Tailored Gallium(III) Chelator NOPO: Synthesis, Characterization, Bioconjugation, and Application in Preclinical Ga-68-PET Imaging
文摘
The bifunctional chelator NOPO (1,4,7-triazacyclononane-1,4-bis[methylene(hydroxymethyl)phosphinic acid]-7-[methylene(2-carboxyethyl)phosphinic acid]) shows remarkably high GaIII complexation efficiency and comprises one carboxylic acid moiety which is not involved into metal ion coordination. An improved synthetic protocol affords NOPO with 45% overall yield. Stepwise protonation constants (log Ka), determined by potentiometry, are 11.96, 5.22, 3.77, and 1.54; the stability constant of the Ga(III) complex is log KGaL = 25.0. Within 5 min, 68Ga(III) incorporation by NOPO is virtually quantitative at room temperature between pH 3 and 4, and at 95聽掳C at pH ranging from 0.5 to 7, at NOPO concentrations of 30聽渭M and 10 渭M, respectively. During amide bond formation at the distant carboxylate using the HATU coupling reagent, an intramolecular phosphinic acid ester (phosphilactone) is formed, which is cleaved during 68Ga complexation or in acidic media, such as trifluoroacetic acid (TFA). Phosphilactone formation can also be suppressed by complexation of Zn2+ prior to conjugation, the resulting zinc-containing conjugates nevertheless being suitable for direct 68Ga-labeling. In AR42J (rat pancreatic carcinoma) xenografted CD-1 nude mice, 68Ga-labeled NOPO鈥揘aI3鈥搊ctreotide conjugate (68Ga鈥揘OPO鈥揘OC) showed high and fully blockable tumor uptake (13.9 卤 5% ID/g, 120 min p.i., compared to 0.9 卤 0.4% ID/g with 5 mg/kg of nonlabeled peptide). Uptake in other tissues was generally below 3% ID/g, except appearance of excretion-related activity accumulation in kidneys. NOPO-functionalized compounds tend to be more hydrophilic than the corresponding DOTA- and NODAGA-conjugates, thus promoting fast and extensive renal excretion of 68Ga-NOPO-radiopharmaceuticals. NOPO-functionalized peptides provide suitable pharmacokinetics in vivo and meet all requirements for efficient 68Ga-labeling even at room temperature in a kit-like manner.
Keywords:
gallium-68; radiopharmaceuticals; phosphinic acid; positron emission tomography; peptides