Tailored Gallium(III) Chelator NOPO: Synthesis, Characterization, Bioconjugation, and Application in Preclinical Ga-68-PET Imaging

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• 作者：Jakub 艩ime膷ek ; Ond艡ej Zemek ; Petr Hermann ; Johannes Notni ; Hans-J眉rgen Wester
• 刊名：Molecular Pharmaceutics
• 出版年：2014
• 出版时间：November 3, 2014
• 年：2014
• 卷：11
• 期：11
• 页码：3893-3903
• 全文大小：542K
• ISSN：1543-8392

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 Ga^III 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 K_a), determined by potentiometry, are 11.96, 5.22, 3.77, and 1.54; the stability constant of the Ga(III) complex is log K_GaL = 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