SAN FRANCISCO, CA—The 50:50mcg rAT:rLukS-PV vaccine formulation had a favorable immunogenicity profile, producing neutralizing antibodies through Day 84, according to a study presented at IDWeek 2013.

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“We previously completed a Phase I/II randomized, double-bind, placebo-controlled, dose escalation clinical trial evaluating the safety and immunogenicity of a Staphylococcus aureus toxoid vaccine, composed of a recombinant alpha-toxoid (rAT) and a recombinant subunit of the Panton Valentine Leukocidin (PVL), rLukS-PV,” stated Tahaniyat Lalani, MBBS, MHS, from Infectious Disease Clinical Research Program, Uniformed Services University, Bethesda, MD. “The vaccine was well-tolerated and resulted in significantly higher anti-rAT and anti-rLukS-PV antibody levels compared to placebo.”

Dr. Lalani and colleagues conducted the present study to evaluate the functionality of the anti-rAT and anti-rLukS-PV antibodies with regards to their in vitro neutralizing antibody activity using serum samples from subjects enrolled in the vaccine trial.

Of the total 176 patients originally enrolled in the vaccine trial, 72 patients were included for the present analysis as randomized to the 25mcg and 50mcg monovalent and bivalent rAT/rLukS-PV cohorts, having received a dose of the active vaccine: monovalent rAT 25mcg (n=12) and 50mcg (n=12); monovalent rLukS-PV 25mcg (n=12) and 50mcg (n=12); bivalent 25mcg rAT/rLukS-PV (n=12); and bivalent 50mcg rAT/rLukS-PV (n=12).

Neutralizing activity was measured using sera collected on Days 0, 28, and 84 for subjects in the monovalent and bivalent 25mcg and 50mcg cohorts. Subjects in the 50:50mcg bivalent treatment cohort received a second injection at Day 84 and an additional serum sample for their neutralizing activity was drawn on Day 84.

The geometric mean titers (GMTs) of rAT and rLukS-PV neutralizing antibody increased in all monovalent and bivalent cohorts, and persisted through Day 84, they concluded. The greatest increase in rAT neutralizing antibody titer was observed in subjects receiving the monovalent 50mcg rAT vaccine, and a lower titer was observed in subjects receiving the bivalent 50mcg rAT/rLukS-PV formulation.

Further, the increase in GMT of rLukS-PV neutralizing antibody was greater in the bivalent cohorts as compared to the monovalent formulations. Similar seroresponse rates (83%; 10/12) to rAT and rLukS-PV were noted in subjects receiving the 50mcg monovalent or bivalent formulations at Day 84. Researchers found there was no booster effect observed following a second dose of vaccine at Day 84.

In regards to the bivalent formulation, a greater increase in the GMT of rAT neutralizing antibody was noted in the monovalent 50mcg rAT group vs. the bivalent 50mcg formulation. Similar increases in GMT of rLukS-PV neutralizing antibody and seroresponse rates were noted in the 50mcg monovalent and bivalent groups.

This data supports the use of the 50:50mcg rAT:rLukS-PV formulation as part of a multivalent vaccine, noted study investigators.