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.