Drug Metabolism Handbook: Concepts and Applications - Hardcover

Nassar, Ala F.

Hardcover

ISBN 10: 0470118032 ISBN 13: 9780470118030

Publisher: Wiley, 2009

Synopsis

A valuable reference tool for professionals involved in the industry, Drug Metabolism in Pharmaceuticals covers new tools such as LC-MS and LC-MS-NMR along with experimental aspects of drug metabolism. This work fills a gap in the literature by covering the concepts and applications of pharmaceutical research, development, and assessment from the point of view of drug metabolism. By providing both a solid conceptual understanding of the drug metabolism system, and a well illustrated, detailed demonstration and explanation of cutting edge tools and techniques, this book serves as a valuable reference tool for bench scientists, medical students, and students of general health sciences.

About this title

About the Author

Ala F. Nassar, PhD, provides technical and operational leadership in characterizing in vitro and in vivo metabolites. His research focuses on understanding how structure modification can improve the ADME profile for new chemical entities as they advance towards clinical candidacy. Recently, he identified and patented pharmacologically active metabolites of Nexavar®. Dr. Nassar serves on the Editorial Board of Drug Metabolism Letters and The Open Drug Metabolism Journal and is also a reviewer for several other journals.

Paul F. Hollenberg, PhD, is Maurice H. Seevers Collegiate Professor and Chair of Pharmacologyat The University of Michigan. His current research focuses on the relationship between thestructures of the active sites of cytochrome P450s and their catalytic function. Dr. Hollenbergwas cofounder and Associate Editor of Chemical Research in Toxicology and has served onnumerous editorial boards and review panels.

Joann Scatina, PhD, has over twenty-four years of drug metabolism experience, and is currentlyVice President of the Drug Metabolism Division, Drug Safety and Metabolism, for WyethResearch. Dr. Scatina's research interests include identification of metabolites, metabolicpathways and enzymes responsible, in vitro-in vivo extrapolation, and prediction of druginteractions. She is a member of the American Society for Pharmacology and ExperimentalTherapeutics and the International Society for the Study of Xenobiotics (ISSX) and is currentlyserving on the Editorial Board of Drug Metabolism Reviews.

From the Back Cover

Explains All The Latest Tools And Technologies In Drug Metabolism

Drug Metabolism Handbook is dedicated to helping bench scientists take full advantage of the latest and most advanced tools and techniques in the field of drug metabolism. From creating new molecular entities to drug development to troubleshooting, the book explains the concepts of drug metabolism and guides readers through its applications in pharmaceutical research, development, and assessment.

Concise and easy to follow, Drug Metabolism Handbook offers:

• Succinct reviews of drug metabolism that underscore the importance of the human metabolic system as well as pharmacokinetics and drug interactions

• Thorough and carefully referenced reviews of most drug metabolism phase I and phase II metabolic enzymes

• Illustrative case studies that highlight the critical role of drug metabolism in the drug development process

• Descriptions of the latest findings and tools such as online H-D exchange, LC-ram, LC-MS/MS, and LC-MS-NMR

• Full coverage of recent FDA regulations and guidelines for drug safety

Each chapter is written by one or more leading experts in the field of drug metabolism. Contributions are based on the authors' own first hand experience at the bench as well as a thorough examination of the literature.

By combining a solid conceptual understanding of the drug metabolism system with detailed explanations of the latest cutting-edge tools and techniques, this book is essential readingfor pharmaceutical scientists, students at universities, and all other researchers involved in drug development.

Drug Metabolism Handbook is dedicated to helping bench scientists take full advantage of the latest and most advanced tools and techniques in the field of drug metabolism. From creating new molecular entities to drug development to troubleshooting, the book explains the concepts of drug metabolism and guides readers through its applications in pharmaceutical research, development, and assessment.

