CD64-Targeted Polymer-Drug Conjugates Exploit Cathepsin K-Dependent Payload Release for Selective Elimination of Immunosuppressive Macrophages

Selective depletion of immunosuppressive macrophages in the tumor microenvironment is a promising strategy in cancer therapy. CD64 is broadly expressed on myeloid cells, including both pro-inflammatory M1-like and immunosuppressive M2-like macrophages that resemble tumor-associated macrophages (TAMs), and thus represents an attractive entry receptor for targeted payload delivery. We developed HPMA-based CD64-targeted polymer-drug conjugates (CD64-TPDCs) that combine multivalent receptor engagement with enzyme-responsive payload release. These copolymers are decorated with the CD64-binding cyclic peptide cp33 and carry the cytotoxic payload mertansine (DM1) bound via cathepsin-cleavable peptide linkers. Multivalent cp33 presentation on the polymer markedly increased the apparent affinity for human CD64, resulting in subnanomolar binding and selective recognition of CD64-expressing cells, significantly improving the binding potency of monovalent cp33 peptide. In polarized M2-like human monocyte-derived macrophages (MDMs), we showed that cytotoxic Gly-Phe-Leu-Gly-DM1 CD64-TPDCs selectively induced apoptosis. In contrast, M1-like MDMs were largely spared despite expressing higher levels of CD64. In M2-like MDMs, CD64-TPDCs rapidly accumulated in lysosomes, whereas in M1-like cells, they remained largely confined to endosomes. To elucidate the basis of this selectivity, we profiled expression of cathepsins in polarized MDMs. We found that M2-like MDMs display substantially higher levels of cathepsin K, establishing a model in which cathepsin K is the major protease responsible for Gly-Phe-Leu-Gly linker cleavage and DM1 release in M2-like macrophages. These findings demonstrate that CD64-TPDCs can be engineered to exploit subset-specific trafficking and cathepsin K-dependent linker cleavage for the selective elimination of M2-like macrophages. This work provides a generalizable design principle for stimuli-responsive PDCs that may actively target immunosuppressive myeloid cells in tumors.