Disposition of the Acyclic Nucleoside Phosphonate ( S )-9(3-Hydroxy-2-Phosphonylmethoxypropyl)Adenine

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Abstract
The acyclic nucleoside phosphonate ( S )-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine [( S )-HPMPA] has been shown to be active against pathogens, like hepatitis B viruses and Plasmodium parasites, that infect parenchymal liver cells. ( S )-HPMPA is therefore an interesting candidate drug for the treatment of these infections. To establish effective therapeutic protocols for ( S )-HPMPA, it is essential that the kinetics of its hepatic uptake be evaluated and that the role of the various liver cell types be examined. In the present study, we investigated the disposition of ( S )-HPMPA and assessed its hepatic uptake. Rats were intravenously injected with [ 3 H]( S )-HPMPA, and after an initial rapid distribution phase (360 ± 53 ml/kg of body weight), the radioactivity was cleared from the circulation with a half-life of 11.7 ± 1.4 min. The tissue distribution of [ 3 H]( S )-HPMPA was determined at 90 min after injection (when >99% of the dose cleared). Most (57.0% ± 1.1%) of the injected [ 3 H]( S )-HPMPA was excreted unchanged in the urine. The radioactivity that was retained in the body was almost completely recovered in the kidneys and the liver (68.4% ± 2.5% and 16.1% ± 0.4% of the radioactivity in the body, respectively). The uptake of [ 3 H]( S )-HPMPA by the liver occurred mainly by parenchymal cells (92.1% ± 3.4% of total uptake by the liver). Kupffer cells and endothelial cells accounted for only 6.1% ± 3.5% and 1.8% ± 0.8% of the total uptake by the liver, respectively. Preinjection with probenecid reduced the hepatic and renal uptake of [ 3 H]( S )-HPMPA by approximately 75%, which points to a major role of a probenecid-sensitive transporter in the uptake of ( S )-HPMPA by both tissues. In conclusion, we show that inside the liver, ( S )-HPMPA is mainly taken up by parenchymal liver cells. However, the level of uptake by the kidneys is much higher, which leads to nephrotoxicity. An approach in which ( S )-HPMPA is coupled to carriers that are specifically taken up by parenchymal cells may increase the effectiveness of the drug in the liver and reduce its renal toxicity.

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