent meta-analysis of 2L+ mUM (7 mo and ̃35% 1-yr OS rate). Conclusions: Pts who progressed on tebe and then received CPI had better OS compared to pts who progressed on IC and then received CPI. Further analysis will explore whether confounding factors are influencing this effect. These exploratory data suggest that tebe, relative to IC, may improve outcomes to subsequent CPI. (1)Yang J. et al. ASCO 2019, J.ClinOncol 37:15_suppl, 9592. Clinical trial information: NCT03070392