Introduction As a primary stability-indicating parameter, potency should be strategically evaluated during each phase of vaccine development. Herein, we present potency testing during the early clinical development of the Schistosoma mansoni ( Sm ) Tetraspanin-2 vaccine formulated on Alhydrogel (Sm -TSP-2/Al). As Sm -TSP-2/Al does not induce sterilizing immunity against its target pathogen ( Sm ) in animal models, potency is measured by “ serological substitution”, a method that can add significant variation to the potency metric, especially when used in a compliance (or ‘ single data point’ ) approach. Methods Potency data were analyzed using the compliance approach to determine if two clinical lots of Sm -TSP-2/Al retained potency over 84 and 36 months post-release, respectively. These same data were also analyzed by: i) least-squares regression with a joinpoint regression analysis; ii) control charting of stability slopes; and iii) bootstrap modeling. Nested-regression and bootstrapping were used to compare the potency of the first (#11-69F-003) and second (#1975) clinical lots of Sm -TSP-2/Al. Results Despite significant variability in the immune assay, both clinical lots of Sm -TSP-2/Al remained potent for 84 and 36 months, respectively, in all four statistical approaches. The first lot of Sm -TSP-2/Al showed a gain in potency starting at 36 months post-release as captured by joinpoint regression. The two clinical lots of Sm -TSP-2/Al had comparable long-term potency. Conclusion While a compliance approach can monitor the long-term stability of Sm -TSP-2/Al, it risks putting this critical stability-indicating parameter out of specification with each time point tested due to statistical multiplicity. Alternative statistical methods, such as joinpoint regression or bootstrapping, do not have this limitation and offer even more precise estimations of potency, with the added benefit of also providing predictive analytics. Nested regression and bootstrapping were shown to be a viable alternatives for lot-to-lot comparisons of the stability of Sm -TSP-2/Al. Instructions for implementing both these potency testing approaches are provided.

Background Recombinant Schistosoma mansoni Tetraspanin-2 formulated on Alhydrogel (Sm-TSP-2/Alhydrogel) is being developed to prevent intestinal and hepatic disease caused by S. mansoni. The tegumentary Sm-TSP-2 antigen was selected based on its unique recognition by cytophilic antibodies in putatively immune individuals living in areas of ongoing S. mansoni transmission in Brazil, and preclinical studies in which vaccination with Sm-TSP-2 protected mice following infection challenge. Methods A randomized, observer-blind, controlled, Phase 1b clinical trial was conducted in 60 healthy adults living in a region of Brazil with ongoing S. mansoni transmission. In each cohort of 20 participants, 16 were randomized to receive one of two formulations of Sm-TSP-2 vaccine (adjuvanted with Alhydrogel only, or with Alhydrogel plus the Toll-like receptor-4 agonist, AP 10–701), and 4 to receive Euvax B hepatitis B vaccine. Successively higher doses of antigen (10 μg, 30 μg, and 100 μg) were administered in a dose-escalation fashion, with progression to the next dose cohort being dependent upon evaluation of 7-day safety data after all participants in the preceding cohort had received their first dose of vaccine. Each participant received 3 intramuscular injections of study product at intervals of 2 months and was followed for 12 months after the third vaccination. IgG and IgG subclass antibody responses to Sm-TSP-2 were measured by qualified indirect ELISAs at pre- and post-vaccination time points through the final study visit. Results Sm-TSP-2/Alhydrogel administered with or without AP-10-701 was well-tolerated in this population. The most common solicited adverse events were mild injection site tenderness and pain, and mild headache. No vaccine-related serious adverse events or adverse events of special interest were observed. Groups administered Sm-TSP-2/Alhydrogel with AP 10–701 had higher post-vaccination levels of antigen-specific IgG antibody. A significant dose-response relationship was seen in those administered Sm-TSP-2/Alhydrogel with AP 10–701. Peak anti-Sm-TSP-2 IgG levels were observed approximately 2 weeks following the third dose, regardless of Sm-TSP-2 formulation. IgG levels fell to low levels by Day 478 in all groups except the 100 μg with AP 10–701 group, in which 50% of subjects (4 of 8) still had IgG levels that were ≥4-fold higher than baseline. IgG subclass levels mirrored those of total IgG, with IgG1 being the predominant subclass response. Conclusions Vaccination of adults with Sm-TSP-2/Alhydrogel in an area of ongoing S. mansoni transmission was safe, minimally reactogenic, and elicited significant IgG and IgG subclass responses against the vaccine antigen. These promising results have led to initiation of a Phase 2 clinical trial of this vaccine in an endemic region of Uganda. Trial registration NCT03110757.