Studies on diphosphonate kinetics. Part I: Evaluation of plasma elimination curves during 24 h.

To improve the understanding of diphosphonate affinity to metabolically active bone, the underlying diphosphonate kinetics have been evaluated and compared to Cr-EDTA kinetics. MDP binds to plasma proteins, varying from 25% initially to approximately 70% after 24 h. The renal clearance of diphosphonate is found to be equal to Cr-EDTA clearance. Using simultaneous bolus injection of 99Tc-MDP and 51Cr-EDTA, it has been possible to obtain a coarse estimate of bone uptake of MDP. This uptake is found to correlate well with s-alkaline phosphatase, but since MDP binding to bone is reversible, the plasma elimination curve is not monoexponential. Therefore it has not been possible to describe the uptake of MDP in bone mathematically.