The in vivo half-life of human interferon alpha-2b (hIFN-alpha-2b) is relatively short, and frequent injections over prolonged periods are required for efficacy. An extended-release formulation of hIFN-alpha-2b (Depo/IFN) was created by encapsulation into a lipid-based drug-delivery system. The capture efficiency was 51% +/- 13% and the release half-life in human plasma at 37 degrees C was 16 days. The pharmacokinetics of Depo/IFN was compared with that of unencapsulated standard hIFN-alpha-2b (Std/IFN) in the peritoneal cavity of male BDF1 mice. Depo/IFN exhibited a 13-fold longer intraperitoneal (i.p.) half-life as compared with Std/IFN (20 vs 1.5 h). The release of free hIFN-alpha-2b from Depo/IFN into the peritoneal cavity was slow and protracted, with a 10-fold lower peak concentration and a 13-fold longer apparent half-life being observed in comparison with Std/IFN. The areas under the curve of free hIFN-alpha-2b in the peritoneal cavity were comparable for Depo/IFN and Std/IFN. hIFN-alpha-2b was detectable in plasma only after the i.p. administration of Std/IFN. These data suggest the possibility that Depo/IFN may be useful as an extended-release formulation of hIFN-alpha-2b.