Quantitative analysis of [99mTc]C2A-GST distribution in the area at risk after myocardial ischemia and reperfusion using a compartmental model

Pharmaceutical Manipulation of Mitochondrial F0F1-ATP Synthase Enables Imaging and Protection of Myocardial Ischemia/Reperfusion Injury Through Stress-induced Selective Enrichment

To rescue ischemic myocardium from progressing to myocardial infarction, timely identification of the infarct size and reperfusion is crucial. However, fast and accurate identification, as well as the targeted protection of injured cardiomyocytes following ischemia/reperfusion (I/R) injury, remain significantly challenging. Here, a near infrared heptamethine dye IR-780 is shown that has the potential to quickly monitor the area at risk following I/R injury by selectively entering the cardiomyocytes of the at-risk heart tissues. Preconditioning with IR-780 or timely IR-780 administration before reperfusion significantly protects the heart from ischemia and oxidative stress-induced cell death, myocardial remodeling, and heart failure in both rat and pig models. Furthermore, IR-780 can directly bind to F0F1-ATP synthase of cardiomyocytes, rapidly decrease the mitochondrial membrane potential, and subsequently slow down the mitochondrial energy metabolism, which induces the mitochondria into a "quiescent state" and results in mitochondrial permeability transition pore inhibition by preventing mitochondrial calcium overload. Collectively, the findings show the feasibility of IR-780-based imaging and protection strategy for I/R injury in a preclinical context and indicate that moderate mitochondrial function depression is a mode of action that can be targeted in the development of cardioprotective reagents.

Understanding the in vivo uptake kinetics of a phosphatidylethanolamine-binding agent (99m)Tc-Duramycin

INTRODUCTION

(99m)Tc-Duramycin is a peptide-based molecular probe that binds specifically to phosphatidylethanolamine (PE). The goal was to characterize the kinetics of molecular interactions between (99m)Tc-Duramycin and the target tissue.

METHODS

High level of accessible PE is induced in cardiac tissues by myocardial ischemia (30 min) and reperfusion (120 min) in Sprague-Dawley rats. Target binding and biodistribution of (99m)Tc-duramycin were captured using SPECT/CT. To quantify the binding kinetics, the presence of radioactivity in ischemic versus normal cardiac tissues was measured by gamma counting at 3, 10, 20, 60 and 180 min after injection. A partially inactivated form of (99m)Tc-Duramycin was analyzed in the same fashion. A compartment model was developed to quantify the uptake kinetics of (99m)Tc-Duramycin in normal and ischemic myocardial tissue.

RESULTS

(99m)Tc-duramycin binds avidly to the damaged tissue with a high target-to-background radio. Compartment modeling shows that accessibility of binding sites in myocardial tissue to (99m)Tc-Duramycin is not a limiting factor and the rate constant of target binding in the target tissue is at 2.2 ml/nmol/min/g. The number of available binding sites for (99m)Tc-Duramycin in ischemic myocardium was estimated at 0.14 nmol/g. Covalent modification of D15 resulted in a 9-fold reduction in binding affinity.

CONCLUSION

(99m)Tc-Duramycin accumulates avidly in target tissues in a PE-dependent fashion. Model results reflect an efficient uptake mechanism, consistent with the low molecular weight of the radiopharmaceutical and the relatively high density of available binding sites. These data help better define the imaging utilities of (99m)Tc-Duramycin as a novel PE-binding agent.

Radiotracers for noninvasive molecular imaging of tumor cell death

The need to monitor cancer therapy-induced cellular and tissue changes using noninvasive imaging techniques continues to stimulate both basic and clinical research. Monitoring changes in cellular proliferative capacity that occur after treatment with radiation and/or chemotherapy has the potential to provide longitudinal information on the cellular dynamics of tumors before, during, and after therapeutic intervention. Cells can lose their reproductive potential through one of several mechanisms, including apoptosis and autophagy (which are forms of programmed cell death), premature senescence, or necrosis. When a tumor responds to therapy, current imaging methods do not provide information about the exact mechanism of cell death executed. We are now beginning to develop the molecular imaging tools that will enable us to noninvasively image cell death mechanisms both in experimental models and in the clinical cancer environment. Studies with these imaging tools will contribute to a better understanding of therapeutic responses and assist in the design and evaluation of more effective treatments. This review examines the state-of-the-art in the use of (radio)tracers for the purpose of imaging mechanisms of tumor cell inactivation (cell death) in animal models and in clinical trials.

