Advancing Bone-Targeted Drug Delivery: Leveraging Biological Factors and Nanoparticle Designs to Improve Therapeutic Efficacy

Designing targeted drug delivery systems to effectively treat bone diseases ranging from osteoporosis to nonunion bone defects remains a significant challenge. Previously, nanoparticles (NPs) self-assembled from diblock copolymers of poly(styrene-alt-maleic anhydride)-b-poly(styrene) (PSMA-b-PS) delivering a Wnt agonist were shown to effectively target bone and improve healing via the introduction of a peptide with high affinity to tartrate-resistant acid phosphatase (TRAP), an enzyme deposited by the osteoclasts during bone remodeling. Despite these promising results, the underlying biological factors governing targeting and subsequent drug delivery system (DDS) design parameters have not been examined to enable the rational design to improve bone selectivity. Therefore, this work investigated the effect of target ligand density, the treatment window after injury, specificity of TRAP binding peptide (TBP), the extent of TRAP deposition, and underlying genetic factors (e.g., mouse strain differences) on TBP-NP targeting. Data based on in vitro binding studies and in vivo biodistribution analyses using a murine femoral fracture model suggest that TBP-NP-TRAP interactions and TBP-NP bone accumulation were ligand-density-dependent; in vitro, TRAP affinity was correlated with ligand density up to the maximum of 200,000 TBP ligands/NP, while NPs with 80,000 TBP ligands showed 2-fold increase in fracture accumulation at day 21 post injury compared with that of untargeted or scrambled controls. While fracture accumulation exhibited similar trends when injected at day 3 compared to that at day 21 postfracture, there were no significant differences observed between TBP-functionalized and control NPs, possibly due to saturation of TRAP by NPs at day 3. Leveraging a calcium-depletion diet, TRAP deposition and TBP-NP bone accumulation were positively correlated, confirming that TRAP-TBP binding leads to TBP-NP bone accumulation in vivo. Furthermore, TBP-NP exhibited similar bone accumulation in both C57BL/6 and BALB/c mouse strains versus control NPs, suggesting the broad applicability of TBP-NP regardless of the underlying genetic differences. These studies provide insight into TBP-NP design, mechanism, and therapeutic windows, which inform NP design and treatment strategies for fractures and other bone-associated diseases that leverage TRAP, such as marrow-related hematologic diseases.