Phenotyping testing considerations

The major advantage of (fractional) oral clearance as a phenotypic trait is that its value is linearly related to the enzyme s catalytic activity, provided that first-order conditions are present. This requirement, along with any safety considerations, is the main reason the dose of an in vivo probe should be as low as possible, consistent with analytical considerations. Furthermore, it is possible to directly extrapolate this type of trait measure to the disposition of other drugs whose metabolism is mediated by the measured enzyme and also to place the trait value within a therapeutic context. On the other hand, estimation of oral clearance requires multiple blood and urine collections, often over many hours, that are an inconvenience for the study subject and require considerable amounts of analytical time and effort. Because of this, simpler and less time-consuming approaches have often been used. However, it is not always appreciated that such phenotyping tests provide only an indirect measure of metabolizing activity and may be affected by factors other than the enzyme s intrinsic clearance. In addition, it is difficult to relate an indirect trait measure to parameters that are of clinical importance, such as the drug s clearance. 

The dominant-skeletal-mutation test requires considerable anatomic knowledge and experience to distinguish true genetic changes in the absence o breeding tests. It is also time-consuming. The cataract test has a more specific phenotype and requires a shorter time per mouse, but it screens for a considerably smaller number of loci. Neither of the tests has been validated by use in several laboratories with a variety of strains and many chemicals. 

The penultimate step in drug development is the testing of the drug candidate in animal models of disease, where all the complex interactions that underlie pathophysiological mechanisms take place. It is important that animal models not only manifest the relevant disease phenotype observed in humans, but that the underlying innate and adaptive immune responses are similar to the human disease. In the case of leishmaniasis, there are reasonably good animal models for some, but not all of the cutaneous forms. The animal models for visceral leishmaniasis are less satisfaaory. Nonetheless, the animal models for leishmaniasis are considerably better than those for other parasitic diseases. 

To date, 12 different 7TMD G-protein-coupled receptors have been identified as capable of coreceptor function for HIV-1, HIV-2, or SIV entry (8-11). Of these, the chemokine receptors CCR5 and CXCR4 are the most important, one or both being used by all viruses tested (9,12,13). To a considerable extent, differential use of these two coreceptors by virus isolates correlates with viral phenotype. Thus, the strains of HIV-1... 

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