1 Assessment of the Effectiveness of Small Pox Vaccines: Immunogenicity Assay Considerations Freyja Lynn Division of Bacterial, Parasitic and Allergenic Products OVRR, CBER The Development and Evaluation of Next-Generation Smallpox Vaccines September 16, 2011

2 Overview Uses of assays to provide evidence of effectiveness Smallpox immunogenicity assay issues relevant to assessment of non-inferiority of human immune response Smallpox immunogenicity assay issues relevant to assessment of animal immune response (Animal Rule)

3 Immunoassay data to provide evidence of effectiveness Surrogate of Protection - Immune measure that explains all the protection afforded by a vaccine - Demonstrated in efficacy studies Correlate of Protection - Immune measure that correlates with but does not explain all of the protection afforded by a vaccine - Demonstrated in efficacy studies Immune measure - Immune response measured that correlates with vaccination or disease, expected to contribute to protection and indicate robustness of immunological response but not necessarily predict protective immunity

4 Smallpox Correlate of protection for licensed vaccine is vaccine “take” - New generation vaccines do not necessarily demonstrate “take” or demonstrate a different “take” than traditional vaccine - Lack of “take” may not indicate lack of efficacy Traditional measure of immunogenicity is the PRNT - Not a correlate of protection - Variola is the human pathogen but impractical for large scale studies What was appropriate for old vaccine may not be appropriate for new vaccines.

5 Challenge – providing evidence of effectiveness of next generation smallpox vaccines Noninferiority to licensed vaccine - Demonstrate immunologic noninferiority to ACM2000 in clinical studies Animal rule - Demonstrate efficacy in animals and bridge to immunogenicity in humans  Models are imperfect but necessary  Use related orthopox rather than variola

6 Noninferiority to licensed vaccine - Comparator would be ACAM What assay? - What are the appropriate immunogenicity endpoints? - What are the appropriate criteria for demonstration of non-inferiority?

7 Diversity of immune responses Human antibody responses to WR proteome after immunization with MVA or Dryvax

8 PRNT using Variola Damon, Davidson, Hughes, et al J Gen Virol. 90:

9 Effect of antigen on vaccine evaluation Groups indicate different vaccination regimens with either MVA or Dryvax Frey, Newman, Kennedy, et al Vaccine. 25:

10 Comparison of neutralization of WR and Dryvax in standardized PRNT Adults immunized with MVA Courtesy of BN and DMID

11 ELISA Groups indicate different vaccination regimens with either MVA or Dryvax Frey, Newman, Kennedy, et al Vaccine. 25:

12 Assessing clinical noninferiority What assay do you choose? - ELISA versus PRNT - Total antibody versus functional What antigen do you choose? - Vaccine strains - Human pathogen What difference is meaningful? - Immunologic difference versus clinical difference - Complexity of virus Mechanism of action between vaccines may be different and one assay may not measure effectiveness equally between vaccines

13 Animal rule - Develop an animal model - Bridge animal efficacy to human dose

14 Possible bridging strategy - Determination of a correlate of protection for the animal model - Bridging correlate of protection in animal model to immunogenicity in humans  Species neutral assay

15 Animal model Correlation between clinical outcome and immune measurement BBRC Study 1023, Courtesy of OBRA/DMID

16 Effects of methodology on PRNT Is PRNT species neutral? Courtesy of BN

17 Assay standardization issues for use as bridge between animal and human PRNT - Choice of virus - Methodology  Methods adapted to each species – is the assay truly species neutral? - Sensitivity ELISA - Species neutrality - Choice of antigen  Crude versus purified material Batch to batch consistency

18 Additional data needed to develop model Vaccine dose ranging to identify a dose that results in both protected and unprotected animals Assessment of immunogenicity in the animal model - Assay sensitivity  ELISA may be more sensitive than PRNT but which assay is more relevant? - Species neutrality  Is PRNT inherently species neutral? - May be product specific

19 Anthrax vaccine Logistic regression between survival and toxin neutralization titer Courtesy of OBRA/DMID

20 Assessment of effectiveness of next generation small pox vaccines: Considerations Noninferiority - Based on clinical data - Comparison to licensed product - Relevance of immunogenicity measures - Selection of assay - Selection of antigen Animal Rule - Demonstrate actual efficacy in animal model - Relevant assay based on correlative data - Relevance of animal model - Assumes the immunoassay in animals and humans is equivalent biologically