Phenotype tests

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. 

In an attempt to further simplify the caffeine phenotyping test, a trait measure based on the plasma or salivary paraxanthine caffeine concentration ratio between three hours and seven hours after administration of the probe has been suggested (56). High linear correlations (>0.89) have been observed between this trait value and caffeine s oral clearance, and if necessary, a predicted caffeine clearance value may be calculated from the ratio (56). Currently, this phenotyping approach appears to be the simplest and most noninvasive means of readily assessing CYP1A2 activity using caffeine as a probe in addition, the method is reproducible and appears to be robust (56), despite the theoretical dependency of the trait value on the urine flow rate (51). 

Sohn DR, Kusaka M, Shin SG, et al. Utility of a one-point (3-hour postdose) plasma metabolic ratio as a phenotyping test using metoprolol in two East Asian populations. Ther Drug Monit 1992 14 184—189. 

Due to intra-individual differences in liver and intestinal CYP3A activity, phenotyping test results are related to the probe drug route of administration. 

Some recent references for other used NAT2 phenotyping tests can be found for dapsone in Alhrevic (2003), O Neil (2000), Queiroz (1997), for sulphamet-hazine in Hadasova (1996) and Meisel (1997), and for procainamide in Okumura (1997) and Mongey (1999). 

To develop appropriate molecular methods for the detection of resistance, it is important to know the gene(s) coding for the altered target proteins and to characterize the alterations. Molecular methods of detection are only of value if there is a very high correlation with the resistant phenotype (tested in bioassays) [2,3]... 

Phenotypic testing may be necessary to detect this genetic variant prior to dosing with a tricyclic/tetracyclic antidepressant, especially in vulnerable populations such as children, elderly, cardiac populations, and those on concomitant medications... 

Phenotypic testing may be necessary in order to detect the 7% of the normal population for whom thioridazine is... 

The fermentation profile is not well adapted to characterize LAB isolated from wine bacteria are in optimal growth conditions and this does not give a true indication of the real metabolism in wine, which is influenced by environmental conditions. In general, the discriminating power is not high and several subcultures are required to obtain a stable profile. Therefore, a clear within-species identification by simple phenotypic tests may, sometimes, be difficult, and these tests are also labour-intensive and time-consuming. 

Two distinct peaks of acid phosphatase activity were detected in each phenotype, but the positions of these peaks differed. For example, in phenotype A, the peaks were approximately at tubes 150 and 190 in B, at about 130, 170, and 265 in BA, at 110 and 155. In these three, the first peaks showed minor enzyme activity. In CA, there was a major peak at about tube 130 and a smaller one at about tube 170. The shape of the curves varied according to the phenotype tested. In general, these results confirmed what gel electrophoresis originally showed, namely, that there are charge differences between the various isoenzymes. The electrophoretic patterns may also be influenced by the type of buffer used to make up the starch gel (K2). 

One of the first standardized phenotypic tests was the HeLa CD4+ plaque reduction assay (4). Although this assay is easy to perform, inexpensive and has a high reproducibility, it is rather labor-intensive and is only suitable for syncytium-inducing (SI) strains. Most patient isolates contain nonsyncytium-inducing (NSI) strains or a mixture of SI and NSI strains. The necessary in vitro cultivation of patient isolates to obtain a high-titer standardized Sl-inocu-lum selects for a subpopulation of SI variants. 

Perhaps the best laboratory test to compare with pharmacogenetic testing is a drug blood level, technically called therapeutic drug monitoring (TDM), which could be considered a phenotypic test of metabolic enzymes [97]. Almost no studies of TDM cost effectiveness exist [98], except for some studies of antibiotics. However, TDM appears to be standard clinical practice (medical insurers routinely reimburse for it) for classic anticonvulsants, theophylline, digoxin, immunosuppressants, and some psychiatric drugs [98]. 

Patients with AAT deficiency develop COPD at an early age (20 to 50 years) primarily owing to an accelerated decline in lung function. Compared with an average annual decline in forced expiratory volume in 1 second (FE Vi) of 25 mL/year in healthy nonsmokers, patients with homozygous Z deficiency have been reported to have declines of 54 mL/year for nonsmokers and 108 mL/year for current smokers. Patients developing COPD at an early age or those with a strong family history of COPD should be screened for AAT deficiency. If the concentration is low, phenotype testing (DNA) should be performed. 

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