Effects of radiotherapy on the cellular uptake of carbon-14 labeled L-methionine in tumor tissue

Amino acid transport system - A substrate predicts the therapeutic effects of particle radiotherapy

L-[methyl-¹¹C]Methionine (¹¹C-Met) is useful for estimating the therapeutic efficacy of particle radiotherapy at early stages of the treatment. Given the short half-life of ¹¹C, the development of longer-lived ¹⁸F- and ¹²³I-labeled probes that afford diagnostic information similar to ¹¹C-Met, are being sought. Tumor uptake of ¹¹C-Met is involved in many cellular functions such as amino acid transport System-L, protein synthesis, and transmethylation. Among these processes, since the energy-dependent intracellular functions involved with ¹¹C-Met are more reflective of the radiotherapeutic effects, we evaluated the activity of the amino acid transport System-A as an another energy-dependent cellular function in order to estimate radiotherapeutic effects. In this study, using a carbon-ion beam as the radiation source, the activity of System-A was evaluated by a specific System-A substrate, alpha-[1-¹⁴C]-methyl-aminoisobutyric acid (¹⁴C-MeAIB). Cellular growth and the accumulation of ¹⁴C-MeAIB or ¹⁴C-Met were evaluated over time in vitro in cultured human salivary gland (HSG) tumor cells (3-Gy) or in vivo in murine xenografts of HSG tumors (6- or 25-Gy) before and after irradiation with the carbon-ion beam. Post 3-Gy irradiation, in vitro accumulation of ¹⁴C-Met and ¹⁴C-MeAIB decreased over a 5-day period. In xenografts of HSG tumors in mice, tumor re-growth was observed in vivo on day-10 after a 6-Gy irradiation dose, but no re-growth was detected after the 25-Gy irradiation dose. Consistent with the growth results, the in vivo tumor accumulation of ¹⁴C-MeAIB did not decrease after the 6-Gy irradiation dose, whereas a significant decrease was observed after the 25-Gy irradiation dose. These results indicate that the activity of energy dependent System-A transporter may reflect the therapeutic efficacy of carbon-ion radiotherapy and suggests that longer half-life radionuclide-labeled probes for System-A may also provide widely available probes to evaluate the effects of particle radiotherapy on tumors at early stage of the treatment.

Positron Emission Tomography Imaging of Tumor Cell Metabolism and Application to Therapy Response Monitoring

Cancer cells do reprogram their energy metabolism to enable several functions, such as generation of biomass including membrane biosynthesis, and overcoming bioenergetic and redox stress. In this article, we review both established and evolving radioprobes developed in association with positron emission tomography (PET) to detect tumor cell metabolism and effect of treatment. Measurement of enhanced tumor cell glycolysis using 2-deoxy-2-[(18)F]fluoro-D-glucose is well established in the clinic. Analogs of choline, including [(11)C]choline and various fluorinated derivatives are being tested in several cancer types clinically with PET. In addition to these, there is an evolving array of metabolic tracers for measuring intracellular transport of glutamine and other amino acids or for measuring glycogenesis, as well as probes used as surrogates for fatty acid synthesis or precursors for fatty acid oxidation. In addition to providing us with opportunities for examining the complex regulation of reprogramed energy metabolism in living subjects, the PET methods open up opportunities for monitoring pharmacological activity of new therapies that directly or indirectly inhibit tumor cell metabolism.

Dynamic 11C-methionine PET analysis has an additional value for differentiating malignant tumors from granulomas: an experimental study using small animal PET

PURPOSE

We evaluated whether the dynamic profile of L-(11)C-methionine (11C-MET) may have an additional value in differentiating malignant tumors from granulomas in experimental rat models by small animal positron emission tomography (PET).

METHODS

Rhodococcus aurantiacus and allogenic rat C6 glioma cells were inoculated, respectively, into the right and left calf muscles to generate a rat model bearing both granulomas and tumors (n=6). Ten days after the inoculations, dynamic 11C-MET PET was performed by small animal PET up to 120 min after injection of 11C-MET. The next day, after overnight fasting, the rats were injected with 18F-2-deoxy-2-fluoro-D-glucose (18F-FDG), and dynamic 18F-FDG PET was performed up to 180 min. The time-activity curves, static images, and mean standardized uptake value (SUV) in the lesions were calculated.

RESULTS

11C-MET uptake in the granuloma showed a slow exponential clearance after an initial distribution, while the uptake in the tumor gradually increased with time. The dynamic pattern of 11C-MET uptake in the granuloma was significantly different from that in the tumor (p<0.001). In the static analysis of 11C-MET, visual assessment and SUV analysis could not differentiate the tumor from the granuloma in all cases, although the mean SUV in the granuloma (1.48±0.09) was significantly lower than that in the tumor (1.72±0.18, p<0.01). The dynamic patterns, static images, and mean SUVs of 18F-FDG in the granuloma were similar to those in the tumor (p=NS).

