Yth. Peserta Diskusi ZOA-BIOTEK-2001, Berikut ini kami sampaikan bagian pertama dari dua kali posting makalah Prof. Ishikawa tentang "Desain Molekul Obat berdasar Mekanisme Transport", Kami mohon maaf, jika gambar tidak dapat tertampilkan dengan baik pada modus teks email. Bagi yang berminat, kami persilakan menengok website ZOA-BIOTEK di http://sinergy-forum.net/zoa/paper/html/papertoshihisaishikawa.html Selamat menikmati makalah Prof. Ishikawa. Moderator Dedy H.B. Wicaksono ================================================== A New Challenge in the Post-Genome Era: Transport Mechanism-Based Drug Molecular Design Toshihisa Ishikawa Department of Biomolecular Engineering Graduate School of Bioecience and Biotechnology, Tokyo Institute of Technology Waldemar Priebe Department of Bioimmunotherapy, University of Texas M.D. Anderson Cancer Centery Abstract In the last decade of the 20th century, the development of high throughput screening (HTS) and high speed analoging (HSA) technologies helped accelerate the drug discovery process. In the 21st century, emerging genomic technologies (i.e., bioinformatics, functional genomics, and pharmacogenomics) may shift the paradigm for drug discovery and development. In the meantime, however, drug discovery and development will remain high-risk, high-stakes ventures with long and costly timelines. The attrition of drug candidates in preclinical and development stages is a major problem in drug design. At least thirty percent of the cases, this attrition is due to poor pharmacokinetics (e.g. , limited absorption. Low plasma concentration levels, high rates of clearance). In the past, pharmaceutical companies have considered such early stage attrition an inevitable cost of doing business; however, as drug development costs have rocketed upward, pharmaceutical companies have begun to seriously reevaluate their current strategies of drug discovery and development. In that light, we propose that a transport mechanism-based design might help to create new, pharmacokinetically advantageous drugs, and as such it should be considered an important component of drug design strategy Terjemahan Abstrak dalam Bhs. Indonesia Tantangan Baru Era Pasca-Genome: Desain Molekul Obat Berdasarkan Mekanisme Transport Pada decade akhir abad 20, perkembangan high-throughput screening (HTS) dan high speed analoging (HAS) telah membantu mempercepat proses penemuan obat baru. Di abad 21, teknologi genomik yang baru bermunculan (seperti bioinformatika, genomic fungsional, dan farmakogenomik) mungkin akan menggeser paradigma pengembangan dan penemuan obat. Sekalipun demikian, untuk sementara ini, pengembangan dan penemuan obat masih merupakan usaha beresiko tinggi dengan modal besar dan membutuhkan waktu yang panjang. Tertolaknya kandidat-kandidat obat pada tahap preklinik dan tahap pengembangan masih menjadi masalah utama dalam deasin obat. Setidaknya dalam 30% kasus, tertolaknya kandidat obat ini akibat farmakokinetika yang jelek (seperti absorpsi yang terbatas, level konsentrasi plasma rendah, dan cepat terbilas habis). Di masa lalu, perusahaan farmasi menganggap bahwa kegagalan suatu kandidat obat di tahap-tahap awal. Introductory Remarks The attrition of drug candidates in preclinical and development stages is a major problem in drug design. At least thirty percent of the cases, this attrition is due to poor pharmacokinetics (e.g., limited absorption. low plasma concentration levels, high rates of clearance). In the past, pharmaceutical companies have considered such early stage attrition an inevitable cost of doing business; however, as drug development costs have rocketed upward, pharmaceutical companies have begun to seriously re-evaluate their current strategies of drug discovery and development. In that light, we propose that a transport mechanism-based design might help to create new, pharmacokinetically advantageous drugs, and as such it should be considered an important component of drug design strategy. Our strategy sprang from the realization that, in the near future, the processes of drug discovery and development will be dramatically changed by the introduction of new research technologies such as bioinformatics, functional genomics, and pharmacogenomics and their use to identify both classical drug targets (e.g., enzymes, membrane- bound receptors, and ion channels) and novel drug targets (e.g., cellular components of signal transduction, orphan nuclear receptors, mRNA, and DNA). As a result organ-specific drug targeting and delivery will become increasingly important in ensuring the site selectivity and pharmacological activity and reducing the side effects of new molecular drug candidates aimed at those targets. Why are Drug Transporters so Important? Drug transporters and drug-metabolizing enzymes are important because they play pivotal roles in