Metabolic changes of glioma following chemotherapy: an experimental study using four PET tracers

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.

The robotic radiosynthesis of 5-[(18)F]fluoro-2'-deoxyuridine and its biological characterization

5-[(18)F]fluoro-2'-deoxyuridine ([(18)F]FUdR) was synthesized using a robotic system as a proliferation probe for PET. [(18)F]FUdR was prepared via radiofluorodestannylation reaction from its organotin precursor. Biodistribution study and microPET imaging of [(18)F]FUdR in NG4TL4 sarcoma-bearing FVB/n mice were performed. The tumor-to-blood and tumor-to-muscle ratio increased steadily from 15 (1.81 and 3.42) to 120min (9.10 and 11.9) post injection. The dynamic microPET imaging demonstrates remarkable radioactivity retention in the tumor, which is consistent with the results of biodistribution study.

Imaging readouts as biomarkers or surrogate parameters for the assessment of therapeutic interventions

Surrogate markers and biomarkers based on imaging readouts providing predictive information on clinical outcome are of increasing importance in the preclinical and clinical evaluation of novel therapies. They are primarily used in studies designed to establish evidence that the therapeutic principle is valid in a representative patient population or in an individual. A critical step in the development of (imaging) surrogates is validation: correlation with established clinical endpoints must be demonstrated. Biomarkers must not fulfill such stringent validation criteria; however, they should provide insight into mechanistic aspects of the therapeutic intervention (proof-of-mechanism) or document therapy efficacy with prognostic quality with regard to the long-term clinical outcome (proof of concept). Currently used imaging biomarkers provide structural, physiological and metabolic information. Novel imaging approaches annotate structure with molecular signatures that are tightly linked to the pathophysiology or to the therapeutic principle. These cellular and molecular imaging methods yield information on drug biodistribution, receptor expression and occupancy, and/or intra- and intercellular signaling. The design of novel target-specific imaging probes is closely related to the development of the therapeutic agents and should be considered early in the discovery phase. Significant technical and regulatory hurdles have to be overcome to foster the use of imaging biomarkers for clinical drug evaluation.

In vivo monitoring of drugs using radiotracer techniques

There is an increasing realization of the role of non-invasive monitoring of drug pharmacology. In this review, we discuss the role of positron emission tomography in such monitoring of tumour and normal tissue drug pharmacokinetics as well as assessment of tumour response, drug-receptor interactions and mechanisms of drug action and resistance. These studies represent a multidisciplinary research effort involving radiochemists, imaging scientists, clinicians, pharmacologists and mathematical modellers. This review evaluates achievements in the field from assessment of commonly used therapeutic agents such as 5-fluorouracil to target specific molecules such as markers for gene expression. It is envisaged that application of this technology will facilitate rational drug design and rapid translation of new ideas to the bedside.

Radiolabelled 5'-iodo-2'-deoxyuridine: a promising alternative to [18F]-2-fluoro-2-deoxy-D-glucose for PET studies of early response to anticancer treatment

We investigated the potential of radiolabelled 5-iodo-2'-deoxyuridine (IUdR) as a pharmacodynamic probe for use with positron emission tomography (PET) in studies of early proliferative response to anticancer treatment. Using the hormone-responsive rat mammary carcinoma OES.HR1, we used a multiple radiotracer method to examine treatment-induced changes in 24 h tumour retention of [131I]IUdR, uptake of [3H]2-deoxy-D-glucose ([3H]DG) together with [99mTc]hexylmethylpropylene amineoxine ([99mTc]HMPAO) uptake as a measure of blood flow. Radiotracer data were compared with macroscopic changes in tumour growth, and cell proliferation as determined by DNA histogram flow cytometry. From 4 days after tumour growth arrest induced by oestrogen ablation, a sustained fall in tumour cell proliferation was demonstrated, which was associated with reduced tumour uptake of each tracer. Whereas reduced levels of tumour [3H]DG could be accounted for by changes in blood flow, this was not the case for [131I]IUdR, which was found to be closely related to percentage S-phase cells within tumour (r = 0.73, p < 0.002). It was also estimated that residual levels of radioiodide may contribute significantly, to the low levels of retained radioactivity associated with responding tumours at 24 h following IUdR administration, suggesting that metabolite correction methods should be implemented as part of IUdR PET imaging protocols. We conclude that [124I]IUdR is a promising alternative to [18F]fluorodeoxyglucose ([18F]FDG) for the early assessment by PET of tumour response to treatments directed at targets associated with cell proliferation.

