Radiosynthesis and ‘click’ conjugation of ethynyl‐4‐[¹⁸F]fluorobenzene — an improved [¹⁸F]synthon for indirect radiolabeling

Abstract

Reproducible methods for [(18)F]radiolabeling of biological vectors are essential for the development of new [(18)F]radiopharmaceuticals. Molecules such as carbohydrates, peptides and proteins are challenging substrates that often require multi-step indirect radiolabeling methods. With the goal of developing more robust, time saving, and less expensive procedures for indirect [(18)F]radiolabeling of such molecules, our group has synthesized ethynyl-4-[(18)F]fluorobenzene ([(18)F]2, [(18)F]EYFB) in a single step (14 ± 2% non-decay corrected radiochemical yield (ndc RCY)) from a readily synthesized, shelf stable, inexpensive precursor. The alkyne-functionalized synthon [(18)F]2 was then conjugated to two azido-functionalized vector molecules via CuAAC reactions. The first 'proof of principle' conjugation of [(18)F]2 to 1-azido-1-deoxy-β-D-glucopyranoside (3) gave the desired radiolabeled product [(18)F]4 in excellent radiochemical yield (76 ± 4% ndc RCY (11% overall)). As a second example, the conjugation of [(18)F]2 to matrix-metalloproteinase inhibitor (5), which has potential in tumor imaging, gave the radiolabeled product [(18)F]6 in very good radiochemical yield (56 ± 12% ndc RCY (8% overall)). Total preparation time for [(18)F]4 and [(18)F]6 including [(18)F]F(-) drying, two-step reaction (nucleophilic substitution and CuAAC conjugation), two HPLC purifications, and two solid phase extractions did not exceed 70 min. The radiochemical purity of synthon [(18)F]2 and the conjugated products, [(18)F]4 and [(18)F]6, were all greater than 98%. The specific activities of [(18)F]2 and [(18)F]6 were low, 5.97 and 0.17 MBq nmol(-1), respectively.

Fluorine-18 labelled building blocks for PET tracer synthesis

“…346 Instead of a direct labelling approach, they reported a two-step method, consisting of first the synthesis of [ 18 F]fluorobenzaldehydes 363a,b,k in 20-75% radiochemical yield (ndc) and subsequent Seyferth-Gilbert Homologation towards ([ 18 F]fluorophenyl)acetylenes 553a-c using the Bestmann-Ohira reagent in 40-60% radiochemical yield (ndc). The two step approach seems more promising than the direct approach employed by Roberts et al, 345 as exemplified by the overall radiochemical yield for (4-[ 18 F]fluorophenyl)acetylene of 26-45% instead of 14 AE 2%. For the purification of (4-[ 18 F]fluorophenyl)acetylene, Krapf et al discovered that radiochemical purities of 498% can be achieved by distillation, thereby avoiding cumbersome HPLC purification.…”

“…To demonstrate the application of (4-[ 18 F]fluorophenyl)acetylene in the synthesis of PET tracers, Roberts et al reacted this building block with a variety of azide precursors (Scheme 129). 345 Compounds 554 and 555 were formed in radiochemical yields of 67% and 56% (analytically determined) respectively. Unfortunately, overall non-decay corrected radiochemical yields, based on starting [ 18 F]fluoride were low due to the low yielding synthesis of 4-([ 18 F]fluorophenyl)acetylene 553a.…”

“…A recent publication by Roberts et al describes the synthesis of another fluorine-18 labelled alkyne building block: (4-[ 18 F]fluorophenyl)acetylene 553a, which can be obtained by direct labelling of its trimethylammonium precursor (Scheme 128). 345 Although the aromatic ring is only marginally electron deficient, the (4-[ 18 F]fluorophenyl)acetylene building block could still be obtained in a radiochemical yield of 14% (ndc). Purification of this building block was done by HPLC, because SPE methods did not lead to desired radiochemical purities of this reagent.…”

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

“…54–56 A 2-cyanobenzothiozole-based 18 F, [ 18 F]-FPyPEGCBT, and an ethynyl-4-[ 18 F]fluorobenzene prosthetic groups were used for conjugation with the terminal cysteine group in a cRGDyK peptide (30 min reaction time, 7 ± 1% End of Bombardment yield) and in matrix-metalloproteinase inhibitor (70 min reaction time, 56 ± 12% yield), respectively. 57,58…”

Sulfur - fluorine bond in PET radiochemistry

“…Following these encouraging preliminary results, it was envisaged that a sterically hindered sulfonyl chloride could be produced as a synthon to be subsequently radiolabelled with [ 18 F] fluoride and conjugated to a peptide or protein. Ideally, the radiosynthon would contain an alkynyl pendant, allowing it to undergo bioconjugation with an azido modified macromolecule using click chemistry (Roberts et al 2015 ). The radiosynthon would contain di- tert butyl groups to provide additional protection against hydrolysis of the sulfur-[ 18 F] fluorine bond and could be formed using a variety of routes (Fig.…”