Preferential accumulation of 11C in human brain tumors after intravenous injection of 11C-1-pyruvate

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Spin echo measurements of the extravasation and tumor cell uptake of hyperpolarized [1-(13) C]lactate and [1-(13) C]pyruvate

PURPOSE

To assess the blood-tissue distribution of hyperpolarized (13) C-labeled molecules in vivo.

METHODS

Spin-echo experiments with simultaneous acquisition of the free induction decay (FID) signal following the excitation pulse and the spin-echo signal, were used to monitor hyperpolarized [1-(13) C]lactate, [1-(13) C]pyruvate, and the perfusion marker, [(13) C]HP001, following their intravenous injection into tumor-bearing mice. Apparent T2 relaxation times and diffusion coefficients were also measured.

RESULTS

An increasing tumor echo/FID ratio was observed for all three molecules, which could be explained by their extravasation into the tumor interstitial space, where T2 relaxation times were longer and diffusion coefficients smaller. Inhibition of the monocarboxylate transporter, which decreased by 40% the label exchange between pyruvate and lactate, reduced the increase in the echo/FID ratio for pyruvate and lactate, but not for HP001, demonstrating that some of the increase in the echo/FID ratio was due to cell uptake of lactate and pyruvate. The different relaxation and diffusion behavior of the intravascular and extravascular signals affected measurements of the apparent label exchange rate constants.

CONCLUSION

Simultaneous collection of both FID and echo signals can provide information on cell uptake thus giving further insight into the kinetics of hyperpolarized (13) C label exchange. Care is needed when comparing exchange rate constants determined in spin-echo-based studies.

Metabolic imaging by hyperpolarized 13C magnetic resonance imaging for in vivo tumor diagnosis

The "Warburg effect," an elevation in aerobic glycolysis, may be a fundamental property of cancer cells. For cancer diagnosis and treatment, it would be valuable if elevated glycolytic metabolism could be quantified in an image in animals and humans. The pyruvate molecule is at the metabolic crossroad for energy delivery inside the cell, and with a noninvasive measurement of the relative transformation of pyruvate into lactate and alanine within a biologically relevant time frame (seconds), it may be possible to quantify the glycolytic status of the cells. We have examined the metabolism after i.v. injection of hyperpolarized (13)C-pyruvate in rats with implanted P22 tumors. The strongly enhanced nuclear magnetic resonance signal generated by the hyperpolarization techniques allows mapping of pyruvate, lactate, and alanine in a 5 x 5 x 10 mm(3) imaging voxel using a 1.5 T magnetic resonance scanner. The magnetic resonance scanning (chemical shift imaging) was initiated 24 seconds after the pyruvate injection and had a duration of 14 seconds. All implanted tumors showed significantly higher lactate content than the normal tissue. The results indicate that noninvasive quantification of localized Warburg effect may be possible.

Functional characterization of brain tumors: an overview of the potential clinical value

Early detection and characterization are still challenging issues in the diagnostic approach to brain tumors. Among functional imaging techniques, a clinical role for positron emission tomography studies with [18F]-fluorodeoxyglucose and for single photon emission computed tomography studies with [201Tl]-thallium-chloride has emerged. The clinical role of magnetic resonance spectroscopy is still being defined, whereas functional magnetic resonance imaging seems able to provide useful data for presurgical localization of critical cortical areas. Integration of morphostructural information provided by computed tomography and magnetic resonance imaging, with functional characterization and cyto-histologic evaluation of biologic markers, may assist in answering the open diagnostic questions concerning brain tumors.

Positron emission tomography using pyruvate-1-11C in two cases of mitochondrial encephalomyopathy

Positron emission tomography (PET) using pyruvate-1-11C was carried out to investigate the in vivo metabolism of pyruvate in the brains of patients with mitochondrial encephalomyopathy and Leigh's disease. Two epileptic patients were studied as control subjects. Radioactivity was eliminated from the brain tissue of the epileptic patients soon after injection of pyruvate-1-11C. PET images of mitochondrial encephalomyopathy patients showed an increase in radioactivity in the cerebral cortex, basal ganglia and thalamus, with elimination of radioactivity being slower than that of epileptic patients. One patient with Leigh's disease showed similar PET images. PET using pyruvate-1-11C is useful for the evaluation of mitochondrial energy metabolism in the brain.

Difference of 14C turnovers in brain and in transplanted glioma after intravenous injection of 14C-1-pyruvate into rats

Carbon 14 from 14C-1-pyruvate injected intravenously into glioma-transplanted rats was incorporated into various compounds in the brain and in the tumor. In the brain the majority of activity was found in CO2 (60%), and minor activities were found in alanine, lactate (15%), glutamate, and aspartate, with decreasing order, 5 min after injection. In the tumor, at 5 min, the largest activity was in lactate (56%), and lower activities were found in CO2 (24%), alanine, glutamate, and aspartate. The total 14C concentration in the tumor was twice that in the brain at 5 min and 15 min. The result was in accordance with the prediction that in brain, where the mitochondrial function is active, 14C-1-pyruvate will be oxidized completely into 14CO2, and that in tumor, where the mitochondrial function is insufficient, 14C-1-pyruvate will be converted only into 14C-lactate and prevent further degradation. It may be assumed that this difference in the turnover of 14C of 14C-1-pyruvate between brain and tumor could constitute a basis for the 'hot' visualization of human brain tumor using cyclotron-produced 11C-1-pyruvate and positron-emission tomography.