AAPS PharmSci^SM Debuts Theme Issue on Pharmacogenetics, Pharmacogenomics Tip Sheet

ARLINGTON, Va. - The American Association of Pharmaceutical Scientists’ (AAPS) online journal, AAPS PharmSci, www.pharmsci.org, published its first theme issue, “Pharmacogenetics-Pharmacogenomics 2000.” The issue contains a review of the current status of “personalized medicine” including its promises and limitations and six original research articles (see abstracts below).

“Overall, this issue of AAPS PharmSci represents a broad spectrum of research in the areas of pharmacogenetics and pharmacogenomics — including the potential economic impact — from several of the world’s top industrial and academic research institutions in the pharmaceutical sciences, including Pfizer, Aclara Biosciences, the University of Washington, Ohio State University and the University of California at San Francisco,” said Wolfgang Sadée, Ph.D., editor-in-chief of AAPS PharmSci. “This issue offers extensive links to cited papers and relevant web sites, and the use of relational databases and interactive graphics; all of these features serve to demonstrate the power of electronic publication.”

The full text of the issue can be viewed at www.pharmsci.org/pharmacogenomics.

Review Abstract
“Pharmacogenomics: The Promise of Personalized Medicine”
Laviero Mancinelli, Maureen Cronin, Wolfgang Sadée

Pharmacogenetics and pharmacogenomics deal with the genetic basis underlying variable drug response in individual patients. The traditional pharmacogenetic approach relies on studying sequence variations in candidate genes suspected of affecting drug response. On the other hand, pharmacogenomic studies encompass the sum of all genes, i.e., the genome. Numerous genes may play a role in drug response and toxicity, introducing a daunting level of complexity into the search for candidate genes. The high speed and specificity associated with newly emerging genomic technologies enable the search for relevant genes and their variants to include the entire genome. These new technologies have essentially spawned a new discipline, termed pharmacogenomics, which seeks to identify the variant genes affecting the response to drugs in individual patients. Moreover, pharmacogenomic analysis can identify disease susceptibility genes representing potential new drug targets. All of this will lead to novel approaches in drug discovery, an individualized application of drug therapy, and new insights into disease prevention. Current concepts in drug therapy often attempt treatment of large patient populations as groups, irrespective of the potential for individual, genetically based differences in drug response. In contrast, pharmacogenomics may help focus effective therapy on smaller patient subpopulations which although demonstrating the same disease phenotype are characterized by distinct genetic profiles. Whether and to what extent this individual, genetics-based approach to medicine results in improved, economically feasible therapy remain to be seen. To exploit these opportunities in genetic medicine, novel technologies will be needed, legal and ethical questions must be clarified, health care professionals must be educated, and the public must be informed about the implications of genetic testing in drug therapy and disease management.

Original Research Article Abstracts

“CYP2D6 Genotyping as an Alternative to Phenotyping for Determination of Metabolic Status in a Clinical Trial Setting”
Suzin McElroy, Christoph Sachse, Jürgen Brockmöller, Jodi Richmond, Maruja Lira, David Friedman, Ivar Roots, B. Michael Silber, and Patrice M. Milos

The emerging application of pharmacogenomics in the clinical trial setting requires careful comparison with more traditional phenotyping methodologies, particularly in the drug metabolism area where phenotyping is used extensively. The research objectives of this study were 1) to assess the utility of cytochrome P450 2D6 (CYP2D6) genotyping as an alternative to traditional phenotyping as a predictor of poor metabolizer status; 2) to identify issues for consideration when implementing CYP2D6 genotyping in clinical trials; and 3) to outline the advantages and disadvantages of CYP2D6 genotyping compared with phenotyping. DNA samples obtained from 558 previously phenotyped individuals were blindly genotyped at the CYP2D6 locus, and the genotype-phenotype correlation was then determined. The CYP2D6 genotyping methodology successfully predicted all but one of the 46 poor metabolizer subjects, and it was determined that this one individual had a novel (presumably inactive) mutation within the coding region. In addition, the authors identified two subjects with CYP2D6 genotypes indicative of poor metabolizers who had extensive metabolizer phenotypes as determined by dextromethorphan/dextrorphan ratios. This finding suggests that traditional phenotyping methods do not always offer 100% specificity. The authors’ results suggest that CYP2D6 genotyping is a valid alternative to traditional phenotyping in a clinical trial setting, and in some cases may be better. The authors also discuss some of the issues and considerations related to the use of genotyping in clinical trials and medical practice.

