A Vero cell method for potency testing of diphtheria vaccines

Area of Science:

• Vaccinology
• Immunology
• In vitro assay development

Background:

• The European Pharmacopoeia (Ph. Eur.) currently mandates an in vivo challenge assay for diphtheria toxoid vaccine potency determination.
• There is a growing need for validated alternative functional assays to replace animal testing in vaccine quality control.
• This study builds upon prior work validating serological methods for tetanus toxoid vaccine potency testing.

Purpose of the Study:

• To evaluate a Vero cell-based functional assay as a viable alternative to the Ph. Eur. in vivo challenge method for diphtheria toxoid vaccine potency.
• To investigate the potential for using sera from the same immunized guinea pigs for simultaneous potency determination of both diphtheria and tetanus toxoid components in combined vaccines.
• To establish optimal operating procedures and vaccine dilutions for guinea pig immunization in Part I of the project.

Main Methods:

• Part I of the collaborative study focused on establishing operating procedures and optimal vaccine dilutions for guinea pig immunization.
• The Vero cell assay was compared against the Ph. Eur. in vivo challenge assay for potency determination.
• Vaccine potency was assessed for four different diphtheria toxoid vaccines with varying activities (approx. 20-200 IU/ml).

Main Results:

• Part I successfully established reliable operating procedures and vaccine dilutions for guinea pig immunization.
• Correlation coefficients between the Vero cell method and the in vivo challenge assay were deemed satisfactory.
• Comparable diphtheria potency estimates were obtained using both the Vero cell assay and the in vivo challenge assay across a range of vaccine activities.

Conclusions:

• The Vero cell assay demonstrates potential as a valid alternative to the Ph. Eur. in vivo challenge assay for diphtheria toxoid vaccine potency determination.
• Preliminary findings suggest that sera from guinea pigs immunized for diphtheria toxoid potency testing may also be suitable for assessing tetanus toxoid components using ELISA.
• The satisfactory results of Part I warrant progression to subsequent phases of the validation study.