Several chemotherapy drugs have been linked to an increased risk of coronary heart disease and the development of hypertension. Of particular interest is doxorubicin, which has been shown in both pre-clinical and clinical settings to lead to heart failure in a subset of subjects. The role of this agent in cardiotoxicity is poorly understood [1,2]. However, there are reports in doxorubicin treated hearts, of increased reactive oxygen species, inadequate handling of calcium and mitochondrial dysfunction, potentially leading to reduced myocardial energetics (with a 30% decrease in ATP in the hearts of treated animals). There is therefore clear evidence that alterations in metabolism play a key role in chemotherapy induced heart failure. The aim of this work was to assess the in vivo metabolic phenotype of the doxorubicin treated heart using hyperpolarized magnetic resonance spectroscopy.

Animals – Seven female Wistar rats (BW = 180-200g) received two IP injections of doxorubicin (6 mg/kg), separated by a week, giving a total cumulative dose of 12 mg/kg. For controls, five female Wistar rats went through the same procedure, but received saline. Animals were monitored daily for signs of distress and scanned at 4 weeks following the first injection.

MR Protocol - [1-¹³C]pyruvate was hyperpolarized and dissolved as previously described [3,4]. An aliquot of 1 ml of 80 mM hyperpolarized [1-¹³C]pyruvate solution was injected over 10 s via a tail vein catheter into an anaesthetised rat positioned in a 7 T MR scanner. Spectra were acquired for 1 min following injection with 1 s temporal resolution using a 15^o RF excitation pulse. Signal was localised to the heart using a home-built ¹³C RF surface coil. Acquired spectra were quantified using jMRUI (NIH, USA) and peak areas were input into a kinetic model described by Atherton et al [5]. The rate of exchange of the ¹³C label between pyruvate and its metabolites was termed ¹³C label incorporation. ¹³C label incorporation into the bicarbonate pool was used as a measure of PDH flux [3]. Following the injection, the ¹³C RF coil was replaced with a ¹H 4-channel phased array coil (Rapid Biomedical, Germany) to assess cardiac function using CINE MRI. A stack of contiguous, 1.5 mm thick, true short axis ECG- and respiration-gated cine/FLASH images were acquired to cover the entire heart. Field of view (FOV) = 51.2 × 51.2 mm, matrix size = 192 × 192, slice thickness = 1.5 mm, echo time (TE)/repetition time (TR) = 1.43/ 4.6 ms, 0.5 ms/17.5° Gaussian RF excitation pulse, and four averages.

Doxorubicin resulted in a reduction in left ventricular mass (10% reduction, p = 0.057, Figure 1), stroke volume (20% reduction, p < 0.01) and end diastolic volume (18% reduction, p < 0.01). PDH flux was significantly reduced in the doxorubicin treated heart (p < 0.01), from 11.6 ± 0.3 to 6.3 ± 0.8 x 10^-3 s^-1 (Figure 2). Alanine was also significantly decreased following 4 weeks of doxorubicin treatment from 5.1 ± 0.5 to 2.8 ± 0.4 x 10^-3 s^-1, whereas ¹³C flux into lactate was unaltered between groups.Doxorubicin treatment resulted in significant wall thinning, and reductions in both cardiac function and volume, all indicative of cardiotoxicity. Interestingly, doxorubicin treatment also resulted in significant metabolic alterations; with reductions in both PDH flux and ¹³C label incorporation into alanine. Such results suggest a move away from glucose oxidation (reduction in PDH flux) and potential reductions in protein synthesis (reduced ¹³C label incorporation into alanine) in the doxorubicin treated heart. An interesting next step would be to attempt to reverse this metabolic phenotype and observe whether there is a compensatory increase in function, providing a potential metabolic therapy to protect against doxorubicin-induced cardiotoxicity.