"Assessing the Cost-Effectiveness of Pharmacogenomics”
David L. Veenstra, Mitchell K. Higashi, and Kathryn A. Phillips

The use of pharmacogenomics to individualize drug therapy offers the potential to improve drug effectiveness, reduce adverse side effects, and provide cost-effective pharmaceutical care. However, the combinations of disease, drug, and genetic test characteristics that will provide clinically useful and economically feasible therapeutic interventions have not been clearly elucidated. The purpose of this paper was to develop a framework for evaluating the potential cost-effectiveness of pharmacogenomic strategies that will help scientists better understand the strategic implications of their research, assist in the design of clinical trials, and provide a guide for health care providers making reimbursement decisions. The authors reviewed concepts of cost-effectiveness analysis and pharmacogenomics and identified five primary characteristics that will enhance the cost-effectiveness of pharmacogenomics: 1) severe clinical or economic consequences can be avoided through the use of pharmacogenomics, 2) drug response using current methods is difficult to monitor, 3) a well-established association between genotype and clinical phenotype exists, 4) there is a rapid and relatively inexpensive genetic test, and 5) the variant gene is relatively common. The authors used this framework to evaluate several examples of pharmacogenomics. The authors found that pharmacogenomics offers great potential to improve patients’ health in a cost-effective manner. However, pharmacogenomics will not be applied to all currently marketed drugs, and careful evaluations are needed on a case-by-case basis before investing resources in research and development of pharmacogenomic-based therapeutics and making reimbursement decisions.

“Human Membrane Transporter Database: A Web-Accessible Relational Database for Drug Transport Studies and Pharmacogenomics”
Qing Yan, Wolfgang Sadée

The human genome contains numerous genes that encode membrane transporters and related proteins. For drug discovery, development, and targeting, one needs to know which transporters play a role in drug disposition and effects. Moreover, genetic polymorphisms in human membrane transporters may contribute to interindividual differences in the response to drugs. Pharmacogenetics, and, on a genome-wide basis, pharmacogenomics, address the effect of genetic variants on an individual’s response to drugs and xenobiotics. However, our knowledge of the relevant transporters is limited at present. To facilitate the study of drug transporters on a broad scale, including the use of microarray technology, the authors have constructed a human membrane transporter database (HMTD). Even though it is still largely incomplete, the database contains information on more than 250 human membrane transporters, such as sequence, gene family, structure, function, substrate, tissue distribution, and genetic disorders associated with transporter polymorphisms. Taking advantage of the features of an electronic journal, this paper serves as an interactive tutorial for using the database, which the authors expect to develop into a research tool.

“Disposition of Acetaminophen and Indocynanine Green in Cystic Fibrosis-Knockout Mice”
Swarupa G. Kulkarni, Anita A. Pegram, Philip C. Smith

Drug treatment poses a therapeutic challenge in cystic fibrosis (CF) because the disposition of a number of drugs is altered in CF. Enhanced clearance of acetaminophen (APAP) and indocyanine green (ICG) have previously been reported in CF patients. The objective of the current study was to investigate if the CF-knockout mouse model (cftr^m1UNC) shows altered pharmacokinetics similar to those seen in CF patients using the two model compounds APAP and ICG. Clearance (CL/F) of APAP and renal (CL_R) and formation (CL_f) clearance of acetaminophen glucuronide (AG) and acetaminophen sulfate (AS) were determined in CF-knockout mice following administration of APAP (50 mg/kg, intraperitoneal). CL_R of AS was 19.5 and 12.9 (mL/min per kg) and CL_f of AS was 10.4 and 6.7 mL/min per kg for homozygous and heterozygous males, respectively, which was significantly different between groups. CL_R of AG was 6.3 and 4.8 mL/min per kg and CL_f of AG was 9.6 and 8.9 mL/min per kg for homozygous and heterozygous males, respectively, although not reaching statistical significance. No significant differences were noted in either Cl_R or CL_f of AG and AS in female CF mice. Plasma concentrations of ICG (10 mg/kg, intravenous) were determined over 0 to 15 minutes. Homozygous females showed a higher apparent volume of distribution (96 mL/kg) relative to heterozygous females (72 mL/kg). Similar to CF patients, a trend toward a lower Cmax was noted in homozygous male and female mice. However, contrary to human data, no significant differences in CL of ICG were noted. These results suggest that the CF-knockout mice have potential as a model for studying altered drug disposition in CF patients.

