BACKGROUND Cutaneous squamous-cell carcinomas and keratoacanthomas are common findings in patients treated with BRAF inhibitors. METHODS We performed a molecular analysis to identify oncogenic mutations (HRAS, KRAS, NRAS, CDKN2A, and TP53) in the lesions from patients treated with the BRAF inhibitor vemurafenib. An analysis of an independent validation set and functional studies with BRAF inhibitors in the presence of the prevalent RAS mutation was also performed. RESULTS Among 21 tumor samples, 13 had RAS mutations (12 in HRAS). In a validation set of 14 samples, 8 had RAS mutations (4 in HRAS). Thus, 60% (21 of 35) of the specimens harbored RAS mutations, the most prevalent being HRAS Q61L. Increased proliferation of HRAS Q61L–mutant cell lines exposed to vemurafenib was associated with mitogen-activated protein kinase (MAPK)–pathway signaling and activation of ERK-mediated transcription. In a mouse model of HRAS Q61L–mediated skin carcinogenesis, the vemurafenib analogue PLX4720 was not an initiator or a promoter of carcinogenesis but accelerated growth of the lesions harboring HRAS mutations, and this growth was blocked by concomitant treatment with a MEK inhibitor. CONCLUSIONS Mutations in RAS, particularly HRAS, are frequent in cutaneous squamous-cell carcinomas and keratoacanthomas that develop in patients treated with vemurafenib. The molecular mechanism is consistent with the paradoxical activation of MAPK signaling and leads to accelerated growth of these lesions. (Funded by Hoffmann–La Roche and others; ClinicalTrials.gov numbers, NCT00405587, NCT00949702, NCT01001299, and NCT01006980.)

Radiotherapy is used to treat many types of cancer, but many treated patients relapse with local tumor recurrence. Tumor-infiltrating myeloid cells (TIMs), including CD11b (ITGAM)+F4/80 (EMR1)+ tumor-associated macrophages (TAMs) and CD11b+Gr-1 (LY6G)+ myeloid-derived suppressor cells (MDSCs), respond to cancer-related stresses and play critical roles in promoting tumor angiogenesis, tissue remodeling and immunosuppression. In this report, we employed a prostate cancer model to investigate the effects of irradiation on TAMs and MDSCs in tumor-bearing animals. Unexpectedly, when primary tumor sites were irradiated we observed a systemic increase of MDSCs in spleen, lung, lymph nodes and peripheral blood. Cytokine analysis showed that the macrophage colony-stimulating factor CSF1 increased by 2-fold in irradiated tumors. Enhanced macrophage migration induced by conditioned media from irradiated tumor cells was completely blocked by a selective inhibitor of CSF1R. These findings were confirmed in prostate cancer patients, where serum levels of CSF1 increased after radiotherapy. Mechanistic investigations revealed the recruitment of the DNA damage-induced kinase ABL1 into cell nuclei where it bound the CSF1 gene promoter and enhanced CSF1 gene transcription. When added to radiotherapy, a selective inhibitor of CSF1R suppressed tumor growth more effectively than radiation alone. Our results highlight the importance of CSF1/CSF1R signaling in the recruitment of TIMs which can limit the efficacy of radiotherapy. Further, they suggest that CSF1 inhibitors should be evaluated in clinical trials in combination with radiotherapy as a strategy to improve outcomes.

Combining immunotherapy and BRAF targeted therapy may result in improved antitumor activity with the high response rates of targeted therapy and the durability of responses with immunotherapy. However, the first clinical trial testing the combination of the BRAF inhibitor vemurafenib and the CTLA-4 antibody ipilimumab was terminated early due to substantial liver toxicities. MEK inhibitors can potentiate the MAPK inhibition in BRAF mutant cells, while potentially alleviating the unwanted paradoxical MAPK activation in BRAF wild type cells that lead to side effects when using BRAF inhibitors alone. However, there is the concern of MEK inhibitors being detrimental to T cell functionality. Using a mouse model of syngeneic BRAFV600E driven melanoma, we tested whether addition of the MEK inhibitor trametinib would enhance the antitumor activity of combined immunotherapy with the BRAF inhibitor dabrafenib. Combination of dabrafenib and trametinib with pmel-1 adoptive cell transfer (ACT) showed complete tumor regression, increased T cell infiltration into tumors and improved in vivo cytotoxicity. Single agent dabrafenib increased tumor-associated macrophages and T regulatory cells (Tregs) in tumors, which decreased with the addition of trametinib. The triple combination therapy resulted in increased melanosomal antigen and MHC expression, and global immune-related gene up-regulation. Given the up-regulation of PD-L1 seen with dabrafenib and/or trametinib combined with antigen-specific ACT, we tested combination of dabrafenib, trametinib with anti-PD1 therapy in SM1 tumors, and observed superior anti-tumor effect. Our findings support the testing of triple combination therapy of BRAF and MEK inhibitors with immunotherapy in patients with BRAFV600E mutant metastatic melanoma.

