Carrier-mediated delivery of 9-(2-phosphonylmethoxyethyl)adenine to parenchymal liver cells: a novel therapeutic approach for hepatitis B

Our aim is to selectively deliver 9-(2-phosphonylmethoxyethyl)adenine (PMEA ) to parenchymal liver cells, the primary site of hepatitis B virus (HBV) i nfection. Selective delivery is necessary because PMEA, which is effective against HBV in vitro, is hardly taken up by the liver in vivo. Lactosylated reconstituted high-density lipoprotein (LacNeoHDL), a lipid particle that is specifically internalized by parenchymal liver cells via the asialoglyco protein receptor, was used as the carrier. PMEA could be incorporated into the lipid moiety of LacNeoHDL by attaching, via an acid-labile bond, lithoc holic acid-3 alpha-oleate to the drug. The uptake of the lipophilic prodrug (PMEA-LO) by the liver was substantially increased after incorporation int o LacNeoHDL. Thirty minutes after injection of [H-3]PMEA-LO-loaded LacNeoHD L into rats, the liver contained 68.9% +/- 7.7% of the dose (free [H-3]PMEA , <5%). Concomitantly, the uptake by the kidney was reduced to <2% of the d ose (free [H-3]PMEA, >45%). The hepatic uptake of PMEA-LO-loaded LacNeoHDL occurred mainly by parenchymal cells (88.5% +/- 8.2% of the hepatic uptake) , Moreover, asialofetuin inhibited the liver association by >75%, indicatin g uptake via the asialoglycoprotein receptor. The acid-labile linkage in PM EA-LO, designed to release PMEA during lysosomal processing of the prodrug- loaded carrier, was stable at physiological pH but was hydrolyzed at lysoso mal pH (half-life, 60 to 70 min). Finally, subcellular fractionation indica tes that the released PMEA is translocated to the cytosol, where it is conv erted into its active diphosphorylated metabolite. In conclusion, lipophili c modification and incorporation of PMEA into LacNeoHDL improves the biolog ical fate of the drug and may lead to an enhanced therapeutic efficacy agai nst chronic hepatitis B.