SOME HISTORICAL REFLECTIONS ON DRUG METABOLISM

Thomas C. Butler, M.D.* Center for Research in Pharmucotogy and Toxicology

University of North Carolina School of Medicine Chapel Hill, N. C.

I am impressed that so many of the topics discussed in this monograph con- cern discoveries that were first made in laboratory animals or techniques first developed with laboratory animals and only subsequently applied in man. That fundamentid work in animals is finding applications in man and that these applications are being emphasized here I find heartening in that it constitutes a public recognition that pharmacology is a medical science. There was a time when such recognition would not have called for comment. When we consider the men who were most influential in the early shaping of pharmacology into its modem forn-Professor Buchheim in Dorpat, later his student, Professor Schmiedeberg in Strassburg and then Professor Abel in this country-we note that these were men trained as physicians, men who never ceased to think of themselves as physicians even when, realizing that the answers to clinical prob- lems must often be sought in the laboratory, they became engrossed in their laboratories.

It would never have occurred to any of these men that there could be any question that the problems on which they worked were related directly to the practice of medicine. It is only in recent years that there has been any desire to make the basic science departments of medical schools so basic as to be isolated from all contact with clinical medicine. This has been a development in a direc- tion that I deplore, and I am inclined to view the clinicians as being in large part culpable. If this monograph can do anything to make pharmacologists realize that they are a part of medicine and clinicians realize that fundamental work with drugs has applicability in practical therapeutics, it will have a salubrious effect.

It was not always that initial study of a drug in animals and subsequent application of the knowledge so gained to human medicine was considered the normal direction of development. We must remember that the earliest studies of drugs were made in man. In all prescientific cultures there must have been a great deal of experimentation on man with all sorts of natural products. From this there emerged much superstitious misinformation but also some sound knowledge of a few valuable remedies and of a good many poisons.

In this preacientific experimentation with natural products in man, there were even some discoveries in the field of drug metabolism. Lewin,' in his book Phantastica, recounts the knowledge of the inhabitants of Eastern Siberia that the active principle of the mushroom, fly agaric, is excreted unchanged in the urine.

Among the exceedingly numerous problems offered by the fly-agaric not the least important is that the Koryaks, Kamchaldales, etc., discovered that the urine of a person intoxicated by agaric also possesses intoxicating properties. Who taught them that the active principle of the mushroom is not destroyed in the organism but is completely excreted in the urine, which then has the same effect on the brain as the

*Recipient of PHS Research Career Program Award 5 KU6 GM 19429 from thz Nation- al Institute of General Medical Sciences.

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agaric itself? As soon as the Koryak notices his inebriety decreasing, he drinks his own urine, if he has no more agarics, or in order to economize. The Koryak women pass a tin receptacle reserved for this purpose to the intoxicated person into which the urine is passed in the presence of all. The urine, frequently still warm, is drunk by the person awakening from sleep and after a few minutes exercises its influence. In this way the action may be renewed several times. If there should be any of the urine left over, it is preserved for a short time to be used on the next occasion.

This same curious type of study of drug metabolism in man by bioassay of urine, accomplished in a nonscientific culture in Siberia, has apparently led to a similar achievement in the antiscientific counterculture of contemporary drug users. In a survey of street drugs carried out in Canada by the Alcoholism and Drug Addiction Research Foundation, one article of commerce discovered was a bottled product purporting to be liquid methamphetamine. It proved to be human urine containing dextroamphetamine and methamphetamine. It was surmised that methamphetamine users were selling their urine for reuse.

With the advent of the modem scientific era of pharmacology in the middle of the last century, drug metabolism began to be studied in a more sophisticated way. I shall use the term “drug metabolism” to include not only the study of the chemical reactions that drugs undergo in the body but also the application of analytical metods to determine concentrations of drugs and their metabolites in blood and tissues for the purpose of studying their distribution and the kinet- ics of their entry into and elimination from the body. Both types of studies we find represented in the work of the early practitioners of the new science of pharmacology in the last century.