Concise and easy to follow, Drug Metabolism Handbook offers:

• Succinct reviews of drug metabolism that underscore the importance of the human metabolic system as well as pharmacokinetics and drug interactions

• Thorough and carefully referenced reviews of most drug metabolism phase I and phase II metabolic enzymes

• Illustrative case studies that highlight the critical role of drug metabolism in the drug development process

• Descriptions of the latest findings and tools such as online H-D exchange, LC-ram, LC-MS/MS, and LC-MS-NMR

• Full coverage of recent FDA regulations and guidelines for drug safety

Each chapter is written by one or more leading experts in the field of drug metabolism. Contributions are based on the authors' own first hand experience at the bench as well as a thorough examination of the literature.

By combining a solid conceptual understanding of the drug metabolism system with detailed explanations of the latest cutting-edge tools and techniques, this book is essential readingfor pharmaceutical scientists, students at universities, and all other researchers involved in drug development.

Excerpt. © Reprinted by permission. All rights reserved.

Drug Metabolism Handbook

Concepts and Applications

John Wiley & Sons

Copyright © 2009 John Wiley & Sons, Inc.
All right reserved.
ISBN: 978-0-470-11803-0

Chapter One

Historical Perspective

ROBERTA S. KING

1.1 CONTROVERSIES SPANNING PAST, PRESENT, AND FUTURE

Two major issues have been debated throughout the history of drug metabolism, and are still disputed to some degree. One is the name itself, the other is the physiological purpose of "drug" metabolism. In the 1800s and early 1900s, the generally agreed purpose of these reactions was reflected in the most widely used name, detoxication mechanisms. However, detoxication became widely recognized as a misnomer because not all parent compounds were toxic and not all metabolites were less or nontoxic. A better term was not invented until the 1950s when the term "drug metabolism" was coined. While handy, this term was still not entirely valid, and it needed silent agreement that "drug" be not restricted to medicinal compounds (Bachmann and Bickel, 1985-86). Thus, xenobiotic metabolism became popular starting in the 1970s, especially in circles studying carcinogens and environmental compounds. Xenobiotic, by definition, included all compounds foreign to the organism, not just medicinal ones. However, even in the early 1900s many examples were already known of metabolism of endogenous compounds, for example, steroids undergoing glucuronidation. These early examples of endogenous substrates were generally dismissed because they typically occurred at much higher concentrations than normally present, so-called "supraphysiological" concentrations. While none of these three terms could be considered ideal, in 1947 R.T. Williams concluded that the field of detoxication included "... all those metabolic processes not specifically covered by the main streams of fat, carbohydrate and protein intermediary metabolism" (Williams, 1947). Williams went on to explain that, "Detoxication is, in fact, the study of the metabolism of organic compounds other than lipids, carbohydrates, proteins and closely related natural compounds, although the lines of demarkation between these two groups is by no means a sharp one ..." (Williams, 1947; Bachmann and Bickel, 1985-86). Thereby, the earliest clear description of the field mainly described what it was not, and later terms were not much more precise.

The physiological purpose of these reactions was also widely debated. Of note is that several of the early theories are still considered at least partially valid. The first theories tried to answer the question of how these transformations were related to detoxication. For example, in 1917, Berczeller published the first article trying to answer why conjugation would result in detoxication (Berczeller, 1917; Bachmann and Bickel, 1985-86). His theory, later to be disproved, was that conjugations such as glucuronidation and sulfonation led to a change in the surface tension of dilute aqueous solutions. He related this to in vivo conditions by indicating that the conjugates, with less surface activity than the parent drug, would be more easily removed from cellular surfaces while the parent drugs would accumulate at the surfaces to toxic concentrations (Bachmann and Bickel, 1985-86). The next hypothesis, proposed in 1922 by Sherwin (Sherwin, 1922), was based on the idea of "chemical defense" against accumulation of foreign compounds. This hypothesis is still considered valid as one role of drug metabolism. Sherwin proposed that the body needed to completely destroy foreign compounds or excrete them in the urine. Thus, for Sherwin, the purpose of oxidation and reduction reactions was to destroy the foreign molecules. If complete destruction was not possible, then conjugation reactions could make the molecule more aqueous soluble and more easily removed through the urine. In 1925, Schller (Schller, 1925) proposed a theory also similar to the modern view of the physiological purpose of drug metabolism. Schller proposed that conjugation reactions led to an increase in the water solubility of compounds leading to a change in the distribution of the compound in the body (Schller, 1925). The third theory, which resonates somewhat with today's metabolism scientists, was proposed by Quick in 1932 (Quick, 1932). His view was that conjugations resulted in an increase in acidity, converting a weak acid parent into a strong acid metabolite which could be eliminated more easily. One benefit of this hypothesis was that it included in its rationale the acetylation reaction, which was ignored by Sherwin and Schller. In his 1947 comprehensive review of metabolism (Williams, 1947), R.T. Williams criticized each of these potential roles of metabolism. Yet rather than proposing his own theory, Williams concluded that "interpretations worthy of the status of theories were lacking at that time" (Bachmann and Bickel, 1985-86).

