In vitro/in vivo characterization of a tramadol HCl depot system composed of monoolein and water.

Various monoolein-water systems containing tramadol HCl, a potent analgesic, were formulated to obtain sustained-release dosage forms which could be administered by subcutaneous, intramuscular or intrathecal injections. They were examined for their in vitro drug-release profiles and in vivo analgesic properties in rats in a 14 h period following intramuscular administration. In order to obtain a lower viscosity, we have substituted a part of monoolein by oleic acid and phospholipids. Both binary (monoolein-water) and quaternary (oleic acid-phospholipid-monoolein-water) formulations exhibited controlled drug-release profiles which were accelerated by surfactant adjunction. This surfactant action was probably due to structural changes in the lipid arrangement and was much more pronounced for the modified formulations. According to the results obtained in vitro, formulations with slower drug release (i.e. the native formulation and the modified one without surfactant) were selected for assessment of their in vivo properties. Both formulations demonstrated prolonged analgesic activities in the rat tail flick test manifested by stable pain relief during more than 10 h compared with the 3 to 4 h analgesia obtained with the commercially available tramadol HCl solution. The sustained-release capabilities were evaluated by using a modified half value duration (HVD) ratio and all sustained-released formulations exhibited a HVD ratio equal or superior to 3.9.