INFLUENCE OF THE PHOSPHATE BALANCE ON THE ACTIVITY DISTRIBUTION OF TC-99M-HYDROXYMETHYLENE DIPHOSPHONATE - EXPERIMENTAL STUDIES IN THE MOUSE

Objective: The purpose was to determine whether changes in the phospha te balance have an influence on the distribution of bone-seeking radio pharmaceuticals.Material and Methods: The biodistribution of Tc-99m-HD P in mice, intravenously administered under varying conditions, was as sessed by removing different organs and estimating their activity in a scintillation counter. Some experiments were also performed with Tc-9 9m-MDP and Tc-99m-DPD. Results: After 1 h and 18 h on phosphate-enrich ed drinking water, the mice showed a strongly increased uptake in all organs/tissues representing background activity and a decrease in the bone uptake. This pattern changed with time. After 6-8 days of phospha te load, we saw a more favourable distribution with a reduction of the background and whole-body activity. Administration of hPTH 1-34 gave rise to an activity distribution similar to that after 6-8 days on pho sphate-enriched water. Changing the phosphate balance had less obvious effects on the distribution of Tc-99m-MDP and Tc-99m-DPD. Conclusion: The activity distribution of bone-seeking radiopharmaceuticals in the mouse is affected by the phosphate balance. The mechanism behind this finding is unknown but it may be partially mediated by PTH. It is pos sible that changes in the phosphate balance, induced by pharmaceutical s or by dietary changes, may affect the image quality at bone scintigr aphy.