Biodistribution and pharmacokinetics of the 99mTc labeled human elastase inhibitor, elafin, in rats

Elafin is a potent reversible inhibitor of the pro-inflammatory proteases leukocyte elastase and protease 3. It is currently in clinical development for the use in postoperative inflammatory diseases. We investigated the pharmacokinetics of 99mTc-labeled elafin (99mTc-Elafin) in blood and individual organs in rat after bolus intravenous injection using the single photon emission tomography (SPECT). 99mTc-Elafin predominantly accumulated in the kidney reaching a maximum of 8.5% ± 0.1% of the injected dose per gram (ID/g) at 5 min post injection (p.i) and decreased only slowly during 24 h. In contrast, the initially high radio activity recorded in the other organs rapidly decreased parallel to the radioactivity detected in blood. The blood kinetics fits to a two compartment kinetics model. The radio activity in the dissected kidney was 4.98 ± 1.24%ID/g 24 h p.i, while in other organs, including the brain, no accumulation of 99mTc-Elafin was detected. At this time point 30% of the detected radioactivity in the kidney was identified to be not metabolized 99mTc-Elafin. In conclusion, the blood and organ-specific kinetic data provide a basis for planning of adequate dosing regimens and the high accumulation of intact elafin in the kidney favors clinical developments targeting inflammatory kidney diseases, such as chronic allograft nephropathy after kidney transplantation.

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