CONCLUSION

Dynamic 11C-MET PET has an additional value for differentiating malignant tumors from granulomatous lesions, which deserves further elucidation in clinical settings.

Usefulness of 11C-methionine for differentiating tumors from granulomas in experimental rat models: a comparison with 18F-FDG and 18F-FLT

Many clinical PET studies have shown that increased (18)F-FDG uptake is not specific to malignant tumors. (18)F-FDG is also taken up in inflammatory lesions, particularly in granulomatous lesions such as sarcoidosis or active inflammatory processes after chemoradiotherapy, making it difficult to differentiate malignant tumors from benign lesions, and is the main source of false-positive (18)F-FDG PET findings in oncology. These problems may be overcome by multitracer studies using 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) or l-(11)C-methionine. However, (18)F-FLT or (11)C-methionine uptake in granulomatous lesions remains unclarified. In this study, the potentials of (18)F-FLT and (11)C-methionine in differentiating malignant tumors from granulomas were compared with (18)F-FDG using experimental rat models.

METHODS

Dual-tracer tissue distribution studies using (18)F-FDG and (3)H-FLT (groups I and III) or (18)F-FDG and (14)C-methionine (groups II and IV) were performed on rats bearing both granulomas (Mycobacterium bovis bacillus Calmette-Guérin [BCG]-induced) and hepatomas (KDH-8-induced) (groups I and II) or on rats bearing both turpentine oil-induced inflammation and hepatomas (groups III and IV). One hour after the injection of a mixture of (18)F-FDG and (3)H-FLT or of (18)F-FDG and (14)C-methionine, tissues were excised to determine the radioactivities of (18)F-FDG, (3)H-FLT, and (14)C-methionine (differential uptake ratio).

RESULTS

Mature epithelioid cell granuloma formation and massive lymphocyte infiltration were observed in the granuloma tissue induced by BCG, histologically similar to sarcoidosis. The granulomas showed high (18)F-FDG uptake comparable to that in the hepatomas (group I, 8.18 +/- 2.40 vs. 9.13 +/- 1.52, P = NS; group II, 8.43 +/- 1.45 vs. 8.91 +/- 2.32, P = NS). (14)C-Methionine uptake in the granuloma was significantly lower than that in the hepatoma (1.31 +/- 0.22 vs. 2.47 +/- 0.60, P < 0.01), whereas (3)H-FLT uptake in the granuloma was comparable to that in the hepatoma (1.98 +/- 0.70 vs. 2.30 +/- 0.67, P = NS). Mean uptake of (18)F-FDG, (3)H-FLT, and (14)C-methionine was markedly lower in the turpentine oil-induced inflammation than in the tumor.

CONCLUSION

(14)C-Methionine uptake was significantly lower in the granuloma than in the tumor, whereas (18)F-FDG and (3)H-FLT were not able to differentiate granulomas from tumors. These results suggest that (14)C-methionine has the potential to accurately differentiate malignant tumors from benign lesions, particularly granulomatous lesions, providing a biologic basis for clinical PET studies.

Differentiation of tumour and inflammation: characterisation of [methyl-3H]methionine (MET) and O-(2-[18F]fluoroethyl)-L-tyrosine (FET) uptake in human tumour and inflammatory cells

PURPOSE

Previous studies suggest that radiolabelled amino acids could be superior to FDG in differentiating tumour and inflammation. Therefore the aim of this study was to investigate the uptake of FET and MET in human tumour and inflammatory cells and to investigate their uptake kinetics.

METHODS

For uptake studies, cells were incubated with 370 kBq FET or 3.7 kBq MET for 15 min. Kinetic studies were performed at variable concentrations of FET and MET. Competitive inhibition studies were done with BCH, MeAIB and L: -serine.

RESULTS

All inflammatory cells incorporated more MET than the tumour cells. The uptake of FET, in contrast, was significantly lower in all inflammatory cells than in the tumour cells. In tumour cells the uptake of MET was about five times the uptake of FET. The competitive inhibitors reduced uptake of both tracers to 20-40% in tumour cells and to 70% in inflammatory cells. Kinetic studies showed that MET and FET transport was saturable in all cells except macrophages and followed a Michaelis-Menten kinetic. Highest capacity (V (max)) and affinity (K (m)) for the uptake of MET was observed in granulocytes. Capacity and affinity for FET uptake were highest in the DHL-4 cells.

CONCLUSION

In contrast to MET, FET accumulated to a significantly greater extent in tumour cells than in inflammatory cells. The marked differences between tumour and inflammatory cells concerning FET and MET uptake suggest that FET and MET are substrates of different subtypes of the L system.

