Delineation of the border zone of ischemic rabbit myocardium by a technetium-labeled nitroimidazole.

Delineation of viable ischemic myocardium is an important problem in nuclear cardiology. To determine the feasibility of using a technetium-labeled nitroimidazole as an indicator of ischemic myocardium at risk of infarction, we characterized the distribution of a 2-nitroimidazole-derivatized PnAO ligand and its 99mTc complex, 99mTcO(PnAO)-1-CH2-(2NI) (BMS-181321) in the ischemic territory of the left anterior descending (LAD) coronary artery of the rabbit. In preliminary experiments, the performance of 14C-deoxyglucose (14C-2DG) and 14C-misonidazole was assessed relative to apparent regional relative myocardial blood flow (rMBF) indicated by 99mTc-teboroxime using double-label autoradiography in the rabbit LAD occlusion model. After demonstrating that 14C-2DG and 14C-misonidazole are selectively retained in the lateral border of the ischemic territory, BMS-181321 was co-injected intravenously, with either 14C-2DG or 14C-misonidazole, 20 min after LAD occlusion. In a separate experiment, 99mTcO(PnAO)-6-CH3, a complex with the same lipophilicity (log k' 0.26 vs. 0.31) as BMS-181321 but which lacks the 2NI moiety, was co-injected with 14C-2DG. After 30 min, the rabbits were sacrificed and 14C/99mTc autoradiograms were obtained from the same tissue sections. The autoradiograms revealed that BMS-181321 was retained with the same microregional distribution as both 14C-2DG and 14C-misonidazole in the border zone of the ischemic LAD territory. The selective retention of BMS-181321 depends on the presence of the nitroimidazole group, since 99mTcO(PnAO)-6-CH3 has a uniformly low myocardial distribution in contrast to the enhanced uptake of co-injected 14C-2DG. These data demonstrate that BMS-181321 is selectively retained in hypoxic myocardium and demarcates the ischemic border zone in a manner similar to 14C-2DG and 14C-misonidazole.

Relationship between in vitro transendothelial permeability and in vivo single-pass brain extraction.

UNLABELLED: In vitro transendothelial permeability was compared to in vivo rat single-pass cerebral extractions to evaluate which method would best estimate the blood-brain barrier (BBB) permeability of several SPECT imaging agents. METHOD: Six 99mTc complexes and seven non-Tc complexes were tested in vitro using monolayers of primary bovine brain microvessel endothelial cells and in vivo using the rat single-pass cerebral extraction model. In vitro transendothelial permeability indices (PI) were determined by measuring the average percent of radioactivity traversing the monolayers as a function of time. In vivo single-pass cerebral extractions were determined using an indicator fractionation method. RESULTS: A positive correlation between extraction and PI was found for the non-Tc complexes (r2 = 0.96). The CBF imaging agents 99mTc-ECD and 99mTc-PnAO have high values for E and PI, demonstrating that these agents penetrate the BBB and have a high membrane permeability, while the heart imaging agent 99mTc-sestamibi had low values for both E and PI. The low PI and E values for 99mTc-sestamibi are consistent with a low brain uptake for this agent, except in cases of disruption of the BBB. In contrast to 99mTc-ECD, 99mTc-PnAO and 99mTc-sestamibi, which had concordant values for E and PI, two highly lipophilic boronic acid adducts of technetium dioxime (BATOs), 99mTc-teboroxime and 99mTcCl(DMG)3(2)MP, had low negative values for PI, but high values for E. In addition, after 3 hr of incubation, the monolayer-to-medium concentration ratio of the BATOs was 642:1 and 744:1, respectively. This compares with values of 89:1 (99mTc-PnAO), 25:1 (99mTc-ECD) and 34:1 (99mTc-sestamibi). CONCLUSION: These data suggest that the high in vivo single-pass extraction of the BATOs may be explained by a hydrophobic interaction with the luminal surface of the capillary endothelial cell plasma membrane. We conclude that a high single-pass extraction cannot necessarily be used to infer high BBB or membrane permeability.

Imaging ischemic tissue at risk of infarction during stroke.

Autoradiograms obtained after middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats show that the 99mTc complex of a 2-nitroimidazole-derivatized propylene amine oxime (BMS-181321) is selectively retained in acutely ischemic brain before disruption of the blood-brain barrier (BBB), but not in the ischemic infarct. BMS-181321 is therefore a marker of ischemic tissue at risk of infarction and its uptake, unlike that of x-ray and magnetic resonance contrast agents, does not require disruption of the BBB. In keeping with this conclusion, we have found that the single-pass cerebral extraction fraction of BMS-181321 is 0.67 at normal rat whole-brain blood flow. Sequential single-photon emission computed tomographic images obtained from cats after MCAO show that the initial distribution of BMS-181321 approximates regional CBF and that selective retention subsequently produces a positive image within the ischemic territory. BMS-181321 is the first Tc complex able to indicate not only ischemia, but also ischemic tissue at risk of infarction. Use of this novel Tc complex to monitor biochemical events during ischemia may contribute to the clinical management of acute stroke.

The single-pass cerebral extraction and capillary permeability-surface area product of several putative cerebral blood flow imaging agents.

