Preparation and Preliminary Evaluation of a Promising 99mTc-Labeled Isonitrile-Containing 6-Thia-Fatty Acid Derivative for Myocardial Metabolism Imaging

For over 40 years, none of the previous ^99mTc-labeled fatty acids for myocardial imaging has potential clinical use. ^99mTc-(C10-6-thia-CO₂H)(MIBI)₅ is the first ^99mTc-labeled fatty acid to exhibit good myocardial uptake (2.06 ± 0.06%ID/g) at 60 min post injection, high heart-to-liver ratio (6.43 ± 1.85 and 9.68 ± 0.76), high heart-to-lung ratio (9.48 ± 1.39 and 11.02 ± 0.89), and high heart-to-blood ratio (164.01 ± 43.51 and 197.36 ± 32.29) at 60 and 120 min in Sprague-Dawley (SD) rats, respectively. It also demonstrated excellent myocardial imaging quality. The above target-to-nontarget ratios exceeded those of [¹²³I]BMIPP and were higher than or close to those of ^99mTc-MIBI at 60 and 120 min. Most of ^99mTc-(C10-6-thia-CO₂H)(MIBI)₅ was partially β-oxidized to protein-bound metabolites in myocardium. Administration of trimetazidine dihydrochloride (TMZ, a fatty acid β-oxidation inhibitor) to rats caused 51% reduction in the myocardial uptake of ^99mTc-(C10-6-thia-CO₂H)(MIBI)₅ and 61% reduction in the distribution of ^99mTc-radioactivity in a residual tissue pellet at 60 min, indicating its considerable sensitivity to myocardial fatty acid β-oxidation.

Synthesis and biodistribution studies of 99m Tc labeled fatty acid derivatives prepared via "Click approach" for potential use in cardiac imaging

¹²³ I-Iodophenylpentadecanoic acid (IPPA) is a metabolic agent used in nuclear medicine for diagnosis of myocardial defects. Efforts are underway worldwide to develop a ^99m Tc substitute of the above radiopharmaceutical for the aforementioned application. Herein, we report synthesis and biodistribution studies of ^99m Tc labeled fatty acids (8, 11, and 15 carbons) obtained via "click chemistry" for its potential use in myocardial imaging. ω-Bromo fatty acids (8C/11C/15C) were synthetically modified at bromo terminal to introduce a heterocyclic triazole with glycine sidearm in a five step procedure. Modified fatty acids were subsequently radiolabeled with preformed [^99m Tc(CO)₃ ]⁺ synthon to yield the desired fatty acid complexes which were evaluated in Swiss mice. All the radiolabeled complexes were obtained with radiochemical purities >80%, as characterized by HPLC. Biodistribution studies of all three complexes in Swiss mice showed myocardial uptake of ~6-9% ID/g at 2 minutes post-injection, close to^* I-IPPA (~9% ID/g). Complexes exhibited significant retention in the myocardium up to 30 minutes (~1% ID/g) but were lower to the standard agent (~7% ID/g). Similar uptake of activity in myocardium for the newly synthesized complexes in comparison to ¹²⁵ I-IPPA along with favorable in vivo pharmacokinetics merits potential for the present "click" design of complexes for myocardial imaging.

Development of 11C-Labeled ω-sulfhydryl fatty acid tracer for myocardial imaging with PET

[¹¹C]-S-methyl-16-thiopalmitic acid (a) was developed with excellent heart-to-background uptake ratios and higher retention in heart. Myocardial uptake and metabolism of the tracer is markedly higher CPT I dependent. When compared to [¹¹C]-S-methyl-14-thiomyristic acid (b), [¹¹C]-S-methyl-12-thiododecanoic acid (c) and [¹¹C]-palmitate, a showed an early high uptake and a significantly slower late clearance in heart and a prolonged myocardial elimination half-life (30 min). Analysis of heart tissue and urine samples showed that a was metabolized via beta-oxidation in myocardium. Small animal PET images of the accumulation of a in the rat myocardium were clearly superior to [¹¹C]-palmitate. These initial studies suggest that a could be a potentially useful clinical PET tracer to assess myocardial fatty acid metabolism.