The in vitro adsorption on hydroxyapatite (HAP) of 99mTc-hydroxymethylene diphosphonate (99mTc-HMDP) and 99mTc-methylene diphosphonate (99mTc-MDP), which are used as bone imaging agents, was studied. The adsorption of both diphosphonates was carried out in the concentration of 1.2×10-5 and 4.7×10-5 mol/dm3. Radioactivities of the filtrate and precipitate taken from the adsorptive solution were measured, and the filtrate was analyzed with radio-thinlayer chromatography. The 99mTc compounds forming polymers, as previously reported, were remained partially on the precipitate paper at the initial stage of adsorption. The amount of 99mTc-HMDP and MDP adsorbed for 1 h was 84% and 77%, respectively, the former adsorbed on HAP to a greater degree than did the latter. The compounds adsorbed on HAP are in a state of adsorption equilibrium with those in the solutions even after adsorbing for 3 h, and seem to be stable.