We have determined cerebral blood flow (CBF) and the single-pass cerebral extraction (E) of several putative agents for external imaging of CBF. Simultaneous measurements of blood flow and extraction were performed in 106 rats. For all agents, comparison of linear and exponential regressions of E on CBF indicates that this relationship can be described as linear over the range of flows studied. Analysis of covariance indicates that the extraction of 123I-IMP, 67Cu-PTSM and 99mTc-HMPAO is higher than that of 99mTc-Cl(DMG)3(2MP) and 99mTc-ECD, particularly at flows above the normal range. Accordingly, for 123I-IMP, 67Cu-PTSM and 99mTc-HMPAO, the slope of the linear regression equation for the relationship between brain capillary permeability surface area product (PS) and CBF is higher than that for 99mTc-Cl(DMG)(3)2MP and 99mTc-ECD. PS varies as a linear function of CBF over the range of flows studied. At a CBF level that corresponds to normal regional CBF for human cortex, 0.5 ml/g/min, all the agents have a single-pass extraction of approximately 70% or greater. While all the agents detected changes in CBF in the normal to ischemic range, at higher flows 123I-IMP, 67Cu-PTSM and 99mTc-HMPAO showed substantially greater fidelity to true CBF than 99mTc-Cl(DMG)(3)2MP and 99mTc-ECD.

Measurement of myocardial blood flow using a co-injection technique for technetium-99m-teboroxime, technetium-96-sestamibi and thallium-201.

We have compared apparent myocardial blood flow (MBFapparent) indicated by 99mTc-teboroxime, 96Tc-sestamibi and 201TI to true MBF indicated by radiolabeled microscopheres using a technique for the co-injection of four radionuclides in the same animal. Studies were performed using rats in a single-pass model to obtain global MBF and using dogs in a multiple-pass model to determine regional MBF. To provide a wide range of MBF, adenosine was administered intravenously and the left anterior descending coronary artery was then ligated in the dogs, or hypercapnia was induced by decreasing respiratory frequency in the rats. The microsphere formula for determining MBF was applied to all agents. When MBFapparent was plotted as a function of true MBF, the ability of each agent to measure changes in true MBF was demonstrated by the proximity of the plotted function to the line of identity. For both the single and multiple-pass studies, statistical analysis of the nonlinear relationship between MBFapparent and true MBF showed that 201TI and 99mTc-teboroxime approximate true MBF better than 96Tc-sestamibi (p less than 0.001) under the conditions used in the present studies. In the single-pass studies, 99mTc-teboroxime approximated true MBF better than 201Tl (p less than 0.05), but in the multiple-pass experiments, 201Tl approximated true MBF better than 99mTc-teboroxime in only one dog (p less than 0.01) with no difference in the other two. Determination of the permeability-surface area product, PS, for each agent shows that the higher fidelity to true MBF obtained with 201Tl and 99mTc-teboroxime is related to substantially greater PS values for these agents relative to 96Tc-sestamibi.

Chloro----hydroxy substitution on technetium BATO [TcCl(dioxime)3 BR] complexes.

The neutral, seven coordinate complexes of technetium known as the BATO (Boronic acid Adducts of Technetium diOximes) complexes have shown their utility as myocardial and cerebral perfusion agents. The axial chloride ligand of the BATO complexes [99mTcCl(dioxime)3 BR] is labile to substitution by a competitive anion; under physiological conditions, the axial chloride ligand can be replaced by a hydroxy group. The chloro and hydroxy analogs have different biodistributions and single-pass cerebral extraction efficiencies. The influence of structure on the rate of the in vitro chloro/hydroxy exchange process has been studied. The mechanism of axial ligand exchange was found to be SN1-CB, which proceeds by way of a transient, neutral six coordinate complex. Evidence is presented which indicates that chloro/hydroxy exchange is not the mechanism by which BATO complexes are retained in the brain.

The relationship between CNS metabolism and cytoarchitecture: a review of 14C-deoxyglucose studies with correlation to cytochrome oxidase histochemistry.

Since the inception of the 14C-deoxyglucose method and its extension to in vivo imaging of regional cerebral glucose metabolism in humans by positron emission tomography, uncertainty has persisted concerning the type of work to which regional metabolism is coupled, as well as the distribution of this work within the neuron. 14C-deoxyglucose studies indicate that functionally-coupled neural metabolism is more apparent in axon terminals and perhaps dendrites than neuronal perikarya. Moreover, it appears that most of the metabolism in axon terminals is accounted for by Na+-K+-ATPase activity. Nevertheless, cytochrome oxidase histochemistry reveals the presence of intensely reactive mitochondria in soma-dendrite regions opposite presynaptic axon terminals, thereby indicating that continuous temporal and spatial summation of postsynaptic graded potentials is associated with increased metabolism. While the situation concerning the relative postsynaptic metabolic prices of EPSP's and IPSP's remains uncertain, the presence of elevated levels of cytochrome oxidase activity within certain classes of presynaptic terminals indicates that active excitation and inhibition is associated with increases in presynaptic metabolism. This observation has been confirmed in 14C-deoxyglucose studies. Nevertheless, studies of neonatal hippocampus indicate that, before metabolic activity shifts to dendritic and telodendritic regions of electrophysiological activity, metabolism is high in somal foci of biosynthesis.