Stereotactic radiotherapy combined with immunotherapy is safe and effective: Results from a phase I clinical trial.

Abstract

9557 Background: There is growing evidence to suggest synergism between stereotactic ablative radiotherapy (SABR) and immunotherapy (IO) against metastatic melanoma. The optimal timing and dosing of SABR for this purpose has not been established. Here, we report results from a Phase I trial, finding the best outcomes with low dose SABR delivered late. Methods: Metastatic melanoma pts with at least two metastases received SABR to a single metastatic site. All pts had standard dose IO with anti-PD1 and/or anti-CTLA4. Following a standard 3+3 design, pts were escalated through three SABR doses (10Gy, 15Gy and 20Gy) delivered at three different time points (Cycle 1, 2, or 3 of IO). Dose limiting toxicity (DLT) were defined as Grade 3 or higher toxicity within 3 months of first treatment and assessed by an independent data safety monitoring committee (IDSMC). Logistic or cox regressions were used to assess the impact of SABR dose, timing and use of combination IO on toxicity, progression-free (PFS) and overall survival (OS) while controlling age, gender and baseline performance status. Results: Between April 2016 and August 2018, 24 pts were enrolled. The median age was 66 years and most were ECOG 0-1 (92%). The median follow-up was 10 months. Three pts (12.5%) developed DLTs (enterocolitis, hepatitis and liver function derangement). None occurred at SABR treated sites and all were in pts receiving 15Gy. DLTs were not associated with SABR timing (p = 0.44) or use of combination IO (p = 0.72). On this basis the IDSMC recommended stopping the trial and maximum tolerated SABR dose was defined at 10 Gy. The median PFS and OS were respectively 5.4m (95% CI 2.1m-NR) and 16.9m (95% CI 7.1m-NR). The median PFS for those receiving 10Gy was numerically higher than those receiving 15Gy (11.8m vs 2.6 m, p = 0.42). The only treatment related factor associated with improved PFS (HR = 0.14, p = 0.02) and OS (HR = 0.09, p = 0.04) was receiving SABR with Cycle 3. SABR dose (PFS p = 0.75, OS p = 0.67) and IO type (PFS p = 0.13, OS p = 0.06) were not significant. Conclusions: SABR combined with IO is generally safe overall. We found the optimal therapeutic index may be achieved with 10Gy delivered with the third cycle of IO; a strategy that warrants further testing. Clinical trial information: ACTRN12616001064493.