Background

Multimodality imaging has rapidly advanced in the last decade, and hyperpolarized ¹³C-pyruvate MRI and ¹⁸F-FDG PET imaging are two imaging techniques that both indirectly report on the cellular glucose metabolism. Combining the two techniques may provide better insight on the metabolic processes that drives diseases such as cancer. However, we hypothesized the relatively high concentrations of pyruvate used in hyperpolarized MRI could interfere with the ¹⁸F-FDG uptake when administrated simultaneously, because their metabolic products compete for the same pathways (1,2). In this study, we examined the effect of injected pyruvate and lactate on ¹⁸F-FDG uptake in healthy rats and in rats with implanted C6 gliomas.

Methods

Healthy male Wistar rats (300-400g) were anesthetized with Isoflurane (1.8%-2% in 0₂) and placed in a Siemens Inveon D-PET, microPET scanner (Siemens Medical Solutions USA, Inc., Knoxville, TN). Pyruvate (125mM in 2 ml, n=5), lactate (125mM in 2 ml, n=4) or saline (2 ml, n=4) was injected as a bolus via a tail vein catheter followed immediately by injection of 1-mCi ¹⁸F-FDG (diluted in saline to 0.9ml) and 1 hour of dynamic PET acquisition. A second study of healthy rats (n=5) was performed using a lactate infusion (100 mmol/Kg/min, 20 min before the PET scan and during the entire scan) instead of the bolus injection protocol. A third study using the lactate infusion protocol was then performed in rats implanted with C6 gliomas (n=4, 200-250g, scanned 10-11 days after C6 implantation). The C6 rats underwent a 3T gadolinium-enhanced T1-weighted MRI scan (GE 3T Signa PET/MR scanner) the day before PET imaging. All rats were fasted for 18-20 hours prior to 18F-FDG injection. The dynamic PET data was reconstructed in 5-s frames (0-3min), in 1-min frames (3-10) and in 10-min frames (20-60min) using a fully 3D TF iterative ordered subsets expectation maximization (OSEM) algorithm. For the C6 animals, hyper-intense regions of contrast-enhanced T1-weighted MRI were used for co-registration of MR and PET images and for identifying the tumor region of interest (ROI). Visualization of radiotracer uptake, co-registration, ROI identification, and analysis of signal differences were performed using the FuseIt tool in PMOD (Version 3.7, PMOD Technologies, Zürich, Switzerland) (Fig. 1). Tumor and tissue TACs were normalized to injected dose, and expressed as %ID/g, assuming 1g/ml. The normalized uptake of radiotracer at 60 min was used for comparison between groups and in the tumor ROIs in contrast to the contralateral normal-appearing brain ROIs.

Results

Healthy rats receiving a bolus injection of either pyruvate or lactate showed no significant effect on ¹⁸F-FDG uptake (p=0.12 and p=0.86 respectively, Fig. 2A and 2B), whereas lactate infusion resulted in a markedly decreased ¹⁸F-FDG signal (-31%, p=0.003, Fig. 2C). For the C6 animals, an expected increased FDG uptake were observed in the tumor versus the contralateral ROI in the saline infused animals (+28%, p=0.03). In contrast, infusion of lactate decreased the ¹⁸F-FDG uptake by 24% (p=0.007) in the healthy regions of the brain but only by 14% (non significant p=0.087) in the tumor, resulting in increased tumor/healthy brain ¹⁸F-FDG contrast (+15%) in the lactate infused animals (Fig. 2D).

Discussion/Conclusion

It is well known that pyruvate, rapidly converted to intracellular lactate when present in excessive amounts, competes with glucose as a substrate for cerebral energy metabolism, and previous studies have shown that exogenous lactate affects brain ¹⁸F-FDG uptake (1,2). The affect is attributed to glucose-6-phosphate accumulation leading to product inhibition of hexokinase. This study showed that lactate infusion, which results in a greater overall dose as compared to bolus injections, could have a measurable effect on any simultaneous ¹⁸F-FDG PET study. Furthermore, a differential effect between normal brain and tumor, as seen here, may be ascribed to tumor tissue having predominately hexokinase-2, which is not subject to product inhibition (3). However, for the typical bolus doses used in in vivo hyperpolarized ¹³C pyruvate and ¹³C lactate studies, the effect on ¹⁸F-FDG uptake in rat brain was not significant. We do note that several sequential injections could potentially have an inhibitory effect as seen with the lactate infusion protocol used in this study.

Acknowledgements

NIH grants R01CA176836, R01EB019018, P41EB01589, S10OD012283, S10OD018130, GE Healthcare and the Lucas Foundation.