Analysis of 13 C and 14 C labeling in pyruvate and lactate in tumor and blood of lymphoma-bearing mice injected with 13 C- and 14 C-labeled pyruvate.

Measurements of hyperpolarized 13 C label exchange between injected [1-13 C]pyruvate and the endogenous tumor lactate pool can give an apparent first-order rate constant for the exchange. The determination of the isotope flux, however, requires an estimate of the labeled pyruvate concentration in the tumor. This was achieved here by measurement of the tumor uptake of [1-14 C]pyruvate, which showed that <2% of the injected pyruvate reached the tumor site. Multiplication of this estimated labeled pyruvate concentration in the tumor with the apparent first-order rate constant for hyperpolarized 13 C label exchange gave an isotope flux that showed good agreement with a flux determined directly by the injection of non-polarized [3-13 C]pyruvate, rapid excision of the tumor after 30 s and measurement of 13 C-labeled lactate concentrations in tumor extracts. The distribution of labeled lactate between intra- and extracellular compartments and the blood pool was investigated by imaging, by measurement of the labeled lactate concentration in blood and tumor, and by examination of the effects of a gadolinium contrast agent and a lactate transport inhibitor on the intensity of the hyperpolarized [1-13 C]lactate signal. These measurements showed that there was significant export of labeled lactate from the tumor, but that labeled lactate in the blood pool produced by the injection of hyperpolarized [1-13 C]pyruvate showed only relatively low levels of polarization. This study shows that measurements of hyperpolarized 13 C label exchange between pyruvate and lactate in a murine tumor model can provide an estimate of the true isotope flux if the concentration of labeled pyruvate that reaches the tumor can be determined.

13C NMR tracers in neurochemistry: implications for molecular imaging.

An overview of 13C nuclear magnetic resonance (NMR) spectroscopy methods and their applications in the study of the metabolism of brain cells in vitro and in the in vivo brain is presented as well as their implications for modern molecular imaging techniques. Various topics will be discussed, such as general properties of the 13C NMR spectrum, 13C NMR spectroscopy acquisition protocols, determination of fractional 13C enrichment, 13C(2H) NMR methodologies, and the use of 13C hyperpolarized substrates for NMR spectroscopy and imaging. Some illustrative applications are described, both in vitro and in vivo.

Targeting of lanthanide(III) chelates of DOTA-type glycoconjugates to the hepatic asyaloglycoprotein receptor: cell internalization and animal imaging studies.

The characterization of a new class of hydrophilic liver-targeted agents for gamma-scintigraphy and MRI, consisting, respectively, of [(153)Sm](3+) or Gd(3+) complexes of DOTA monoamide or bisamide linked glycoconjugates (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), is reported. In vitro studies show high uptake of radiolabeled [(153)Sm]-DOTAGal(2) by the human hepatocyte carcinoma cell line Hep G2 containing the asialoglycoprotein receptor (ASGP-R), which is decreased to less than 50% by the presence of its high-affinity ligand asialofetuin (ASF). In vivo biodistribution, gamma-imaging and pharmacokinetic studies on Wistar rats using the [(153)Sm](3+)-labeled glycoconjugates show a high uptake in the receptor-rich organ liver of the radiolabeled compounds containing terminal galactosyl groups, but very little uptake for those compounds with terminal glycosyl groups. Blocking the receptor in vivo reduced liver uptake by 90%, strongly suggesting that the liver uptake of these compounds is mediated by their binding to the asyaloglycoprotein receptor (ASGP-R). This study also demonstrated that the valency increase improves the targeting capability of the glycoconjugates, which is also affected by their topology. However despite the specific liver uptake of the radiolabeled galactose-bearing multivalent compounds, the animal MRI assessment of the corresponding Gd(3+) chelates shows liver-to-kidney contrast effects which are not significantly better than those shown by GdDTPA. This probably results from the quick wash-out from the liver of these highly hydrophilic complexes, before they can be sufficiently concentrated within the hepatocytes via receptor-mediated endocytosis.