Synthesis and Radiolabeling of 18F-LW223

A 4-step synthetic strategy was developed for the preparation of the chloride precursor and LW223 (Supplemental Fig. 1; supplemental materials are available at http://jnm.snmjournals.org). Initially, the starting material, 3-methyl-4-phenylquinoline-2-carboxylic acid, was prepared as previously reported by us (14 (link)). This was converted to the (R)-sec-butylamide by coupling with (R)-sec-butylamine using the coupling agent hexafluorophosphate benzotriazole tetramethyl uronium. Under standard basic conditions, the amide was subjected to an N-methylation. The key step then involved a radical-mediated bromination of the 3-methyl substituent using N-bromosuccinimide and the radical initiator benzoyl peroxide. The resulting bromide intermediate was then converted to the chloride or LW223 using lithium chloride or sodium fluoride, respectively. All intermediates and final compounds were purified by column chromatography and characterized using a combination of nuclear magnetic resonance spectroscopy and mass spectrometry (organic chemistry section within supplemental materials (14 (link),15 (link))). The 2 amide rotamers of LW223 were separated and characterized by liquid chromatography–mass spectrometry (Supplemental Fig. 2).
¹⁸F-LW223 was prepared as shown in Figure 1, using the GE Healthcare TRACERlab FX_FN synthesizer. The radiotracer was purified by semipreparative high-performance liquid chromatography using the following conditions: C18 Synergi Hydro-RP 80 Å, 150 × 10 mm, 4-μm column (Phenomenex), acetonitrile/water (70:30 v/v), and flow rate of 3 mL/min. ¹⁸F-LW223 was formulated in 10% ethanol in saline. The average radioactivity yield was 50% ± 4% (starting from 22 ± 3 GBq of ¹⁸F-fluoride, n = 34) after a total synthesis time of 55 min. The identity of ¹⁸F-LW223, radiochemical purity (>99%), and molar activity (89 ± 12 GBq/μmol, n = 34) were determined by high-performance liquid chromatography analysis at the end of synthesis.