MITO END-3: efficacy of avelumab immunotherapy according to molecular profiling in first-line endometrial cancer therapy

Background Immunotherapy combined with chemotherapy significantly improves progression-free survival compared to first-line chemotherapy alone in advanced endometrial cancer, with a much larger effect size in microsatellite-instability high (MSI-H) cases. New biomarkers might help to select patients that may have benefit among those with a microsatellite-stable (MSS) tumor. Methods In a pre-planned translational analysis of the MITO END-3 trial, we assessed the significance of genomic abnormalities in patients randomized to standard carboplatin/paclitaxel without or with avelumab. Results Out of 125 randomized patients, 109 had samples eligible for next-generation sequencing (NGS) analysis, and 102 had MSI tested. According to The Cancer Genome Atlas (TCGA), there were 29 cases MSI-H, 26 MSS TP53 wild-type (wt), 47 MSS TP53 mutated (mut), and one case with POLE mutation. Four mutated genes were present in more than 30% of cases: TP53, PIK3CA, ARID1A, and PTEN. Eleven patients (10%) had a BRCA1/2 mutation (five in MSI-H and six in MSS). High TMB (≥10 Muts/Mb) was observed in all MSI-H patients, in four out of 47 MSS/TP53 mut, and no case in the MSS/TP53 wt category. The effect of avelumab on progression-free survival significantly varied according to TCGA categories, being favorable in MSI-H and worst in MSS/TP53 mut (P interaction=0.003); a similar non-significant trend was seen in survival analysis. ARID1A and PTEN also showed a statistically significant interaction with treatment effect, which was better in the presence of the mutation (ARID1A P interaction=0.01; PTEN P interaction=0.002). Conclusion The MITO END-3 trial results suggest that TP53 mutation is associated with a poor effect of avelumab, while mutations of PTEN and ARID1A are related to a positive effect of the drug in patients with advanced endometrial cancer.