Comparing PD-L1 scores for predicting pembrolizumab efficacy in first-line treatment of unresectable cutaneous squamous cell carcinomas

e21593 Background: In the CARSKIN study (NCT02883556), 1L pembrolizumab demonstrated promising activity with durable responses and manageable safety in patients (pts) with unresectable locally advanced or metastatic CSCC. Here we report best ORR and survival endpoints of the entire cohort by PD-L1 status. Methods: The imaging assessment per RECIST v1.1 was blinded with independent central review. Best ORR, PFS, DOR OS, in the overall sample and by PD-L1 status was centrally assessed by 2 blinded independent pathologists, one using the anti–PD-L1 E1L3N clone (TPS E1L3N ), the other using the 22C3 antibody (TPS 22C3 and CPS 22C3 ). Among 57 pts, 6 were excluded from the analysis: 2 untreated pts, 1 early unrelated death and 3 pts tested only with 1 assay). ROC curve analysis was used to compare assays. Results: With a median follow-up of 26 mo, best ORR was 51% with 14 PR (27%) and 12 CR (24%). Relapse occurred in 3 PR and 1 PD-L1 – CR pts. Respective median PFS, DOR, and OS were 13.7, NR, and 25.3 mo; respective 1-year PFS, DOR and OS were 50% [38-66], 80% [65-97] and 73% [61-86]. With a cutoff of 1%, best ORR was higher either for TPS E1L3N + pts vs TPS E1L3N – pts (60% vs 18%, p = 0.02) and for CPS 22C3 + pts (59% vs 20%, p = 0.038), but not TPS 22C3 + pts (p = 0.76). The optimal cutoff of CPS 22C3 for ORR using a ROC curve was estimated to be ≥ 7% with a sensitivity of 0.70 and a specificity of 0.75. For best ORR, area under the ROC curve was higher for CPS 22C3 (0.72) than TPS E1L3N (0.62) but the difference was not significant (P = 0.21). Pts with TPS E1L3N ≥ 1% had a higher 1-y PFS (57% vs 27%, p = 0.004) and OS (82% vs 42%, P = 0.01). Pts with CPS 22C3 ≥ 1%, had a better 1-y OS (79% vs 50%, p = 0.03), but not 1y-PFS. Conclusions: CPS 22C3 ≥ 7% appeared equivalent to TPS E1L3N ≥ 1% for predicting ORR and OS in pts receiving pembrolizumab in 1L treatment of advanced or metastatic CSCCs. Replications are warranted for validation. Pts with low scores might beneficiate from combined treatments. Clinical trial information: NCT02883556 .