Calciprotein Particles (CPPs) Are Cleared Predominantly by Liver, Spleen, and Bone in a Rat Model of CKD

Background: Calciprotein particles (CPPs) provide an efficient mineral buffering system to prevent the complexation of phosphate and calcium in the circulation. However, in chronic kidney disease (CKD), the phosphate load exceeds the mineral buffering capacity, resulting in the formation of crystalline CPP2 particles. CPP2 have been associated with cardiovascular events and mortality. Moreover, CPP2 have been demonstrated to induce calcification in vitro. In this study, we examined the fate of CPP2 in a rat model of CKD. Methods: Calcification was induced in Sprague Dawley rats by a 5/6-nephrectomy (5/6-Nx) combined with a high phosphate diet. Control rats received a sham surgery and high phosphate diet. Twelve weeks after surgery kidney failure was significantly induced in 5/6-Nx rats as determined by enhanced creatinine and urea plasma levels and abnormal kidney histological architecture. Subsequently, radioactive and fluorescent (FITC)-labeled CPP2 ([89Zr]Zr-CPP2-FITC) were injected i.v. to determine clearance invivo. Results: Using positron emission tomography scans and radioactive biodistribution measurements, it was demonstrated that [89Zr]Zr-CPP2-FITC are mainly present in the liver, spleen, and bones in both 5/6-Nx and sham rats. Immunohistochemistry showed that [89Zr]Zr-CPP2-FITC are predominantly taken up by Kupffer cells and macrophages. However, [89Zr]Zr-CPP2-FITC could also be detected in hepatocytes. In the different parts of the aorta and in the blood, low values of [89Zr]Zr-CPP2-FITC were detectable, independent of the presence of calcification. Conclusion: CPP2 are cleared rapidly from the circulation by the liver, bones, and spleen in a rat model of CKD. In the liver, Kupffer cells, macrophages, and hepatocytes contribute to CPP2 clearance. Our results suggest that targeting liver CPP clearance may reduce the burden of crystalline CPP in the development of vascular calcification.