The pilot phase I dose escalation clinical trial of RiVax, completed by investigators at the University of Texas Southwestern Medical Center (UT Southwestern), marks the first time a ricin toxin vaccine has ever been clinically tested in humans.
The trial enrolled 15 volunteers in groups of five who were vaccinated with three successive monthly injections of the same dose level of RiVax. Three dose levels of RiVax were evaluated.
The vaccine was prepared without an adjuvant to determine whether the subunit itself was immunogenic and safe. Even without an adjuvant, RiVax induced antibodies in all five of the individuals who received the highest dose, four out of five who received the intermediate dose, and one out of five who received the lowest dose levels.
The functional activity of the antibodies was confirmed by transferring serum globulins from the vaccinated individuals along with active ricin toxin to sensitive mice, which then survived the treatment.
“The fact that the animals survived in the presence of antibodies from the vaccines shows that the antibodies were functionally active,” stated Dr Ellen Vitetta, director of the Cancer Immunobiology Center at UT Southwestern. “Based on the levels of antibodies made by the volunteers, we expect that the human volunteers would be protected against ricin toxin exposure.”