Pharmacokinetic Analysis of Compounds Instilled into the Mouse Lung: Study with In Vivo Electron Spin Resonance Spectroscopy and Nitroxyl Spin Probes as Model Drugs.

Abstract

Introduction: Detailed understanding of the absorption mechanisms of compounds in the lungs is important for developing effective systems for the pulmonary administration of drugs. This study analyzed the pharmacokinetics of model compounds instilled into the mouse lungs using in vivo electron spin resonance (ESR) spectroscopy. Methods: An aqueous solution of a nitroxyl probe with cationic, anionic, or neutral groups was instilled into the lungs of healthy mice at high concentrations as model drugs, and the behavior of the probes was assessed using concentration-dependent changes in the linewidths of ESR signals obtained in live mice. Results: When solutions of nitroxyl probes at high concentrations, which produce broad ESR signals, were instilled into the mouse lungs, sharp ESR signals originating from diluted probes were superimposed onto broad signals. Broad signals decreased at various rates for all probes depending on their lipophilicity. Sharp signals for neutral probe and anionic probe immediately increased after instillation and then decreased. Sharp signals for cationic probe with a quaternary ammonium group continued to increase after instillation. Pharmacokinetic analysis of the blood concentrations of these probes suggests that the probes are distributed to lung tissues in addition to the blood. The concentration dependence of the initial velocity of broad signal decay suggests the possibility that the transfer of charged probes from the alveolar space to the bloodstream may be mediated by transporters, whereas a neutral probe may be transferred via passive diffusion. Conclusion: The differences in pharmacokinetic behavior in lungs could be examined in vivo among model compounds with different charge states. In vivo ESR spectroscopy is a powerful tool for the in vivo analysis of pulmonary pharmacokinetics, in combination with nitroxyl probes as model drugs.