Validation of transport measurements in skeletal muscle with N-13 amino acids using a rabbit isolated hindlimb model

Comparative evaluation of 18F-labeled glutamic acid and glutamine as tumor metabolic imaging agents

(18)F-labeled (2S,4R)-4-fluoro-l-glutamine (4F-GLN) has demonstrated high uptake in tumor cells that undergo high growth and proliferation. Similar tumor targeting properties have also been observed for (18)F-labeled (2S,4R)-4-fluoro-l-glutamate (4F-GLU), suggesting that both are useful imaging agents. A new labeling procedure facilitates the preparation of (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU with confirmed radiochemical and enantiomeric purity. Here, we report the preparation and comparative evaluation of (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU as tumor metabolic imaging agents.

METHODS

Uptake of enantiomerically pure (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU was determined in 3 tumor cell lines (9L, SF188, and PC-3) at selected time points. The in vitro cell uptake mechanism was evaluated by inhibition studies in 9L cells. In vivo biodistribution and PET studies were performed on male F344 rats bearing 9L tumor xenografts.

RESULTS

In vitro cell uptake studies showed that (18)F-(2S,4R)4F-GLN displayed higher uptake than (18)F-(2S,4R)4F-GLU. Amino acid transport system ASC (alanine-serine-cysteine-preferring; in particular, its subtype ASCT2 [SLC1A5 gene]) and system X(c)(-) (SLC7A11 gene) played an important role in transporting (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU, respectively, across the membrane. After being transported into cells, a large percentage of (18)F-(2S,4R)4F-GLN was incorporated into protein, whereas (18)F-(2S,4R)4F-GLU mainly remained as the free amino acid in its original form. In vivo studies of (18)F-(2S,4R)4F-GLN in the 9L tumor model showed a higher tumor uptake than (18)F-(2S,4R)4F-GLU, whereas (18)F-(2S,4R)4F-GLU had a slightly higher tumor-to-background ratio than (18)F-(2S,4R)4F-GLN. Imaging studies showed that both tracers had fast accumulation in 9L tumors. Compared with (18)F-(2S,4R)4F-GLU, (18)F-(2S,4R)4F-GLN exhibited prolonged tumor retention reflecting its incorporation into intracellular macromolecules.

CONCLUSION

Differences in uptake and metabolism in tumor cells were found between (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU. Both agents are potentially useful as metabolic tracers for tumor imaging.

Preparation and characterization of L-[5-11C]-glutamine for metabolic imaging of tumors

Recently, there has been a renewed interest in the study of tumor metabolism above and beyond the Warburg effect. Studies on cancer cell metabolism have provided evidence that tumor-specific activation of signaling pathways, such as the upregulation of the oncogene myc, can regulate glutamine uptake and its metabolism through glutaminolysis to provide the cancer cell with a replacement of energy source.

METHODS

We report a convenient procedure to prepare l-[5-(11)C]-glutamine. The tracer was evaluated in 9L and SF188 tumor cells (glioma and astrocytoma cell lines). The biodistribution of l-[5-(11)C]-glutamine in rodent tumor models was investigated by dissection and PET.

RESULTS

By reacting (11)C-cyanide ion with protected 4-iodo-2-amino-butanoic ester, the key intermediate was obtained in good yield. After hydrolysis with trifluoroacetic and sulfonic acids, the desired optically pure l-[5-(11)C]-glutamine was obtained (radiochemical yield, 5% at the end of synthesis; radiochemical purity, >95%). Tumor cell uptake studies showed maximum uptake of l-[5-(11)C]-glutamine reached 17.9% and 22.5% per 100 μg of protein, respectively, at 60 min in 9L and SF188 tumor cells. At 30 min after incubation, more than 30% of the activity appeared to be incorporated into cellular protein. Biodistribution in normal mice showed that l-[5-(11)C]-glutamine had significant pancreas uptake (7.37 percentage injected dose per gram at 15 min), most likely due to the exocrine function and high protein turnover within the pancreas. Heart uptake was rapid, and there was 3.34 percentage injected dose per gram remaining at 60 min after injection. Dynamic small-animal PET studies in rats bearing xenografted 9L tumors and in transgenic mice bearing spontaneous mammary gland tumors showed a prominent tumor uptake and retention.

CONCLUSION

The data demonstrated that this tracer was favorably taken up in the tumor models. The results suggest that l-[5-(11)C]-glutamine might be useful for probing in vivo tumor metabolism in glutaminolytic tumors.

A new method for assessment of drug absorption from muscle: application of a local perfusion system

Drug absorption from muscle has been examined using the rabbit hind leg, isolated by cannulating the femoral artery and vein, and perfused with or without bovine serum albumin (BSA) using a single-pass technique. A model compound, [14C]sucrose, was injected into the musculus gastrocnemius, and its appearance in the venous outflow was monitored and kinetically analysed by statistical moment theory. The fraction absorbed and mean arrival time in the venous outflow were calculated to be 103% and 50.5 min, respectively, in the BSA-containing system. The results of perfusion experiments without BSA were not significantly different, suggesting possible use of the simpler BSA-free perfusion system to obtain information on the muscular absorption of drugs.