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Characterizing the tumor and immune landscape of melanoma patients treated with combined checkpoint blockade and MAPK targeted therapy.

9522 Background: Melanoma therapy has been revolutionized by two novel therapeutic approaches: mitogen activated protein kinase (MAPK) targeted therapy (MTT) and immune checkpoint blockade therapy (ICB). Less than half of patients respond to ICB monotherapy, in part due to non-responsive tumor microenvironment (TME). It previously has been shown that MTT enhances anti-tumor immunity within the TME, thus providing a strong rationale for its combination with immunotherapy. Regimens combining MTT with ICB have had mixed results, and which patients should be treated with these combinations is unknown. Methods: The first arm (NCT03149029) of a planned two stage design was to enroll 14 patients (pts) harboring BRAFV600 mutation treated with 2 weeks (wks) of MTT (dabrafenib plus trametinib) then 6 wks of concomitant MTT and pembrolizumab, followed by single-agent pembrolizumab thereafter. The primary endpoint is clinical benefit (CB) defined as partial/complete response or stable disease (per RECIST1.1) persisting at 24 wks. If 9 of 14 pts had CB, then 11 more pts would be enrolled for a total cohort of 25. Serial biopsies were performed prior to MTT, following the 2-week lead-in of MTT, and following six wks of combination immune therapy and MTT. Single-cell RNA-seq profiling of CD45+ and CD45- cells was performed using both the smart-seq2 plate-based protocol and 10x genomics platform. Results: Sixteen pts were enrolled, with 14 receiving both MTT and ICB. Two pts did not receive ICB due to MTT toxicity. Only 5 had CB, and the second stage did not open. A 6th pt had CB extracranially with a new small brain met at wk 24 scans was considered CB for tumor analysis. A clustering analysis of 25 samples (n = 9 pts) showed that following MTT the abundance of CD8 T-cells as well as tumor IFNγ levels were significantly elevated in CB vs. no CB (NCB) patients. In addition, tumor associated macrophages (TAM) in NCB patients possessed mainly an M2 phenotype and expressed a significantly higher level of immune suppressor genes, such as HLA-G and CD52. Interestingly, NCB pts had a significantly higher expression of tumor TGFβ, which is a strong inducer of M2 macrophages. In contrast, most of the TAMs occupying the tumor of the CB pts had the M1 phenotype, and significantly expressed CD9, CD81 and CD82, important factors during antigen recognition and immunological synapse formation. Conclusions: Abbreviated MTT with ICB did not lead to increased clinical benefit at 24 wks in this small study. It is theorized that the tumor’s ability to create a unique microenvironment by producing certain factors (e.g. TGFβ), modifies the immune system and may tilt its path into immune suppression thereby reducing the efficacy of this combinatorial therapy in melanoma pts with metastatic disease. These results may help identify pts most likely to benefit from combined MTT plus ICB and new targets to overcome resistance to these regimens. Clinical trial information: NCT03149029.