determining the pharmacokinetic profiles of drugs and, by extension. their overall pharmacological effects (i.e., drug absorption, drug distribution, drug metabolism and elimination, drug concentration at the target site, and the number and morphology of target receptors) (Fig. 1), If a drug's concentration is too low at the target site (time course a), one can expect no pharmacological effect. If it is too high, one can expect side effects (time course c). Thus, in the best case, a drug's concentration at its target site (time course b) should be well controlled for sufficient periods by the action of factors controlling drug transport (influx and efflux) and drug metabolism. Figure 1. Drug transporters are critically involved in the pharmacokinetics of drugs. For example, transporter proteins expressed in epithelial cells of the small intestine and in brain capillary endothelial cells, greatly affect the oral bioavailability of drugs and their penetration into the central nervous system (CNS). From cancer chemotherapy studies, it is known that overexpression of the transporter proteins P-glycoprotein (also called MDR1) and multidrug resistance-associated protein 1 (MRP1 ) can render cancer cells resistant to anticancer drugs. Interestingly, both P-glycoprotein and MRP1 belong to the ATP-binding cassette (ABC)-transporter family. This family of human ABC transporters is now known to contain more than 40 members, most of whose genes have been identified and sequenced. Evidence is also accumulating that human ABC proteins are involved in the transport of both drugs and endogenous substances and ions 1). (For detailed information, see URL http://www.gene.ucl.ac.uk/users/hester/abc. html) P-glycoprotein, in particular, is gaining attention for its involvement in drug absorption by the small intestine and drug penetration into the brain; it is expressed in a variety of normal cells and organs, and its modulation in these tissues can influence the activity and bioavailability of drugs. In the intestine, for instance, modulation of P-glycoprotein may control the degree of drug uptake after drug ingestion. At the blood-brain barrier, high P-glycoprotein levels can limit the uptake of desired drugs into the brain; conversely, low P-glycoprotein activity can lead to abnormally increased accumulation and undesirable side effects. Genetic Polymorphism of Drug Transporters The effects of drug transporters on the pharmacokinetic profile of a drug depend on their expression and functionality. Indeed, the expression of drug transporters can be modulatcd by endogenous and exogenous factors, including drugs, themselves. It is also now known that inherited differences among individuals may also affect drug efficacy and toxicity. Such inherited differences include genetic polymorphisms in drug targets and drug-metabolizing enzymes, as well as in drug transporters. Hitherto, pharmacogenetics, the field dealing with such inherited differences and their effect on pharmacokinetics, has significantly contributed to our understanding of genetic causes underlying differences in drug metabolism (e.g., cytochrome P-450 mediated drug metabolism). In fact, recent technological advances allowing massive molecular sequencing have in turn allowed a consortium of researchers to identify single nucleotide polymorphisms (SNPs) as one possible cause of variable drug response among individuals. The SNP consortium plans to complete a high-density map of 300,000 to 600,000 SNPs by mid 2001 and make it openly available to the public (http://snp/cshl/org) . In light of such advances, it is important to carefully examine the clinical significance, if any, of polymorphisms in drug transporter genes. Several preclinical reports have already noted naturally occurring polymorphisms in P-glycoprotein and their effects on drug absorption, distribution and elimination; although the clinical relevance of these and other drug transporter polymorphisms has not yet been fully elucidated. Recently, however, Hoffmeyer et al. provided evidence of multiple polymorphisms in the MDR1 gene encoding human P-glycoprotein 2). One of those mutations in particular, a C-to-T variant in the exon 26 of the MDR1 gene, was significantly correlated with P-glycoprotein expression and function; as a result, individuals homozygous for the polymorphism expressed significantly less duodenal P-glycoprotein and significantly more plasma digoxin. Hoffmeyer et al. also inferred that this polymorphism might affect the absorption and tissue concentrations of numerous other substrates of P-glycoprotein. From the discovery and characterization of drug transporter gene polymorphisms such as this one in the MDR1 gene may come a diagnostic test for discriminating between different MDR I alleles and better strategies for designing molecular anticancer drugs.