Nucleophilic ring-opening of activated aziridines: A one-step method for labeling biomolecules with fluorine-18
Graphical abstract
A one-step radiolabeling of model compounds and biomolecules with fluorine-18 has been achieved via nucleophilic ring-opening of activated aziridines (see Scheme). High to moderate yields of 18F-incorporation were achieved under mild labeling conditions. One-step 18F-labeling of model compounds and biomolecules via nucleophilic ring-opening of activated aziridines.
Introduction
Molecular imaging has the potential to detect disease progression or therapeutic effectiveness earlier than most conventional methods in the fields of oncology, neurology, and cardiology. Of the several promising molecular imaging technologies, PET is of particular interest for diagnosis and drug development because of its high sensitivity and ability to provide quantitative and kinetic data [1], [2], [3], [4], [5], [6], [7]. Positron emitting isotopes such as carbon-11, nitrogen-13, and oxygen-15 can replace their non-radioactive counterparts in target compounds to produce tracers for PET imaging that are chemically identical to the original molecules and have the same pharmacological properties. Of all the PET isotopes, fluorine-18 is the most convenient labeling isotope due to its relatively long physical half-life (110 min). The physical half-life of 110 min allows for more complex multi-step radiosynthesis and distribution to PET centers that lack radiochemistry facilities. In addition, its low ß+ energy of 635 keV results in high resolution and a low radiation dose to patients [8].
The direct labeling of peptides and other biomolecules is difficult to accomplish due in part to the harsh reaction conditions employed during nucleophilic substitution reactions. Thus, the fluorine-18 labeling of peptides such as bombesin [9], [10], somatostatin [11], [12], [13], neurotensin [14], and RGD [15], [16] have been accomplished using indirect methods. The indirect methods involve the coupling of 18F-labeled prosthetic groups to biomolecules via appropriate functionalities [17]. This requires a multi-step procedure which is time consuming. A direct labeling method is therefore highly desirable.
Schirrmacher et al. [18] described the direct labeling of 18F-Tyr3-Octreotate via 18F–19F isotope exchange on a silicon atom under mild conditions, but reported low specific activity due to high levels of non-radioactive fluorinated compound. This problem was solved only by utilizing an indirect method, employing an efficient two-step labeling procedure and using very small amounts of precursor in the isotope exchange reaction [19]. Recently, Mu et al. [20] developed a silicon-based direct and facile 18F-labeling of peptides using hydroxyl or hydrogen as a leaving group.
Very few publications are known which describe the nucleophilic ring-opening of aziridines using 18F-fluoride [21], [22]. The preparation of 18F-labeled 2-fluoroethylamines, -amides and -sulfonamides is performed by at least a two-step procedure applying 18F-2-fluoroethylamine [23], [24] or 18F-2-bromofluoroethane [25], [26]. The ring-opening of appropriate aziridine may deliver such structural motifs by a single-step synthesis. Very recently, Vasdev et al. [27] developed a highly regioselective method which involves the ring-opening of N-benzyloxycarbonyl-protected 2-methylaziridine with [18F] fluoride. This method allows the generation of new 18F-labeled amines for incorporation into radiopharmaceuticals.
Nucleophilic ring-opening of aziridines with halides is well known [28], [29], however, relatively few reports are found for the ring-opening of aziridines with fluoride as the nucleophile. The most often used fluorination reagents are BF3·OEt2 [29], [30], [31], HF*Pyridine (Olah's Reagent) [32], [33], diethylaminosulfur trifluoride (DAST) and LiBF4 [34], TBAF [35], [36], KHF2 [37] or KF [38]. Unfortunately, for nucleophilic fluorine-18 radiolabeling, 18F-fluorination reagents are limited. Among the above mentioned non-radioactive fluorination reagents, only [18F]-TBAF and [18F]-KF are mostly used for direct nucleophilic 18F-fluorination reactions. Here, we present the nucleophilic ring-opening of appropriately activated aziridines as an option for direct labeling of biomolecules such as peptides and nucleosides under mild conditions.
Section snippets
Synthesis of aziridine-based model compounds
First, several 2-carboxylic acid benzyl amide substituted aziridines as model compounds were prepared and fluorinated under non-radioactive conditions in order to evaluate the potential of the envisaged approach (Scheme 1, Table 1). In this study, no questions on stereochemistry were pursued and as such racemic compounds were used as starting materials except for the thymidine derivatives. Preliminary investigations were performed in order to get an idea about the reactivity of the prepared
Conclusion
Several aziridine-based model compounds with different activating groups were synthesized and labeled with fluorine-18 in moderate to high yields. The nucleophilic ring-opening of the aziridine derivatives was highly regioselective since only 3-fluoro-2-amino-amides were obtained as indicated by NMR. The application of the most promising building block for the labeling of biomolecules was demonstrated. Compared to the multi-step synthetic procedures, the aziridine approach is simple and
General
Analytical thin-layer chromatography (TLC) was performed on Merck silica gel, NH2 or RP-18 (60 F254) plates (0.25 mm) precoated with a fluorescent indicator. Preparative thin-layer chromatography (preparative TLC) was performed on silica gel (60F-254) plates (0.5 mm with concentrating zone) precoated with a fluorescent indicator. Preparative HPLC purifications of peptides were performed on a Luna C18 column (5 μm, 150 mm × 15 mm) with H2O (+0.1% TFA)/CH3CN (+0.1% TFA) and a gradient depending on
Acknowledgments
We thank Marion Kuzora, Bärbel Bennua Skalmowski, Marion Slopianka and Miljen Martic for their support. We are also grateful to Thomas Brumby for critical reading of the manuscript and productive discussions.
References (43)
J. Fluorine Chem.
(2006)- et al.
Nucl. Med. Biol.
(1994) - et al.
Nucl. Med. Biol.
(1997) - et al.
Nucl. Med. Biol.
(2002) - et al.
J. Fluorine Chem.
(2000) - et al.
Appl. Rad. Isot.
(2002) - et al.
Tetrahedron Lett.
(2009) Tetrahedron
(2004)- et al.
J. Fluorine Chem.
(1990) - et al.
Carbohyd. Res.
(2003)