Purpose: Diethylenetriamine-pentaacetic acid (DTPA)-coupled minigastrins are unsuitable for therapeutic application with the available β-emitting radiometals due to low complex stability. Low tumour-to-kidney ratio of the known radiopharmaceuticals is further limiting their potency. We used macrocyclic chelators for coupling to increase complex stability, modified the peptide sequence to enhance radiolytic stability and studied tumour-to-kidney ratio and metabolic stability using 111In-labelled derivatives. Methods: Gastrin derivatives with decreasing numbers of glutamic acids were synthesised using 111In as surrogate for therapeutic radiometals for in vitro and in vivo studies. Gastrin receptor affinities of the natIn-metallated compounds were determined by receptor autoradiography using 125I-CCK as radioligand. Internalisation was evaluated in AR4-2J cells. Enzymatic stability was determined by incubating the 111In-labelled peptides in human serum. Biodistribution was performed in AR4-2J-bearing Lewis rats. Results: IC50 values of the natIn-metallated gastrin derivatives vary between 1.2 and 4.8nmol/L for all methionine-containing derivatives. Replacement of methionine by norleucine, isoleucine, methionine-sulfoxide and methionine-sulfone resulted in significant decrease of receptor affinity (IC50 between 9.9 and 1,195nmol/L). All cholecystokinin receptor affinities were >100nmol/L. All 111In-labelled radiopeptides showed receptor-specific internalisation. Serum mean-life times varied between 2.0 and 72.6h, positively correlating with the number of Glu residues. All 111In-labelled macrocyclic chelator conjugates showed higher tumour-to-kidney ratios after 24h (0.37-0.99) compared to 111In-DTPA-minigastrin 0 (0.05). Tumour wash out between 4 and 24h was low. Imaging studies confirmed receptor-specific blocking of the tumour uptake. Conclusions: Reducing the number of glutamates increased tumour-to-kidney ratio but resulted in lower metabolic stability. The properties of the macrocyclic chelator-bearing derivatives make them potentially suitable for clinical purpose
