Carbon-11 Isotopic Radiolabeling of CP31398 and Development of a Fluorine-18 Derivative to Target Protein p53 with...

Mutant protein p53, a central driver of pro-oncological deregulations, is widely recognized as a biomarker of cancer aggressiveness and therapy resistance. In a personalized medicine perspective, positron emission tomography (PET) imaging of mutant p53 would be a powerful tool for patient stratification and drug development. However, to date, no PET radiotracers directly targeting mutant p53 have been reported. Inspired by the CP31398 drug, which stabilizes p53 conformation and treats tumors expressing mutant p53, we designed two novel PET radiotracers labeled with either carbon-11 by isotopic labeling or fluorine-18, namely [¹¹C]CP31398 and [¹⁸F]FG-CP31398. The nonradioactive fluorinated analogue was synthesized, as well as the two radiolabeling precursors. Optimization of the radiomethylation with carbon-11 of the phenol precursor was achieved, and automated radiosynthesis of [¹¹C]CP31398 afforded the ready-to-inject radiotracer with 40 ± 13% radiochemical yield and 39 ± 12 GBq/μmol (n = 6) molar activity after quality control. Automated radiofluorination with ¹⁸F-fluoride by aliphatic S_N2 of a tosylate precursor afforded the ready-to-inject [¹⁸F]FG-CP31398 in 10 ± 4% radiochemical yield (RCY) and 80 ± 39 GBq/μmol (n = 4) molar activity after quality control. Binding experiments performed with [¹⁸F]FG-CP31398 on HEK-293T cells transfected with a green fluorescent protein-p53 wild-type plasmid, with or without presaturation with CP31398, demonstrated that despite the chemical modification performed on this compound, [¹⁸F]FG-CP31398 was still able to bind specifically to the cell with ca. 20% nonspecific binding. However, autoradiography experiments performed after the incubation of either [¹¹C]CP31398 or [¹⁸F]FG-CP31398 on H358 (p53 positive) and A549 (p53 negative) tumor slices derived from human lung cancer cells revealed that both tracers were not able to bind the p53-positive cells. Surprisingly, a specific fixation demonstrated by presaturation experiments was observed for both radiotracers on the p53-negative A549 cells. Overall, our findings indicate that neither CP31398 nor FG-CP31398 binds directly to p53 but instead interacts with an unidentified target. While unsuitable for p53 imaging, these radiotracers may serve as valuable tools to unravel the controversial mechanism of action of CP31398.