Comparison of 18F-labeled fluoroalkylphosphonium cations with 13N-NH3 for PET myocardial perfusion imaging

Despite substantial advances in the diagnosis of cardiovascular disease, there is a need for 18F-labeled myocardial perfusion agents for the diagnosis of ischemic heart disease because current PET tracers for myocardial perfusion imaging have a short half-life that limits their widespread clinical use in PET. Thus, 18F-labeled fluoroalkylphosphonium derivatives (¹⁸F-FATPs), including (5-¹⁸F-fluoropentyl)triphenylphosphonium cation (¹⁸F-FPTP), (6-¹⁸F-fluorohexyl)triphenylphosphonium cation (¹⁸F-FHTP), and (2-(2-¹⁸F-fluoroethoxy)ethyl)triphenylphosphonium cation (¹⁸F-FETP), were synthesized. The myocardial extraction and image quality of the ¹⁸F-FATPs were compared with those of 13N-NH3 in rat models. Methods: The first-pass extraction fraction (EF) values of the ¹⁸F-FATPs (¹⁸F-FPTP, ¹⁸F-FHTP, ¹⁸F-FETP) and 13N-NH3 were measured in isolated rat hearts perfused with the Langendorff method (flow velocities, 0.5, 4.0, 8.0, and 16.0 mL/min). Normal and myocardial infarction rats were imaged with small-animal PET after intravenous injection of 37 MBq of ¹⁸F-FATPs and 13N-NH3. To determine pharmacokinetics, a region of interest was drawn around the heart, and time-activity curves of the ¹⁸F-FATPs and 13N-NH3 were generated to obtain the counts per pixel per second. Defect size was analyzed on the basis of polar map images of ¹⁸F-FATPs and 13N-NH3. Results: The EF values of ¹⁸F-FATPs and 13N-NH3 were comparable at low flow velocity (0.5mL/min), whereas at higher flows EF values of ¹⁸F-FATPs were significantly higher than those of 13N-NH3 (4.0, 8.0, and 16.0 mL/min, P, 0.05). Myocardium-to-liver ratios of ¹⁸F-FPTP, ¹⁸F-FHTP, ¹⁸F-FETP, and 13N-NH3 were 2.10 ± 0.30, 4.36 ± 0.20, 3.88 ± 1.03, and 0.70 ± 0.09, respectively, 10 min after injection, whereas myocardium-to-lung ratios were 5.00 ± 0.25, 4.33 ± 0.20, 7.98 ± 1.23, and 2.26 ± 0.14, respectively. Although ¹⁸F-FATPs and 13N-NH3 sharply delineated myocardial perfusion defects, defect size on the 13N-NH3 images was significantly smaller than on the ¹⁸F-FATP images soon after tracer injection (0-10min, P 5 0.027). Conclusion: ¹⁸F-FATPs exhibit higher EF values and more rapid clearance from the liver and lung than 13N-NH3 in normal rats, which led to excellent image quality in a rat model of coronary occlusion. Therefore, ¹⁸FFATPs are promising new PET radiopharmaceuticals for myocardial perfusion imaging.