Glucose and methionine uptake by rat brain tumor treated with prodrug-activated gene therapy

The effect of acyclovir (ACV) on the metabolism of rat 9L-gliosarcoma cells expressing the herpes simplex virus-thymidine kinase gene was studied using 2-deoxy-2-[18F]fluoro-D-glucose (FDG) and L-[methyl-11C]methionine. Though the average weight of the tumors treated with ACV was significantly lower than that of the saline-injected control group, FDG and methionine uptake per weight of tumor tissue was not different between the two groups. This result exhibits a striking contrast to the metabolic pattern observed after radiation therapy, suggesting the different pathways regarding tumor cell death between the therapies.

The in vivo metabolic pattern of low-grade brain gliomas: a positron emission tomographic study using 18F-fluorodeoxyglucose and 11C-L-methylmethionine

OBJECTIVE

The object of the present study was to identify metabolic differences between low-grade astrocytomas and oligodendrogliomas and to improve their diagnosis and noninvasive assessment, because both types of tumors look very similar from the point of view of clinical and radiological data (as assessed by computed tomography and magnetic resonance imaging).

METHODS

Before any aggressive treatment, 22 patients with primary low-grade gliomas (astrocytomas in 12 patients and oligodendrogliomas in 10) were investigated with positron emission tomography for both glucose metabolism (18F-fluorodeoxyglucose) and amino acid uptake (11C-L-methylmethionine). An original software that allows a full metabolic analysis of the tumor region of interest (defined from the T1-weighted magnetic resonance image) and compares tumor tissue uptake tracer concentrations with average healthy tissue values has been implemented for data processing. Heterogeneity of each individual tumor has been taken into account and was expressed in histograms, which provided data about the mean and also extreme and intermediate values of tracer concentrations and the way these values are distributed among the full tumor mass.

RESULTS

It has been shown that both tumor types exhibit a glucose hypometabolism (slightly more pronounced with astrocytomas), whereas they strongly differ in methionine uptake, which is high in all oligodendrogliomas and either decreased, normal, or moderately increased in astrocytomas. This latter metabolic difference between both tumor populations may be partially explained by their different cell densities.

CONCLUSION

This study suggests that despite similar radiological and clinical presentations, these two kinds of low-grade gliomas are metabolically different and could therefore have specific responses to different therapies. Moreover, their in vivo metabolic follow-up with positron emission tomography should rely on different parameters, depending on their histological type; methionine uptake may be more relevant than glucose metabolism in the follow-up of oligodendrogliomas.

Clinical application of 18F-FUdR in glioma patients--PET study of nucleic acid metabolism

Positron emission tomography was used to investigate the metabolism of nucleic acids by 18F-fluoro-2'-deoxyuridine (18F-FUdR) in 22 patients with gliomas. Sixteen cases of high grade glioma clearly demonstrated a region of high activity with a differential absorption rate (DAR) of 0.64 +/- 0.34. Six cases of low grade glioma failed to reveal a positive image of the tumor and the DAR in tumor was 0.21 +/- 0.042 (p < 0.01). This PET-18F-FUdR study succeeded in differentiating high and low grade gliomas from the view point of nucleic acid metabolism.

Multicentric glioma studied with positron emission tomography: a case report

Positron emission tomography (PET) studies of a multicentric glioma case were undertaken using 11C-methionine (Met) and 18F-fluorodeoxyuridine (FUdR). Met-PET revealed high accumulating lesion in both the left and right hemispheres, whereas in the FUdR-PET, the lesion on the right showed marked accumulation, but not that on the left. For both foci, the histologic diagnoses were anaplastic astrocytoma, but the lesion on the right showed higher density of undifferentiated tumor cells than that on the left. PET studies using Met and FUdR are effective in delineating the proliferative potential of glioma cells, which otherwise cannot be determined by histopathologic studies.

Brain tumor accumulation and plasma pharmacokinetic parameters of 2'-deoxy-5-18F-fluorouridine

Using positron emission tomography and radio-high performance liquid chromatography, the accumulation of 2'-deoxy-5-18F-fluorouridine in the brain tumors and plasma pharmacokinetic parameters were investigated in 20 patients. High accumulation of the tracer in high grade gliomas and meningiomas and very rapid degradation of the tracer in the plasma were found. Very large variations were observed in both tumor accumulation and pharmacokinetic data. The tumor accumulation, however, did not correlate with any of the plasma pharmacokinetic parameters: area under the plasma concentration-time curve, mean residence time, total body clearance and steady-state volume of distribution. The results suggest that the accumulation of the tracer reflects the metabolic activity of the brain tumor tissues and that the effect of the rapid metabolic change in the tracer in the plasma on the tumor accumulation may be minor.