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

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.

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.

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.

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.