Technetium-99m direct radiolabeling of lanreotide: a somatostatin analog

Lanreotide, a synthetic octapeptide analog of a native hormone somatostatin, was labeled with a commonly available, inexpensive radionuclide 99mTc. Labeling was accomplished by reduction of the cysteine bridge, which provided sulfhydryl groups for chelation with 99mTc. Stannous chloride was used as reducing agent, while tartrate acted as transchelating agent. Lanreotide (100 microg), stannous chloride dihydrate (100 microg) and tartaric acid (64 microg) were dissolved in acetate/acetic acid buffer (pH 2.8). After overnight (approximately 18 h) incubation, approximately 444 MBq (12 mCi) 99mTc was added and kept in boiling water for 30 min. More than 97% labeling efficiency was confirmed by RP-HPLC, ITLC-SG and C18 cartridge analysis. Radiolabeling results in one major peak when analyzed by reverse-phase HPLC. The stability of the 99mTc-peptide bond was evaluated by cysteine challenge studies.