Evaluation of a Mitochondrial Voltage Sensor, (¹⁸F-Fluoropentyl)Triphenylphosphonium Cation, in a Rat Myocardial Infarction Model

Abstract

Radiolabeled lipophilic cationic compounds, such as 18 F-labeled phosphonium salt, accumulate in the mitochondria through a negative inner transmembrane potential. The purpose of this study was to develop and evaluate ( 18 F-fluoropentyl)triphenylphosphonium salt ( 18 F-FPTP) as a myocardial PET agent. Methods: A reference compound of 18 F-FPTP was synthesized via 3-step nucleophilic substitution reactions and was radiolabeled via 2-step nucleophilic substitution reactions of no-carrier-added 18 F-fluoride. Accumulations of 18 F-FPTP, 3 H-tetraphenylphosphonium, and 99m Tc-sestamibi were compared in a cultured embryonic cardiomyoblast cell line (H9c2). The biodistribution of 18 F-FPTP was assessed using BALB/c mice. The 18 F-FPTP small-animal PET study was performed in Sprague-Dawley rats with or without left coronary artery (LCA) ligation. Results: 18 F-FPTP was synthesized with a radiochemical yield of 15%-20% and radiochemical purity of greater than 98%. Specific activity was greater than 6.3 TBq/mmol. Cell uptake of 18 F-FPTP was more than 15-fold higher in H9c2 than in normal fibroblasts (human normal foreskin fibroblasts). Selective collapse of mitochondrial membrane potential substantially decreased cellular uptake for 18 F-FPTP and 3 H-tetraphenylphosphonium, compared with that for 99m Tc-sestamibi. The biodistribution data in mice (n 5 24) showed rapid blood clearance and high accumulation in the heart. Heart-to-blood ratios at 10 and 30 min were 54 and 133, respectively. Heart-to-lung and heart-to-liver ratios at 10, 30, and 60 min were 4, 4, and 7 and 4, 5, and 7, respectively. Dynamic small-animal PET for 60 min after injection of 18 F-FPTP showed an initial spike of radioactivity, followed by retention in the myocardium and rapid clearance from the background. 18 F-FPTP small-animal PET images in LCA-occluded rats demonstrated sharply defined myocardial defects in the corresponding area of the myocardium. The myocardial defect size measured by 18 F-FPTP small-animal PET correlated closely with the hypoperfused area measured by quantitative 2,3,5-triphenyltetrazolium chloride staining (r 2 5 0.92, P , 0.001). Conclusion: The excellent pharmacokinetics of 18 F-FPTP and its correlation with 2,3,5-triphenyltetrazolium chloride staining in normal and LCAoccluded rats suggest that this molecular probe may have a high potential as a mitochondrial voltage sensor for PET. This probe may also allow high throughput, with multiple daily studies and a wide distribution of PET myocardial imaging in the clinic.

Comparison of 18F-Labeled Fluoroalkylphosphonium Cations with 13N-NH3 for PET Myocardial Perfusion Imaging

“…Furthermore, the lipophilic interaction between the triphenylphosphonium cation and the lipid layer is attractive because the alkyl group increases lipophilicity (30,31). Thus, we synthesized several radiolabeled phosphonium cations and assessed lipophilicity at different carbon chain lengths and with different functional groups to derive the uptake kinetics (9,10,12,15,16). The three 18 F-FATP derivatives in this study ( 18 F-FPTP, 18 F-FHTP, and 18 F-FETP) were easily synthesized from the reaction of triphenylphosphine with a precursor and exhibited appropriate cationic activity and lipophilicity for penetration of the mitochondrial membrane and accumulation within (log P 5 1.31 6 0.02, 1.78 6 0.05, and 0.89 6 0.02, respectively).…”

“…The perfusion value of each segment was measured as the average value of all pixels in the segment. The infarct area, which was defined as the fraction of the polar map segment with a perfusion value of 60% or less relative to that of the segment with the highest value, was expressed as a percentage of the left ventricular myocardium (16).…”

“…To address this unmet clinical need, several publications have proposed 18 F-labeled phosphonium cations as myocardial imaging agents (9)(10)(11)(12)(13)(14)(15)(16). Similar to SPECT tracers such as 99m Tc-sestamibi and 99m Tc-tetrofosmin, phosphonium cations accumulate in cardiomyocytes as a result of the higher mitochondrial membrane potential (11,(17)(18)(19)(20).…”

“…Similar to SPECT tracers such as 99m Tc-sestamibi and 99m Tc-tetrofosmin, phosphonium cations accumulate in cardiomyocytes as a result of the higher mitochondrial membrane potential (11,(17)(18)(19)(20). Previously, we reported the synthesis and characterization of novel 18 F-labeled fluoroalkylphosphonium derivatives ( 18 FFATPs), including (5-18 F-fluoropentyl)triphenylphosphonium cation ( 18 F-FPTP), (6-18 F-fluorohexyl)triphenylphosphonium cation ( 18 F-FHTP), and (2-(2-18 F-fluoroethoxy)ethyl)triphenylphosphonium cation ( 18 F-FETP), as a voltage sensor for myocardial imaging (9,10,16). In vivo biodistribution and imaging studies of 18 F-FATPs demonstrated intense initial myocardial uptake and retention of these tracers, with rapid clearance from liver and lung.…”

Comparison of Reconstruction Methods in a Small Animal Cardiac Positron Emission Tomography Study Using a 18F-Labeled Myocardial Agent, [18F] FPTP

“…A CRC-712MH radioisotope calibrator was used for measurement of radioactivity. Normal rats were imaged using dedicated microPET (Inveon following the described method (13). In brief, activated 18 F fluoride was added to 4.0 mg of pentane-1, 5-diyl bis (4-methylbenzenesulfonate) dissolved in acetonitrile.…”