“Human Proton/Oligopeptide Transporter (POT) Genes: Identification of Putative Human Genes Using Bioinformatics”
Christopher W. Botka, Thomas W. Wittig, Richard C. Graul, Carsten Uhd Nielsen, Kazutaka Higaki, Gordon L. Amidon, and Wolfgang Sadée

The proton-dependent oligopeptide transporters (POT) gene family currently consists of ~70 cloned cDNAs derived from diverse organisms. In mammals, two genes encoding peptide transporters, PepT1 and PepT2 have been cloned in several species including humans, in addition to a rat histidine/peptide transporter (rPHT1). Because the Candida elegans genome contains five putative POT genes, we searched the available protein and nucleic acid databases for additional mammalian/human POT genes, using iterative BLAST runs and the human expressed sequence tags (EST) database. The apparent human orthologue of rPHT1 (expression largely confined to rat brain and retina) was represented by numerous ESTs originating from many tissues. Assembly of these ESTs resulted in a contiguous sequence covering ~95% of the suspected coding region. The contig sequences and analyses revealed the presence of several possible splice variants of hPHT1. A second closely related human EST-contig displayed high identity to a recently cloned mouse cDNA encoding cyclic adenosine monophosphate (cAMP)-inducible 1 protein (gi:4580995). This contig served to identify a PAC clone containing deduced exons and introns of the likely human orthologue (termed hPHT2). Northern analyses with EST clones indicated that hPHT1 is primarily expressed in skeletal muscle and spleen, whereas hPHT2 is found in spleen, placenta, lung, leukocytes, and heart. These results suggest considerable complexity of the human POT gene family, with relevance to the absorption and distribution of cephalosporins and other peptoid drugs.

“Determination of Membrane Protein Glycation in Diabetic Tissue” Eric Zhang, Peter Swaan

Diabetes-associated hyperglycemia causes glycation of proteins at reactive amino groups, which can adversely affect protein function. Although the effects of glycation on soluble proteins are well characterized, there is no information regarding membrane-associated proteins, mainly because of the lack of reproducible methods to determine protein glycation in vivo. The current study was conducted to establish such a method and to compare the glycation levels of membrane-associated proteins derived from normal and diabetic tissue. The authors present a detailed sample preparation protocol based on the borohydride-periodate assay, modified to allow manipulation of animal tissue. Assay noise associated with extraction protocols and nonproteinaceous buffer components was eliminated by the using 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS) as a membrane detergent, applying desalting columns, and including a protein precipitation step. The glycation level of membrane proteins from diabetic rats is elevated to 4.89 nmol/mg protein (standard deviation [SD] 0.48) compared with normoglycemic control tissue (2.23 nmol/mg protein, SD 0.64). This result is consistent with and correlated to the total glycated hemoglobin levels in diabetic and normoglycemic rats. Using <100 µg protein, the described methods allow further study of protein glycation effects on the function of individual transporter proteins and the role of these modifications in diabetes.

About AAPS PharmSci
AAPS PharmSci^SM, www.pharmsci.org, is AAPS’ electronic-only, peer-reviewed journal, covering the areas of drug discovery, development and therapy.

About AAPS
AAPS is a professional, scientific society of more than 11,000 members employed in academia, industry, government and other research institutes worldwide. Founded in 1986, AAPS aims to advance science through the open exchange of scientific knowledge, serve as an information resource, and contribute to human health through pharmaceutical research and development. For more information about AAPS, visit AAPS Pharmaceutica at www.aapspharmaceutica.com.

* * *