Syntheses of [¹¹C]2- and [¹¹C]3-trifluoromethyl-4-aminopyridine: potential PET radioligands for imaging demyelinating diseases

Objectives: Trifluoromethyl groups are of great interest in radiopharmaceuticals as they can enhance chemical and metabolic stability and allow for the manipulation of physicochemical properties such as pK_a, lipophilicity and bioavailability¹ while providing a handle that is amenable to labeling with carbon-11² or fluorine-18^3-7. Although most available methods are focused on ¹⁸F-labeling with [¹⁸F]fluoroformor its copper(I) derivative, the main limitation for PET radiotracer development lies in the low to moderate molar activities achievable by these methods. To address this problem, the synthesis of [¹¹C]fluoroform from cyclotron-produced [¹¹C]methane was recently developed, allowing for the retention of high molar activity and the labeling of small molecules with [¹¹C]trifluoromethyl groups with molar activities exceeding 5.4 Ci/μmol². Aminopyridines are of interest for the development of PET radiotracers. For example, radiolabeled derivatives of 4-aminopyridine (4AP), an FDA-approved drug for multiple sclerosis that binds to voltage-gated K⁺ channels present in demyelinated axons, have been proposed for PET imaging of demyelinating diseases. Here, we describe methods for producing [¹¹C]trifluoromethyl derivatives of 4AP and present the first imaging results with [¹¹C]3-trifluoromethyl-4-aminopyridine ([¹¹C]3-CF₃‑4AP) in a Rhesus macaque.

Methods: The [¹¹C]fluoroform production method² was adapted to generate [¹¹C]methane from cyclotron-produced [¹¹C]carbon dioxide using a GE Tracerlab FX MeI synthesizer. Subsequently, [¹¹C]methane was converted into [¹¹C]fluoroform by passing it through a heated reactor containing CoF₃. This was then converted into the trifluoromethylating agent [¹¹C]CuCF₃ for the systematic investigation of its use to produce [¹¹C]CF₃-4AP derivatives from pyridines donning a halide group (iodo or bromo) at 2- or 3-position and an ester, amide or amine group at the 4-position. Finally, [¹¹C]3-CF₃-4AP was produced for preliminary imaging in a healthy Rhesus macaque. Dynamic PET data was acquired for 120 min after a slow bolus injection of 1.93 mCi of [¹¹C]3-CF₃‑4AP.

Results: Production of [¹¹C]CHF₃ compared well to previous reports. While the overall radiochemical yield from [¹¹C]carbon dioxide, 52 ± 8% (n=15), was comparable to that from [¹¹C]methane, changes to the gas flow to increase pressure and contact time between [¹¹C]methane and CoF₃ further improved the radiochemical purity of the [¹¹C]fluoroform from about 80% to 94 ± 4% (n=15). Trifluoromethylation of bromo- and iodopyridine substrates containing electron-withdrawing (CO₂Me) and electron-donating groups (NHBoc and NH₂) with [¹¹C]fluoroform resulted in a wide range of yields (5-97%), with iodo-precursors giving higher yields than their bromo counterparts. Dynamic PET imaging of [¹¹C]3-CF₃-4AP showed a fast high uptake of radioactivity in brain followed by a fast washout, similar to that reported for [¹⁸F]3-F-4AP⁸.

Conclusions: This work illustrates the versatility of labeling pyridines with [¹¹C]fluoroform and provides initial evidence for the potential use of [¹¹C]3-CF₃‑4AP as a PET radioligand for demyelination. References: ¹Purser S, Moore PR, Swallow S, et al. Chem Soc Rev 37, 320-330. ²Haskali MB, Pike VW. Chem Eur J 23, 8156-8160. ³Huiban M, Tredwell M, Mizuta S, et al. Nat Chem 5, 941-944. ⁴van der Born D, Sewing C, Herscheid JK, et al. Angew Chem Int Ed Engl 53, 11046-11050. ⁵Ruhl T, Rafique W, Lien VT, et al. Chem Commun (Camb) 50, 6056-6059. ⁶Ivashkin P, Lemmonier G, Cousin J, et al. Chemistry 20, 9514-9518. ⁷Carroll L, Evans HL, Spivey AC, et al. Chem Commun (Camb) 51, 8439-8441. ⁸Brugarolas P, Sánchez-Rodríguez JE, Tsai HM, et al. Sci Rep 8, 607.