16-Cyclopentadienyl Tricarbonyl 99mTc 16-Oxo-hexadecanoic Acid: Synthesis and Evaluation of Fatty Acid Metabolism in Mouse Myocardium

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 Bioevaluation of the Cyclopentadienyl Tricarbonyl Technetium-99m 2-Nitroimidazole Derivatives for Tumor Hypoxia Imaging

Hypoxia imaging agents can play an important role in the tumor treatment by avoiding the worse effect of radiotherapy and chemotherapy due to the tumor hypoxia. Due to the small size and easy coordination, tricarbonyl technetium-99m can be used to label a wide range of imaging agents. In this work, the tricarbonyl ^99mTc labeled small-sized hypoxia imaging agents containing 2-nitroimidazoles were prepared, which have different carbon chain lengths between cyclopentadienyl and 2-nitroimidazole, and which have one or two 2-nitroimidazole groups. The results of S180 cell experiment and biodistribution indicated that these molecules have different hypoxic selectivity. When contains one 2-nitroimidazole, as the carbon chain lengthens, which means the molecular volume becomes larger, hypoxia cellular uptake and selectivity decrease in S180 cell uptake experiment. In biodistribution study in mice bearing S180 tumor, Tc-2 (1-cyclopentadienyl-5-(2-nitro-1H-imidazol-1-yl)-pentan-1-one tricarbonyl ^99mTc complex), which has intermediate carbon chain, is better due to the more complex factors. Its tumor/blood (T/B) ratio is 3.56±0.25, tumor/muscle(T/M) ratio is 1.73±0.29 and tumor uptake is 2.23±0.24 %ID/g at 2 h. Comparing to other tricarbonyl technetium complexes containing one 2-nitroimidazole, the complexes in this work have an advantage in tumor/blood ratio and tumor uptake. This suggests that the small-volume cyclopentadienyl may have an advantage when used as a ligand. When contains two 2-nitroimidazole groups, the complex, 1-cyclopentadienyl-5-di(2-(2-nitro-1H-imidazol-1-yl)ethyl)amino-pentan-1-one tricarbonyl ^99mTc complex (Tc-4), has the better results in the cell experiment than those which contain one 2-nitroimidazole group. Thus the hypoxia imaging agent contains two 2-nitroimidazole groups is more advantageous, but further modifications of Tc-4 are needed to improve its clearance rate in the blood, because the increased lipophilicity leads to a decrease in the T/B ratio of Tc-4. In conclusion, small volume hypoxia imaging agents with two 2-nitroimidazole groups may be the trend of development.

Cysteine Derivatized 99mTc-Labelled Fatty Acids as β-Oxidation Markers

With the aim of developing 99mTc-labeled fatty acids intended for myocardial metabolism imaging we report herein the synthesis and characterization of two novel derivatives of undecanonoic and hexadecanonoic acid that have been functionalized at the ω-site by cysteine through the formation of a thioether bond (Cys–FA11 and Cys–FA16). Equimolar amounts of each ligand and the [NEt4]2[Re(CO)3Br3] precursor generated the respective hexacoordinated neutral complexes in which the ligand coordinated to the metal through the SNO donor system of cysteine. The rhenium complexes were characterized by elemental analysis, IR and NMR spectroscopies. The analogous technetium-99m complexes, 99mTc–Cys–FA11 and 99mTc–Cys–FA16 were prepared by incubation of the ligand with the precursor [99mTc(CO)3(H2O)3]+ (radiochemical yield ≥98%). Their structure was established by comparative HPLC techniques. In vivo studies in mice showed high initial heart uptake for both 99mTc complexes (7.4 ± 0.53 and 7.07 ± 0.73 percentage of injected dose (%ID)/g at 1 min post injection. Rapid clearance (0.60 ± 0.02 %ID/g) was observed for 99mTc–Cys–FA11 while the clearance of the longer fatty acid 99mTc–Cys–FA16 was slower (2.31 ± 0.09 %ID/g at 15 min p.i.). Metabolite analysis study indicated that complexes were catabolized through the β-oxidation process.

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.

