(173f) The Hydroxyapatite Affinity and Binding Kinetics of Peptides Modified with Bisphosphonates, Poly(Aspartic Acid), and Poly(Glutamic Acid)

In an effort to specifically target osteogenic (bone-forming) drugs to bone tissue, the conjugation of peptides to bone-binding moieties was examined. Model peptides were covalently linked to pamidronate (an FDA approved, nitrogen-containing bisphosphonate), poly(aspartic acid) (Asp₈, eight consecutive aspartic acid residues), or poly(glutamic acid) (Glu₈, eight consecutive glutamic acid residues). These moieties were investigated due to their documented attraction to calcium ions in hydroxyapatite (HA), the mineral component of bone. To study the effects of peptide molecular weight on the bone-binding capabilities of each type of conjugate, peptides of variable size were synthesized and modified with the respective HA-targeting agents. Model peptides followed the structure of Y-[GNAEGNAR]_m-X, using the single letter amino acid code and where m = 0.5, 1, 2, and 4 and X represents the bone-binding moiety (pamidronate, Asp₈, Glu₈, or blank for the control group). The [GNAEGNAR] repeat unit was selected to balance hydrophilic and hydrophobic properties of the peptide, as well as ionic charges present on the molecule. Each peptide is dissolved in a phosphate buffered saline solution saturated with HA and shaken at physiological conditions. The concentration of peptide is determined at each time point using ultra-violet absorption on high performance liquid chromatography. Peptide concentrations are measured at specific time points during the first 24 hours to determine the HA binding kinetics for each type of modified peptide. The ability of pamidronate, Asp₈, and Glu₈ to significantly enhance the binding of the conjugated peptides to HA in vitro is promising for the future of targeted osteogenic drug delivery in bone tissue engineering and regenerative medicine.