18F‐Labeling of Peptides by means of an Organosilicon‐Based Fluoride Acceptor

Abstract

Fluorine-18 is among the most commonly used radionuclides for positron emission tomography (PET). This non-invasive imaging technique is capable of providing in vivo information about the distribution of radiolabeled biomolecules by 1808 coincidence detection of two simultaneously emitted photons from positron–electron annihilation. Although a number of different radiotracers have been successfully employed in PET, only a few, such as 2-[F]fluoro-2-deoxy-d-glucose (FDG) and [F]fluorodopa, have gained widespread application in nuclear medicine. The reason for this is that the regioselective introduction of F into tracer molecules is often non-specific and radiochemical yields (RCY) of the Flabelled product are low. The introduction of F into tracer molecules requires high temperatures and often leads to undesired by-products. The research over the last decade clearly indicates that the success of PET in nuclear medicine justifies the intense search for versatile labeling formulations for the syntheses of F-radiopharmaceuticals. Especially the development of rational F-labelling strategies for peptides, until now characterized by multistep procedures, is considered to be one of the most important tasks. As an alternative to conventional F-labelling chemistry, the use of [F]fluorosilanes as labeling synthons was first proposed by Rosenthal et al. who treated chlorotrimethylsilane with n.c.a. (no carrier added) F in aqueous acetonitrile isolating the corresponding [F]fluorosilane in 65% yield. A preliminary in vivo evaluation revealed fast hydrolysis of the compound accompanied by high radioactivity (F) uptake by the bone making it unsuitable as a labeling synthon. Another approach is based on the work by Pilcher et al. who fluorinated organosilanoles with nonradioactive HF in high yields. An analogous labeling strategy was proposed in a symposium abstract. However, so far no labeling experiments using aqueous F /[F]HF solutions have been reported. Most recently Ting et al. used organotriethoxysilanes as labeling precursors for the synthesis of [F]fluorosilanes but no practical application for the synthesis of potential radiopharmaceuticals has been demonstrated. Herein we report the syntheses of substituted [F]organofluorosilanes using organochlorosilanes as labeling precursors and their in vitro and in vivo stability. As an alternative labeling approach we also describe the F–F isotopic exchange using [F]di-tert-butylphenyl fluorosilane as a highly efficient silicon-based fluoride acceptor (SiFA compound). As proof of applicability we transferred the SiFA approach to the development of a simple and practical formulation for the synthesis of a F-labelled SiFA derivatized Tyr-octreotate, a peptide used in oncology for the visualization of neuro-endocrine tumors. We synthesized three [F]organofluorosilanes, namely [F]fluorotriphenylsilane (1), [F]fluoro-tert-butyldiphenylsilane (2), and [F]fluorodi-tert-butylphenylsilane (3), and evaluated their in vitro stability in human serum as well as their in vivo stability in rats, studied by animal-PET. These data are essential for finding the most suitable compound and for evaluating the labeling concept. The reaction in acetonitrile of the triorganochlorosilanes (5–11.8 mmolmL ) Ph3SiCl, tBuPh2SiCl, and tBu2PhSiCl, with the azeotropically dried complex F /Kryptofix2.2.2./ K at room temperature provided almost quantitatively the corresponding [F]triorganofluorosilanes 1–3 (Figure 1), as demonstrated by means of radio-HPLC. Their identities were confirmed by coelution of the radioactive probes spiked with the related nonradioactive F-analogues. The specific activity of 1, 2, and 3 was determined using UV-calibration curves and was in the range 1500–1700 GBqmmol . To investigate the applicability of the F-labelled compounds for the development of Si–F containing radiopharmaceuticals, their in vitro stability in human serum was investigated (Figure 1). In agreement with previously published data, the [F]triphenylfluorosilane 1 was found to be stable for 4 h in neutral water (data not shown) but unstable at pH 7.4–7.6 in human serum. In contrast, the tert[*] Dr. R. Schirrmacher, Dr. E. Schirrmacher, J. Tillmanns, Dr. T. Siessmeier, Dipl.-Ing. H. G. Buchholz, Prof. Dr. P. Bartenstein Klinik und Poliklinik f1r Nuklearmedizin Johannes Gutenberg Universit6t Mainz 55131 Mainz (Germany) Fax: (+49)6131-172386 E-mail: schirrmacher@klinik.nuklearmedizin.uni-mainz.de Dipl.-Chem. G. BradtmBller, Prof. Dr. K. Jurkschat Lehrstuhl f1r Anorganische Chemie Universit6t Dortmund 44221 Dortmund (Germany) Fax: (+49)231-755-5048 E-mail: klaus.jurkschat@uni-dortmund.de