Molecular predictors of response among patients with MMRd tumors treated on NCI-MATCH Arm Z1D.

2616 Background: On arm Z1D of the NCI-MATCH trial, the PD-1 inhibitor nivolumab was found to have activity among patients with mismatch repair-deficient (MMRd) tumors as defined by complete loss of MLH1 or MSH2 nuclear expression determined by immunohistochemistry, with 6-month progression free survival of 51%. We aimed to identify molecular predictors of response in this population. Methods: Among patients treated on NCI-MATCH Z1D, we evaluated genomic and tissue predictors of clinical benefit (CB), defined as patients with RECIST v1.1 complete or partial response or stable disease for ≥ 6months. WES files were processed and filtered using GATK best practices preceding TMB and MSI calculations according to MSI sensor score, a WES-based MSI rating system. Cutoffs were set to define TMB (TMB-Low: ≤10 mutation/Mb; TMB-High: >10) and MSI (MSS: ≤10% unstable loci; MSI-Low: 10 > x ≤ 20; MSI-High: >20). Multiplex immunofluorescence (mIF) used formalin-fixed paraffin-embedded slides stained using a BOND RX automated stainer. Expression analyses followed normalization in DEseq2's median of ratios method. Gene set enrichment analysis was conducted by “empirical phenotype-based permutation test.” Additional RNA, WES, and mIF comparisons used the Wilcoxon rank-sum test. Results: Among 36 patients accrued to NCI-MATCH Z1D with pretreatment correlative samples available, 7 were unevaluable for response, and 1 was misclassified as having an MMRd tumor. Of the remaining 28, 15 had CB (2 CR, 10 PR, 3 SD ≤ 6 months) and correlative data were available for 26 (WES), 27 (RNAseq), and between 10-20 for mIF based on the marker assessed. According to MSI-sensor score, 11 were MSI-high, 8 were MSI-low, and 7 were MSS. MSI-sensor status, but not TMB was associated with CB (p=0.037 and p=0.185, respectively). Similar results were seen when using CR+PR vs SD+PD evaluation. Using RNAseq gene set enrichment analyses, CB patients had increased expression of interferon alpha (p=0.01), interferon gamma (p=0.03), PI3K-AKT-mTOR (p=0.02), cytotoxicity (p=0.05) and antigen processing (p=0.01) gene sets, while hedgehog signaling genes were increased in non-CB patients (p=0.04). The ESTIMATE immune index and infiltration of CD4+/PD1+/Ki67+ cell populations as determined by mIF were nominally higher in patients with CB (p=0.051 and p=0.075). Conclusions: Among patients with MMRd tumors treated with PD-1 checkpoint blockade, correlative analyses demonstrate associations between CB and MSI-sensor score as well as biomarkers indicative of immune infiltration and antigen presentation. This suggests that these measures may help differentiate patient response in MSI tumors. Clinical trial information: NCT02465060.