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The reproducibility and diagnostic value of local bone to soft tissue uptake ratio of 99mTc(Sn)-MDP as a bone tracer was examined in a prospective study in 35 patients who were under investigation and/or treatment for postmenopausal osteoporosis. The ratio of tracer uptake in the second lumbar vertebra (L2) and both femoral shafts was calculated from the number of counts in suitable regions of interest. Results obtained with settings and calculations in the routine practice were compared to the results obtained by revision of all raw data in one run by one person. The results were compared to the serum alkaline phosphatase activity (AP) and to local bone mineral mass as determined by dual photon absorptiometry (DPA). In 15 patients serial measurements during fluoride therapy were also compared to serum osteocalcin values and to bone histomorphometric data. The precision error of the calculation of uptake ratios from raw counts (including selection of region of interest) was 13.9% for the femoral shaft and 14.7% for L2. The mean difference between left and right femoral shaft in individuals was not significant and its variance was small P greater than 0.1). There was a weak but significant linear correlation between local uptake ratio in the spine and AP in the total material (r = 0.328 P less than 0.01). However, changes in local uptake ratio during therapy with fluoride in 15 patients were too small to be of any value and did not correlate with changes in alkaline phosphatase or osteocalcin or trabecular surface covered with osteoblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

The localization of 99mTc(Sn)-MDP in bone tissue was compared with 99mTc-MDP by means of microautoradiography of undecalcified bone sections. Sections of good histological quality were obtained by a rapid embedding method in methylmethacrylate. No differences were found in the localization of these radiopharmaceuticals in fetal rat calvariae after incubation in vitro or in rat femora after administration in vivo. In the incubation experiment, hydrolyzed 99mTc was formed. The uptake was high in areas of new bone formation. No uptake was seen in cells or in resorbing areas. In compact bone 99mTc(Sn)-MDP was predominantly taken up in the vicinity of blood vessels.

The HSA-binding of 99mTc incubated as 99mTc(113Sn)-MDP depends in vitro on the presence of competing anions normally present in plasma and on the MDP and Tc concentration, but is independent of the Sn concentration. The HSA-binding of 113Sn depends on the MDP and Sn concentration; not on the Tc concentration. 99mTc(Sn)-MDP binds to HSA as a unit. The binding of 99mTc(Sn)-MDP to plasma proteins in vivo is much lower as calculated from in vitro experiments because of competition between 99mTc(Sn)-MDP and endogenous anions for the same binding-sites.

In an incubation experiment the Human Serum Albumin (HSA) binding of 99mTc-MDP (electrolytically labeled) and 99mTc(Sn)-MDP is established. During the incubation some pertechnetate is formed and in the case of 99mTc(Sn)-MDP also some hydrolyzed 99mTc. The HSA binding of 99mTc-MDP is less than the HSA binding of 99mTc(Sn)-MDP as established with gel chromatography, TCA-precipitation, ammonium sulfate precipitation and ultrafiltration. TCA-precipitation seems to be an insufficient method for determining the protein binding of 99mTc(Sn)-MDP. The urinary excretion in rats shows only one 99mTc-compound in both cases. The bone seeking properties of the urine-excreted 99mTc-compound were confirmed in another rat.