Bisphosphonate-Modified Serum Albumin as an Effective Inhibitor of Calciprotein Particle Maturation with Long Plasma Retention Time

Purpose: Inhibitors of the calciprotein particle (CPP) maturation have been developed so far as therapeutics for vascular calcification. However, the short blood half-life limited their application. Here we designed the conjugate of a CPP maturation inhibitor, alendronate (ALN) with human serum albumin (HSA) to utilize the long blood retention nature of HSA. Method: The HSA-ALN conjugates with different modification numbers of ALN per HSA were prepared. The inhibitory effect of the conjugates on the CPP maturation was evaluated using a reported cell-free system as a time of conversion from CPPI to CPPII. The CPP binding of fluorescent-labeled conjugates was carried out using flow cytometry. The plasma half-life of the conjugates was evaluated in mice after intravenous injection. Results: We found that the HSA-ALN conjugates bound to CPP via the specific interaction between ALN and calcium phosphate of CPP. As a result, the conjugates showed a much higher inhibition effect of CPP maturation than those of free ALN and intact HSA. A modification ratio of two ALN molecules per HSA was found to be significant enough to inhibit the CPP maturation. Such a small modification ratio minimized the impact on the long blood retention nature of HSA. Conclusion: This study showed that HSA-ALN conjugates not only have superior CPP growth inhibition effects but also possess significantly higher blood half-lives compared to those of free ALN. These findings suggest that HSA-ALN conjugates are promising therapeutics for vascular calcification associated with the deposition of CPP.