Assessment of ¹⁸F-fluoroglucaric acid uptake in lung and brain cancer cell lines

Introduction: Tumor necrosis, which can represent the aggressive features of cancers with poor prognosis, is an important prognostic factor for cancer staging and treatment efficacy evaluation. Noninvasive evaluation of tumor necrosis after a certain anticancer therapy may prove a valuable method for assessment of response. Early detection of treatment induced necrosis is a critical unmet clinical need. Glucaric acid is a non-toxic, natural compound with a linear 6-carbon backbone and dicarboxylic acid functionality. ¹⁸F-Fluoroglucaric acid (FGA) has been shown to accumulate in isoproterenol-induced myocardial injury in a rat model. The objective of this study was to assess whether FGA targeting tumor necrosis is more effective and suitable PET tracer for detection of tumor response to therapy than FDG which is considered to be the gold standard in PET imaging.

Methods: FGA was obtained by a simple one-step oxidation of FDG using 4-Acetamido-TEMPO, sodium bromide and sodium hypochlorite. The final product was used directly without purification. Both radiochemical yield and purity were > 99%. For the in vitro uptake experiment, FGA was used for a direct comparison with FDG in 3 lung cancer cells (H2009, A549, and H2030) and 2 brain cancer cells (LN-18 and U87). Cells were incubated with different concentrations of chemotherapy drugs (0, 34 and 134 µM cisplatin for lung cancer cells; 0, 125 and 500 µM carboplatin for brain cancer cells) for 72 h to induce different degrees of necrosis. The degree of necrosis was measured by an automatic cell viability counter. 1-2 µCi of each tracer was added to each well of a 12 well plate and incubated for 1 h at 37 ^o C (n=4). Afterward, cells were harvested, centrifuged, washed twice, and then counted in a gamma well counter. Normalized uptake was calculated by (counts in cells*10^6)/(total counts*cell number). Uptake ratios between treated and non-treated group (T/N) were calculated for each cancer cell line.

Results: All three lung cancer cells and two brain cancer cells showed similar degree of necrosis at the same drug concentration of cisplatin and carboplatin respectively. The average values were: 4±2, 27±5 and 50±5% for H2009, A549, and H2030 at 0, 34 and 134 µM cisplatin; and 3±2, 43±4 and 75±8% for LN-18 and U87 at 0, 125 and 500 µM carboplatin respectively. Similar patterns were observed for all five cancer cell lines examined in this study: FGA uptake increased with increasing drug concentration, whereas change in FDG uptake was very small from one drug concentration to another. The T/N ratios for FGA and FDG were 10.5±0.1, 4.2±0.2, and 3.2±0.2 vs.1.1±0.3, 0.8±0.2, and 1.0±0.2 in H2009, A549, and H2030; and 4.2±0.2 and 3.1±0.2 vs. 0.8±0.2 and 1.0±0.1 in LN-18 and U87.

Conclusions: Out of the five cancer cell lines assessed in this study, the T/N ratio was the highest for FGA uptake in H2009 (lung cancer), and it was determined that FGA is superior to FDG for distinguishing different levels of necrosis. Therefore, FGA can potentially be a preferred choice over FDG for detection of tumor response to therapy using PET/CT imaging.