Synthesis of a lipophilic prodrug of 9-(2-phosphonylmethoxyethyl)adenine (PMEA) and its incorporation into a hepatocyte-specific lipidic carrier.

Purpose: 9-(2-Phosphonylmethoxyethyl)adenine (PMEA), a potent inhibitor of Hepatitis B virus replication, is in vivo hardly taken up by parenchymal liver cells (the site of infection). Our aim is to examine whether lactosylated reconstituted HDL (LacNeoHDL), a lipidic particle that is specifically internalized by parenchymal liver cells, is a suitable carrier for the selective delivery of PMEA to this cell type.

Methods: To incorporate PMEA into LacNeoHDL, we synthesized a lipophilic prodrug (PMEA-LO) by coupling PMEA via an acid-labile phosphonamidate bond to lithocholic acid-3alpha-oleate.

Results: The yield of the synthesis was 52% ([3H]PMEA-LO: 24%). [3H]PMEA-LO readily incorporated into LacNeoHDL (13 molecules/particle) without affecting the size and net negative charge of the carrier. Further, incubation studies at lysosomal pH showed [3H]PMEA was completely released from the carrier whereas, at neutral pH or in plasma, appreciable release was not observed.

Conclusions: The conjugation of PMEA with lithocholic acid-3alpha-oleate results in a lipophilic prodrug that readily associates with Lac-NeoHDL. The association of the prodrug does not affect the physicochemical properties of the particle, and PMEA is released from the carrier at lysosomal pH. These findings indicate that by using the prodrug approach, LacNeoHDL is a suitable carrier to deliver PMEA to parenchymal liver cells.