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INTRODUCTION | DEFINITION | METHODS OF TESTING (I) | METHODS OF TESTING (II)
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D R U G R E S I S T A N C E I N M A L A R I A
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METHODS OF ASSESSING ANTIMALARIAL DRUG RESISTANCE
The problems associated with the assessment of antimalarial drug resistance in vivo led to the introduction of a number of in vitro tests for the measurement of antimalarial drug susceptibility in the late 1970s.
IN VITRO. The two most commonly used traditional in vitro assays are the WHO schizont maturation assay and the isotopic microtest (Desjardins et al., 1979). Both in vitro tests are based on the assessment of the parasite's metabolic processes in short or long term culture. The data derived from in vitro tests have to be put in relation with in vivo and pharmacological tests to determine individual susceptibiliy levels for the drugs tested. There is generally a clear relationship between in vivo and in vitro response with well-absorbed drugs such as chloroquine or quinine in the absence of vomiting or diarrhoea. With poorly or variably absorbed drugs, however, such as halofantrine, pharmacokinetic factors may produce drug failure in vivo in the presence of sensitive isolates. All in vitro tests have in common, that they are based on the determination of the effect of drugs on the growth and development of malaria parasites. Drugs are mixed with blood-medium-mixture in the wells of microtiter plates in serial dilutions and incubated at around 37°C in the presence of increased CO2 and decreased O2 levels. The difference of the methods essentially lies in the assessment of the parasite growth inhibition data (see "Other In Vitro Tests" for more details on the methodology).
From the point of view of a researcher interested in pure drug resistance, in vitro tests avoid many of the confounding factors which influence in vivo tests by removing parasites from the host and placing them into a controlled experimental environment. These tests therefore more accurately reflect intrinsic antimalarial drug resistance. Multiple tests can be performed on isolates, several drugs can be assessed simultaneously, and experimental drugs can be tested (Bloland, 2001). The in vitro assays not only yield quantitative results, they also determine the phenotype of the parasite independently of the immune and physiopathological conditions of the host. The correlation of in vitro response with clinical response in patients, on the other hand, is neither clear nor consistent, and the correlation appears to depend on the level of acquired immunity within the population tested. All results obtained from in vitro tests have therefore to be put in relation to clinical findings when determining cut-off points for resistance and when the results are used to develop treatment guidelines. In addition both assays have a number of individual drawbacks, which may limit their usefullness (see "Other In Vitro Tests" for more details).
H. Noedl 2002
http://malaria.farch.net
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© 2002 H. Noedl
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