In 2008, the field is described as an "elaborate defense system against foreign compounds and against the accumulation of potentially toxic endogenous molecules" (Meyer, 2007). However, the same author also adds recognition of the modern "concept of molecular links between xenobiotic metabolism and endogenous pathways of sterol, lipid, bile acid and energy homeostasis." To complicate matters, the current generation of genetic technologies has revealed that all of the enzyme families contributing to drug metabolism include both members with selectivity for endogenous compounds and members with selectivity for exogenous compounds, and that homologues are present throughout diverse species from bacteria to human. Indeed, gene knockout studies have shown that multiple members of the "drug"-metabolizing enzyme families are essential to life or reproductive processes, as well as serving as a means to defend against xenobiotics (Sheets, 2007). Thus, in 2008, the field has no more specific name than the popular term "drug metabolism," yet its physiological function has broadened to include endogenous compounds and endogenous regulatory pathways.

1.2 1800s: DISCOVERY OF MAJOR DRUG METABOLISM PATHWAYS (CONTI AND BICKEL, 1977)

Many current students of metabolism are surprised to learn that drug metabolism experiments were first conducted and published more than 180 years ago, first in dogs in 1824 and next in humans in 1841 (Whler, 1824; Ure, 1841). While these experiments were quite rudimentary by today's standards, they laid the foundation for all of metabolism by establishing the paradigm that the body could take up exogenous compounds, perform chemical reactions on them, and then remove them via the urine in a chemically altered form. Also noteworthy is that by 1893, one or more examples of each major metabolism pathway had been published (Table 1.1, Fig. 1.1). Nearly all of this early work was published in German, often by German scientists. This was a time of significant advances in chemistry and medicine in German laboratories. However, the metabolism studies were limited by the difficulty in purifying and proving structure identification of the proposed metabolites. Advances in organic chemistry and analytical methods often directly led to advances in the metabolism studies.

Glycine conjugation has the honor of being the first-described metabolism reaction. In 1841 and 1842, two scientists independently ingested benzoic acid and observed a compound in the urine "in copious amounts," and "without any apparent unhealthy effects" (Ure, 1841; Keller, 1842; Conti and Bickel, 1977). The excreted compound was similar to benzoic acid, but also contained nitrogen. Identification of the compound as hippuric acid, the glycine conjugate of benzoic acid, was made in 1845 by a French scientist named Dessaignes (Dessaignes, 1845).

The benzoic acid studies soon led to the observation of oxidation as a preliminary step to conjugation, by observing that ingestion of cinnamic acid also caused excretion of hippuric acid into the urine (Erdmann and Marchand 1842a, b; Whler and Frerichs, 1848). We now call this biotransformation of cinnamic acid to benzoic acid, -oxidation. Whler and Frerichs (1848) also discovered aldehyde oxidation when they found that dogs and cats excreted a "considerable amount" of benzoic acid after treatment with benzaldehyde (oil of bitter almonds) (Conti and Bickel, 1977, p. 11).

