Evaluation of [¹⁸F]Mitophos_04 as an imaging agent for changes in mitochondrial membrane potential

Objectives Mitochondria are the powerhouses of living cells, producing cellular energy in the form of ATP. The collapse of the mitochondrial membrane potential is an early step in apoptosis, a form of programmed cell death and common target pathway of anti-cancer drugs. The ability to image changes in mitochondrial membrane potential in vivo could indicate early treatment response. Lipophilic cations, such as phosphonium salts, accumulate in active mitochondria and this is exacerbated in cancer cells. This work assessed [¹⁸F]Mitophos_04 as a probe for imaging apoptosis.

Methods [¹⁸F]MitoPhos_04 was synthesized through a copper mediated 1,3-cycloaddition reaction between between [¹⁸F]fluoroethyl azide and the terminal alkyne group of the corresponding precursor. Radioligand uptake assays were performed with murine B-cell lymphoma, naïve and treated with 100μM Cisplatin. FLICA and propidium iodide co-stained flow cytometry assessed the cellular state post-treatment. SD rats received bolus iv administrations of 8.9±1MBq [¹⁸F]MitoPhos_04 to determine ligand biodistribution, dosimetry and metabolite analysis.

Results [¹⁸F]MitoPhos_04 was successfully synthesised via a rapid two-step radiosynthesis (≤1h), resulting in high radiochemical purity (≥99%) and suitable radiochemical yields (0.6-1.5GBq). Metabolite studies showed good tracer stability (50% parent tracer at 90min) with the liver acting as the predominant organ of excretory metabolism. Dosimetry estimates yielded a total effective dose of 21μSv/MBq. In vitro cellular uptake was good (40% of administered dose), stable after 30min incubation time and significantly decreased by ca. 60% at 12h and 70% at 24h post-Cisplatin treatment. Decreases correlated with increased cellular staining, supporting a pro-apoptotic state. In vivo, activity accumulated in tissues with a high proliferative capacity, such as the hematopoietic system and thyroid.

Conclusions The tracers promising in vitro results warrant further in vivo evaluation.