Initial correlative studies from a trial of cetuximab and pembrolizumab in metastatic colorectal cancer (mCRC).

Abstract

4062 Background: Cetuximab is an EGFR-targeting IgG1 mAb. Pre-clinical data suggests cetuximab induces CD8+ cytotoxic T-cell (CTL) infiltration of tumors. We hypothesized that augmentation of CTLs in the tumor microenvironment (TME) may provide the proper milieu for effective PD-1 inhibition in metastatic colorectal cancer (mCRC). We conducted a phase Ib/II study of cetuximab with the PD-1 antibody, pembrolizumab, in mCRC. Correlative blood and tissue samples were collected to assess the impact of this treatment on CTLs, as well as potential compensatory alterations in regulatory T-cells (Tregs) and suppressive MDSCs (NCT02713373). Methods: 3 week treatment cycles included pembrolizumab at 200 mg on day 1 and cetuximab 250 mg/m2 following the 400 mg/m2 loading dose. Tumor biopsies were obtained at baseline and at c4d1 (Day 64). Peripheral blood (PB) was drawn at baseline, c2d1 (day 22) and c4d1 (Day 64). Flow cytometry was performed within 24 hours with additional samples stored for future analysis. In the present analysis, we assessed changes in levels of the cellular populations of interest between cycle 4 and cycle 1. Results: Forty-two RAS-wt patients were enrolled through October 2019. Paired tumor tissues were successfully analyzed for 16 patients and PB for 38. Intratumoral CTLs (CD3+CD8+) increased significantly (+47%, p < .05). In PB, there was a slight overall decrease in PB CTLs (-5%, p = NS) and a significant decrease in CD8+CD45RO+PD1+ cells (-42%, p < .05). We saw simultaneous decreases in PD-1+ CTLs in the tumor and PB. There was a trend for increase in Tregs (CD4+ Cd25+ FoxP3+) in PB (+11%, p = NS), but an overall increase in the Teff:Treg ratio (+30%, p = NS). CD4+CTLA4+ cells significantly increased (+37%, p < .05). Granulocytic MDSCs (CD11b+CD14−CD33+HLADR−) in PB decreased significantly on-treatment (-30%, p < 0.5). The sample size and tissue limitations prohibited meaningful evaluation of tissue Tregs and MDSCs via the present methods. Conclusions: Cetuximab and pembrolizumab induced dynamic changes in the TME and PB. The treatment associated increase in intratumoral CTLs was particularly pronounced, consistent with their local expansion and/or influx. This was accompanied by a decrease in PB CTLs. Decreases in multiple PD-1+ lymphoid populations were observed in both tumor and PB, notably PD-1+ CTLs. Of note, we saw a synchronous increase in immunosuppressive CD4+CTLA4+ T-cells in PB. Patient outcomes are pending maturation. Further analyses are planned, coupled with integration of clinical data. Clinical trial information: NCT02713373 .