Development of Leptospira in vitro potency assays: EU/industry experience and perspectives

Nobivac® Lepto (MSD Animal Health) is a non-adjuvanted canine leptospirosis vaccine containing inactivated whole cells of Leptospira interrogans serogroup Canicola serovar Portlandvere and L. interrogans serogroup Icterohaemorrhagiae serovar Copenhageni. The current standard in vivo potency test is a hamster challenge test associated with major drawbacks such as animal suffering and poor reproducibility. Here, the quantification of antigenic mass by ELISA as a new in vitro potency test is described, supporting the 3Rs concept (replacement, reduction, and refinement of animal tests) and in accordance with European Pharmacopoeia Monograph 0447 (Canine Leptospirosis Vaccine [Inactivated]). The two corresponding sandwich ELISAs are based on monoclonal antibodies specific for immunodominant leptospiral lipopolysaccharide epitopes. Protection in passive immunization experiments demonstrate that these monoclonal antibodies recognize key protective antigens in currently licensed human and veterinary whole cell Leptospira vaccines. The high precision and robustness renders the two ELISAs much more reliable correlates of potency in dogs than the hamster potency test. The recent approval of these assays for a new canine leptospirosis vaccine is an important contribution to the 3Rs in quality control testing of Leptospira vaccines.