Distant metastasis-free survival results from the randomized, phase 2 mRNA-4157-P201/KEYNOTE-942 trial

LBA9503 Background: mRNA-4157 is a novel mRNA-based personalized cancer vaccine which encodes up to 34 patient-specific tumor neoantigens. The open-label randomized Phase 2 mRNA-4157-P201/Keynote-942 trial met its primary endpoint of recurrence free survival (RFS) in patients with resected high-risk stage IIIB/C/D and IV melanoma. The study has shown a statistically significant and clinically meaningful improvement in RFS in the combination therapy compared to pembrolizumab monotherapy, with a reduction in the risk of recurrence or death by 44% (HR = 0.561; 95% CI: (0.309, 1.017); 1-sided p-value of 0.0266). This report provides the first analysis of the secondary efficacy endpoint of distant metastasis-free survival (DMFS). Methods: mRNA-4157-p201 is an ongoing multicenter, open-label, randomized Phase II trial in patients with completely resected, high-risk Stage IIIB/C/D and IV cutaneous melanoma. Patients were randomized 2:1 (stratified by stage) to receive mRNA-4157 in combination with pembrolizumab or pembrolizumab alone. mRNA-4157 (1mg) was administered intramuscularly every 3 weeks for a total of 9 doses and pembrolizumab (200mg) intravenously was given every 3 weeks for up to 18cycles. The primary endpoint was investigator-assessed RFS, defined as local, locoregional, distant recurrence, or new primary melanoma. The secondary endpoint of DMFS was pre-specified and hierarchically tested following positive RFS. All tests were performed at 1-sided alpha = 0.99. Results: 157 patients were randomized to the combination of mRNA-4157 with pembrolizumab (n = 107) or pembrolizumab monotherapy (n = 50). The primary analysis for the primary endpoint occurred after all patients completed a minimum of 12 months on study and 44 RFS events were observed. At a median follow-up of 23 (combination) and 24 (pembrolizumab) months in the primary analysis, RFS events were reported in 22.4% (24/107) of patients in the combination arm and in 40% (20/50) of patients in the monotherapy arm. The 18-month RFS rates (95% CI) were 78.6% (69.0%, 85.6%) vs 62.2% (46.9%,74.3%) in the combination and monotherapy arms respectively. There was also a statistically and clinically significant improvement in DMFS for the combination versus pembrolizumab monotherapy (HR = 0.347; 95% CI: (0.145, 0.828); 1-sided p-value 0.0063). Distant recurrence or death was reported in 8.4% (9/107) and 24% (12/50) of patients, with 18-month DMFS rates (95% CI) were 91.8% (84.2%, 95.8%) vs 76.8% (61.0%, 86.8%) in the combination and monotherapy arm, respectively. Conclusions: mRNA-4157 in combination with pembrolizumab as adjuvant therapy for resected high-risk melanoma significantly prolonged DMFS compared to pembrolizumab. These results provide further evidence that a personalized neoantigen approach is potentially beneficial for cancer patients. A phase 3 randomized study will be initiated in patients with melanoma. Clinical trial information: NCT03897881 .