Hyperpolarized pyruvate has previously been used to probe primary brain cancer. Through imaging the delivery and metabolism of both hyperpolarized [1-13C]pyruvate and ethyl-[1-13C]pyruvate in a rodent model of cancer metastasis to the brain, we show that the transport of [1-13C]pyruvate across the blood brain barrier may be limiting until it is compromised by metastatic cell infiltration.
Animal model: Female BD-IX rats (n=4) were anaesthetised (isoflurane) and stereotactically injected intra-cerebrally with ENU-1564 cells to provide a well-characterised model of secondary brain metastases;[5] controls were injected with saline. Rats were scanned weekly post-surgery with both hyperpolarized probes and post-gadolinium proton gradient echo T1-weighted images (80×80mm FOV; 192×192 matrix; 1.5mm thick; 120ms TR; 3.8ms TE; 7T Varian system) were acquired to assess tumour burden. Animals were provided with analgesia and monitored daily; all experiments were performed following ethical approval.
[1-13C]Pyruvate imaging: A flyback 3D spectral-spatial EPI sequence (37.5mm slab excite; 64×64mm2 FOV; volume 13C transmit/surface receive; TR 150ms; TE 12.18ms) was designed and reconstructed as described previously[6] at 2×2×4mm3. 31mg of [1-13C]pyruvate with OX063 radical was polarized in a prototype hyperpolarizer for 45 minutes prior to dissolution as described previously,[7] and injection of 2ml 80mmol pyruvate over 20s via a tail vein cannula. Hyperpolarized pyruvate/bicarbonate/lactate volumes were acquired interleaved (TR=1.8s/volume; total FA=17∘,61∘,61∘).
Ethyl-[1-13C]Pyruvate imaging: A spiral multiecho sequence was designed with echo times to maximise the total effective number of signal averages (30mm sinc excitation; 80×80mm2 FOV; 0.5s TR; 8 echoes; TE=1.05,1.59,2.13,2.67,3.21,3.75,4.29,4.83, with FID 64ms, FA/shot=15∘). Ethyl-[1-13C]pyruvate (Isotec) was doped with 5μL/10mM Gd-DPTA in ethanol and 15mM AH111501 radical (GE Healthcare), which was found to be more soluble than OX063. Ethyl-[1-13C] pyruvate was then hyperpolarized in a prototype hyperpolarizer at 94GHz for approximately 2.5hours prior to dissolution. 2mL was injected over 20s via tail vein cannula. Data were reconstructed by a pre-measured gradient impulse response function[7] followed by NUFFT[8] for a reconstructed resolution of 3.125×3.125mm2 (PSF FWHM).
Hyperpolarized [1-13C]pyruvate was visible predominantly in the cerebral circulation (Fig. 1). Lactate following pyruvate injection predominantly colocalized to and near major vessels in healthy animals, and in tumour once the BBB was disrupted.
The use of AH111501 meant that ethyl-[1-13C]pyruvate polarized to a higher degree than reported previously,[2] with limiting solid-state polarization ∼70% that of pyruvate. Uniform perfusion of ethyl-[1-13C]-pyruvate was observed in the brain, in contrast to pyruvate (Fig. 2), indicating greater transport. Downstream production of [1-13C]pyruvate from hyperpolarized ethyl-[1-13C]-pyruvate was also visible approximately homogeneously within the brain. Lactate production from ethyl-[1-13C]-pyruvate was only visible in the region of the tumour, and not elsewhere, in contrast to the vascular production following infusion of [1-13C]-pyruvate. It was not possible to detect hyperpolarized lactate following infusion of either probe until tumours had become angiogenic (at 4 weeks), and therefore also visible to gadolinium.
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