Toxicological evaluation of a synthetic QS-21 adjuvant in a tuberculosis vaccine candidate—Demonstrating safety and immunological equivalence to natural reference

Synthetic saponin-based adjuvants hold promise for overcoming the supply instability and purity limitations inherent to naturally derived QS-21. However, establishing their toxicological and functional equivalence to the natural reference is a necessary step before clinical use. This study systematically evaluated the safety and immunogenicity of a recombinant tuberculosis vaccine (named LYB007) formulated with a synthetic QS-21-Api compared to an identical formulation using a naturally-derived reference via single- and repeated-dose toxicity assessments in Sprague-Dawley (SD) rats. Both synthetic and naturally-derived formulations exhibited highly comparable safety and immunological profiles. Injection site reactogenicity was self-limiting, with complete histological resolution by Day 58. Systemic physiological adaptations, including transient thermal shifts and acute-phase protein fluctuations, were consistent with the potent immunostimulatory activity of QS-21 and did not have long-term toxicological significance. Notably, both formulations induced robust, uniform (100%) seroconversion with identical kinetics of antibody development and persistence, with peak geometric mean titers (GMTs) consistently attained between Day 29 and Day 43. The strict seronegativity in negative and mock-vaccine control groups confirmed that the humoral response was exclusively driven by the LYB007-adjuvant complex. Histopathological findings, such as splenic germinal center expansion and extramedullary hematopoiesis, were identified as expected immunological adaptations rather than manifestations of systemic toxicity. In conclusion, these findings confirm that the synthetic QS-21-Api possesses a safety and efficacy profile equivalent to the naturally-derived reference. The use of a chemically defined and standardized adjuvant component overcomes key constraints in vaccine scalability and quality control, supporting the development of future high-efficacy subunit vaccines.