Imaging the Warburg effect with 23Na MRI and 82Rb positron emission tomography

It is widely accepted that hypoxia in tumours or myocardium with underdeveloped or obstructed vasculature, induces a switch from aerobic respiration to anaerobic glycolysis as the main fuel for ATP production. Tumours display a preference for glycolysis over oxidative metabolism even in the presence of oxygen. This observation was first described by Warburg and has since been recognized as one of the hallmarks of cancer. The expression and/or activity of glycolytic enzymes underlying this switch are controlled by several cell signalling pathways that results in upregulated glycolysis and lactate excretion and downregulated pyruvate entry into the TCA cycle. Developing inhibitors of these pathways is of enormous current interest for molecularly targeted cancer therapeutics and for developing clinically translatable molecular imaging approaches to report on these processes in vivo.

Na/K ATPase (EC 3.6.1.37) is the membrane transporter responsible for pumping 3Na out and 2K into the cell for every molecule of ATP converted, of which it is a major consumer. The ATP supply for Na/K ATPase has been suggested to be preferentially derived from glycolysis since it is spatially co-localised with the glycolytic enzymes pyruvate kinase, phosphoglycerate kinase and glyceraldehyde 3-phosphate dehydrogenase. Furthermore, in the human erythrocyte, which possess no mitochondria, previous work using 13C flux analysis of glucose consumption following treatment with the Na ionophore monensin showed that 6 ions of Na are pumped out of the cell per molecule of glucose consumed, suggesting a tight coupling between glycolytic flux and Na/K ATPase activity, while studies in a range of cancer cells have shown that activation of the Na/K pump with gramicidin (or inhibition with ouabain), leads to an increase (or decrease) in glycolysis, respectively. In this context there is evidence that Na/K ATPase could be a target for cancer therapy. Breast cancer patients receiving cardiac glycosides such as digoxin show distinct tumour cell morphologies, smaller tumour volumes at diagnosis, reduced metastasis after two years follow-up and showed reduced recurrence rate five years after mastectomy. The mechanisms underlying these observations are unclear, but there is emerging evidence that Na/K ATPase, which is the principal inhibitory target for these glycosides, could be a therapeutic target for cancer treatment.

The relationship between altered metabolism during tumorigenesis, Na/K ATPase activity and pathology is not well understood and there are currently no non-invasive clinically validated imaging tools of Na/K ATPase activity in cancer or the heart. Furthermore despite significant advances in our understanding, the mechanisms underlying the Warburg effect remain poorly understood and imaging techniques still largely rely on 18FDG PET which reports on glucose transport and not downstream bioenergetics. In this CDT project we propose to develop a combination of in vivo imaging techniques employing 23Na MRI for probing intracellular and extracellular sodium ion concentration in combination with 82Rb PET for probing Na/K ATPase activity in tumours and in the heart to offer a hitherto unexplored window on dysregulated metabolism in vivo.

Strains on the healthcare system in the UK create an acute need for finding more effective, efficient, safe, and accurate non-invasive imaging solutions for clinical decision-making, both in terms of diagnosis and prognosis, and to reduce unnecessary treatment procedures and associated costs. Medical imaging is currently undergoing a step-change facilitated through the advent of artificial intelligence (AI) techniques, in particular deep learning and statistical machine learning, the development of targeted molecular imaging probes and novel "push-button" imaging techniques. There is also the availability of low-cost imaging solutions, creating unique opportunities to improve sensitivity and specificity of treatment options leading to better patient outcome, improved clinical workflow and healthcare economics. However, a skills gap exists between these disciplines which this CDT is aiming to fill.

Consistent with our vision for the CDT in Smart Medical Imaging to train the next generation of medical imaging scientists, we will engage with the key beneficiaries of the CDT: (1) PhD students & their supervisors; (2) patient groups & their carers; (3) clinicians & healthcare providers; (4) healthcare industries; and (5) the general public. We have identified the following areas of impact resulting from the operation of the CDT.

- Academic Impact: The proposed multidisciplinary training and skills development are designed to lead to an appreciation of clinical translation of technology and generating pathways to impact in the healthcare system. Impact will be measured in terms of our students' generation of knowledge, such as their research outputs, conference presentations, awards, software, patents, as well as successful career destinations to a wide range of sectors; as well as newly stimulated academic collaborations, and the positive effect these will have on their supervisors, their career progression and added value to their research group, and the universities as a whole in attracting new academic talent at all career levels.

- Economic Impact: Our students will have high employability in a wide range of sectors thanks to their broad interdisciplinary training, transferable skills sets and exposure to industry, international labs, and the hospital environment. Healthcare providers (e.g. the NHS) will gain access to new technologies that are more precise and cost-efficient, reducing patient treatment and monitoring costs. Relevant healthcare industries (from major companies to SMEs) will benefit and ultimately profit from collaborative research with high emphasis on clinical translation and validation, and from a unique cohort of newly skilled and multidisciplinary researchers who value and understand the role of industry in developing and applying novel imaging technologies to the entire patient pathway.

- Societal Impact: Patients and their professional carers will be the ultimate beneficiaries of the new imaging technologies created by our students, and by the emerging cohort of graduated medical imaging scientists and engineers who will have a strong emphasis on patient healthcare. This will have significant societal impact in terms of health and quality of life. Clinicians will benefit from new technologies aimed at enabling more robust, accurate, and precise diagnoses, treatment and follow-up monitoring. The general public will benefit from learning about new, cutting-edge medical imaging technology, and new talent will be drawn into STEM(M) professions as a consequence, further filling the current skills gap between healthcare provision and engineering.

We have developed detailed pathways to impact activities, coordinated by a dedicated Impact & Engagement Manager, that include impact training provision, translational activities with clinicians and patient groups, industry cooperation and entrepreneurship training, international collaboration and networks, and engagement with the General Public.