Development of 99mTc labelled somatostatin analogues and evaluation of their radiochemical and biological behaviour

Conjugates of two somatostatin analogues, octreotide and RC-160, with HYNIC were synthesized, characterized and purified by reverse phase HPLC. Radiolabelling of the conjugates with 99mTc was achieved using tricine as co-ligand. High labelling efficiencies were obtained and 99mTc peptides with high radiochemical purity were found when analysed both by ITLC and HPLC. In vitro stability of 99mTc-peptides in human serum and towards cysteine challenge was determined by Cellogel electrophoresis and HPLC after ultrafiltration of serum solution through a 20 kDa cut off membrane. Biodistribution studies were performed in healthy mice at 5 and 30 minutes and 1, 2, 4 and 24 h post-injection. Urine and blood samples were collected at sacrifice time. Samples of urine and ultrafiltrate murine serum were analysed by electrophoresis and reverse phase HPLC in order to get some information about radiocompounds metabolism. Biological distribution of 99mTc octreotide was also assayed in mice pre-treated with an excess of unlabelled peptide. From our results we conclude that this labelling method led to stable 99mTc complexes both in vitro and in vivo when high specific activities (37-72 GBq/μmole) were used. Biodistribution studies of both 99mTc-peptides indicated a radioactivity distribution profile with significant differences especially in the liver uptake that is higher for 99mTc RC-160. However, a rapid blood clearance was obtained for both radiolabelled peptides, and the urine analysis indicated that 99mTc peptide is mostly excreted as the initial complex. Pre-treatment with unlabelled peptide did not affect renal excretion of 99mTOC but pancreas and intestine radioactive uptake was significantly lower, indicating saturation of somatostatin receptors and selective uptake. (author)