Preclinical Evaluation of 3-¹⁸F-Fluoro-2,2-Dimethylpropionic Acid as an Imaging Agent for Tumor Detection

Abstract

Deregulated cellular metabolism is a hallmark of many cancers. In addition to increased glycolytic flux, exploited for cancer imaging with ¹⁸F-FDG, tumor cells display aberrant lipid metabolism. Pivalic acid is a short-chain, branched carboxylic acid used to increase oral bioavailability of prodrugs. After prodrug hydrolysis, pivalic acid undergoes intracellular metabolism via the fatty acid oxidation pathway. We have designed a new probe, 3-¹⁸F-fluoro-2,2-dimethylpropionic acid, also called ¹⁸F-fluoro-pivalic acid (¹⁸F-FPIA), for the imaging of aberrant lipid metabolism and cancer detection. Methods: Cell intrinsic uptake of ¹⁸F-FPIA was measured in murine EMT6 breast adenocarcinoma cells. In vivo dynamic imaging, time course biodistribution, and radiotracer stability testing were performed. ¹⁸F-FPIA tumor retention was further compared in vivo to ¹⁸F-FDG uptake in several xenograft models and inflammatory tissue. Results: ¹⁸F-FPIA rapidly accumulated in EMT6 breast cancer cells, with retention of intracellular radioactivity predicted to occur via a putative ¹⁸F-FPIA carnitine-ester. The radiotracer was metabolically stable to degradation in mice. In vivo imaging of implanted EMT6 murine and BT474 human breast adenocarcinoma cells by ¹⁸F-FPIA PET showed rapid and extensive tumor localization, reaching 9.1% ± 0.5% and 7.6% ± 1.2% injected dose/g, respectively, at 60 min after injection. Substantial uptake in the cortex of the kidney was seen, with clearance primarily via urinary excretion. Regarding diagnostic utility, uptake of ¹⁸F-FPIA was comparable to that of ¹⁸F-FDG in EMT6 tumors but superior in the DU145 human prostate cancer model (54% higher uptake; P = 0.002). Furthermore, compared with ¹⁸F-FDG, ¹⁸F-FPIA had lower normal-brain uptake resulting in a superior tumor-to-brain ratio (2.5 vs. 1.3 in subcutaneously implanted U87 human glioma tumors; P = 0.001), predicting higher contrast for brain cancer imaging. Both radiotracers showed increased localization in inflammatory tissue. Conclusion: ¹⁸F-FPIA shows promise as an imaging agent for cancer detection and warrants further investigation.