Background: One of the most invasive forms of breast cancer is the Triple-Negative Breast Cancer (TNBC). Despite the combination of chemotherapy and immunotherapy among other treatment modalities, patients show limited response mostly due to the highly immunosuppressive tumor microenvironment (TME). In addition, TNBC is almost three times as prevalent in Black women as it is in non-Hispanic White women and affects them more aggressively. One of the biological factors involved in this racial health disparity is the presence of the highly immunosuppressive (M2) tumor-associated macrophages (TAMs) in the TME. Agonists of the Stimulator of Interferon Genes (STING) receptor in TAMs have been reported to produce tumor regression in murine TNBC models by reverting the M2 phenotype of TAMs to an immunostimulating (M1) phenotype. The M1 phenotype induces cell-mediated cancer cell killing via the production of the chemokine CXCL10 which is also a positive prognostic factor for patient survival in TNBC. The whole-body distribution of macrophages and low TME bioavailability of STING agonists are barriers to this STING agonist-mediated M2-to M1 reversal strategy, and the selective and targeted delivery of STING agonists to TAMs in the TME can help circumvent these challenges. While lipoproteins are natural carriers and interact readily with macrophages, TAMs highly express the mannose receptor CD206. Moreover, reconstituted high-density lipoprotein nanoparticles (rHDL NPs) accumulate in the TME. In this study, we characterize a formulation of a STING agonist, DMXAA, encapsulated in mannose-decorated rHDL NPs.
Methods: For the targeting of TAMs, the mannose moiety was introduced in rHDL NPs via DSPE-PEG-mannose (DPM). Dynamic light scattering (DLS) and transmission electron micrograph (TEM) were used to estimate the diameter size of the particles. Fluorescence and absorbance measurements of DMXAA were used to assess the encapsulation of DMXAA in the particles. In vitro TAMs were generated with incubation of macrophages in tumor-conditioned media. Western blots and ELISA assays were used to quantify proteins and cytokines.
Results: The rHDL-DPM NPs displayed a slight increase in diameter size compared to the conventional rHDL NPs. A higher anisotropy was recorded with the DMXAA formulated with the rHDL-DPM NPs compared with the free DMXAA. The rHDL-DPM-DMXAA elicited higher CXCL10 levels in TAMs compared to the free DMXAA.
Conclusion: The increase in diameter size of the rHDL-DPM NPs suggests that the DPM is incorporated unto the nanoparticles, and the higher anisotropy of DMXAA indicates that the DMXAA is encapsulated in the nanoparticles. The higher expression of CXCL10 produced by the rHDL-DPM-DMXAA may elicit an enhanced infiltration of CD8T cells in vivo and future studies will assess the effect of the rHDL-DPM-DMXAA NPs in tumor regression in TNBC models. This novel formulation may be utilized in combination with existing TNBC immunotherapies and help address biological factors contributing to health disparities in TNBC.
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