UC San Diego

Recognition and cellular uptake by macrophages and ensuing clearance by the mononuclear phagocyte system (MPS) stands as a significant biological barrier for nanoparticle (NP) therapeutics. Here, we report a readily deployable method to prepare peptide brush polymers (PBPs) by ROMP, which are used to examine the effects of charge, size, and composition on macrophage uptake using RAW 264.7 cells. In our long-standing interest of developing enzyme-responsive systems, the polymers were encoded with multiple copies of a peptide substrate (GPLGLAG) for proteolytic enzymes specific to diseased or inflamed tissues. The incorporation of a permanent cation (quaternary amine) into the peptide sequence further permitted water-solubility for homopolymers and facilitated formulation of discrete, uniform NPs formulated from block copolymers, which were also investigated. An enzyme-activatable cell uptake strategy is also reported by way of a zwitterionic-to-cationic charge switch using a model system.

Similarly, enzyme hydrolysis of N-terminus conjugated peptide brush polymers (NPBPs) was shown to promote the assembly of discrete, nanoscale cylindrical micelles, upon exposure of carboxylates on the polymer after proteolytic cleavage. In contrast, C-terminus conjugated PBPs (CPBPs), which are structural isomers of NPBPs and expose protonated amines when subjected to proteolysis, remain as fully dispersed polymers, as indicated by TEM. As an initial assessment of their behavior in vivo, intravenous (IV) and intratumoral (IT) injections of N- and CPBP analogues were performed in a metastatic and solid tumor model known to have elevated levels of matrix metalloproteases capable of cleaving peptide substrates on NPBP and CPBP. These systems incorporated a Gd-DOTA label for tracking as well as assessment of relaxivity properties by MRI.

In a separate application, peptide polymer amphiphiles (PPAs) were used to decorate liquid crystal (LC) microdroplets comprised of the nematic 5CB, in order to provide the basis for multi-length scale response systems. The PPAs were designed with biphenyl, mesogen-like side-chains to promote assembly at the LC microdroplet interfaces and peptidic moieties (GPLGLAG) capable of enzymatic processing by thermolysin. Enzymatic cleavage of the PPAs was shown to trigger changes in PPA-surfactant complexes formed at the interface of the LC, thus giving rise to an easily measured optical response. To improve the design of PPAs for triggering LC interfaces, a library of norbornyl mesogens was synthesized to study the nature of homopolymer and copolymer anchoring at the LC/aqueous interface. From these collective experiments, a set of design principles emerged that was then used to design an optimized PPA copolymer.