Nothing of the mechanism of these transformations was yet understood. During this era, the body was considered simply a chemical reaction container. Indeed, the chemists found many transformations "absolutely puzzling" (Conti and Bickel, 1977, p. 8) because, outside the organism, they could only be reproduced only under very harsh conditions, if at all. For example, oxidation of benzene to phenol had never been accomplished when, in 1867, Schultzen and Naunyn published their very clear determination of phenol in the urine of humans and dogs after ingestion of benzene. Even in the twenty-first century, only rather harsh and non-physiological conditions are known to chemically transform benzene to phenol (March, 1992).

In 1844 Erdmann observed that euxanthic acid isolated from urine of cows fed mango leaves could be hydrolyzed to euxanthone, but it took until 1870 to characterize the sugar moiety as an oxidized form of glucose. Similarly, conjugated phenols were observed in 1851, but it took until 1876 to confirm that the conjugate was a sulfate producing sulfuric acid upon hydrolysis (Staedeler, 1851; Baumann, 1876). Baumann is known as the "father of sulfation" and subsequently showed that many other ingested chemicals were also excreted as sulfates in the urine, often after preliminary oxidation to a phenol (Baumann, 1876). Methylation was first described in 1887 after feeding pyridine acetate to dogs and isolating the N-methyl product from the urine (His, 1887). Also in 1887, Jaffe and Cohn first observed a product that appeared to be conjugated with acetic acid (Jaffe and Cohn, 1887), although it took until 1893 for Cohn to confirm N-acetylation as a major conjugation reaction (Cohn, 1893).

While biochemists, chemists, physiologists, and pharmacologists contributed to these discoveries, the emphasis was on studying the metabolism of "foreign" compounds. More importantly, the emphasis was the fate of the chemical compound, rather than the organism that transformed it. Indeed, Bachmann and Bickel concluded that before circa 1870, "the whole matter was a biochemical curiosity rather than a physiologically meaningful process" (Bachmann and Bickel, 1985-86, p. 213). Evidence that this view began to change comes from Nencki's thesis of 1870, foreseeing that, "... one will on the one hand be able to establish laws allowing predictions on the fate of new compounds, and on the other hand gain increasing insight into the organism as a 'chemical agent'." (Nencki, 1870) Beginning in about 1876 with Baumann's phenyl sulfate (Baumann, 1876), it was often found that the excreted compounds were much less toxic than their parent compounds. And by 1893, enough evidence existed for introduction of the term "detoxication" in a textbook of physiological chemistry (Neumeister, 1893; Bachmann and Bickel, 1985-86, p. 213). Detoxication still stands as one major physiological role of metabolism, but is no longer recognized as the only role.

1.3 1900-1950s: CONFIRMATION OF MAJOR PATHWAYS AND MECHANISTIC STUDIES

As better techniques for identification of compounds were developed, the major reactions were confirmed and put upon somewhat stronger structural foundations; and in two cases the active cofactors for the reactions were elucidated. For example, Lipmann earned the 1953 Nobel Prize in Medicine, in part, for his 1945 publication of the role of coenzyme A in acetylation of sulfanilamides (Lipmann, 1945). In 1953, Cantoni published evidence for S-adenosyl methionine as the active cofactor for methylation reactions (Cantoni, 1953). The biosynthetic source of glycine was elucidated in 1946 (Shemin, 1946), but it was not yet understood how the conjugation reaction occurred under physiological conditions.

