Direct labeling studies of octapeptides with rhenium-188

The somatostatin octapeptide lanreotide and octreotide were labeled with rhenium-188 by direct method. the optimization of the labeling, the in vitro stability of the complexes and the biodistribution were evaluated. Labeling efficiency was generally greater than 95%. the best time for reaction was 2 hours. Re-188-lanreotide gave better radiochemical yields than Re-188-octreotide, and it was stable for a longer time (24 vs. 4 hours), but the mass of lanreotide had to be larger than that for octreotide (350 vs. 250 mug), and the required ratio between mass of stannous chloride and peptide was also greater for lanreotide. Use of tartrate/phthalate buffer with Re-188-octreotide gave better radiochemical results and showed more labeling stability than the use of acetate/acetic acid buffer. the results of Cys challenge for Re-188-peptides revealed higher chelation strength for Re-188-octreotide than for Re-188-lanreotide for concentrations of cysteine below 1 mM. for a concentration of cysteine of 100 mM the rate of transchelation of Re-188 to cysteine was the same for both peptides. the CD50 value for Re-188-octreotide was 10 mM of cysteine and for Re-188-lanreotide, it was 55 mM.Re-188-labeled peptides were distributed in the gastrointestinal tract or in the kidneys and rapid blood clearance was observed in the first hour. Results indicated that the products can be obtained with high radiochemical purity, appropriate for in vivo studies to assess their efficacy in radioimmunotherapy.CNEN, IPEN, Radiopharm Ctr, São Paulo, BrazilUniversidade Federal de São Paulo, Sch Med, Div Radiol, São Paulo, BrazilUniversidade Federal de São Paulo, Sch Med, Div Radiol, São Paulo, BrazilWeb of Scienc