Richard M. Weinshilboum

Catechol-O-methyltransferase (COMT) inactivates catecholamines and catechol drugs such as L-DOPA. A common genetic polymorphism in humans is associated with a three-to-four-fold variation in COMT enzyme activity and is also associated with individual variation in COMT thermal instability. We now show that this is due to G-->A transition at codon 158 of the COMT gene that results in a valine to methionine substitution. The two alleles can be identified with a PCR-based restriction fragment length polymorphism analysis using the restriction enzyme Nla III. The identification of a gentic marker associated with significant alterations in enzyme activity will facilitate the analysis of a possible role for the COMT gene in neuropsychiatric conditions in which abnormalities in catecholamine neurotransmission are believed to occur, including mood disorders, schizophrenia, obsessive compulsive disorder, alcohol and substance abuse, and attention deficit hyperactivity disorder. In addition, this polymorphism may have pharmacogenetic significance in that it will help make it possible to identify patients who display altered metabolism of catechol drugs.

Thiopurine methyltransferase (TPMT) catalyzes thiopurine S-methylation, an important metabolic pathway for drugs such as 6-mercaptopurine. Erythrocyte (RBC) TPMT activity was measured in blood samples from 298 randomly selected subjects. Of the subjects, 88.6% were included in a subgroup with high enzyme activity (13.50 +/- 1.86 U, mean +/- SD), 11.1% were included in a subgroup with intermediate activity (7.20 +/- 1.08 U), and 0.3% had undetectable activity. This distribution conforms to Hardy-Weinberg predictions for the autosomal codominant inheritance of a pair of alleles for low and high TPMT activity, TPMT(L) and TPMT(H), with gene frequencies of .059 and .941, respectively. If RBC TPMT activity is inherited in an autosomal codominant fashion, then subjects homozygous for TPMT(H) would have high enzyme activity, subjects heterozygous for the two alleles would have intermediate activity, and subjects homozygous for TPMT(L) would have undetectable activity. The segregation of RBC TPMT activity among 215 first-degree relatives in 50 randomly selected families and among 35 members of two kindreds and one family selected because they included probands with undetectable RBC enzyme activity were also compatible with the autosomal codominant inheritance of RBC TPMT. For example, in eight matings between subjects with intermediate activity (presumed genotype TPMT(L)TPMT(H)) and subjects with high activity (presumed genotype TPMT(H)TPMT(H)), 47% (8/17) of the offspring had intermediate activity. This value is very similar to the 50% figure expected on the basis of autosomal codominant inheritance (chi(2) ([1]) = .059). Further experiments are required to determine whether this genetic polymorphism for an important drug metabolizing enzyme may represent one factor in individual variations in sensitivity to thiopurines.

he promise of pharmacogenetics, the study of the role of in heritance in the individual variation in drug response, lies in its potential to identify the right drug and dose for each patient. Even though individual differences in drug response can result from the effects of age, sex, disease, or drug interactions, genetic factors also influence both the efficacy of a drug and the likelihood of an adverse reaction. 1-3 This article briefly reviews concepts that underlie the emerging fields of pharmacogenetics and pharmacogenomics, with an emphasis on the pharmacogenetics of drug metabolism. Although only a few examples will be provided to illustrate concepts and to demonstrate the potential contribution of pharmacogenetics to medical practice, it is now clear that virtually every pathway of drug metabolism will eventually be found to have genetic variation. The accompanying article by Evans and McLeod 4 expands on many of the themes introduced here. Once a drug is administered, it is absorbed and distributed to its site of action, where it interacts with targets (such as receptors and enzymes), undergoes metabolism, and is then excreted. 5,6 Each of these processes could potentially involve clinically significant genetic variation. However, pharmacogenetics originated as a result of the observation that there are clinically important inherited variations in drug metabolism. Therefore, this article — and the examples highlighted — focuses on the pharmacogenetics of drug metabolism. However, similar principles apply to clinically significant inherited variation in the transport and distribution of drugs and their interaction with their therapeutic targets. The underlying message is that inherited variations in drug effect are common and that some tests that incorporate pharmacogenetics into clinical practice are now available, with many more to follow. The concept of pharmacogenetics originated from the clinical observation that there were patients with very high or very low plasma or urinary drug concentrations, followed by the realization that the biochemical traits leading to this variation were inherited. Only later were the drug-metabolizing enzymes identified, and this discovery was followed by the identification of the genes that encoded the proteins and the DNA-sequence variation within the genes that was associated with the inherited trait. Most of the pharmacogenetic traits that were first identified were monogenic — that is, they involved only a single gene — and most were due to genetic polymorphisms; in other words, the allele or alleles responsible for the variation were relatively common. Although drug effect is a complex phenotype that depends on many factors, early and often dramatic examples involving succinylcholine and isoniazid facilitated acceptance of the fact that inheritance can have an important influence on the effect of a drug. Today there is a systematic search to identify functionally significant variations in DNA sequences in genes that influence the effects of various drugs. 4 t

