Direct labeling studies of octapeptides with rhenium-188

Abstract

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 2hours. 188Re-lanreotide gave better radiochemical yields than 188Re-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. 250μg), and the required ratio between mass of stannous chloride and peptide was also greater for lanreotide. Use of tartrate/phthalate buffer with 188Re-octreotide gave better radiochemical results and showed more labeling stability than the use of acetate/acetic acid buffer. The results of Cys challenge for 188Re-peptides revealed higher chelation strength for 188Re-octreotide than for 188Re-lanreotide for concentrations of cysteine below 1mM. For a concentration of cysteine of 100mM the rate of transchelation of 188Re to cysteine was the same for both peptides. The CD50 value for 188Re-octreotide was 10mM of cysteine and for 188Re-lanreotide, it was 55mM.188Re-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.