Showing papers by "Richard M. Weinshilboum published in 2003"

Inheritance and Drug Response

Abstract: 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

Intensification of Mercaptopurine/Methotrexate Maintenance Chemotherapy May Increase the Risk of Relapse for Some Children With Acute Lymphoblastic Leukemia

Abstract: Purpose: Thioguanine nucleotides (TGNs) mediate the cytotoxicity of mercaptopurine (MP). Methylated MP metabolites (formed by thiopurine methyltransferase [TPMT]) and methotrexate (MTX) polyglutamates can inhibit de novo purine synthesis. We explored whether dose adjustment of MP and MTX by erythrocyte (E) levels of TGN and MTX (including polyglutamates) could improve outcome in childhood acute lymphoblastic leukemia (ALL). Patients and Methods: A total of 538 children with ALL were randomly assigned to have their oral MP/MTX maintenance therapy adjusted by white cell counts (WBC), E-TGN, and E-MTX (pharmacology group), or by WBC only (control group). Results: After a median follow-up of 7.8 years, 79 patients had relapsed. Cox regression analysis showed an increased risk of relapse for boys (P = .00003), high WBC at diagnosis (P = .03), pharmacology arm (6.6 times increased relapse hazard for girls), high TPMT activity (P = .002), and high average neutrophil counts during maintenance therapy (P = .0009),...

Thiopurine S-methyltransferase pharmacogenetics: chaperone protein association and allozyme degradation.

Abstract: Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs such as 6-mercaptopurine. A common genetic polymorphism for TPMT is associated with large individual variations in thiopurine drug toxicity and therapeutic efficacy. TPMT*3A, the most common variant allele in Caucasians, has two alterations in amino acid sequence, resulting in striking decreases in TPMT protein levels. This phenomenon results, in part, from rapid degradation through a ubiquitin-proteasome-mediated process. We set out to test the hypothesis that chaperone proteins might be involved in targeting TPMT for degradation. As a first step, hsp90, hsp70 and the cochaperone hop were immunoprecipitated from a rabbit reticulocyte lysate (RRL) that included radioactively labelled *3A and wild-type TPMT. TPMT*3A was much more highly associated with all three chaperones than was the wild-type enzyme. The RRL was also used to confirm the accelerated degradation of *3A compared to wild-type TPMT. Treatment of RRL with the hsp90 inhibitor geldanamycin resulted in enhanced association of hsp90 with wild-type TPMT, an observation that correlated with accelerated ubiquitin-dependent degradation of wild-type TPMT. Geldanamycin treatment of COS-1 cells transfected with FLAG-tagged wild-type also resulted in a time and geldanamycin concentration-dependent decrease in TPMT activity and protein, which was compatible with results obtained in the RRL. These observations indicate that TPMT is a client protein for hsp90 and suggest that chaperone proteins, especially hsp90, are involved in targeting both TPMT*3A and, in the presence of geldanamycin, the wild-type allozyme for degradation. Therefore, chaperone proteins play an important mechanistic role in this clinically significant example of pharmacogenetic variation in drug metabolism.

Human estrogen sulfotransferase (SULT1E1) pharmacogenomics: gene resequencing and functional genomics.

Abstract: Estrogens are used as drugs and estrogen exposure is a risk factor for hormone-dependent diseases such as breast cancer. Sulfate conjugation is an important pathway for estrogen metabolism. The sulfotransferase (SULT) enzyme SULT1E1 has the lowest Km values for estrogens and catecholestrogens of the 10 known human SULT isoforms. We previously cloned and characterized the human SULT1E1 cDNA and gene as steps toward pharmacogenetic studies. In the present experiments, we set out to determine whether common, functionally significant genetic polymorphisms might exist for SULT1E1. As a first step, we ‘resequenced’ the eight SULT1E1 exons and exon–intron splice junctions as well as portions of the 5′-flanking region using DNA from 60 African-American and 60 Caucasian-American subjects. In all, 23 polymorphisms, 22 single nucleotide polymorphisms (SNPs) and one insertion deletion were observed. There were three nonsynonymous coding SNPs (cSNPs) that altered the following encoded amino acids: Asp22Tyr, Ala32Val and Pro253His. Among these, 12 pairs of SNPs were tightly linked. In addition, 12 unambiguous SULT1E1 haplotypes were identified, including six that were common to both populations studied. Transient expression in COS-1 cells of constructs containing the three nonsynonymous cSNPs showed significant decreases in SULT1E1 activity for the Tyr22 and Val32 allozymes, with corresponding decreases in levels of immunoreactive protein. There were no changes in levels of either activity or immunoreactive protein for the His253 allozyme. Apparent Km values of the Val32 allozyme for the two cosubstrates for the reaction, 17β-estradiol and 3′-phosphoadenosine 5′-phosphosulfate, were not significantly different from those of the wild-type enzyme, but there was a two- to three-fold increase in Km values for the His253 allozyme and a greater than five-fold increase for the Tyr22 allozyme. These observations raise the possibility that genetically determined variation in SULT1E1-catalyzed estrogen sulfation might contribute to the pathophysiology of estrogen-dependent diseases as well as variation in the biotransformation of exogenously administered estrogens. British Journal of Pharmacology (2003) 139, 1373–1382. doi:10.1038/sj.bjp.0705369

Pharmacogenomics and reducing the frequency of adverse drug events.

