For application in positron emission tomography (PET), PrP9, a N,N',N''-trisubstituted triazacyclononane with methyl(2-carboxyethyl)phosphinic acid pendant arms, was developed as (68)Ga(3+) complexing agent. The synthesis is short and inexpensive. Ga(III) and Fe(III) complexes of PrP9 were characterized by single-crystal X-ray diffraction. Stepwise protonation constants and thermodynamic stabilities of metal complexes were determined by potentiometry. The Ga(III) complex possesses a high thermodynamic stability (log K([GaL])=26.24) and a high degree of kinetic inertness. (68)Ga labeling of PrP9 is possible at ambient temperature and in a wide pH range, also at pH values as low as 1. This means that for the first time, the neat eluate of a TiO(2)-based (68)Ge/(68)Ga generator (typically consisting of 0.1 M HCl) can be directly used for labeling purposes. The rate of (68)Ga activity incorporation at pH 3.3 and 20 degrees C is higher than for the established chelators DOTA and NOTA. Tris-amides of PrP9 with amino acid esters were synthesized to act as models for multimeric peptide conjugates. These conjugates exhibit radiolabeling properties similar to those of unsubstituted PrP9.

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