Radio-HPLC analysis of aliquots of serum, taken at 0 min, 30 min and 2 h incubation times, indicated that a small fraction of 201 Tl(I) was released from K222-PSMA immediately upon addition of the serum to [ 201 Tl(K222-PSMA)] + (Supporting Information Figure ). However, after that, no significant changes were observed, indicating that [ 201 Tl]Tl(I) K222 chelates are relatively stable in human serum over 2 h.