Optimizing Ligand Valency to Maximize Tendon Accumulation of Peptide-Targeted Nanoparticles

In many tissues, including musculoskeletal tissues such as tendon, systemic delivery typically results in poor targeting of free drugs. Hence, we previously developed a targeted drug delivery nanoparticle (NP) system for tendon healing, leveraging a tartrate resistant acid phosphatase (TRAP) binding peptide (TBP) ligand. The greatest tendon targeting was observed with NPs functionalized with 30 000 TBP ligands per NP at day 7 during the proliferative healing phase, relative to the inflammatory (day 3) and early remodeling (day 14) phases of healing. Nevertheless, TRAP activity varies throughout healing and, therefore, may offer an opportunity for optimizing temporal therapeutic targeting through multivalent interactions. Hence, in this study, we hypothesized that the ligand density (9000-55,000 TBPs per NP) can optimize tendon accumulation on the basis of variable TRAP levels. The multivalent nanoparticles were loaded with three different fluorophores. In vitro, the ligand density and fluorophore had no effect on the physicochemical properties of the NPs, including size, charge, polydispersity index, or dye loading efficiency; however, the TRAP binding affinity correlated positively with the ligand density. In vivo, the ligand density correlated positively with NP homing and retention in the tendon, establishing opportunities to leverage ligand density for tendon targeting across the tendon healing cascade, during aging, and in other tendon pathologies, including tendinopathies.