Abstract
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Objectives Heart failure is a leading cause of death worldwide. As a high energy-demanding organ, dysfunction in myocardial energy metabolism is closely linked with cardiac pathophysiology. Mitochondria, as a central organelle in cell energetics, play a vital role in cardiac homeostasis. Loss of the mitochondrial membrane potential is the first irreversible stage of cell death, leading to tissue damage and cardiac remodelling. Lipophilic cations are sensitive to the mitochondrial membrane potential, as such, they have the potential to quantify changes in cardiac energetics as a result of pharmacological intervention or in (models of) disease. We compared the utility of two novel radiolabelled phosphonium cations, [18F]Mitophos_04 and [18F]Mitophos_07, to detect rotenone-induced reductions in mitochondrial membrane potential.
Methods Radioligands were synthesized through a copper-catalysed cycloaddition reaction between [18F]fluoroethyl azide and the terminal alkyne group of the corresponding precursor. As a proof of mechanism, in vitro radioligand assays were performed using murine B-cell lymphoma; cells were treated with the mitochondrial complex 1 inhibitor, rotenone (2 h, 1-10 nM). To evaluate the tracers in vivo, naïve SD rats (n=3-4) received iv administrations of circa. 9 MBq of [18F]Mitophos_04 or [18F]Mitophos_07 and dynamic PET:CT scans were performed. Images were reconstructed (FBP) and regions of interest drawn on the heart to generate time activity curves.
Results In vitro, cellular uptake of [18F]Mitophos_07 was reduced in a dose-dependent manner with a maximum decrease of 30% following rotenone treatment (1-10nM). These results suggest that [18F]Mitophos_07 is sensitive to changes in mitochondrial membrane potential due to rotenone-induced inhibition of mitochondrial complex 1. No significant changes in cellular uptake were observed for [18F]Mitophos_04 with rotenone treatment. In vivo, both tracers showed good metabolic stability with 60 % and 40 % of the [18F]Mitophos_04 and [18F]Mitophos_07 parent compound (60 min). [18F]Mitophos_07 showed greater uptake in the myocardium and thyroid (SUV60min: 4.7±1.5 and 17.4±13 respectively) compared with [18F]Mitophos_04 (SUV60min: 0.6±0.06 and 4.2±2 respectively). Both tracers reached a rapid pseudo-equilibrium with plasma in the myocardium, bone marrow and thyroid after 20min.
Conclusions In vitro, [18F]Mitophos_07 was sensitive to changes in mitochondrial membrane potential induced by rotenone-inhibition of mitochondrial complex 1. The greater accumulation of [18F]Mitophos_07 in rat myocardium, along with the in vitro data, indicates this tracer may be suitable for imaging cardiac energetics and dysfunction, meriting further investigation. In vivo studies, with an intravenous rotenone challenge to inhibit mitochondrial complex 1, will be used to support the proof of mechanism.