# Effects of Vemurafenib ± Cobimetinib on Intratumoral and Host Immunity in Patients With BRAFV600 Mutant Melanoma: Implications for Combination With Immunotherapy

## TL;DR

This study examines how BRAF and MEK inhibitors affect immune cells in melanoma patients, finding that these drugs increase T cell infiltration but do not generate new immune responses.

## Contribution

The study provides new insights into the dynamic changes in tumor-infiltrating lymphocytes and their clonality during BRAF/MEK inhibitor therapy in melanoma patients.

## Key findings

- BRAF/MEK inhibitors increased CD8+ and CD4+ TIL density in most patients by day 8 or 15.
- Gene expression showed upregulation of immune-related pathways, including MHC and chemokine genes.
- TCR clonal expansion was observed in three patients but mostly diminished after day 15.

## Abstract

Prior studies in patients with BRAF‐mutant melanoma have shown increased density of tumor infiltrating lymphocytes (TIL) after 2 weeks of BRAF (BRAFi) ± MEK inhibition (MEKi), but did not characterize the functional state or clonal diversity of TIL over time. We evaluated sequential tumor biopsies during therapy to test the hypotheses that BRAF/MEKi would increase TIL to day 29, with increases in IFNγ signatures and T‐cell homing receptor ligands and expansion of functional intratumoral tumor‐reactive CD8 T‐cells and TIL clonality in the tumor microenvironment.

Subjects with biopsy‐accessible BRAF‐mutant advanced melanoma received vemurafenib+/−cobimetinib. Tumor biopsies were obtained at baseline and days 8, 15, and 29 on therapy. Tumors were analyzed by quantitative immunofluorescence (QIF), NanoString, and TCRseq.

Five patients were enrolled. All had an initial tumor response followed by subsequent progression. In four patients, both CD8+ and CD4+ TIL density increased by day 8 or 15 per QIF and continued to increase at day 29 in two. Gene expression data showed upregulation of genes/pathways associated with immunologic rejection of cancer, including Class I and II MHC expression, antigen processing/presentation, and critical T‐cell attracting chemokines. TCRvβ clonal expansion was observed in 3 patients, but most diminished after day 15.

Data from this study provides provocative evidence that, while BRAF+/−MEK inhibitor therapy produces an increase in overall and clonal T cell infiltrates, there is limited evidence for generation of new or persistent tumor immunity. Thus, BRAFi/MEKi therapy may enable tumor‐reactive T cells to infiltrate tumors but tumor control does not appear to depend on priming new immune responses.

We evaluated sequential tumor biopsies during therapy to test the hypotheses that BRAF/MEKi would increase TIL and lead to an expansion of functional intratumoral tumor‐reactive CD8 T‐cells and TIL clonality in the tumor microenvironment. Data from this study provides provocative evidence that, while BRAF+/−MEK inhibitor therapy produces an increase in overall and clonal T cell infiltrates, there is limited evidence for generation of new or persistent tumor immunity. Thus, BRAFi/MEKi therapy may enable tumor‐reactive T cells to infiltrate tumors but tumor control does not appear to depend on priming new immune responses.