Uptake and biodistribution of technetium-99m-MD32P during rat tibial bone repair

The present study was carried out in order to test the hypothesis that intravenously injected Tc-MDP separates into its technetium and methylene diphosphonate components in the bone, and that the technetium is preferentially taken-up by the newly-formed osteoid, while the methylene diphosphonate is taken up by the forming mineral. Uptake of Tc-MDP was studied in a rat model of primary bone formation following tibial bone marrow ablation. Each of five radiopharmaceuticals (99mTCO4, 99mTc-MDP, Tc-MD32P, 99mTc-MD32P or MD32P) was injected and their uptake was followed in the whole bone as well as in the organic and inorganic phases of the bone. Irrespective of the radionuclides injected, 99mTc was always taken-up preferentially by the organic phase, while the 32P was preferentially taken-up by the inorganic phase. When 99mTcO4 was injected, it was not taken up by the bone at all. These results indicate that the increased incorporation of 99mTc, when administered as 99mTc-MDP during bone healing, reflects an enhancement in the formation of the organic matrix and not of the calcification process. The study also suggests that the 99mTc-MDP dissociates into its technetium and methylene diphosphonate moieties, which are then adsorbed onto the organic and inorganic phases respectively.