o-BF3-Phosphonium pincer moieties in the design of delocalized lipophilic cation based tracers for PET imaging of mitochondrial function - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Journal of Nuclear Medicine Année : 2017

o-BF3-Phosphonium pincer moieties in the design of delocalized lipophilic cation based tracers for PET imaging of mitochondrial function


Objectives: Delocalized lipophilic cations, such as triphenylphosphonium (TPP), are able to cross cell membranes and are known to accumulate in the mitochondria in a membrane potential (MMP) dependent manner. MMP is a reliable indicator of mitochondrial and, consequently, cell and tissue function, hence MMP dependent uptake radiotracers are of interest in cardiac and tumour imaging. Conventional C-18F radiofluorination methods require strict aprotic conditions and heating, moreover, C-18F fluorinated tracers may suffer from metabolic defluorination reducing the quality of the images. The objective of this study was to investigate a suitability of o-B[18F]F3-phosphonium pincer moiety, easily accessible via radiofluorination of boronic ester precursors in aqueous mixtures to give high stability, for use as MMP dependent tracers. Methods: Required boronic ester precursors and cold 19F-fluorine labelled reference compounds 1-4 (Fig. 1) were prepared and purified following modified published synthetic protocols. Radiofluorination was carried out in a 0.2 mg scale with carrier added 18F-fluoride at 55oC in 30 min in 20% acetonitrile / water mixture acidified by addition of concentrated hydrochloric acid. LogD7.4 values were determined via the shake flask method. Stability in human serum was assessed using radio-HPLC analysis. The isolated mitochondria uptake assay was performed on freshly isolated mitochondria from Sprague-Dawley rat heart tissue. Cell uptake of the tracer was measured in normal H9c2 cardiac and U87 tumour cells in presence and absence of a membrane depolarizing agent 2-[2-(3-chlorophenyl)hydrazineylyidene]propanedinitrile (CCCP). PET/CT imaging and biodistribution studies were performed in CD-1 nude mice bearing U87 tumour xenografts. Overall charge distributions were calculated using fully optimised geometries at TPSS-D3/def2-TZVPP theory level. 18FHTP (6-18F-fluorohexyltriphenylphosphonium) was prepared in an original one step radiolabelling procedure from an iodinated precursor and was used to validate all assays and as a benchmark for the novel tracers. Results: 18FHTP and 18F-1-4 were obtained in 20-25% decay corrected radiochemical yields within 60 min from the end of bombardment. The LogD7.4 values ranged between 1.78±0.06 (18FHTP and 18F-2) and 0.52±0.01 (18F-1), with 0.88±0.09 and 1.11±0.08 for 18F-3 and 18F-4, respectively. All radiotracers showed 蠅 95% stability in human serum for 3 h. Tracers 18FHTP and 18F-2 showed a comparable MMP dependent uptake in isolated mitochondria (~8%), with the uptake of 18F-3 and 18F-4 below 4% and MMP independent, indicating the importance of the lipophilicity. U87 cell uptake of all tracers was low and MMP independent (~ 0.1% and 1% for 18FHTP and 18F-1-4 respectively). 18F-2 showed the highest and the only MMP dependent cell uptake from the series of new compounds(~ 4% vs ~7% observed for 18FHTP). None of BF3 group containing tracers showed any cardiac or tumour uptake in vivo. Overall charge distribution analysis in a computational study revealed a localised positive charge at the boron atom with partial negative charges carried by fluorine atoms or an ipso carbon of one of the unsubstituted phenyl rings. Conclusion: A series of o-BF3 substituted triphenylphosphonium tracers were successfully synthesized and radiolabelled with fluorine-18. A preliminary in vitro investigation confirmed the importance of the lipophilicity for mitochondrial uptake of the tracer, however, this was not sufficient to ensure cell membrane transport and consequent in vivo localization. The comparison of overall charge distribution maps suggests that addition of a BF3 group introduces a localized charge which is not compensated for by the charge of phosphonium and cannot be delocalised into the phenyl rings.


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hal-01566444 , version 1 (21-07-2017)


  • HAL Id : hal-01566444 , version 1


Shubhanchi Nigam, Juozas Domarkas, Julie Bernard, Goncalo Clemente, Benjamin Burke, et al.. o-BF3-Phosphonium pincer moieties in the design of delocalized lipophilic cation based tracers for PET imaging of mitochondrial function . Journal of Nuclear Medicine, 2017, 58 (Supplément 1), pp.482. ⟨hal-01566444⟩
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