The aim of this work was to characterize the in vitro behavior of N4- and N3S-RGDS-derivative peptides labeled with 99mTc. Peptides AGGG-Abu-GRGDSPK-NH2 (F22) and C(acm)-GGG-Abu-GRGDSPK-NH2 (SMA1) were synthesized by solid phase. The stability of 99mTc-labeled peptides was assessed in a 30-fold molar excess of cysteine and in plasma. The affinity for plasma proteins was
also evaluated. Labeling yield was >95% for both peptides. 99mTc-F22 was not stable in presence of cysteine, but 63% of 99mTc remained chelated to SMA1 up to 24 hours. Both peptides showed low affinity to plasma proteins. N3S-RGDS-derivative peptide
(SMA1) showed more stable coordination binding with 99mTc and a higher stability in plasma with regard to N4-RGDS-derivative peptide (F22).