CBER Presentation
Malaria antibody test development
FDA Workshop on Testing for Malarial Infections in Blood Donors
July 12, 2006
Nigel Appleton
Newmarket Laboratories Ltd
Malaria antibody test development
The Problem
Blood transfusion services losing too many donors under long deferral system
Deferral system not 100% effective anyway
Malaria antibody test development
The Task
To develop a test to allow quicker return of deferred donors
To (at least) maintain or (preferably) improve safety of blood supply
Replace laborious IFAT
NOT a diagnostic, but an aid to a strategy for maintaining donor numbers
Malaria antibody test development
Development targets - for users
Compatible with transfusion laboratory strategy and practise
Simple and cost-effective
High sensitivity and specificity
Reactive with all four Plasmodium spp infecting humans
Malaria antibody test development
Development targets - for developer
Make as much use as possible of existing knowledge and reagents (for speed of development)
Commercially viable with respect to costs and IP protection
Microplate format to start
Malaria antibody test development
The problems
Relatively few samples available from known cases
Many samples with questionable storage histories
Many samples with unknown or inadequate case history - particularly with respect to time of exposure/infection relative to that of sample draw
No seroconversion panels available
No true "Gold Standard" test - only comparator IFAT
Only P falciparum cultivable
Malaria antibody test development
Some solutions
Collaborations with academic and research institutions - identify highly conserved and immunogenic antigens
Make use of prior work in vaccine research
Merozoite surface proteins (MSPs) showed most initial promise, and recombinants were already available
"Common" antigens - aldolase, dehydrogenases, not very immunogenic - antibody titres low and variable
MSPs are immunogenic and repeatedly exposed to the immune system, with portions being shed into the circulation.
Malaria antibody test development
Technical Considerations
"Recombinant antigen sandwich" format (recombinants on solid phase AND in conjugate) preferred
Very low backgrounds, high S/N ratios
Very economic use of materials
Good stability, and robust in "guard-band" studies
Undiluted samples can be used - eliminates one diluent, and sample presence in well can be confirmed by OD reading
Detects all Ig classes
Malaria antibody test development
Need for multiple antigens
| One recombinant | Sensitivity | 69% |
| Two recombinants | Sensitivity | 73% |
| Three recombinants | Sensitivity | 82% |
| Four recombinants | Sensitivity | 99% |
Now use 3 P. falciparum-derived and one P. vivax-derived recombinants
More can be added in when suitable antigens identified
Malaria antibody test development
Performance
Pre-launch
P. falciparum 94.4 %
P. vivax 100.0 %
P. malariae 80.0 %
P. ovale 67.0 %
However, numbers for P malariae and P ovale very small, so results not statistically valid
Post- launch
| P. falciparum | 106/108 film positive | 98.1%(CI 93.5 - 99.5%) |
| P. vivax | 12/12 | 100.0% (CI 75.7 - 100.0%) |
Seed C.R. et al; Vox Sanguinis (2005) Vol 88, pp 98-106
These authors calculated that the extra risk imposed by using this test in their operating environment (Australia) was
One infectious P falciparum donation per 175 years
One infectious P. vivax donation per 4.2 years
Malaria antibody test development
Ongoing Development
Improve performance
Ongoing collaborations with researchers, using bioinformatics to help identify more useful antigens for gene sequencing and recombinant construction.
(Several candidates identified)
Possibility also of identifying antigens useful as markers in improved antigen-detection diagnostic
Malaria antibody test development
Ongoing Problems
Still limited numbers of samples, particularly from P malariae and P ovale infections
More intensive work on genomes of these two rarer species only just beginning
Malaria antibody test development
Acknowledgements
Dr Alan Kitchen
Professor Tony Holder and colleagues
Dr. Jana McBride and colleagues
Professor P. Chiodini and colleagues
American Red Cross
Researchers at The London School of Hygiene and Tropical Medicine
Many others.