H NMRS of Carnosine Combined with P NMRS to Better Characterize Skeletal Muscle PH Dysregulation in Duchenne Muscular Dystrophy

In recent years, quantitative nuclear magnetic resonance imaging and spectroscopy (NMRI and NMRS) have been used more systematically as outcome measures in natural history and clinical trial studies for Duchenne muscular dystrophy (DMD). Whereas most of these studies have emphasized the evaluation of the fat fraction as an assessment for disease severity, less focus has been placed on metabolic indices measured by NMRS. P NMRS in DMD reveals an alkaline inorganic phosphate (P ) pool, originating from either leaky dystrophic myocytes or an increased interstitial space. H NMRS, exploiting the pH-sensitive proton resonances of carnosine, an intracellular dipeptide, was used to distinguish between these two hypotheses. NMR data were obtained in 23 patients with DMD and 14 healthy subjects on a 3-T clinical NMR system. Both P and H NMRS data were acquired at the level of the gastrocnemius medialis muscle. A multi-slice multi-echo imaging acquisition was performed for the determination of water T and fat fraction in the same region of interest. Whereas nearly all patients with DMD showed an elevated pH compared with healthy controls when using P NMRS, H NMRS-determined pH was not systematically increased. As expected, the carnosine-based intracellular pH was never found to be alkaline in the absence of a concurrent P -based pH elevation. In addition, abnormal intracellular pH, based on carnosine, was never associated with normal water T values. We conclude that, in one group of patients, both H and P NMRS showed an alkaline pH, originating from the intracellular compartment and reflecting ionic dysregulation in dystrophic myocytes. In the other patients with DMD, intracellular pH was normal, but an alkaline P pool was still present, suggesting an extracellular origin, probably revealing an expanded interstitial volume fraction, often associated with fibrotic changes. The data demonstrate that H NMRS could serve as a biomarker to assess the normalization of intramyocytic pH and sarcolemmal permeability following therapy inducing dystrophin expression in patients with DMD.