BRAF and MEK Inhibitors Increase PD-1-Positive Melanoma Cells Leading to a Potential Lymphocyte-Independent Synergism with Anti–PD-1 Antibody

Abstract

<b>Purpose:</b> BRAF and MEK inhibitors (BRAF/MEKi) favor melanoma-infiltrating lymphocytes, providing the rationale for current combinatorial trials with anti-PD-1 antibody. A portion of melanoma cells may express PD-1, and anti-PD-1 antibody could have a direct antitumor effect. Here, we explore whether BRAF/MEKi modulate rates of PD-1⁺ melanoma cells, supporting an additional-lymphocyte-independent-basis for their therapeutic combination with anti-PD-1 antibody.<b>Experimental Design:</b> With data mining and flow cytometry, we assessed PD-1, PD-L1/2 expression on melanoma cell lines (CCLE, <i>N</i> = 61; validation cell lines, <i>N</i> = 7) and melanoma tumors (TCGA, <i>N</i> = 214). We explored <i>in vitro</i> how BRAF/MEKi affect rates of PD-1⁺, PD-L1/2⁺ melanoma cells, and characterized the proliferative and putative stemness features of PD-1⁺ melanoma cells. We tested the functional lymphocyte-independent effect of anti-PD-1 antibody alone and in combination with BRAF/MEKi <i>in vitro</i> and in an <i>in vivo</i> immunodeficient murine model.<b>Results:</b> PD-1 is consistently expressed on a small subset of melanoma cells, but PD-1⁺ cells increase to relevant rates during BRAF/MEKi treatment [7.3% (5.6-14.2) vs. 1.5% (0.7-3.2), <i>P</i> = 0.0156; <i>N</i> = 7], together with PD-L2⁺ melanoma cells [8.5% (0.0-63.0) vs. 1.5% (0.2-43.3), <i>P</i> = 0.0312; <i>N</i> = 7]. PD-1⁺ cells proliferate less than PD-1^- cells (avg. 65% less; <i>t</i> = 7 days) and are preferentially endowed with stemness features. <i>In vivo</i>, the direct anti-melanoma activity of PD-1 blockage as monotherapy was negligible, but its association with BRAF/MEKi significantly delayed the development of drug resistance and tumor relapse.<b>Conclusions:</b> BRAF/MEKi increase the rates of PD-1⁺ melanoma cells that may sustain tumor relapse, providing a lymphocyte-independent rationale to explore combinatory strategies with anti-PD-1 antibody. <i>Clin Cancer Res; 24(14); 3377-85. ©2018 AACR</i>.