Selective HDAC6 inhibitors improve anti-PD-1 immune checkpoint blockade therapy by decreasing the anti-inflammatory phenotype of macrophages and down-regulation of immunosuppressive proteins in tumor cells

Abstract

Histone deacetylases (HDACs) are involved in diverse cellular regulatory mechanisms including non-canonical functions outside the chromatin environment. Several publications have demonstrated that selective HDAC inhibitors (HDACi) can influence tumor immunogenicity and the functional activity of specific immune cells. In particular, the selective inhibition of HDAC6 has been reported to decrease tumor growth in several malignancies. However, there is still no clarity about the cellular components mediating this effect. In this study, we evaluated the HDAC6i Nexturastat A as a priming agent to facilitate the transition of the tumor microenvironment from “cold” to “hot”, and potentially augment immune check-point blockade therapies. This combination modality demonstrated to significantly reduce tumor growth in syngeneic melanoma tumor models. Additionally, we observed a complete neutralization of the up-regulation of PD-L1 and other immunosuppressive pathways induced by the treatment with anti-PD-1 blockade. This combination also showed profound changes in the tumor microenvironment such as enhanced infiltration of immune cells, increased central and effector T cell memory, and a significant reduction of pro-tumorigenic M2 macrophages. The evaluation of individual components of the tumor microenvironment suggested that the in vivo anti-tumor activity of HDAC6i is mediated by its effect on tumor cells and tumor-associated macrophages, and not directly over T cells. Overall, our results indicate that selective HDAC6i could be used as immunological priming agents to sensitize immunologically “cold” tumors and subsequently improve ongoing immune check-point blockade therapies.

Indocyanine Green-Nexturastat A-PLGA Nanoparticles Combine Photothermal and Epigenetic Therapy for Melanoma

“…To ensure our epigenetic therapy with NextA elicited only immunomodulation in combination with PTT, we performed subsequent studies with free NextA at 5 µM to minimize any cytotoxicity effects in our combination. An additional rationale for selecting this dose is that previously published studies utilized 5 µM NextA for immunomodulatory effects in melanoma cells [24,25]. Overall, these results demonstrate that the INAPs heat in a concentration-and laser power-dependent manner and can be used for a single administration of PTT, after which they exhibit negligible PTT properties.…”

“…Group 6 received NextA-PLGA boosters to maintain sufficient concentrations of NextA within the tumor microenvironment (TME) to complement the effects of PTT. Additionally, the dosing for free NextA in group 2 was based on the optimal doses previously determined to elicit antitumor activity [23][24][25]. The average final tumor temperature achieved during PTT measured by the thermal imaging camera was ~95°C and the corresponding thermal doses were…”

“…In preclinical studies with melanoma murine models, HDAC6 inhibitors exhibited antitumor activity and immune marker modulation to increase melanoma immunogenicity through major histocompatibility complex Class I (MHC-I) and tumor antigen expression on melanoma cells [23][24][25]. Other groups have also demonstrated that HDAC6 promotes melanoma proliferation and migration, suggesting its potential as a therapeutic target to control tumor growth [26,27].…”

Immunological Effects of Epigenetic Modifiers

“…This durable and stable upregulation is facilitated by the histone acetylation of the PD-L1 and PD-L2 genes and was observed in vitro and in vivo in different solid tumor models [85,86]. Tumor-bearing mice receiving class I HDACi combined with PD-1 blockade exhibited a significant reduction in tumor growth and a better overall survival compared to mice receiving single agents [85][86][87][88] (Table 5). In vivo: Murine renal cell carcinoma (RENCA) luciferase-expressing cells implanted in BALB/c mice.…”

“…SM1 mouse melanoma cells. [88] Ricolinostat + bromodomain inhibitor JQ1 NSCLC Enhanced activation of tumor infiltrating CD8 T cells and secretion of effector cytokine IFNγ, Increased CD8/Treg ratio.…”