Purpose: The aim of this study was to investigate the metabolism of hyperpolarized [1-13C]pyruvate in a well-established hormone-responsive breast cancer model in a manner that preserves the 3D architecture and the extracellular matrix of the tumor, without the effects of metabolites wash-in from other body organs.
Materials and Methods: MCF7 human breast cancer cells were injected subcutaneously to immune deficient female mice. When the tumor reached the size of 1 cm3, it was resected and cut to 500 µm precision-cut tissue slices. The slices were maintained in a 10 mm NMR tube within an NMR spectrometer under controlled perfusion conditions, which kept the slices viable for more than 7 hours. Hyperpolarized [1-13C]pyruvate was introduced to the tumor slices within the NMR tube and the metabolism was monitored in real-time. 13C acquisition was performed using product-selective saturating RF excitations, (selective excitations) which fully excited the metabolite of interest ([1-13C]lactate) while the precursor ([1-13C]pyruvate) was excited to a much lower degree. Therefore, only newly synthetized metabolites were detected in the consecutive excitation, and these signals were used to determine the lactate dehydrogenase (LDH) activity. Two different approaches of introduction of the hyperpolarized solution to the slices were used: arrested vs. continuous perfusion. While the first is more common and allows for calculation of enzymatic rate under constant concentration of the hyperpolarized substrate, the latter better simulates the physiological state.
Results: The LDH activity, which was manifested as hyperpolarized [1-13C]lactate production in the tumor slices, was 4.0 ± 5.1 and 3.8 ± 3.5 nmole/nmole ATP in 1 min in the arrested and continuous perfusion conditions, respectively. This rate was converted to an expected LDH activity in a mass using the ATP level of these tumors, resulting in 3.3 and 3.4 µmole/g in 1 min respectively.
Conclusion: The feasibility of monitoring the metabolism of hyperpolarized substrate in a small amount of tumor tissue was demonstrated. The LDH activity in breast cancer was quantified and suggested feasibility of translating this type of enzymatic activity determination to the clinical setting. The LDH activity was similar when determined under arrested or continuous perfusion of the slices. This study may be useful as guidance for treatment response assessment in a large number of tumor types and therapies ex vivo. Further studies are needed to validate these results in vivo.
