The Effect of Chirality on the Application of 5-[¹⁸F]Fluoro-Aminosuberic Acid ([¹⁸F]FASu) for Oxidative Stress Imaging

The cystine transporter, system x_C^-, plays a crucial role in sustaining redox homeostasis and is reported to be overexpressed in several cancer subtypes. 5-[¹⁸F]Fluoroaminosuberic acid ([¹⁸F]FASu) is a novel positron emission tomography (PET) tracer, which exhibits specific uptake via system x_C^-. [¹⁸F]FASu synthesis by the commonly used Kryptofix 2.2.2/K₂CO₃-facilitated fluorination method results in four diastereomers, as a result of 2 chiral centers at positions 2- and 5- of the tracer. We recently reported the synthesis of the optically pure 2S-[¹⁸F]FASu from chiral precursors. Our preliminary results indicated preferential uptake of the 2S-isomer by tumor cells compared to 2R-[¹⁸F]FASu. Few studies have investigated the biodistribution of chiral ¹⁸F-labeled amino acids. The aim of this study was to evaluate the imaging utility and biodistribution of the 5-position diastereomers as well as the racemic (2S,5R/S-) mixture in three different tumor models.

All three tracer conformations allowed for the visualization of tumor xenografts at 1 h (for U-87 and PC-3 tumors) or 2 h (in the case of MDA-MB-231 xenografts) post-injection, with the racemate (2S,5R/S-) displaying similar image contrast as compared to the 5- position diastereomers and the 2S,5S-[¹⁸F]FASu conformation exhibiting relatively higher contrast for imaging U-87 and PC-3 xenografts. Tumor uptake of the isomers was blocked by an excess of the non-radioactive standard, aminosuberic acid (ASu), confirming target specificity. All three isomers were excreted via the renal pathway. Biodistribution analyses showed that PC-3 tumors had the highest tracer uptake, and the accumulation (%ID/g) of the 2S,5R/S-, 2S,5S-, and 2S,5R- isomers was 9.19 ± 1.14, 8.00 ± 1.41, and 7.16 ± 2.13 at 1 h post-injection, respectively. This gave corresponding tumor-to-muscle ratios of 33.68 ± 9.52, 31.42 ± 4.54, and 25.33 ± 4.97, respectively.

Our data suggest that pure 2S-[¹⁸F]FASu can be used to noninvasively image system x_C^- in a variety of cancers, either as the racemic mixture (2S,5R/S-) or optically pure form. Furthermore, this work shows potential utility of [¹⁸F]FASu for detection of glioblastoma and prostate cancer.