The concentration of sodium ions (Na +) is raised in solid tumours and can be measured at the cellular, tissue and patient levels. At the cellular level, the Na + gradient across the membrane powers the transport of H + ions and essential nutrients for normal activity. The maintenance of the Na + gradient requires a large proportion of the cell's ATP. Na + is a major contributor to the osmolarity of the tumour microenvironment, which affects cell volume and metabolism as well as immune function. Here, we review evidence indicating that Na + handling is altered in tumours, explore our current understanding of the mechanisms that may underlie these alterations and consider the potential consequences for cancer progression. Dysregulated Na + balance in tumours may open opportunities for new imaging biomarkers and re-purposing of drugs for treatment.

Background Breast cancer remains a leading cause of death in women and novel imaging biomarkers are urgently required. Here, we demonstrate the diagnostic and treatment-monitoring potential of non-invasive sodium (23Na) MRI in preclinical models of breast cancer. Methods Female Rag2−/−Il2rg−/− and Balb/c mice bearing orthotopic breast tumours (MDA-MB-231, EMT6 and 4T1) underwent MRI as part of a randomised, controlled, interventional study. Tumour biology was probed using ex vivo fluorescence microscopy and electrophysiology. Results 23Na MRI revealed elevated sodium concentration ([Na+]) in tumours vs non-tumour regions. Complementary proton-based diffusion-weighted imaging (DWI) linked elevated tumour [Na+] to increased cellularity. Combining 23Na MRI and DWI measurements enabled superior classification accuracy of tumour vs non-tumour regions compared with either parameter alone. Ex vivo assessment of isolated tumour slices confirmed elevated intracellular [Na+] ([Na+]i); extracellular [Na+] ([Na+]e) remained unchanged. Treatment with specific inward Na+ conductance inhibitors (cariporide, eslicarbazepine acetate) did not affect tumour [Na+]. Nonetheless, effective treatment with docetaxel reduced tumour [Na+], whereas DWI measures were unchanged. Conclusions Orthotopic breast cancer models exhibit elevated tumour [Na+] that is driven by aberrantly elevated [Na+]i. Moreover, 23Na MRI enhances the diagnostic capability of DWI and represents a novel, non-invasive biomarker of treatment response with superior sensitivity compared to DWI alone.

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Eslicarbazepine acetate (ESL) is a dibenzazepine anticonvulsant approved as adjunctive treatment for partial-onset epileptic seizures. Following first pass hydrolysis of ESL, S-licarbazepine (S-Lic) represents around 95% of circulating active metabolites. S-Lic is the main enantiomer responsible for anticonvulsant activity and this is proposed to be through the blockade of voltage-gated Na + channels (VGSCs). ESL and S-Lic both have a voltage-dependent inhibitory effect on the Na + current in N1E-115 neuroblastoma cells expressing neuronal VGSC subtypes including Na v 1.1, Na v 1.2, Na v 1.3, Na v 1.6, and Na v 1.7. ESL has not been associated with cardiotoxicity in healthy volunteers, although a prolongation of the electrocardiographic PR interval has been observed, suggesting that ESL may also inhibit cardiac Na v 1.5 isoform. However, this has not previously been studied. Here, we investigated the electrophysiological effects of ESL and S-Lic on Na v 1.5 using whole-cell patch clamp recording. We interrogated two model systems: (1) MDA-MB-231 metastatic breast carcinoma cells, which endogenously express the “neonatal” Na v 1.5 splice variant, and (2) HEK-293 cells stably over-expressing the “adult” Na v 1.5 splice variant. We show that both ESL and S-Lic inhibit transient and persistent Na + current, hyperpolarise the voltage-dependence of fast inactivation, and slow the recovery from channel inactivation. These findings highlight, for the first time, the potent inhibitory effects of ESL and S-Lic on the Na v 1.5 isoform, suggesting a possible explanation for the prolonged PR interval observed in patients on ESL treatment. Given that numerous cancer cells have also been shown to express Na v 1.5, and that VGSCs potentiate invasion and metastasis, this study also paves the way for future investigations into ESL and S-Lic as potential invasion inhibitors.

11Eslicarbazepine acetate (ESL) is a dibenzazepine anticonvulsant approved as adjunctive treatment for 12 partial-onset epileptic seizures. Following first pass hydrolysis of ESL, S-licarbazepine (S-Lic) 13represents around 95 % of circulating active metabolites. S-Lic is the main enantiomer responsible 14for anticonvulsant activity and this is proposed to be through the blockade of voltage-gated Na + 15 channels (VGSCs). ESL and S-Lic both have a voltage-dependent inhibitory effect on the Na + current 16 in N1E-115 neuroblastoma cells expressing neuronal VGSC subtypes including Nav1.1, Nav1.2, 17Nav1.3, Nav1.6 and Nav1.7. ESL has not been associated with cardiotoxicity in healthy volunteers, 18although a prolongation of the electrocardiographic PR interval has been observed, suggesting that 19ESL may also inhibit cardiac Nav1.5 isoform. However, this has not previously been studied. Here, 20we investigated the electrophysiological effects of ESL and S-Lic on Nav1.5 using whole-cell patch 21 clamp recording. We interrogated two model systems: (1) MDA-MB-231 metastatic breast 22 carcinoma cells, which endogenously express the 'neonatal' Nav1.5 splice variant, and (2) HEK-293 23 cells stably over-expressing the 'adult' Nav1.5 splice variant. We show that both ESL and S-Lic 24 inhibit transient and persistent Na + current, hyperpolarise the voltage-dependence of slow 25 inactivation, and slow the recovery from channel inactivation. These findings highlight, for the first 26 time, the potent inhibitory effects of ESL and S-Lic on the Nav1.5 isoform, suggesting a possible 27 explanation for the prolonged PR interval observed in patients on ESL treatment. Given that 28 numerous cancer cells have also been shown to express Nav1.5, and that VGSCs potentiate invasion 29 and metastasis, this study also paves the way for future investigations into ESL and S-Lic as potential 30 invasion inhibitors. 31