Comparison of the Stability of Technetium-Labeled Peptides to Challenge with Cysteine

We have labeled a series of short peptides with technetium-99m either by direct labeling at pH 11 or by exchange from [99mTc]technetium glucoheptonate. Typical labeling yields obtained were as follows: N-acetyl-Gly-Cys(S-Acm)-Gly-Cys(S-Acm)-Gly-NH2 (Acm = acetamidomethyl) 99%, S-benzoyl-mercaptoacetyltriglycine (S-Benzoyl-MAG3) 95%, mercaptoacetyldiglycine-NH2 (MAG2-NH2) 94%, MAG3 92%, N-acetyl-Aib-Aib-Cys-NH2 (Aib = aminoisobutyric acid) 91%, Gly-Gly-Gly-Gly 90%, N-acetyl-Gly-Gly-Cys-Gly 87%, cyclo-1,4-(Gly-Gly-Gly-Gln) 40%, S-methyl-MAG2-NH2 0%. In the absence of cysteine, all of the labeled peptides were quite stable in solution, with at least 80−90% of the labeled peptide remaining at 24 h. The order of stability of the labeled peptides to challenge with cysteine was found to be MAG3 > S-benzoyl-MAG3 > N-acetyl-Gly-Cys(S-Acm)-Gly-Cys(S-Acm)-Gly-NH2 >N-acetyl-Aib-Aib-Cys-NH2 > MAG2-NH2 > N-acetyl-Gly-Gly-Cys-Gly > cyclo-1,4-(Gly-Gly-Gly-Gln) > Gly-Gly-Gly-Gly. The peptides without a sulfur donor were least stable to challenge with cysteine.