Measuring extracellular human brain pH and amino acid metabolism with hyperpolarized [1-13C]pyruvate

Hyperpolarized carbon-13 MRI has shown promise for non-invasive assessment of the cerebral metabolism of [1-13C]pyruvate in both healthy volunteers and in patients. Exchange of pyruvate to lactate catalyzed by lactate dehydrogenase (LDH), and pyruvate flux to bicarbonate through pyruvate dehydrogenase (PDH), are the most widely studied reactions in vivo . Here we show the potential of the technique to probe other metabolic reactions in the human brain. Approximately 50 s after intravenous injection of hyperpolarized pyruvate, high flip angle pulses were used to detect cerebral 13C-labelled carbon dioxide (13CO2), in addition to the 13C-bicarbonate (H13CO2-) subsequently formed by carbonic anhydrase. Brain pH weighted towards the extracellular compartment was calculated from the ratio of H13CO3- to 13CO2 in seven volunteers using the Henderson-Hasselbalch equation, demonstrating an average pH ± S.D. of 7.40 ± 0.02, with inter-observer reproducibility of 0.04. In addition, hyperpolarized [1-13C]aspartate was also detected in four of nine volunteers demonstrating irreversible pyruvate carboxylation to oxaloacetate by pyruvate carboxylase (PC), and subsequent transamination by aspartate aminotransferase (AST), with this flux being approximately 6% of that through PDH. Hyperpolarized [1-13C]alanine signal was also detected within the head but this was localized to muscle tissue in keeping with skeletal alanine aminotransferase (ALT) activity. The results demonstrate the potential of hyperpolarized carbon-13 MRI to assess cerebral and extracerebral [1-13C]pyruvate metabolism in addition to LDH and PDH activity. Non-invasive measurements of brain pH could be particularly important in assessing cerebral pathology given the wide range of disease processes that alter acid-base balance. ### Competing Interest Statement Dr Rolf F Schulte is employed by GE Healthcare ### Funding Statement We have funding from Cancer Research UK (CRUK; C19212/A27150; C19212/A16628), the Lundbeck Foundation, the Multiple Sclerosis Society (2015 35), the National Institute of Health Research (NIHR) Cambridge Biomedical Research center (BRC-1215 20014), CRUK Cambridge center, the Cambridge Experimental Cancer Medicine center, the Evelyn Trust, Addenbrooke's Charitable Trust, and the NIHR/Wellcome Trust Cambridge Clinical Research Facility. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The East of England - Cambridge South Research Ethics Committee gave ethical approval for this work (Molecular Imaging and Spectroscopy with Stable Isotopes in Oncology and Neurology: MISSION-MIMS; REC: 15/EE/0255). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors