Long-acting delivery systems for peptides: reduced plasma testosterone levels in male rats after a single injection

The study of a biocompatible w/o microemulsion for parenteral administration of peptides was performed. The behaviour of an LH-RH analog, [d-Trp-6]LH-RH, in a w/o microemulsion was evaluated both in vitro and in vivo. After a single intramuscular injection in rats of the microemulsion containing 500 μg/ml of the drug, the plasma levels of testosterone fell; between 10 and 20 days after injection, the plasma testosterone level was lower than that observed after daily injections of 100 μg/kg of the LH-RH analog. The results suggested that prolonged action and protection of biodegradable molecules could be achieved by using w/o microemulsions administered parenterally.