Abstract: ‘Application of pharmacogenetic and pharmacogenomic information should allow the potential for ADEs to be premanaged in the out-patient setting.’ Drug toxicity and adverse drug events Pharmacogenomics, as applied to medical practice, offers the promise of reduction in adverse drug events (ADEs), enhanced drug efficacy and selection of patients able to respond to specific agents. This editorial will focus on the history and evolving role of pharmacogenetics and pharmacogenomics as it moves from the basic research laboratory into clinical practice to reduce the occurrence of ADEs. In 1994, 2,216,000 patients either reported to the hospital or were in the hospital as a result of ADEs. Of these, 106,000 patients experienced fatal ADEs [1]. In 2000, 2,168,248 toxic events due to all forms of poisons and drugs were reported in the US. Toxin-related events in 2000 affected 8% of the US population. Of these events [2]:

Thiopurine S‐Methyltransferase (TPMT) Pharmacogenetics: Role of Chaperone Proteins in variant Allozyme Degradation

Abstract: Clinical Pharmacology & Therapeutics (2003) 73, P29–P29; doi:

Human Catecholamine Sulfotransferase (SULT1A3) Pharmacogenetics: Common Functional Genetic Polymorphism in African-American Subjects

Abstract: Clinical Pharmacology & Therapeutics (2003) 73, P29–P29; doi:

Sulfotransferase (sult) 1A1 pharmacogenetics: Functional 5′‐flanking region (5′‐FR) polymorphisms

Abstract: Clinical Pharmacology & Therapeutics (2003) 73, P77–P77; doi:

L-DOPA biotransformation: correlations of dosage, erythrocyte catechol O-methyltransferase and platelet SULT1A3 activities with metabolic pathways in Parkinsonian patients.

Abstract: The objectives of this study were to determine (1) the effects of dose and drug absorption on pathways of biotransformation of L-DOPA in Parkinsonian patients treated with Sinemet, and (2) the extent to which genetically-determined variations in the activities of erythrocyte catechol O-methyltransferase and/or platelet phenol sulfotransferase might be reflected in individual differences in L-DOPA metabolism. In the 19 patients studied, there were negative correlations between dosage or absorption and extent of O-methylation and of sulfation of L-DOPA or its metabolites. Levels of activity for erythrocyte COMT were also reflected in individual variation in the metabolism of L-DOPA. In contrast, differences in platelet phenol sulfotransferase were not reflected in differences in sulfation of L-DOPA or of its metabolites. If such a relationship did exist, it might have been obscured by the effects of high dosage of L-DOPA, effects which might have resulted from a deficiency of the sulfation cosubstrate 3'-phosphoadenosine 5'-phosphosulfate in patients taking higher doses of drug.

Intensification o f M ercaptopurine/M ethotrexate M aintenance Chemotherapy M ay I ncrease t he R isk o f R elapse f or S ome Children W ith A cute L ymphoblastic L eukemia

Abstract: Purpose: Thioguanine nucleotides (TGNs) mediate the cytotoxicity of mercaptopurine (MP). Methylated MP metabolites (formed by thiopurine methyltransferase [TPMT]) and methotrexate (MTX) polyglutamates can inhibit de novo purine synthesis. We explored whether dose adjustment of MP and MTX by erythrocyte (E) levels of TGN and MTX (including polyglutamates) could improve outcome in childhood acute lymphoblastic leukemia (ALL). Patients and Methods: A total of 538 children with ALL were randomly assigned to have their oral MP/MTX maintenance therapy adjusted by white cell counts (WBC), ETGN, and E-MTX (pharmacology group), or by WBC only (control group). Results: After a median follow-up of 7.8 years, 79 patients had relapsed. Cox regression analysis showed an increased risk of relapse for boys (P .00003), high WBC at diagnosis (P .03), pharmacology arm (6.6 times increased relapse hazard for girls), high TPMT activity (P .002), and high average neutrophil counts during maintenance therapy (P .0009), with a significant interaction between sex and randomization group (P .0007). For girls, the relapse risk was 5% in the control group and 19% in the pharmacology group (P .001) because of an increased relapse hazard during the first year after cessation of therapy. TPMT activity was the most significant predictor of relapses among girls in the pharmacology arm (P < .0001). Overall, the TPMT activity was higher for patients who relapsed after cessation of therapy compared with those who stayed in remission (girls 19.5 v 17.4 U/mL, P .03; boys 19.3 v 18.0 U/mL, P .04). Conclusion: Adding pharmacologically guided treatment intensification to dose adjustments by blood counts may not be warranted for girls, whereas new approaches to optimize maintenance therapy are needed for boys. J Clin Oncol 21:1332-1339. © 2003 by American Society of Clinical Oncology.