Synthesis and evaluation of 68Ga labeled palmitic acid for cardiac metabolic imaging

This work evaluates the potential of a ⁶⁸Ga labeled long chain 16C fatty acid for cardiac metabolic imaging. For radiolabeling with ⁶⁸Ga, hexadecanedioic acid was coupled with the chelator p-NH₂-Bn-NOTA. Under the optimized conditions, NOTA-hexadecanoic acid could be radiolabeled with ⁶⁸Ga in ≥95% yields. In biodistribution studies carried out in Swiss mice, ⁶⁸Ga-NOTA-hexadecanoic acid showed low myocardial uptake at 2 min p.i. (3.7 ± 1.3%ID/g). While ⁶⁸Ga-NOTA-hexadecanoic acid cleared rapidly from non-target organs such as blood, lungs, intestine and kidney, wash out from liver was slow. Radio-HPLC analyses of myocardial extracts of rats injected with ⁶⁸Ga-NOTA-hexadecanoic acid confirmed its metabolic transformation in the myocardium.

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.

68Ga labeled fatty acids for cardiac metabolic imaging: Influence of different bifunctional chelators

Development of ⁶⁸Ga labeled fatty acids is of immense interest due to the availability of ⁶⁸Ga through a generator and its superiority over SPECT based tracers in carrying out dynamic imaging on a PET scanner. Our present work explores the influence of different chelators on the cardiac uptake and pharmacokinetics of the ⁶⁸Ga-labeled fatty acids. Two new ⁶⁸Ga labeled fatty acids were synthesized by conjugation of 11-aminoundecanoic acid with the bifunctional chelators (BFCs) viz. p-SCN-Bn-DTPA (S-2-(4-isothiocyanatobenzyl)-diethylenetriaminepentaacetic acid) and p-SCN-Bn-NODAGA (S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1-glutaric acid-4,7-acetic acid) and their comparison was carried out with the previously reported ⁶⁸Ga-NOTA-undecanoic acid. Both the conjugates were radiolabeled with ⁶⁸Ga in high yields and purities (>95%). Their formation was established by preparation and characterization of their inactive analogs with ^natGa at macroscopic levels. Biodistribution studies of the complexes in Swiss mice showed lower initial myocardial uptake for ⁶⁸Ga-NODAGA-undecanoic acid (3.8±0.6%ID/g) and ⁶⁸Ga-DTPA-undecanoic acid (1.3±0.5%ID/g) complexes in comparison to previously reported ⁶⁸Ga-NOTA-undecanoic acid complex (7.4±2.8%ID/g) at 2min p.i. However, significant retention of the tracer in the myocardium was observed in the case of ⁶⁸Ga-NODAGA-undecanoic complex, which led to improved heart/non-target ratios of the complex over time in comparison to the other ⁶⁸Ga complexes. Similarly, the DTPA complex exhibited increased washout from the liver in comparison to other ⁶⁸Ga derivatives. The β oxidation mechanism in myocytes was investigated by isolating the myocardial extract post intravenous injection of the respective ⁶⁸Ga complexes and analyzing them by radio-HPLC, which showed metabolic transformation of the parent fatty acid complex peak in all the three complexes. This study has provided an insight into the design characteristics of ⁶⁸Ga labeled fatty acids to achieve the desired myocardial imaging characteristics.

Preliminary studies of a novel cyclopentadienyl tricarbonyl technetium-99m fatty acid derivative for myocardical imaging

This study reports the synthesis and evaluation studies of 6'-cyclopentadienyl tricarbonyl technetium-99m 6'-oxo-11-(hexanamide)undecanoic acid (1). 1 was prepared with 26.5 ± 4.3% of radiochemical yield and more than 98% of radiochemical purity. Tissue distribution in mice showed that high radioactivity accumulated in the heart with moderate clearance. However, unfortunately, similar to those of other technetium-labeled fatty acid analogs, the biodistribution studies of 1 in mice showed poor heart-to-blood ratios, which suggested that 1 cannot be used as myocardial imaging agent, and it may provide a theoretical basis or a lab experience for corresponding fatty acid tracers studies.