Subcellular binding of 99Tcm-gluconate in infarcted myocardium in dogs

Imaging of the third gasotransmitter hydrogen sulfide using 99mTc-labeled alpha-hydroxy acids

INTRODUCTION

Hydrogen sulfide (H₂S) defined as the third gasotransmitter after nitric oxide (NO) and carbon monoxide (CO) is an important mediator of various physiological functions. Although various H₂S-imaging techniques using fluorescence and luminescence have been developed, only one radionuclide imaging using ⁶⁴Cu-labeled cyclen complex was reported. Thus, we tried to develop ^99mTc-labeled H₂S imaging agents.

METHODS

Various α-hydroxy acids such as glycolate, L-lactate, D-lactate, D-gluconate, D-glucoheptonate, D-glucuronate, D-glucarate, and citrate were labeled with ^99mTc in the presence of stannous chloride. The labeled compounds were incubated with 0.2 mM of NaHS, and reactive sulfur species and then analyzed by ITLC/normal saline to detect the formation of insoluble complex. Matrigels containing various concentrations of NaHS were xenografted on the shoulder of normal mice, and an imaging study was performed after intravenous injection of [^99mTc]Tc-gluconate. We also obtained autoradiography image of a rat brain with a temporary brain ischemia after intravenous injection of [^99mTc]Tc-gluconate.

RESULTS

[^99mTc]Tc-gluconate showed the highest formation of insoluble complex (87.8 ± 3.6%) after incubation with 0.2 mM NaHS. The other reactive species such as glutathione, cysteine, sulfite, sulfate, thiosulfate, and NO did not form insoluble complex representing the reaction being specific to H₂S. The Matrigel containing 2 μmol NaHS showed uptake of [^99mTc]Tc-gluconate, which proved the feasibility as the specific H₂S imaging agent in vivo. Temporary ischemic lesion of rat brain showed high radioactivity accumulation representing the feasibility as endogenous H₂S imaging agents.

CONCLUSION

We proved that ^99mTc-labeled α-hydroxy acid especially [^99mTc]Tc-gluconate is a novel endogenous H₂S imaging agent, which might contribute to study and diagnosis of various diseases related with inflammation and hypoxia.

Very early noninvasive detection of acute experimental nonreperfused myocardial infarction with 99mTc-labeled glucarate

BACKGROUND

99mTc glucarate has recently been reported to be an infarct-avid agent. The feasibility of imaging with 99mTc glucarate was evaluated for the early diagnosis of nonreperfused and reperfused myocardial infarction and compared with localization of simultaneously administered 111In anti-myosin.

METHODS AND RESULTS

Four groups of six rabbits each were studied. The left anterior descending coronary artery (LAD) was kept persistently occluded (n = 6) or reperfused after 40 minutes (n = 6) in rabbits. After confirmation of LAD occlusion by 201Tl scintigraphy, a mixture of 99mTc glucarate (15.7 +/- 1.6 mCi) and 111In anti-myosin (0.53 +/- 0.03 mCi) was administered intravenously. Another group of rabbits (n = 6) with 5 or 15 minutes of LAD occlusion were used to assess the affinity of 99mTc glucarate for the ischemic myocardium. The remaining 6 rabbits with reperfused myocardial infarction were used for the assessment of subcellular localization of 99mTc glucarate. 99mTc glucarate cleared rapidly from circulation (elimination t1/2, 36 minutes). Infarcts were visualized within 10 minutes in reperfused and within 30 minutes in nonreperfused coronary territories after intravenous administration. 111In anti-myosin delineated reperfused infarcts within 1 to 3 hours, but no uptake was seen in persistently occluded rabbits. 99mTc glucarate uptake in reperfused and nonreperfused infarct centers was 28 and 12 times greater, respectively, than that in normal myocardium (P = .0001). A direct correlation between glucarate and anti-myosin localization (r = .60 for nonreperfused; 0.76 for reperfused; P < .0001) was observed. Ischemic hearts showed no glucarate uptake. Subcellularly, 99mTc glucarate localized predominantly in the nuclear fraction of the infarct, with lesser extents in the mitochondrial and cytoplasmic fractions.

CONCLUSIONS

Noninvasive imaging of myocardial infarcts with 99mTc glucarate is possible within minutes in persistently occluded or reperfused myocardial infarcts. Early detectability results from the rapid blood clearance and high avidity of glucarate for the acutely necrotic myocardial tissue.

Quantification of ocular inflammation with technetium-99m glucoheptonate

Histological and morphometric evaluation of ocular inflammation is difficult, particularly when there is extensive ocular involvement with abscess formation and necrosis. A quantitative imaging procedure applicable to humans would be important clinically. To establish such a procedure, turpentine-induced ocular inflammation was obtained by subconjunctival injection in the right eye of 55 rabbits. The left eye was used as control and injected with a volume of saline equal to the volume of turpentine in the right eye. Volumes of turpentine or saline were 0.02, 0.04, 0.06, 0.2 and 0.6 ml, and the rabbits were divided into groups 1-5, according to these volumes. Imaging was performed 48 h after turpentine injection and 6 h after intravenous injection of 10 mCi of technetium-99m glucoheptonate (99mTc-GH). An inflammatory reaction index (IRI), defined as the ratio of counts of the right eye divided by counts of the left eye, was used. IRIs were proportional to the degree of inflammation and allowed the distinction of 3 subgroups: one represented by group 4, one by group 5 and one by groups 1, 2 and 3. This method of quantification of ocular inflammatory processes using 99mTc-GH is original, rapid, non-invasive, reproducible and safe, although unable to differentiate inflammatory processes caused by doses of turpentine which are very small and close to each other. It is conceivable that its application to humans will bring new insight into the ocular inflammatory process and response to therapy.

Accumulation of 99Tcm-gluconate in daunorubicin-treated neonatal heart cells in culture

The cardiotoxic effect of daunorubicin was investigated using cultured heart cells from neonatal rat hearts. Exposure of the cells to low but toxic concentrations of daunorubicin led to drastically altered permeability properties of the plasma membrane as measured by release of lactate dehydrogenase. Simultaneously, 99Tcm-gluconate, an isotope complex used in scintigraphic imaging of myocardial infarcts, was accumulated in the cells. The results suggest that daunorubicin-induced cardiac damage may be estimated by isotope uptake.

Binding of 99mTc-gluconate to heart mitochondria

99mTc-gluconate has previously been shown to bind to isolated rat heart mitochondria. In the presence of potassium cyanide this binding is enhanced several fold, an effect which has been shown to be correlated to the efficiency of the in vivo binding of the isotope complex in ischemic dog hearts. The present investigation shows that the potassium cyanide-induced binding was mainly localized to the mitochondrial inner membrane; in the presence of cyanide other organelles, e.g., nuclei and liver microsomes also showed some binding. Boiling, lipid extraction, or addition of N-ethylmaleimide caused a variable inhibition of the binding of 99mTc-gluconate to rat heart mitochondria, which also was markedly influenced by temperature, pH, time, and concentrations of protein and isotope complex, but not by conditions affecting energy levels or calcium transport. Fractionation of submitochondrial particles exposed to 99mTc-gluconate in the presence of potassium cyanide indicated that the 99mTc-gluconate-binding component cofractionated with cytochrome oxidase. It is postulated that a protein component localized in the mitochondrial inner membrane, possibly cytochrome oxidase, is responsible for the binding of 99mTc-gluconate.