Safety and immunogenicity of a Shigella flexneri 2a Invaplex 50 intranasal vaccine in adult volunteers

Shigellosis is a leading cause of diarrhea worldwide prompting vaccine development. The Shigella flexneri Invaplex 50 is a macromolecular complex containing IpaB, IpaC, and LPS, formulated from an aqueous extract of virulent Shigella delivered via nasal administration. Preclinical vaccine testing demonstrated safety, immunogenicity and efficacy. An open-label dose-escalating phase 1 study evaluated a 3-dose (2-week intervals) regimen via nasal pipette delivery. Thirty-two subjects were enrolled into one of four vaccine dose groups (10, 50, 240, or 480 μg). The vaccine was well tolerated with minor short-lived nasal symptoms without evidence of dose effect. Antibody-secreting cell (ASC) responses were elicited at doses ≥50 μg with the highest IgG ASC, Invaplex 50 (100%) and S. flexneri 2a LPS (71%), as well as, serologic responses (43%) occurring with the 240 μg dose. Fecal IgA responses, Invaplex 50 (38.5%) and LPS (30.8%), were observed at doses ≥240 μg. The Invaplex 50 nasal vaccine was safe with encouraging mucosal immune responses. Follow-on studies will optimize dose, delivery mechanism and assess efficacy in a S. flexneri 2a challenge study.