CHAPTER 1
Historical Perspective

ROBERTA S. KING

Department of Biomedical and Pharmaceutical Sciences, College of Pharmacy, University of Rhode Island, Kingston, RI, USA

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 (Bachman 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; Bachman 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 (Bachman and Bickel, 1985-86; Berczeller, 1917). 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 (Bachman and Bickel, 1985-86). The next hypothesis, proposed in 1922 by 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, Schüller (1925) proposed a theory also similar to the modern view of the physiological purpose of drug metabolism. Schüller 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 (Schüller, 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 Schüller. 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" (Bachman 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 200?years ago, first in dogs in 1824 and next in humans in 1841 (Woehler, 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 and Figure 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, 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; Woelher and Frerichs, 1848). We now call this biotransformation of cinnamic acid to benzoic acid, ß-oxidation. Woelher 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 nonphysiological 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,...