PEG chains modulate electrostatic interactions between PAMAM and articular cartilage

Osteoarthritis is a debilitating disease of synovial joints affecting millions of people worldwide. Despite this, there are currently no disease modifying osteoarthritis drugs (DMOADs), mainly due to poor retention within the joint space. Previously, our group has shown that partially PEGylated cationic poly(amido amine) (PAMAM) dendrimers electrostatically bind to articular cartilage and improve therapy retention times, resulting in reduced severity of osteoarthritis symptoms. Here, we develop a greater understanding of how PEGylation influences dendrimer-cartilage interactions through systematic modulation of PEG chain length and accessible charged amines, defined as PAMAM primary amines freely accessible to the physiological environment and void of any interactions with PEG. Utilizing various models, we found that cartilage binding strength and binding kinetics increased with increasing accessible charged amines, while biocompatibility decreased, independent of PEG chain length. Conversely, decreasing accessible charged amines or increasing PEG chain length enhanced diffusion through cartilage explants. We found accessible charged amines controlled electrostatic binding strength while PEG chain length controlled reversibility of binding. When PEG-PAMAM conjugates were compared to other cationic molecules studied for their cartilage binding properties, all dendrimers tested exhibited significantly greater binding affinities and binding site densities but reduced Donnan partitioning coefficients. Finally, we found controlling binding strength and cartilage diffusion is critical to achieving extended retention within healthy joints in vivo. Future studies can use the enhanced mechanistic understanding of dendrimer-cartilage interactions established here to optimize PEG-PAMAM conjugates as a cartilage drug delivery platform.