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

Jakub Šimeček, Ondřej Zemek, Petr Hermann, Johannes Notni, und Hans-Jürgen Wester

03.11.2014 [Original Artikel

The bifunctional chelator NOPO (1,4,7-triazacyclononane-1,4-bis[methylene(hydroxyl-methyl)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 °C at pH ranging from 0.5 to 7, at NOPO concentrations of 30 μM and 10 μM, 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–NaI3–octreotide conjugate (68Ga–NOPO–NOC) showed high and fully blockable tumor uptake (13.9 ± 5% ID/g, 120 min p.i., compared to 0.9 ± 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.

A facile direct nucleophilic synthesis of O-(2-[18F]fluoroethyl)-l-tyrosine ([18F]FET) without HPLC purification

Olga Fedorova, Olga Kuznetsova, Maria Stepanova, Victor Maleev, Yuri Belokon, Hans-Juergen Wester, Raisa Krasikova

13.04.2014 [Orginal Artikel]

Due to favourable in vivo characteristics, its high specificity and the longer half-life of 18F (109.8 min) allowing for remote-site delivery, O-(2-[18F]fluoroethyl)-l-tyrosine ([18F]FET) has gained increased importance for molecular imaging of cerebral tumors. Consequently, the development of simple and efficient production strategies for [18F]FET could be an important step to further improve the cost-effective availability of [18F]FET in the clinical environment. In the present study [18F]FET was synthesized via direct nucleophilic synthesis using an earlier developed chiral precursor, the NiII complex of an alkylated (S)-tyrosine Schiff base, Ni-(S)-BPB-(S)-Tyr-OCH2CH2OTs. The purification method has been developed via solid phase extraction thereby omitting cumbersome HPLC purification. The suggested SPE purification using combination of reverse phase and strong cation exchange cartridges provided [18F]FET in high chemical, radiochemical and enantiomeric purity and 35 % radiochemical yield (decay-corrected, 45 min synthesis time). The method was successfully automated using a commercially available synthesis module, Scintomics Hotboxone. Based on the current results, the proposed production route appears to be well suited for transfer into an automated cassette-type radiosynthesizers without using HPLC.