The rapidly advancing field of cancer immunotherapy is currently limited by the scarcity of noninvasive and quantitative technologies capable of monitoring the presence and abundance of CD8+ T cells and other immune cell subsets. In this study, we describe the generation of 89Zr-desferrioxamine-labeled anti-CD8 cys-diabody (89Zr-malDFO-169 cDb) for noninvasive immuno-positron emission tomography (immuno-PET) tracking of endogenous CD8+ T cells. We demonstrate that anti-CD8 immuno-PET is a sensitive tool for detecting changes in systemic and tumor-infiltrating CD8 expression in preclinical syngeneic tumor immunotherapy models including antigen-specific adoptive T cell transfer, agonistic antibody therapy (anti-CD137/4-1BB), and checkpoint blockade antibody therapy (anti-PD-L1). The ability of anti-CD8 immuno-PET to provide whole body information regarding therapy-induced alterations of this dynamic T cell population provides new opportunities to evaluate antitumor immune responses of immunotherapies currently being evaluated in the clinic.

Colony stimulating factor-1 (CSF-1) recruits tumor-infiltrating myeloid cells (TIMs) that suppress tumor immunity, including M2 macrophages and myeloid derived suppressor cells (MDSC). The CSF-1 receptor (CSF-1R) is a tyrosine kinase that is targetable by small molecule inhibitors such as PLX3397. In this study, we used a syngeneic mouse model of BRAFV600E-driven melanoma to evaluate the ability of PLX3397 to improve the efficacy of adoptive T-cell therapy (ACT). In this model, we found that combined treatment produced superior anti-tumor responses compared with single treatments. In mice receiving the combined treatment, a dramatic reduction of TIMs and a skewing of MHCIIlow to MHCIIhi macrophages was observed. Further, mice receiving the combined treatment exhibited an increase in tumor-infiltrating lymphocytes (TILs) and T cells, as revealed by real-time imaging in vivo. In support of these observations, TILs from these mice released higher levels of IFN-γ. In conclusion, CSF-1R blockade with PLX3397 improved the efficacy of ACT immunotherapy by inhibiting the intratumoral accumulation of immune suppressive macrophages.

Blocking oncogenic signaling induced by the BRAFV600E mutation is a promising approach for melanoma treatment. We tested the anti-tumor effects of a specific inhibitor of Raf protein kinases, PLX4032/RG7204, in melanoma cell lines. PLX4032 decreased signaling through the MAPK pathway only in cell lines with the BRAFV600E mutation. Seven out of 10 BRAFV600E mutant cell lines displayed sensitivity based on cell viability assays and three were resistant at concentrations up to 10 μM. Among the sensitive cell lines, four were highly sensitive with IC50 values below 1 μM, and three were moderately sensitive with IC50 values between 1 and 10 μM. There was evidence of MAPK pathway inhibition and cell cycle arrest in both sensitive and resistant cell lines. Genomic analysis by sequencing, genotyping of close to 400 oncogeninc mutations by mass spectrometry, and SNP arrays demonstrated no major differences in BRAF locus amplification or in other oncogenic events between sensitive and resistant cell lines. However, metabolic tracer uptake studies demonstrated that sensitive cell lines had a more profound inhibition of FDG uptake upon exposure to PLX4032 than resistant cell lines. In conclusion, BRAFV600E mutant melanoma cell lines displayed a range of sensitivities to PLX4032 and metabolic imaging using PET probes can be used to assess sensitivity.

Purpose To evaluate the immunomodulatory effects of CTLA-4 blockade with tremelimumab in peripheral blood mononuclear cells (PBMC). Experimental Design We used next generation sequencing to study the complementarity determining region 3 (CDR3) from the rearranged T cell receptor (TCR) variable beta (V-beta) in PBMC of 21 patients, at baseline and 30–60 days after receiving tremelimumab. Results After receiving tremelimumab there was a median of 30% increase in unique productive sequences of TCR V-beta CDR3 in 19 out of 21 patients, and a median decrease of 30% in only 2 out of 21 patients. These changes were significant for richness (p=0.01) and for Shannon index diversity (p=0.04). In comparison, serially collected PBMC from four healthy donors did not show a significant change in TCR V-beta CDR3 diversity over one year. There was a significant difference in the total unique productive TCR V-beta CDR3 sequences between patients experiencing toxicity with tremelimumab compared to patients without toxicity (p=0.05). No relevant differences were noted between clinical responders and non-responders. Conclusions CTLA4 blockade with tremelimumab diversifies the peripheral T cell pool, representing a pharmacodynamic effect of how this class of antibodies modulates the human immune system.

Combining immunotherapy with targeted therapy blocking oncogenic BRAFV600 may result in improved treatments for advanced melanoma. Here, we developed a BRAFV600E-driven murine model of melanoma, SM1, which is syngeneic to fully immunocompetent mice. SM1 cells exposed to the BRAF inhibitor vemurafenib (PLX4032) showed partial in vitro and in vivo sensitivity resulting from the inhibition of MAPK pathway signaling. Combined treatment of vemurafenib plus adoptive cell transfer (ACT) therapy with lymphocytes genetically modified with a T cell receptor (TCR) recognizing chicken ovalbumin (OVA) expressed by SM1-OVA tumors, or pmel-1 TCR transgenic lymphocytes recognizing gp100 endogenously expressed by SM1, resulted in superior antitumor responses compared with either therapy alone. T cell analysis demonstrated that vemurafenib did not significantly alter the expansion, distribution, or tumor accumulation of the adoptively transferred cells. However, vemurafenib paradoxically increased MAPK signaling, in vivo cytotoxic activity, and intratumoral cytokine secretion by adoptively transferred cells. Together, our findings, derived from two independent models combining BRAF-targeted therapy with immunotherapy, support the testing of this therapeutic combination in patients with BRAFV600 mutant metastatic melanoma.