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

Abstract

Measurements of hyperpolarized 13C label exchange between injected [1‐13C]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 [1‐14C]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 13C label exchange gave an isotope flux that showed good agreement with a flux determined directly by the injection of non‐polarized [3‐13C]pyruvate, rapid excision of the tumor after 30 s and measurement of 13C‐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 [1‐13C]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 [1‐13C]pyruvate showed only relatively low levels of polarization. This study shows that measurements of hyperpolarized 13C 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.

Pulse sequence considerations for quantification of pyruvate‐to‐lactate conversion kPL in hyperpolarized 13C imaging

“…Since angiogenesis is a hallmark of cancer, analogous phenomena need to be considered in patient studies . For instance, if a measured AIF is to be extracted for modeling, differential flow suppression between tumor and blood may impact the accuracy of k PL estimation …”

“…Therefore, the magnetization spoiling of the AIF depends on the RF pulses seen by the inflowing HP spins through major arteries, while the signal loss due to crusher‐induced flow suppression is a function of the distribution of spin vector velocities within any given voxel. Another interesting factor to explore in the future is how the blood lactate pool interacts with the pulse sequences, and how this affects k PL estimation …”

“…41,42 For instance, if a measured AIF is to be extracted for modeling, differential flow suppression between tumor and blood may impact the accuracy of k PL estimation. 13,14,43,44 While both DSE magnetization spoiling and crusher-induced flow suppression seemingly increase k PL artificially by reducing pyruvate signal, there are a few fundamental differences. In general, RF spoiling, including DSE spoiling, acts on longitudinal magnetization, creating an effective loss similar to T 1 relaxation.…”

“…For instance, this can include an arterial input function (AIF) that characterizes the size, shape, and timing of the injected pyruvate bolus . Moreover, the bolus delivery and clearance parameters describe perfusion to the tumor microvasculature, permeability to interstitial space, cancer cellular uptake, and release and removal of HP compounds . Additionally, using kinetic modeling enables incorporation of hyperpolarized signal loss mechanisms, primarily the RF pulses used to interrogate HP magnetization and the intrinsic T 1 relaxation of HP biomarkers.…”

Challenges and Opportunities in Cancer Metabolomics

“…Investigators need to guard against this phenomenon as well. However, to circumvent such pitfalls nuclear magnetic resonance (NMR) spectroscopy‐based smart approaches that use 13 C and 14 C labeling in pyruvate and lactate in tumor and blood of lymphoma‐bearing mice alongside NMR‐based analysis can be implemented. Sarcosine, an N‐methyl derivative of the amino acid glycine, was identified as a differential metabolite that was highly increased during prostate cancer progression to metastasis and is detected noninvasively in urine .…”