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U.S. Department of Health and Human Services

Vaccines, Blood & Biologics

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Malaria antibody test development

Printable Version

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 recombinantSensitivity69%
Two recombinantsSensitivity73%
Three recombinantsSensitivity82%
Four recombinantsSensitivity99%

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. falciparum106/108 film positive98.1%(CI 93.5 - 99.5%)
P. vivax12/12100.0% (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.