Uptake of bone-seekers is solely associated with mineralisation! A study with 99mTc-MDP, 153Sm-EDTMP and 18F-fluoride on osteoblasts

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Abstract
Although polyphosphonates (PPs) were introduced as bone imaging agents in nuclear medicine in the early 1970s, the mechanisms involved in their uptake still remain unclear. Suggested mechanisms range from mineral adsorption with disputed binding to the organic phase, over incorporation into the mineralisation process to a combination of both mechanisms. Thus, our investigations aimed to: (1) evaluate adsorption parameters of 99mTc-MDP, 153Sm-EDTMP and 18F-fluoride on mineralising osteoblast cultures, (2) correlate the radiotracer binding measured in the cell cultures with binding values from our previously presented mineral model and (3) compare binding with cell number. Primary osteoblasts were obtained by sequential digestion of foetal mice calvariae. The cells were incubated with 0.3 μmol of radiolabelled PPs or 25 MBq 18F-fluoride for 120 min. Gamma signals from labelled samples were detected with a Millennium Hawkeye SPECT camera or with a dedicated Advance full-ring PET scanner and the binding percentages were calculated. From days 8 to 15 of culture, the percent binding of all evaluated tracers increased significantly, whereas the protein concentration showed insignificant changes. Additional comparisons of the binding values with our recently published pre-vivo model revealed remarkable agreement, suggesting solely bone-forming minerals to be responsible for radiotracer binding. This study provides evidence that binding of the evaluated radiotracers is not associated with osteoblast numbers but only with the concentration of bone-forming minerals. The presented correlations substantiate our recently presented pre-vivo model for the evaluation of bone-seekers: mechanisms associated with the uptake of bone-seekers are irreversible and mineral-associated processes.