Binding studies of [18F]-fluoride and polyphosphonates radiolabelled with [111In], [99mTc], [153Sm], and [188Re] on bone compartments: a new model for the pre vivo evaluation of bone seekers?

Introduction. Although the first polyphosphonates were already introduced in the early 1970s, mechanisms involved in uptake still remain speculative. The present work aimed to establish a new method to rate the influence of various factors on the uptake and to evaluate new bone-seekers on these bone compartments. Methods. Radioactive-labelled diphosphonates and [18F]-fluoride were added to a vial containing hydroxyapatite (HA), collagen, or amorphous calcium phosphate (ACP) in 3 ml of Hanksʹ Balanced Salt Solution (HBSS). After incubation, these suspensions were filtered, the radioactivity was measured in the gamma-counter, and the percentage of irreversibly bound radioactivity was calculated. Results. Kinetic experiments revealed uptake increase over time for [99mTc]-MDP and [18F]-fluoride on various amounts of matrix. After 120 min, static studies on HA yielded: [99mTc]-EDTMP < [188Re]-/Re-EDTMP < [99mTc]-/11 μl Re-EDTMP < [99mTc]-/In-EDTMP < [99mTc]-/15 μl Re-EDTMP < nca [188Re]-EDTMP < [111In]-/Re-EDTMP < [111In]-EDTMP < [111In]-/In-EDTMP < [99mTc]-DPD < [99mTc]-/80 μl Re-EDTMP < [99mTc]-EDTMP “boiled” < [99mTc]-/150 μl Re-EDTMP < [153Sm]-EDTMP < [99mTc]-/11 μl Re-EDTMP “boiled” < [18F]-ions < [99mTc]-MDP. Collagen showed very low uptake. Reincubation experiments suggest that bone tracers are irreversibly bound. Conclusion. The presented method is rapid and feasible to examine the adsorption of radioactive-labelled substances on bone components. Correlations between our findings and published in vivo data support the application as a simple model.