Efficacy Of Hypofractionated Radiotherapy (Rx) In Melanoma Patients Who Failed Anti-Pd-1 Monotherapy: Assessing The Abscopal Effect.

9537 Background: Radiotherapy (Rx) and anti-PD-1 mAb are potentially synergistic. No study has tested this combination only in pts who failed on anti-PD-1 mAb, which allows to assess the abscopal effect. We evaluated this combination in a cohort of advanced melanoma pts after failure of anti-PD-1 monotherapy. Methods: Analysis of a prospective database in a referral center searching for advanced melanoma pts with confirmed (2 CT-scans) progressive (PD) or stable (SD) disease on anti-PD-1 monotherapy, who later received concurrent Rx without modification of anti-PD-1 mAb regimen. Radiologists performed independent tumor evaluations (RECIST 1.1) every 3 m, both on radiated and non-radiated lesions, with abscopal effect defined as a partial (PR) or complete (CR) response outside radiated fields. Results: 26 pts (21 achieving PD, 5 SD, 10 pt ≥3 involved organs), mean age 70 Y, were included. Anti-PD-1 mAb was first line in 50% of pts. Rx, consisting of hypofractionnated Rx (3-5 sessions, 26 Gy), standard palliative Rx, or gamma-knife in respectively 23, 2, and 1 pts, was begun on a single site in 73% of pts or on 2 sites after a median of 5 m after beginning anti-PD-1 mAb. Median follow-up after onset of anti-PD-1 mAb was 17 (7-35) m, with 65% of pts alive at last follow-up. Best response was 7 CR (27%, including CR in 4 pts with prior PD) 1 PR, 3 SD (12%), 15 PD (58%). Abscopal effect was seen in 10 pts (38%). No correlation between the occurrence of CR and BRAF/NRAS mutation status, number of metastatic sites, presence or absence of brain metastases, and LDH level was seen. Anti-PD-1 mAb could be discontinued in 6 pts with CR, without relapse to date. No unusual adverse event was recorded. Conclusions: In pts who have previously failed on anti-PD-1 mAb, obtaining with concurrent Rx and without modifying anti-PD-1 mAb, CR or PR in 30% of pts, median OS not achieved, and abscopal effect in > 1/3 of pts is probably not due only to late efficacy of anti-PD-1 mAb but suggests a synergy with RT. Release after radiation of tumor neoantigens may stimulate immune response. Hypo-fractionated radiotherapy may enhance anti-PD1 monotherapy efficacy in melanoma pts who failed on anti-PD-1 mAb. Controlled studies are needed.