The fate of four 75Se-labeled amino acids: studies of some unsuccessful pancreas-scanning agents.

The Radiochemical Centre developed a number of alpha-amino acids labeled with 75Se in the hope of finding a pancreas-scanning agent superior to L-selenomethionine. Six were submitted for clinical trial and we tested four of these: D-selenomethionine and three derivatives of selenocysteine. Dynamic scintillation camera studies showed that liver uptake was substantial in all cases but that the time course of accumulation and removal differed between agents. Plasma radioactivity curves demonstrated differences in initial removal rates. Two of the agents showed evidence of incorporation of activity into protein and release into the circulation just as with L-selenomethionine. Whole-body counter measurements showed that the retention of activity from the agents exhibiting early plasma protein incorporation was similar to that of L-selenomethionine. Much of the activity from the other two agents was excreted faster but small proportion was retained for a long time. Although the cummulated activity and absorbed dose were substantially less for at least two of the agents than for 75Se-L-selenomethionine, the lack of sufficient pancreatic concentration renders them inferior as scanning agents.