Understanding the in vivo uptake kinetics of a phosphatidylethanolamine-binding agent ^99mTc-Duramycin

Introduction: ^99mTc-Duramycin is a peptide-based molecular probe that binds specifically to phosphatidylethanolamine (PE). The goal was to characterize the kinetics of molecular interactions between ^99mTc-Duramycin and the target tissue. Methods: High level of accessible PE is induced in cardiac tissues by myocardial ischemia (30min) and reperfusion (120min) in Sprague-Dawley rats. Target binding and biodistribution of ^99mTc-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 180min after injection. A partially inactivated form of ^99mTc-Duramycin was analyzed in the same fashion. A compartment model was developed to quantify the uptake kinetics of ^99mTc-Duramycin in normal and ischemic myocardial tissue. Results: ^99mTc-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 ^99mTc-Duramycin is not a limiting factor and the rate constant of target binding in the target tissue is at 2.2ml/nmol/min/g. The number of available binding sites for ^99mTc-Duramycin in ischemic myocardium was estimated at 0.14nmol/g. Covalent modification of D15 resulted in a 9-fold reduction in binding affinity. Conclusion: ^99mTc-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 ^99mTc-Duramycin as a novel PE-binding agent.