A monoreactive DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) prochelator (4,7,10-tricarboxymethyl-tert-butyl ester 1,4,7,10-tetraazacyclododecane-1-acetate) was synthesised which is useful in solidphase and solution-phase peptide synthesis; it was coupled to the somatostatin analogue Tyr -octreotide. The new compound delineates SRIF-receptor positive tumours very favourably and shows distinctly lower uptake by the kidneys. Evidently, the differences in the coordination chemistry of the metals causes the differences in the biological behaviour. Indeed, a crystallographic study of the Ga 3 and Y 3 complexes of the model peptide DOTA-d-PheNH 2 showed differences in the geometry of the complexes. The gallium complex is hexacoordinated with pseudooctahedral cis geometry and a folded macrocyclic unit. The equatorial plane is formed by two transannular nitrogens of the cyclen ring and two oxygens of the corresponding carboxylate groups. The two axial positions are formed by the two remaining ring nitrogen atoms. The amide carboxy oxygen is not bound to the metal and one carboxylate group is free and most likely contributes to the favourable handling of the radiopeptide by the kidneys. In contrast, the structure of Y-DOTA-dPheNH 2 has eight-fold coordination, and includes the amide carboxy oxygen. The geometry is a compact and somewhat distorted square-antiprism with two almost perfect planes (N4 and O4) with a maximum deviation of 0.025 . The dihedral angle between the two planes is only 0.368.
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