Immunogenicity of single-dose diphtheria vaccines based on PLA/PLGA microspheres in guinea pigs

Abstract

Biodegradable polyester microspheres (MS) have shown potential for single-dose vaccines. This study examined the immunogenicity of diphtheria toxoid (Dtxd) microencapsulated in different types of poly(lactide) (PLA) and poly(lactide- co-glycolide) (PLGA) MS prepared by the methods of spray-drying and coacervation. We investigated the influence of polymer type (PLGA 50:50 of low M w; PLA of high M w; end-group stearylated PLAs of low M w) and co-encapsulated excipients (BSA and/or trehalose) on Dtxd content, in vitro release and immunogenicity in guinea pigs. The co-encapsulated trehalose lowered the Dtxd entrapment efficiency in the spray-dried particles from 75 to 56%, whereas albumin alone had no effect in the spray-drying, but improved the encapsulation in the coacervation process. With the hydrophobic, end-group stearylated PLAs, Dtxd could only be encapsulated in the presence of albumin. Guinea pigs immunised with Dtxd-MS made with the relatively hydrophilic PLGA 50:50 exhibited specific and sustained antibody responses over 40 weeks, comparable to the responses to alum-adjuvanted toxoid. In contrast, undetectable or very low antibody responses were determined after immunisation with MS made with hydrophobic polymers. Surprisingly, large (15–60 μm) and small (1–5 μm) MS gave comparable primary antibody responses. In conclusion, the data presented confirm the feasibility of MS vaccines to induce strong, long-lasting protective antibody responses after a single immunisation.