Abstract Tumor-associated macrophages (TAMs) represent a potentially promising therapeutic target in cancer because they have been shown to facilitate tumor growth, invasiveness, and metastasis. However, methods to specifically target therapies to TAMs are lacking. To address this problem, we designed and synthesized mannosylated micellar nanoparticles (ManNPs), composed of tri-block co-polymers. The three polymer blocks include (1) an azido-displaying block for functionalization with biomolecules via azide-alkyne ‘click’ chemistry, (2) a cationic block for the condensation of polyanions such as siRNA, and (3) a pH-responsive terpolymer block that facilitates endosomal disruption. This terpolymer is hydrophobic at pH 7.4, allowing these polymers to self-assemble into 25 nm micellar nanoparticles under physiologic conditions. However, they become protonated at lower pH ranges representative of endosomal compartments (5.8 - 6.2), leading to disassembly of the nanoparticles, and increased ability to disrupt endosomal membranes and enable cytoplasmic delivery. This pH-dependent behavior has been validated using a red blood cell hemolysis assay. This environmentally-responsive behavior facilitates improved cytoplasmic delivery of siRNA, access to the intracellular silencing machinery, and consequently, knockdown of target gene expression. Further, we demonstrate that mannosylation of these nanoparticles via ‘click’ chemistry significantly enhances their ability to deliver siRNA into murine bone marrow-derived primary macrophages (BMDMs), relative to control, untargeted nanoparticles. Targeted nanoparticle uptake is mediated specifically through the macrophage mannose receptor (CD206), as validated through competition experiments with free mannose, or by pre-incubating BMDMs with lipopolysaccharide to downregulate CD206 expression. This is particularly important for cancer applications because CD206 is upregulated in tumor-suppressed and non-activated macrophages, enabling more specific targeting of TAMs versus healthy macrophages in other tissues. The ManNPs described here present new opportunities to target TAMs in various cancers, providing an enabling technology for the modification of the immunosuppressive tumor environment by targeting TAM activity. Studies are pending to demonstrate this behavior in an in vivo murine model of metastatic breast cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2894. doi:1538-7445.AM2012-2894