Association of prior immune checkpoint blockade (ICB) with longer progression-free survival (PFS) in patients treated with intermittent versus continuous dabrafenib and trametinib: A post-hoc analysis of S1320

10039 Background: S1320 is a phase 2 randomized clinical trial presented at the 2020 AACR Annual Meeting in April demonstrating that continuous dosing of dabrafenib and trametinib yields longer PFS than intermittent dosing of these agents in patients with BRAFV600E/K melanoma. Here we look at the association between prior exposure to ICB and PFS in patients randomized to either intermittent or continuous dosing on S1320. Methods: Patients without disease progression after 8 weeks of dabrafenib and trametinib were randomized 1:1 to proceed with intermittent therapy (3-week-off, 5-week-on) or to stay on the continuous daily dosing schedule. The design called for 206 randomized patients with the primary outcome of PFS. Response assessments were made using RECIST v1.1 at 8-week intervals. A post-hoc analysis assessed differences in PFS in the pool of all randomized patients based on prior exposure to anti-PD1 antibodies, a randomization stratification factor. Kaplan-Meier estimates and multivariable Cox regression models (controlling for pre-randomization age, Zubrod performance status, LDH, unknown primary, M-Stage) were used to evaluate the association between this stratification factor and PFS. Results: Of 242 patients treated on study, 105 were randomized to continuous dosing, 101 to intermittent dosing, and 36 were not randomized due to disease progression at 8 weeks or other factors. 37% of the 242 enrolled and 37% of the 206 randomized patients had previously been treated with ICB. Among all randomized patients, there were no differences in baseline characteristics comparing patients with and without prior immune checkpoint inhibitor exposure: age median 62 vs 59, LDH elevation 37% vs 39%,, stage IVB/C 73% vs 64%, Zubrod performance status 0, 57% vs 58%. PFS was longer in patients with prior ICB exposure (hazard ratio = 0.60, 95% confidence interval 0.41,-0.98, median = 6 vs 9 months from randomization, 8 vs 11 months from starting treatment). There was no difference in the association between prior ICB exposure and PFS between arms (interaction p-value = 0.62). Conclusions: In patients without early progression on dabrafenib and trametinib, PFS was longer with prior to exposure to ICB . Although the groups had similar baseline characteristics and rates of randomization, these results could still be influenced by non-controlled factors influencing clinicians’ decisions to start a patient on immune versus targeted therapy. Clinical trial information: NCT02196181.