Microsuspension of fatty acid esters of entecavir for parenteral sustained delivery.

Entecavir (EV), an anti-viral agent for hepatitis B infection, should be administered under fasted state, as intestinal absorption of this hydrophilic compound is markedly decreased under post-prandial conditions. Herein, in order to improve therapeutic adherence, a parenteral sustained delivery system was constructed, by synthesizing water-insoluble ester prodrugs of the nucleotide analogous with fatty acids. EV-3-palmitate (named EV-P), exhibited the lowest solubility in phosphate buffered saline (pH 7.4, 1.1 μg/ml), with extended release profile compared with EV, EV-3-myristate, and EV-3-stearate, was selected as a candidate to formulate drug suspension. The crystalline suspension was fabricated using anti-solvent crystallization technique, with a mean particle size of 7.7 μm. After subcutaneous (SC) injection in beagle dogs (0.43 mg/kg as EV), the plasma concentrations of EV were markedly protracted with lowered maximum plasma concentration (C, 4.7 ng/ml), extended time required to reach C (T, 9.0 days), and lengthened elimination half-life (T 129.3 h) compared with those after oral administration (0.0154 mg/kg, C, 15.4 ng/ml; T, 0.01 days; T, 4.1 h). The systemic exposure of the lipidic prodrug was below 0.1% compared with that of EV following SC injection, denoting that EV-P was rapidly converted into the parent compound in blood. Therefore, SC delivery of EV-P microsuspension can be an alternative to oral EV therapy, offering prolonged pharmacokinetic profile after single injection.