Efficient site-specific radiolabeling of a modified C2A domain of synaptotagmin I with [99mTc(CO)3]+: a new radiopharmaceutical for imaging cell death

We describe the design and synthesis of a new Tc-99m labeled bioconjugate for cell-death imaging, based on C2A, the phosphatidylserine (PS)-binding domain of rat synaptotagmin I. Since several lysine residues in this protein are critical for PS binding, we engineered a new protein, C2AcH, to include the C-terminal sequence CKLAAALEHHHHHH, incorporating a free cysteine (for site-specific covalent modification) and a hexahistidine tag (for site-specific radiolabeling with [99mTc(CO)3(OH2)3]+). We also engineered a second derivative, C2Ac, in which the C-terminal sequence included only the C-terminal cysteine. These proteins were characterized by electrospray mass spectrometry, SDS/PAGE, and size exclusion chromatography and radiolabeled with [99mTc(CO)3(OH2)3]+. Conjugates of the proteins with the rhenium analogue [Re(CO)3(OH2)3]+ were also synthesized. Site-specific labeling was confirmed by performing a tryptic digest of rhenium tricarbonyl-labeled C2AcH, and only peptides containing the His-tag contained the [Re(CO)3]+. The labeled proteins were tested for binding to red blood cells (RBC) with exposed PS in a calcium dependent manner. Labeling 100 microg of C2AcH with [99mTc(CO)3(OH2)3]+ at 37 degrees C for 30 min gave a radiochemical yield of > 96%. However, C2AcH that had first been conjugated with fluorescein maleimide or iodoacetamide via the Cys residue gave only 50% and 83% radiochemical yield, respectively, after incubation for 30 min at 37 degrees C. Serum stability results indicated that >95% of radiolabeled C2AcH remained stable for at least 18 h at 37 degrees C. Site-specifically labeled C2AcH exhibited calcium-dependent binding to the PS on the RBC, whereas a nonspecifically modified derivative, C2AcH-B, in which lysines had been modified with benzyloxycarbonyloxy, did not. We conclude that (i) the combination of Cys and a His-tag greatly enhances the rate and efficiency of labeling with [99mTc(CO)3(OH2)3]+ compared to either the His-tag or the Cys alone, and this sequence deserves further evaluation as a radiolabeling tag; (ii) non-site-specific modification of C2A via lysine residues impairs target binding affinity; (iii) 99mTc-C2AcH has excellent radiolabeling, stability and PS binding characteristics and warrants in vivo evaluation as a cell-death imaging agent.

99mTc-labeled duramycin as a novel phosphatidylethanolamine-binding molecular probe

With only 19 amino acids, duramycin is the smallest known polypeptide that has a defined 3-dimensional binding structure. Duramycin binds phosphatidylethanolamine (PtdE) at a 1:1 ratio with high affinity and exclusive specificity. As an abundant binding target, PtdE is a major phospholipid and accounts for about 20% of the phospholipid content in mammalian cellular membranes. PtdE is externalized to the surface of apoptotic cells and also becomes accessible in necrotic cells because of compromised plasma membrane integrity. Given the unique physicochemical properties of duramycin and the availability of PtdE in acute cell death, the goal of this study was to develop and evaluate 99mTc-duramycin as a novel molecular probe for imaging PtdE.

METHODS

Duramycin is covalently modified with succinimidyl 6-hydrazinonicotinate acetone hydrazone (HYNIC) and labeled with 99mTc using a coordination chemistry involving tricine-phosphine coligands. The retention of PtdE-binding activities was confirmed using competition assays with PtdE-containing liposomes. The blood clearance, pharmacokinetics, and biodistribution of 99mTc-duramycin were measured in rats. Finally, 99mTc-duramycin binding to acute cell death in vivo was demonstrated using a rat model of acute myocardial infarction induced by ischemia and reperfusion and confirmed using autoradiography and histology.

RESULTS

HYNIC-derivatized duramycin with 1:1 stoicheometry was synthesized and confirmed by mass spectrometry. The radiolabeling efficiency was 80%-85%, radiochemical purity was 78%-89%, and specific activity was 54 GBq. The radiotracer was purified with high-performance liquid chromatography radiodetection before use. The specific uptake of 99mTc-duramycin in apoptotic cells, compared with that in viable control cells, was enhanced by more than 30-fold. This binding was competitively diminished in the presence of PtdE-containing liposomes but not by liposomes consisting of other phospholipid species. Intravenously injected 99mTc-duramycin has favorable pharmacokinetic and biodistribution profiles: it quickly clears from the circulation via the renal system, with a blood half-life of less than 4 min in rats. The hepatic and gastrointestinal uptake were very low. 99mTc-duramycin is completely unmetabolized in vivo, and the intact agent is recovered from the urine. Combined with a fast clearance and low hepatic background, the avid binding of 99mTc-duramycin to the infarcted myocardium quickly becomes conspicuous shortly after injection. The uptake of radioactivity in infarcted tissues was confirmed by autoradiography and histology.

CONCLUSION

99mTc-duramycin is a stable, low-molecular-weight PtdE-binding radiopharmaceutical, with favorable in vivo imaging profiles. It is a strong candidate as a molecular probe for PtdE imaging and warrants further development and characterization.