Synthesis, characterization and in vitro evaluation of a bone targeting delivery system for salmon Calcitonin

Synthetic salmon Calcitonin (sCT) is currently used to treat and manage conditions associated with low bone mass, and elicits its antiresorptive effect by acting upon Calcitonin receptors (CTRs) located on bone-resorbing osteoclast cells. However, CTRs are also widely distributed in many non-skeletal tissues (such as kidney, brain, and lung), and the competitive uptake of available sCT amongst such CTRs likely reduces sCT availability for bone resident osteoclast cells, particularly if the drug is administered systemically and not specifically targeted to bone. Hence, the objective of this study was to synthesize and characterize a bisphosphonate (BP)-mediated bone targeting delivery system for sCT and to determine whether the bioactivity of sCT was retained after BP conjugation. BP–sCT conjugates were synthesized by initially reacting sCT with sulfosuccinimidyl-4-[N-maleimidomethyl]cyclohexane-1-carboxylate (sulfo-SMCC) in dimethyl formamide in the presence of triethylamine (TEA) at room temperature. Thiolated (Thiol)-BP was then reacted with the sCT–sulfo-SMCC conjugates to generate sCT–BP conjugates, which were purified by dialysis and assayed using the micro-BCA protein assay. Non-BP containing control sCT–Cysteine conjugates were also synthesized using the same procedure. Reactions were monitored and characterized using matrix-assisted laser desorption ionization time-of-flight mass spectrometer (MALDI-TOF) analysis and Tris–Tricine SDS-PAGE. Conjugates were evaluated for in vitro bone mineral affinity using a hydroxyapatite binding test, for bone mineral specificity using different calcium salt binding affinity assays, and for continued sCT bioactivity after conjugation using an intracellular cAMP stimulation in human T47D breast cancer cells. Our results confirmed that BP-conjugated sCT exhibited significantly greater affinity and specificity for bone mineral over unmodified sCT, and that sCT–BP conjugates retained strong CT bioactivity after conjugation. Our conjugation strategy holds the promise of facilitating the delivery of sCT preferentially to skeletal bony tissues, thereby increasing its local concentration to bone surfaces. This peptide hormone–bisphosphonate drug system represents a new class of antiresorptive drug that has not previously been attempted, nor has a bone targeting formulation of sCT been reported.