Histone Deacetylase 6 Inhibitors as Adjuvants for Targeted and Immune Therapies in Melanoma
Open AccessMelanoma is the least common but deadliest type of skin cancer. The standard of care for melanoma patients includes surgery, targeted therapy, immunotherapy, chemotherapy, and radiation. Although the development and approval of targeted and immune therapies was a major breakthrough for melanoma patients, a substantial subset of patients do not respond or become resistant to these therapies. Therefore, researchers are investigating the potential of epigenetic drugs, such as histone deacetylase inhibitors (HDACi), as adjuvants for targeted and immune therapies. However, pan-HDACi have been shown to induce cytotoxicity and side effects in patients in early clinical trials. Therefore, it is necessary to use isoform specific HDACi for these purposes, as they are less cytotoxic and can be used as immunomodulatory agents. Among the different HDAC isoforms, HDAC6 has been shown by our group and others to be an important modulator of the immune system as well as play a critical role in different cellular processes, such as proliferation, cell signaling, and metastasis, among others. HDAC6i have the potential to serve as adjuvants for currently available therapies due to the wide breadth of cellular processes they participate in. Therefore, in this dissertation, we evaluate the use of highly selective HDAC6i as adjuvants for immunotherapy (i.e., anti-CD47), as an enhancer of macrophage-based adoptive cell therapies, and as adjuvants for BRAF inhibitors.The first study, presented in Chapter 2, evaluates the HDAC6i Nexturastat A (NextA) in regulating the innate immune checkpoint comprising the CD47/SIRPα axis. CD47, overexpressed by cancer cells, interacts with SIRPα on macrophages to prevent phagocytosis. Blocking CD47 blockade is a novel immunotherapeutic approach to enhance innate anti-tumor immunity. Its efficacy is limited unless combined with other therapies. In this study, we demonstrate that HDAC6 inhibition modulates the macrophage phenotype, diminishes SIRPα on macrophages, increases expression of pro-phagocytic signals, and downregulates CD47 expression in melanoma cells. These findings led to the overall conclusion that macrophages treated with NextA were more phagocytic than untreated macrophages, an effect further enhanced with anti-CD47. Furthermore, we observed that the combination of NextA and anti-CD47 decreased SM1 melanoma growth in vivo by increasing immune cell infiltration in the tumor microenvironment (TME). Altogether, our results indicate that HDAC6i synergizes with CD47 blockade to reduce tumor growth and enhance innate anti-tumor immunity. Given the results presented in Chapter 2 on the potential of HDAC6 inhibition in controlling the macrophage phenotype and phagocytic function, we then investigated whether HDAC6i-treated macrophages would be a viable macrophage-based cell therapy for melanoma. As shown in Chapter 3, we showed that the M1/M2 ratio is an important predictor of the immunological state of the TME. With our therapy, we aimed to increase the M1/M2 ratio as well as prevent phenotype switch of the transplanted macrophages. We observed that HDAC6i treated pro-inflammatory and anti-tumoral (M1) macrophages were resistant to phenotype switch towards the anti-inflammatory and pro-tumoral (M2) phenotype, a major limitation of past and current macrophage-based cell therapies. Furthermore, we observed that M1 NextA treated macrophages decreased tumor growth in vivo in syngeneic and humanized melanoma models by enhancing CD8 T cell infiltration in the TME, as well as increasing the M1/M2 ratio. In our last study (Chapter 4), we assessed the role of NextA in sensitizing multiple mouse melanoma cell lines harboring different genetic alterations to Vemurafenib. Vemurafenib is a BRAFi commonly used to treat melanoma patients whose therapeutic efficacy is limited by the emergence of resistance. Through cytotoxicity, viability, and apoptosis assays, our findings suggest that the combination therapy of Vemurafenib and NextA decreases viability and increases cytotoxicity in some melanoma cells compared to either drug alone. However, the effects of this combination on cell death and overcoming resistance need to be further investigated. Altogether, our results suggest that HDAC6i can be used as adjuvants to enhance the therapeutic efficacy of immunotherapy (i.e., anti-CD47), macrophage-based cell therapies, and targeted therapies such as BRAFi. This is due to the multiple cellular pathways that HDAC6 plays a role in, whether they are intrinsic or extrinsic to the tumor.
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GraciaHernandez_gwu_0075A_16213.pdf | 2023-11-14 | Open Access |
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