Influence of the phosphate balance on the activity distribution of 99mTc-hydroxy-methylene diphosphonate. Experimental studies in the mouse

• Objective: The purpose was to determine whether changes in the phosphate balance have an influence on the distribution of bone-seeking radiopharmaceuticals. Material and Methods: The biodistribution of 99mTc-HDP in mice, intravenously administered under varying conditions, was assessed by removing different organs and estimating their activity in a scintillation counter. Some experiments were also performed with 99mTc-MDP and 99mTc-DPD. Results: After 1 h and 18 h on phosphate-enriched 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 phosphate 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 phosphate-enriched water. Changing the phosphate balance had less obvious effects on the distribution of 99mTc-MDP and 99mTc-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 possible that changes in the phosphate balance, induced by pharmaceuticals or by dietary changes, may affect the image quality at bone scintigraphy.

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