Nencki's (1870) prediction of gaining insight into the organism as a chemical agent was also developed in significant ways. In the 1800s, the blood was often considered the localized source of the transformations, and the emphasis was on animal or human ingestion of a compound and isolation of excretion products from the urine. In the early 1900s, techniques such as hepatectomy and perfusion of livers and kidneys of laboratory animals proved the alternative paradigm of organ-based metabolism. One novel approach (Hemingway, Pryde and Williams, 1934) used serially perfused dog organs (such as liver or spleen) in combination with the kidney to demonstrate that the liver was the main site of glucuronic acid conjugation. In 1936, Potter and Elvehjem described an improved alternative to the commonly used tissue slice and tissue mince methods (Potter and Elvehjem, 1936). Their device used a glass test tube as mortar with a motor-driven blown-glass pestle for nondestructive tissue homogenization. Claude added differential centrifugation to the improved homogenization technique, and made possible subcellular localization of metabolism via isolation of individual tissue organelles (Claude, 1940). He also coined the still-popular term "microsome" (Claude, 1943).

Of note is that during the early twentieth century, most metabolism studies were conducted within biochemistry departments of university medical schools. Also during this time, because of (or in spite of) the World Wars, metabolism research expanded across Europe to England and North America, and by 1950 the United States had replaced Germany as the dominant origin of metabolism publications.

Approximately 100 years after the first published discoveries, "Modern" metabolism science was founded by the Welshman R.T. Williams, who in 1947 published the first text devoted to metabolism entitled Detoxication Mechanisms: The Metabolism of Drugs and Allied Organic Compounds. This text was expanded in a second edition in 1959 with a modified title, The Metabolism and Detoxication of Drugs, Toxic Substances, and Other Organic Compounds. In these texts, Williams brought systematic organization and clarity to what had previously been broad and disconnected research, "so that working hypotheses (could) be advanced" (Williams, 1947, quoted from Caldwell, 2006). It was Williams who proposed that metabolism occurred through reactions representing two distinct phases leading to the still popular terms, "phase I" and "phase II" metabolism reactions. Another revolutionary, B.B. Brodie of the United States National Institutes of Health, first published in 1948 and "led the field into its modern phase"(Bachmann and Bickel, 1985-86, p.188). As foreseen by Nencki in 1870 (Nencki, 1870), predictive rules for functional group transformations began to emerge, and the enzymology of the reactions began to be elucidated.

1.4 1950s-1980: MODERN DRUG METABOLISM EMERGES, WITH ENZYMATIC BASIS

Because of advances in analytical technologies and biochemical methods, metabolism studies took off starting in about 1950. For example, partition chromatography improved separations and allowed differentiation of drug versus metabolites. Isotope-tracer methods (mostly 14C and 15N) allowed metabolites of foreign compounds to be detected and quantified at nontoxic doses. Absorption spectrophotometry also improved both quantification and identification of drug versus metabolites. For the first time, the cofactors of the enzymatic reactions were fully elucidated, and the enzymatic biosynthesis pathways of the cofactors were established. For example, the molecular mechanism of glucuronidation utilizing the reactive cofactor, uridine-3',5'-diphosphate glucuronic acid (UDPGA) was published in 1954 (Dutton and Storey, 1954). Advances in tissue fractionation methods provided the means to prove the enzymatic basis of metabolism. Indeed, Brodie and coworkers published the first review of this emphasis in 1958 entitled, "Enzymatic metabolism of drugs and other foreign compounds" (Brodie, Gillette and La Du, 1958). In the late 1950s and early 1960s, the cytochrome P450 enzyme system was discovered and characterized as the source of microsomal oxidations of several drugs and steroid hormones (Mason, 1957a, b; Klingenberg, 1958; Omura and Sato, 196; Omura et al., 1965). By the end of this period, the role of cytochrome P450 in drug metabolism was well established and the important modifying factors (inhibition, induction, and polymorphisms) were beginning to be understood (Conney et al., 1980; Netter, 1980; Orrenius, Thor and Jernstrm, 1980; Ritchie et al., 1980).

(Continues...)

Excerpted from Drug Metabolism Handbook Copyright © 2009 by John Wiley & Sons, Inc.. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Wiley
• Publication date: 2009
• Language: English
• ISBN 10: 0470118032
• ISBN 13: 9780470118030
• Binding: Hardcover
• Edition number: 1
• Number of pages: 1064