BACKGROUND: The efficacy of tamoxifen therapy for the treatment of breast cancer varies widely among individuals. Plasma concentrations of the active tamoxifen metabolite endoxifen are associated with the cytochrome P450 (CYP) 2D6 genotype. We examined the effects of concomitant use of selective serotonin reuptake inhibitor antidepressants, which are CYP2D6 enzyme inhibitors commonly prescribed to treat hot flashes in women who take tamoxifen, and genotypes for genes that encode tamoxifen-metabolizing enzymes on plasma concentrations of tamoxifen and its metabolites. METHODS: Eighty patients with newly diagnosed with breast cancer who were beginning tamoxifen therapy (20 mg/day orally), 24 of whom were taking CYP2D6 inhibitors, were genotyped for common alleles of the CYP2D6, CYP2C9, CYP3A5, and sulfotransferase (SULT) 1A1 genes. Plasma concentrations of tamoxifen and its metabolites were measured after 1 and 4 months of tamoxifen therapy. Differences in plasma concentrations of tamoxifen and its metabolites between genotype groups were analyzed by the Wilcoxon rank sum test. All statistical tests were two-sided. RESULTS: Among all women, plasma endoxifen concentrations after 4 months of tamoxifen therapy were statistically significantly lower in subjects with a CYP2D6 homozygous variant genotype (20.0 nM, 95% confidence interval [CI] = 11.1 to 28.9 nM) or a heterozygous genotype (43.1 nM, 95% CI = 33.3 to 52.9 nM) than in those with a homozygous wild-type genotype (78.0 nM, 95%CI = 65.9 to 90.1 nM) (both P = .003). Among subjects who carried a homozygous wild-type genotype, the mean plasma endoxifen concentration for those who were using CYP2D6 inhibitors was 58% lower than that for those who were not (38.6 nM versus 91.4 nM, difference = -52.8 nM, 95% CI = -86.1 to -19.5 nM, P = .0025). The plasma endoxifen concentration was slightly reduced in women taking venlafaxine, a weak inhibitor of CYP2D6, whereas the plasma endoxifen concentration was reduced substantially in subjects who took paroxetine (a potent inhibitor of CYP2D6). Genetic variations of CYP2C9, CYP3A5, or SULT1A1 had no statistically significant associations with plasma concentrations of tamoxifen or its metabolites. CONCLUSION: Interactions between CYP2D6 polymorphisms and coadministered antidepressants and other drugs that are CYP2D6 inhibitors may be associated with altered tamoxifen activity.

Metabolomics is the study of metabolism at the global level. This rapidly developing new discipline has important potential implications for pharmacologic science. The concept that metabolic state is representative of the overall physiologic status of the organism lies at the heart of metabolomics. Metabolomic studies capture global biochemical events by assaying thousands of small molecules in cells, tissues, organs, or biological fluids-followed by the application of informatic techniques to define metabolomic signatures. Metabolomic studies can lead to enhanced understanding of disease mechanisms and to new diagnostic markers as well as enhanced understanding of mechanisms for drug or xenobiotic effect and increased ability to predict individual variation in drug response phenotypes (pharmacometabolomics). This review outlines the conceptual basis for metabolomics as well as analytical and informatic techniques used to study the metabolome and to define metabolomic signatures. It also highlights potential metabolomic applications to pharmacology and clinical pharmacology.