A Tc-99m-Labeled Long Chain Fatty Acid Derivative for Myocardial Imaging

C-11- and I-123-labeled long chain fatty acid derivatives have been reported as useful radiopharmaceuticals for the estimation of myocardial fatty acid metabolism. We have reported that Tc-99m-labeled N-[[[(2-mercaptoethyl)amino]carbonyl]methyl]-N-(2-mercaptoethyl)-6-aminohexanoic acid ([ 9 9 m Tc]-MAMA-HA), a medium chain fatty acid derivative, is metabolized by β-oxidation in the liver and that the MAMA ligand is useful for attaching to the omega-position of fatty acid derivatives as a chelating group for Tc-99m. On the basis of these findings, we focused on developing a Tc-99m-labeled long chain fatty acid derivative that reflected fatty acid metabolism in the myocardium. In this study, we synthesized a dodecanoic acid derivative, MAMA-DA, and a hexadecanoic acid derivative, MAMA-HDA, and performed radiolabeling and biodistribution studies. [ 9 9 m Tc]MAMA-DA and [ 9 9 m Tc]MAMA-HDA were prepared using a ligand-exchange reaction. Biodistribution studies were carried out in normal mice and rats. Then, a high initial uptake of Tc-99m was observed, followed by a rapid clearance from the heart. The maximum heart/blood ratio was 3.6 at 2 min postinjection of [ 9 9 m Tc]MAMA-HDA. These kinetics were similar to those with postinjection of p-[ 1 2 5 I]iodophenylpentadecanoic acid. Metabolite analysis showed [ 9 9 m Tc]MAMA-HDA was metabolized by β-oxidation in the body. In conclusion, [ 9 9 m Tc]MAMA-HDA is a promising compound as a long chain fatty acid analogue for estimating β-oxidation of fatty acid in the heart.