Radioiodinated fatty acid analogs for myocardial imaging.

Abstract

Fatty acids are the preferred substrate for the normoxic heart. About sixty percent of the energy required by the myocardium is provided by fatty acid $\beta$-oxidation. Many scientists have focused on the alterations in fatty acid metabolism in the ischemic heart for the development of radiolabelled fatty acids for functional imaging of the heart. Three main categories of compounds were synthesized: tetrazoles (1 and 2), glycidic and $\alpha$-methylene acids (3-5), and analogs of oleic acid (6, 7 and 7A). The tetrazole group has a similar pKa and size to that of a carboxyl group; however, such fatty acid analogs cannot undergo normal fatty acid metabolism. Glycidic and $\alpha$-methylene analogs are potential irreversible inhibitors of fatty acid metabolism. Oleic acid analogs were investigated to assess the affect of stereochemical consequences on biodistribution. The key intermediates in the synthesis of the target compounds were $\omega$-nitrophenyl alkylcarboxylic acids and alcohols, which were made using a variety of cross-coupling reactions. The Wittig reaction, which was used in the synthesis of tetrazole 1 and glycidic acid 3, gave low yields of the cross-coupled products. Therefore, the remaining target compounds were synthesized by condensation of appropriate RCu (CN) ZnI and substituted benzyl bromides or by Pd$\sp{\rm II}$ catalyzed cross-coupling of substituted arylhalides with suitable alkynes. The latter two reactions produced much higher yields of the desired products. All of the target compounds were radiolabeled with $\sp{125}$I by various Cu(I) catalyzed radioiodine exchange procedures and were then subjected to tissue biodistribution (TD) studies in rats. Except for the 15-(4-iodophenyl)-2-methylene-pentadecanoic acid (5), all of the fatty acid analogs failed to surpass clinically-used 15-(4-iodophenyl)pentadecanoic acid (IPPA) in their ability to be taken up and retained by the rat myocardium. Tissue distribution of 5 indicated that it was taken up and cleared by the heart in a manner similar to that of IPPA. Moreover, scintigraphic studies performed in rats and rabbits showed both compounds to have identical imaging properties.