Defining the optimal formulation and schedule of a candidate toxoid vaccine against Clostridium difficile infection: A randomized Phase 2 clinical trial

Clostridium difficile, a major cause of nosocomial and antibiotic-associated diarrhea, carries a significant disease and cost burden. This study aimed to select an optimal formulation and schedule for a candidate toxoid vaccine against C. difficile toxins A and B. Randomized, placebo-controlled, two-stage, Phase 2 study in a total of 661 adults aged 40–75 years. Stage I: low (50μg antigen) or high (100μg antigen) dose with or without aluminum hydroxide (AlOH) adjuvant, or placebo, administered on Days 0–7–30. Stage II: Days 0–7–30, 0–7–180, and 0–30–180, using the formulation selected in Stage I through a decision tree defined a priori and based principally on a bootstrap ranking approach. Administration was intramuscular. Blood samples were obtained on Days 0, 7, 14, 30, 60 (Stage I and II), 180, and 210 (Stage II); IgG to toxins A and B was measured by ELISA and in vitro functional activity was measured by toxin neutralizing assay (TNA). Safety data were collected using diary cards. In Stage I the composite immune response against toxins A and B (percentage of participants who seroconverted for both toxins) was highest in the high dose+adjuvant group (97% and 92% for Toxins A and B, respectively) and was chosen for Stage II. In Stage II the immune response profile for this formulation through Day 180 given on Days 0–7–30 ranked above the other two administration schedules. There were no safety issues. The high dose+adjuvant (100μg antigen+AlOH) formulation administered at 0–7–30 days elicited the best immune response profile, including functional antibody responses, through Day 180 and was selected for use in subsequent clinical trials