Subcellular Distribution and Metabolism Studies of the Potential Myocardial Imaging Agent [99mTc(N)(DBODC)(PNP5)]+

99m Tc(N)-DBODC(5) is the lead compound of a new series of monocationic 99m Tc(N)-based potential myocardial imaging agents that exhibit original biodistribution properties. This study was addressed to elucidate the mechanisms of distribution, retention, and elimination of this promising 99m Tc(N)-agent. Methods: The sex-related in vitro and in vivo stability and the subcellular distribution of 99m Tc(N)-DBODC(5) were investigated. Studies were performed by considering binding to the serum proteins; stability in rat serum, human serum, and rat liver homogenates; and the chemical integrity of the complex after extraction from rat tissues such as heart, liver, and kidney, as well as from intestinal fluids and urine. The effect of cyclosporin A on the in vivo pharmacokinetic properties of 99m Tc(N)-DBODC(5) was also evaluated. Subcellular distribution of 99m Tc(N)-DBODC(5) in ex vivo rat heart was determined by standard differential centrifugation techniques. Results: No significant in vitro serum protein binding and no notable biotransformation of the native compound into different species by the in vitro action of the serum and liver enzymes was evidenced. In vivo experiments showed that sex affects the pharmacokinetic profile of the 99m Tc(N)-complexes including metabolism and excretion. Chromatographic profiles of 99m Tc(N)-radioactivity extracted from tissues and fluids of female rats were always coincident with the control. Conversely, a small percentage of metabolized species was detected by high-performance liquid chromatography in liver extracts of male rats. Furthermore, administration of cyclosporin A caused a significant reduction of lung, liver, and kidney washout along with a considerable variation in activity distribution in the intestinal tract in both male and female rats, thus indicating a possible implication of Pgp transporters in determining the biologic behavior of 99m Tc(N)-DBODC(5). However, this phenomenon was more pronounced in females. Subcellular distribution studies showed that 86.3% ± 7.4% of 99m Tc(N)-DBODC(5) was localized into mitochondrial fraction as a result of the interaction with the negative membrane potential. Conclusion: Evidence showing that the new 99m Tc(N)-myocardial tracers behave as multidrug resistance-associated protein P-glycoprotein substrates, combined with their selective mitochondrial accumulation, strongly supports the possibility that diagnostic application of 99m Tc(N)-DBODC(5) can be extended to tumor imaging and noninvasive multidrug resistance studies.