Immunomodulating property of MAPK inhibitors: from translational knowledge to clinical implementation

Laboratory Investigation (2017) 97, 166–175 © 2017 USCAP, Inc All rights reserved 0023-6837/17 MINI REVIEW Immunomodulating property of MAPK inhibitors: from translational knowledge to clinical implementation Mario Mandalà1, Francesco De Logu2, Barbara Merelli1, Romina Nassini2 and Daniela Massi3 Treatment of metastatic melanoma was radically changed by the introduction of inhibitors of BRAF, an oncogene mutated in 40–50% of patients. Another area of advancement was the use of immunotherapy, and specifically, immune checkpoint inhibitors. There is compelling evidence that oncogenic BRAF, in addition to driving melanoma proliferation, differentiation and survival, induces T-cell suppression directly through the secretion of inhibitory cytokines or through membrane expression of co-inhibitory molecules such as the PD-1 ligands PD-L1 or PD-L2. Furthermore, the presence of oncogenic BRAF leads to an immune suppressive phenotype characterized by the presence of inhibitory immune cells such as regulatory T cells, myeloid-derived suppressor cells, or tumor-associated macrophages, which can in turn inhibit the function of tumor-infiltrating T cells. Growing evidence suggests that, in addition to their established molecular mechanism of action, the therapeutic efficacy of BRAF inhibitors and MEK inhibitors relies on additional factors that affect the tumor–host interactions, including the enhancement of melanoma antigen expression and the increase in immune response against tumor cells. Focus of the present review is to summarize the off target mechanisms of response to BRAF inhibitors and MEK inhibitors and the synergy between targeted therapy and immunotherapy as the biological source to open a window of strategic opportunities for the design of new exciting clinical trials. Laboratory Investigation (2017) 97, 166–175; doi:10.1038/labinvest.2016.132; published online 19 December 2016 The therapy of metastatic melanoma (MM) was radically changed by the introduction of inhibitors of BRAF, an oncogene mutated in 40–50% of patients. The BRAF inhibitors (BRAFi) proved to be more successful than conventional chemotherapy in the treatment of these patients in terms of activity and efficacy, achieving partial and complete remissions in many instances instances.1,2 Seminal clinical trials have shown that treatment of MM patients with BRAFi (vemurafenib, dabrafenib) is associated with improved response rate, progression free survival, and overall survival compared with conventional chemotherapy.1–3 However, while clinical responses to BRAFi may be dramatic, with some patients maintaining remission for several months or years, the median duration of response is 6–7 months.1–3 This finding was the rationale for the introduction of MEK inhibitors (MEKi).4 The combination of BRAFi with MEKi was proposed as a strategy in MM patients to delay or even prevent the onset of resistance, without increasing the risk of developing secondary cancers. Three large, prospective, randomized clinical trials indicate that combined therapy is significantly more effective than either drug used alone and that resistance occurs at a significant later stage, proposing this combination as the new standard treatment in MM patients.5–7 Another area of improvement in melanoma treatment involves the use of immunotherapy, and specifically, the immune checkpoint inhibitors. Monoclonal antibodies targeting immunomodulatory molecules such as cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed death receptor-1 (PD-1), represent a further advance, with objective response rates ranging between 15% and 40%, respectively, when used as monotherapy, and up to 60% when combined in large randomized clinical trials.5,7,8 Although BRAF/MEK inhibitors are effective in xenograft models with impaired or depleted immune system, growing evidence suggests that the therapeutic efficacy of BRAFi and MEKi could rely on additional factors that affect the tumor–host interactions, including the enhancement of melanoma antigen expression and the increase in immune response against tumor cells.9,10 Consistently, preclinical data show that oncogenic BRAF 1 Unit of Medical Oncology, Department of Oncology and Hematology, Papa Giovanni XXIII Cancer Center Hospital, Bergamo, Italy; 2Unit of Clinical Pharmacology and Oncology, Department of Health Sciences, University of Florence, Florence, Italy and 3Division of Pathological Anatomy, Department of Surgery and Translational Medicine, University of Florence, Florence, Italy Correspondence: Associate Professor D Massi, MD, Division of Pathological Anatomy, Department of Surgery and Translational Medicine, University of Florence, Largo Brambilla, 3, Florence 50134, Italy. E-mail: Received 5 August 2016; revised 6 November 2016; accepted 7 November 2016 166 Laboratory Investigation | Volume 97 February 2017 | www.laboratoryinvestigation.org Immunomodulatory effects of targeted therapies M Mandalà et al contributes to immune evasion, and that targeting this mutation may increase the melanoma immunogenicity.11,12 Herein, we review the preclinical and clinical data supporting that BRAFi and MEKi are also immunomodulating drugs and suggest that combining targeted therapy with immunotherapy is potentially able to boost the immune responses or to overcome immune mediated mechanisms of resistance. Oncogenic BRAF as a Mechanism of Immunotolerance In the last years, extensive information emerged concerning the molecular basis of melanoma pathogenesis, progression, and response to therapy. There is evidence that cutaneous melanomas exhibit markedly elevated base mutation rates compared to nearly all other solid tumors,13,14 which is almost entirely attributable to increased abundance of the cytidine to thymidine (C4T) transitions a typical UV-lightinduced mutational signature. Among somatic mutations, activation of the mitogen-activated protein kinase (MAPK) pathway by oncogenic mutations was found in up to 90% of melanoma cases.15,16 Among them, BRAF mutations were demonstrated in 40–50% of melanomas. Most BRAF mutations localize to the kinase domain and increase the kinase activity of BRAF toward MEK. A valine-to-glutamate substitution in the glycine-rich loop is the most frequent BRAF mutation (V600E). This gain-of-function BRAF mutation accounts for more than 80% of the BRAF alterations described in melanoma, with alternative point mutations at the same position (V600D, V600K, V600R) Figure 1 BRAFV600 mutation in melanoma cells increases the production of immunosuppressive factors (IL-10, VEGF, or IL-6) that, in turn, can promote recruitment of myeloid-derived suppressor cells and regulatory T cells in the tumor microenvironment. Another immunosuppressive effect of mutant BRAF is related to the downregulation of MHC class I (MHC-I) molecules and decrease in CD8+ T/FoxP3+CD4+ T cell ratio and NK cells. www.laboratoryinvestigation.org | Laboratory Investigation | Volume 97 February 2017 c (...truncated)