Ratiometric pH-Responsive ¹⁹F Magnetic Resonance Imaging Contrast Agents Based on Hydrazone Switches

Abstract

Hydrazone-based molecular switches serve as efficient ratiometric pH-sensitive agents that can be tracked with 19F NMR/MRI and 1H NMR. Structural changes induced between pH 3 and 4 lead to signal appearance and disappearance at 1H and 19F NMR spectra allowing ratiometric pH measurements. The most pronounced are resonances of the CF3 group shifted by 1.8 ppm with 19F NMR and a hydrazone proton shifted by 2 ppm with 1H NMR.

Tuning the pH of Activation of Fluorinated Hydrazone-Based Switches─A Pathway to Versatile 19F Magnetic Resonance Imaging Contrast Agents

“…Procedure 1. Ethyl 2-(pyridin-2-yl)acetate derivatives were synthesized following the literature procedure: 36 n -BuLi (2.5 M solution in hexanes, 2.05 equiv) was added dropwise to a stirred solution of diisopropylamine (2.10 equiv) in THF at −78 °C under Ar. The resulting solution was warmed for 1 h at room temperature to 0 °C and stirred at 0 °C for 1 h. Then, the solution was transferred to a stirred solution of modified 2-picoline (1.0 equiv) and diethyl carbonate (3.0 equiv) in THF at −78 °C under Ar.…”

“…Hydrazone molecular switches (1a–1g, 2a, 2b, 3a, 3b, 4a, Figure 1 ) were obtained according to the modified literature procedure. 36 Trifluoromethylaniline (1 equiv) was dissolved in a mixture (1:5 ratio) of 38% HCl and anhydrous (99.9%) EtOH and stirred in an ice bath for 30 min. A cold (≈0 °C) aqueous solution of NaNO 2 (1 equiv) was then added dropwise over a period of 30 min.…”

“…As a result of the configuration change, the hydrazone molecular switch changes its absorption band, 1 H NMR, and, if a fluoroorganic group is appropriately introduced, also 19 F NMR spectrum, 36 which facilitates 19 F MR imaging of the pH gradient. In the case of a native hydrazone switch bearing a single CF 3 or F functional group, the switching process takes place in the pH range of 3−4, 9 which is adequate for the gastric environment 42 but insufficient for tumor imaging (5.0−6.5).…”

“…Additionally, we decided to change the position of the −CF 3 group from para to ortho to increase the difference in chemical shifts between the E and Z−H + states. Such a change should have minimal effect on the pK a of the resulting switch because of only 0.1 differences in Hammett constants of o-CF 3 and p-CF 3 groups and nearly identical pK a of CF 3 -substituted hydrazone switches regardless of the position of the −CF 3 group 36.…”

“…Many pH-sensitive agents have been proposed for 19 F MRI such as PEGylated nanogels containing perfluorocarbons, fluorinated pyridoxine, C 6 F 6 -loaded Au-fluorescent mesoporous silica nanoparticles, or copolymers − that rely on the volume phase transition, decomposition, or isomerization as mechanisms of signal activation. Recently, we proposed molecular switches as potential 19 F MRI agents . The main advantage is that no external reference either for chemical shift or for concentration is necessary, as they act as ratiometric probes not affected by differences in the pharmacokinetic properties of a probe and a reference.…”