Assessment of the sensitivity of 2H MR spectroscopy measurements of [2,3-2H2]fumarate metabolism for detecting tumor cell death

Imaging the metabolism of [2,3-H-2(2)]fumarate to produce malate can be used to detect tumor cell death post-treatment. Here, we assess the sensitivity of the technique for detecting cell death by lowering the concentration of injected [2,3-H-2(2)]fumarate and by varying the extent of tumor cell death through changes in drug concentration. Mice were implanted subcutaneously with human triple negative breast cancer cells (MDA-MB-231) and injected with 0.1, 0.3, and 0.5 g/kg [2,3-H-2(2)]fumarate before and after treatment with a multivalent TRAlL-R2 agonist (MEDI3039) at 0.1, 0.4, and 0.8 mg/kg. Tumor conversion of [2,3-H-2(2)]fumarate to [2,3-H-2(2)]malate was assessed from a series of 13 spatially localized H-2 MR spectra acquired over 65 min using a pulse-acquire sequence with a 2-ms BIR4 adiabatic excitation pulse. Tumors were then excised and stained for histopathological markers of cell death: cleaved caspase 3 (CC3) and DNA damage (terminal deoxynucleotidyl transferase dUTP nick end labeling [TUNEL]). The rate of malate production and the malate/fumarate ratio plateaued at tumor fumarate concentrations of 2 mM, which were obtained with injected [2,3-H-2(2)]fumarate concentrations of 0.3 g/kg and above. Tumor malate concentration and the malate/fumarate ratio increased linearly with the extent of cell death determined histologically. At an injected [2,3-H-2(2)]fumarate concentration of 0.3 g/kg, 20% CC3 staining corresponded to a malate concentration of 0.62 mM and a malate/fumarate ratio of 0.21. Extrapolation indicated that there would be no detectable malate at 0% CC3 staining. The use of low and nontoxic fumarate concentrations and the production of [2,3-H-2(2)]malate at concentrations that are within the range that can be detected clinically suggest this technique could translate to the clinic.

Automated summary

The sensitivity of a metabolism imaging technique for detecting tumor cell death was assessed by lowering the concentration of injected [2,3-H-2(2)]fumarate and varying the extent of cell death through changes in drug concentration. The results showed that the production of [2,3-H-2(2)]malate plateaued at a tumor fumarate concentration of 2 mM, and the tumor malate concentration and malate/fumarate ratio increased linearly with the extent of cell death determined histologically.