Validation of 99mTc-labeled “4+1” fatty acids for myocardial metabolism and flow imaging

2 + 1 Tricarbonyl Complexes of Technetium(I) with a Combination of N,N-Bidentate Ligands and Ethyl Isocyanoacetate: How Strong Is the Interfering Effect of Chloride Ions on Their Formation?

Technetium(I) 2 + 1 tricarbonyl complexes with a combination of N,N-bidentate ligands (2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) and ethyl isocyanoacetate were prepared and characterized by NMR, IR, UV/visible, and luminescence spectroscopies and by high-performance liquid chromatography (HPLC). The crystal structures of [99Tc(CO)3(bipy)(CNCH2COOEt)](ClO4) (in the form of a solvate with 0.5CH2Cl2) and [99Tc(CO)3(phen)(CNCH2COOEt)](ClO4) (in the form of an adduct with an outer-sphere phen molecule) were determined by single-crystal X-ray diffraction. To evaluate the interfering effect of chloride ions on the formation of the 2 + 1 complexes, the kinetics of the replacement of labile monodentate ligand X in the complexes [MX(CO)3(N∧N)] (M = Re, 99Tc; N∧N = bipy, phen; X = Cl-, ClO4-) by CNCH2COOEt in ethanol were compared. The 99Tc bipy complexes with X = ClO4- (according to the IR data, perchlorate anion in ethanol is displaced from the coordination sphere by the solvent molecule) and X = Cl- are characterized by close ligand replacement rates. In the case of the 99Tc complexes with phen and Re complexes with both phen and bipy, the chloride complexes are appreciably less reactive than the chloride-free complexes. The technetium complexes are considerably more reactive in ligand replacement than their rhenium analogues. In the chloride-containing medium (saline), the complex [99mTc(CO)3(bipy)(CNCH2COOEt)]+ can be prepared under the conditions acceptable for nuclear medical applications, although higher isonitrile concentrations are required as compared to the chloride-free system.

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.

Myocardial accumulation of bupivacaine and ropivacaine is associated with reversible effects on mitochondria and reduced myocardial function

BACKGROUND

Mechanisms of local anesthetic cardiac toxicity are still not completely understood. In this study, we analyzed whether concentrations of local anesthetics found in clinical toxicity affect myocardial mitochondrial structure and oxygen consumption.

METHODS

Guinea pig isolated heart Langendorff preparations were exposed to bupivacaine (3.0 and 7.5 μg/mL) and ropivacaine (3.6 and 9.0 μg/mL) for 10 minutes. Heart rate, systolic blood pressure, the first derivative of left ventricular pressure (+dP/dt), electrocardiogram, and coronary flow were recorded. The local anesthetic tissue concentration was measured either immediately after local anesthetic exposure, or after 20- and 60-minute washout periods. In addition, electron microscopy of myocardial mitochondria was performed using a scoring system for structural damage of mitochondria. Cardiomyocyte cell culture was incubated with bupivacaine, and oxygen consumption ratio, extracellular acidification, and relative amounts of PGC-1α mRNA, a regulator of cellular energy metabolism, were determined.

RESULTS

Bupivacaine and ropivacaine induced reversible PR interval and QRS prolongation, and left ventricular pressure and +dP/dt reduction. Myocardial tissue concentration of local anesthetics was 3-fold the arterial concentration. Mitochondria showed a significant concentration-dependent morphological swelling after local anesthetic application. These changes were reversed by a 20-minute washout period for ropivacaine and by a 60-minute washout for bupivacaine. Bupivacaine reduced mitochondrial oxygen consumption and increased PGC-1α expression in neonatal cardiomyocyte cell cultures, whereas fatty acid metabolism remained unaffected.

CONCLUSIONS

Bupivacaine and ropivacaine accumulate in the myocardium. Reversible local anesthetic-induced mitochondrial swelling occurs at concentrations that induce a negative inotropic effect. Bupivacaine reduces cellular metabolism, whereas this reduction is reversible by fatty acids. Interaction with mitochondria may contribute to the negative inotropic effect of local anesthetics.