The short-term fixation of radiopharmaceuticals in bone

The relationship between local blood flow and bone clearance of 18 F, 32 P-P (orthophosphate), 32 P-PPi (pyrophosphate), 45 Ca, 85 Sr, 99mTc- and 113 Sn-EHDP was evaluated in the non-adult rat. During one single blood passage nearly 100% of 18 F was extracted in the femoral diaphysis, the extraction efficiency for the other radiopharmaceuticals was about 40%. Blood flow and bone clearance was higher and the extraction efficiency lower in the distal femoral epiphysis including the growth plate. The short-term bone uptake of 32 P-P was lowered considerably by the simultaneous application of inactive orthophosphate, whereas the bone uptake of 32 P-PPi was not suppressed. It is suggested that the rapid deposition of radiopharmaceuticals in bone is not caused by an ionic exchange process on crystal surfaces of already existing mineral but is due to an ionic exchange during the precipitation of calcium phosphate in the calcifying bone matrix. Only 18 F is exchanged with hydroxyl groups of already precipitated hydroxyapatite, whereas the calcium and phosphate ions in the precipitated hydroxyapatite are not further accessible to an exchange process.