An experimental study onO-[18F]fluoromethyl-L-tyrosine for differentiation between tumor and inflammatory tissues

OBJECTIVE

O-[18F]fluoromethyl-L-tyrosine (18F-FMT) is a recently developed tumor-detecting agent with simple preparation and high radiochemical yields. The aim of this study was to assess the potency of 18F-FMT for differentiating tumor and inflammatory tissues using an animal model with an implanted tumor and experimentally induced inflammatory foci.

METHODS

An ascites hepatoma cell line, AH109A, turpentine oil and Staphylococcus aureus were inoculated subcutaneously into Donryu rats as a tumor model, aseptic inflammation model and bacterial infection model, respectively. The biodistribution of radioactivity was assessed in rats at 5, 10, 30, 60, and 120 min after injection with 18F-FMT. Dual tracer whole-body and macro autoradiographies were performed 60 min after injection with a mixture of 18F-FMT and 2-deoxy-D-[1-14C]glucose (14C-DG).

RESULTS

Tumor uptake of 18F-FMT was on average 1.27% injected dose per gram of tissue (%ID/g) and 1.43% ID/g at 30 min and 60 min, respectively and significantly higher than that in other normal tissues, except the pancreas (3.48% ID/g at 60 min). The uptakes in the aseptic and bacterial inflammatory tissues were very low and were not different from those of the background tissues. Dual tracer whole-body and macro autoradiographic studies showed that tumor uptake of 18F-FMT was clearly higher than uptake by the other tissues, while 18F-FMT accumulated much less both in aseptic and bacterial inflammatory tissues. In contrast, the 14C-DG images showed high accumulations not only in tumors but also in aseptic and bacterial inflammatory tissues.

CONCLUSION

18F-FMT seems to be a promissing tracer for the differentiation between tumor and inflammation because of higher specificity to tumor.

PET imaging for differentiating recurrent brain tumor from radiation necrosis

The exact incidence of true radiation necrosis is largely unknown. It is probably much less frequent than indicated by MR or CT findings. Differentiating radiation necrosis from recurrent tumor is a diagnostic challenge, however, and has important implications for the patient's management. Even though the first results were published 20 years ago, the total number of case studies using FDG-PET in this indication remains limited. Several reports are also hampered by methodologic limitations. The technique has been largely criticized, notably in articles that themselves were not completely free of methodological flaws. Overall however, FDG-PET seems to be a valuable clinical tool. As a general rule, suspicious lesions on MR imaging that show increased FDG uptake (ie, uptake equal to or great than that in normal cortex) are likely to represent tumor recurrence. Sensitivity is an issue, especially but not exclusively with low-grade gliomas. Although false-positive results may occur, specificity is usually high in routine clinical practice. Coregistration with MR imaging surely improves the diagnostic performances of FDG-PET because it helps delineate the suspicious area. Another important aspect is the prognostic value of FDG uptake, which is now well established. It seems clear that only the combination of FDG with a radiolabeled amino acid analogue (MET or a more recent fluorinated compound) can provide a comprehensive characterization of suspected brain tumor recurrence.

Imaging gliomas with positron emission tomography and single-photon emission computed tomography

Over the last two decades the large volume of research involving various brain tracers has shed invaluable light on the pathophysiology of cerebral neoplasms. Yet the question remains as to how best to incorporate this newly acquired insight into the clinical context. Thallium is the most studied radiotracer with the longest track record. Many, but not all studies, show a relationship between (201)Tl uptake and tumor grade. Due to the overlap between tumor uptake and histologic grades, (201)Tl cannot be used as the sole noninvasive diagnostic or prognostic tool in brain tumor patients. However, it may help differentiating a high-grade tumor recurrence from radiation necrosis. MIBI is theoretically a better imaging agent than (201)Tl but it has not convincingly been shown to differentiate tumors according to grade. MDR-1 gene expression as demonstrated by MIBI does not correlate with chemoresistance in high grade gliomas. Currently, MIBI's clinical role in brain tumor imaging has yet to be defined. IMT, a radio-labeled amino acid analog, may be useful for identifying postoperative tumor recurrence and, in this application, appears to be a cheaper, more widely available tool than positron emission tomography (PET). However, its ability to accurately identify tumor grade is limited. 18 F-2-Fluoro-2-deoxy-d-glucose (FDG) PET predicts tumor grade, and the metabolic activity of brain tumors has a prognostic significance. Whether FDG uptake has an independent prognostic value above that of histology remains debated. FDG-PET is effective in differentiating recurrent tumor from radiation necrosis for high-grade tumors, but has limited value in defining the extent of tumor involvement and recurrence of low-grade lesions. Amino-acid tracers, such as MET, perform better for this purpose and thus play a complementary role to FDG. Given the poor prognosis of patients with gliomas, particularly with high-grade lesions, the overall clinical utility of single photon emission computed tomography (SPECT) and PET in characterizing recurrent lesions remains dependent on the availability of effective treatments. These tools are thus mostly suited to the evaluation of treatment response in experimental protocols designed to improve the patients' outcome.