Abstract
Conventional inhibitors of immune checkpoints such as anti-programmed death-1 and its ligand (anti-PD-1/PD-L1) and anti-cytotoxic T lymphocyte-associated protein 4 (anti-CTLA4) have revolutionized therapeutic approaches to cancer, establishing immunotherapy as the standard of care for many cancers. A significant number of cancers, however, remain refractory to the inhibition of these immune checkpoints, leading to the search for alternative immune checkpoints that are more relevant to those diseases. Tumor-associated macrophage (TAM)-mediated efferocytosis is an increasingly appreciated immune checkpoint with a profound impact on the phenotype of the tumor microenvironment (TME). TAMs perform their efferocytic function through the receptor MerTK, and MerTK activity correlates with tumor progression. To combat efferocytosis in the TME, we developed poly[[2-(diisopropylamino)ethyl methacrylate]-b-poly(methacrylamidomannose)] nanoparticles (PMAM NPs) capable of encapsulating and preferentially delivering UNC2025 (a MerTK inhibitor) to TAMs. The NPs had suitable physicochemical properties, such as a size of 130 nm and a neutral surface charge. The PMAM NPs encapsulated hydrophobic cargo and released them in a pH-dependent manner, showing suitability for cytosolic delivery. Moreover, the PMAM NPs showed 12-fold greater macrophage internalization than traditional PEGMA NPs. Macrophage internalization was shown to be dependent on the mannose receptor CD206, as the blockade of CD206 led to a significant decrease in PMAM NP internalization. Furthermore, PMAM NPs had a lower internalization than PEGMA NPs in 4T1 cancer cells that do not express CD206, further confirming macrophage selectivity. In vivo biodistribution studies showed the PMAM NPs were capable of internalization by TAMs in the TME. Lastly, UNC2025-PMAM NPs significantly reduced tumor volume compared to free UNC2025, showing greater therapeutic efficacy in a model of triple-negative breast cancer. These glycopolymer-based, efferocytosis-blocking NPs have promise both as a class of standalone cancer immunotherapy and as an adjuvant to improve response rates to checkpoint immunotherapy.