Evaluation of novel phosphonium cations as imaging agents for changes in mitochondrial membrane potential

Objectives Changes in mitochondrial bioenergetics and the loss of mitochondrial membrane potential can be early indicators of pathophysiology. Lipophilic phosphonium cations have been shown to accumulate within mitochondria and offer a route to image mitochondrial bioenergetics in vivo. We compare the kinetics of two radiolabelled phosphonium cations, [¹⁸F]MitoPhos_04 [A] and [¹⁸F]MitoPhos_07 [B].

Methods Radioligands were synthesized through a copper-catalysed cycloaddition reaction between [¹⁸F]fluoroethyl azide and the terminal alkyne group of the corresponding precursor. In vitro radioligand assays were performed in murine B-cell lymphoma, naïve and treated with Cisplatin (24hrs,100μM). In vivo evaluations were performed using naïve SD rats (n=3-4), which received iv administrations of 9MBq of [¹⁸F][A] or [¹⁸F][B] to determine biodistribution and ligand kinetics.

Results In vitro, cellular uptakes of 50% and 5% were observed for [¹⁸F][A] and [¹⁸F][B] respectively. The uptake of both ligands was decreased by 70-80% after Cisplatin treatment. In vivo, [¹⁸F][A] was excreted by a hepatobillary route, whereas [¹⁸F][B] was also excreted by a renal route. For both radioligands, activity accumulated in tissues with a high proliferative capacity, such as the hematopoietic system. [¹⁸F][B] demonstrated a greater accumulation in the myocardium and thyroid (SUV_60min:4.7±1.5 and 17.4±13 respectively) compared with [¹⁸F][A] (SUV_60min:0.6±0.06 and 4.2±2 respectively). Both ligands reached a rapid pseudo-equilibrium (<20mins) in the bone marrow, thyroid and myocardium.

Conclusions In vitro data is consistent with the theory that uptake of [¹⁸F][A] and [¹⁸F][B] is sensitive to treatment with pro-apoptotic agents. Both ligands rapidly reached a pseudo-equilibrium in vivo. The initial differences in cellular accumulation and the preferential accumulation of [¹⁸F][B] in myocardial and thyroid tissues warrant further investigation, as well as a comparison in isolated mitochondrial preparations.

Research Support GSK Imanova Ltd