Evaluation of splenic apoptosis with 4-[¹⁸F]Fluorophenyl)triphenyl phosphonium ion (¹⁸F-TTP)

Objectives Both the “intrinsic” and “extrinsic” apoptotic pathways are usually associated with mitochondrial membrane permeabilization and loss of membrane potential, which can be measured by a voltage sensor. Recently we developed ¹⁸F-TTP (Shoup et al, Mol Imaging Biol 2011;13(3):511-517). We have studied this tracer in a variety of systems including burn injury where we observed reduced uptake in a variety of tissues - spleen showing the most prominent effects. In this report we studied the details of the apoptotic effect of burn injury in spleen.

Methods Groups of 6 C57/BL6 mice were studied. The animals received 30 % total body surface area full-thickness thermal injury on their dorsal surface or sham treatment. At 24 h after injury, the animals were studied with three systems. 100 μCi of ¹⁸F-TTP was injected via tail-vein and biodistribudtion was measured after 20 min. The spleens were harvested for apoptosis assessment by the Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) technique. The number (percentage) of TUNEL positive apoptotic cells was scored.

Results The results of the ¹⁸F-TTP biodistribution studies demonstrated that peak uptake in the spleen occurred at between 20 - 40 min after injection. Tracer uptake in burned animals was reduces to 1.13 ± 0.17 compared with 3.28 ± 1.24 (p<0.005) in sham treated controls. The results of TUNEL staining revealed that 26.8±6.4% of the cells in white medulla of spleen were apoptotic in burned animal compared with 4.6 ± 2.1% (p<0.005) in sham treated controls.

Conclusions ¹⁸F-TPP is a promising new voltage sensor for detecting mitochondrial dysfunction, and burn-induced apoptotic status in spleen.These results demonstrate that ¹⁸F-TTP is a useful tracer for the in vitro evaluation of apoptosis. In the future this tracer could be of value for studying for quantifying apoptosis non invasively with PET.