Although there had been a few earlier studies of drugs by men who thought of themselves primarily as physiologists, modem pharmacology can be thought of as being founded by Rudolf Buchheim at the University of Dorpat (now Tartu). Dorpat was in Russian territory, but the University was German in language and spirit. During the period of Buchheim’s professorship at Dorpat, from 1847 to 1867, he directed the research of numerous students and did much through his efforts to bring about recognition of pharmacology as a scientific discipline in its own right. Oswald Schmiedeberg, the most illustrious and influential of Buchheim’s students, has published an assessment of Buchheim’s role in the founding of scientific pharmacology.’

It was during Schmiedeberg’s long tenure of the chair at the University of Strassburg over the period from the Franco-Prussian War to World War I that the new science of pharmacology enjoyed its greatest growth and greatest public esteem. New departments of pharmacology were being established all over Europe and America, and many of them were to be headed by men trained by Schmiedeberg.

These two men, Buchheim and Schmiedeberg, who did the most to establish pharmacology as a recognized discipline, were both interested in drug metabo- lism.

Textbooks of pharmacology universally attribute the discovery of the hyp- notic action of chloral hydrate to Oscar Liebreich in 1869. This is cited as a clas- sic example of an erroneous theory of drug metabolism leading to a useful result. Liebreich thought that chloral hydrate would be slowly decomposed by the alka- line blood to chloroform and formic acid and that the chloroform so produced would exert its depressant effect.

It is a little-known fact that Buchheim discovered the hypnotic action of chloral hydrate eight years earlier than Liebreich, and through the same curiously erroneous line of theoretical rea~oning.~ He too thought that the com-

504 Annals New York Academy of Sciences

pound would decompose to chloroform and formic acid, but his objective was to find something that would form an acid for the purpose of neutralizing what he thought to be excessive alkalies in the blood in certain diseased conditions. He thought it likely that chloroform would undergo further decomposition, with formation of hydrochloric acid and formic acid, this additional acid being desir- able for the objective he had in mind.

Buchheim took chloral hydrate himself and observed the hypnotic action, an action that was confirmed by administration to patients in the Dorpat clinic. Buchheim accepted without question, as Liebreich did later, the demonstration of the hypnotic action of chloral hydrate as proof of the correctness of the pre- diction that it would decompose to chloroform. That the depressant action that he attributed to chloroform was apparent he took to be an indication that chloro- form does not quickly decompose, as he had expected. Buchheim considered the unpleasant taste of chloral hydrate an obstacle to its acceptance in practice and hoped to find something better. He did not publish at the time, and Liebreich’s later discovery was entirely independent. Buchheim’s experiments with chloral hydrate in himself and the Dorpat patients can be considered an early study of drug metabolism in man, even if his interpretation of the results in terms of metabolic pathways has proved to be incorrect.

The spectacular popularity that chloral hydrate attained almost immedi- ately after Liebreich’s introduction of the drug quite understandably directed attention to the analogous bromine compound. Bromal hydrate is decomposed by alkali to bromoform and formic acid. It occurred independently to at least four workers that if its metabolic fate were analogous to that proposed by Lie- breich for chloral hydrate, it should also be hypnotic. Bromal hydrate received a limited human trial by three investigators, with results distressingly different from those produced by its chlorine analogue. A Scottish physician, Dougall, who took the drug himself and was fortunate enough to survive, published a gruesome account of his experience in 1870.‘ This again was a study of drug metabolism in man, although it was many years later before it could be so inter- preted.‘ Rather than undergoing the fate that Buchheim and Liebreich thought that chloral hydrate should or the fate that it actually does undergo (reduction to the alcohol), bromal hydrate, by contrast to its chlorine analogue, is a very reactive positive halogen compound.

The study of drug metabolism by identification of drug metabolites in urine also appears early in the modem era of pharmacology. I t is a curious fact that some of the earliest achievements in isolation of drug metabolites were on sub- stances that would today be considered the most difficult to isolate, the glucu- ronides. Despite the lack of the modem methods that would facilitate their isola- tion, glucuronides were in fact isolated from urine over a hundred years ago. The first glucuronide identified as a drug metabolite was the glucuronide of trichlo- roethanol, which was isolated by von Mering from the urine of patients receiving chloral hydrate.

Josef von Mering was a physician of versatile talents who made other notable contributions in diverse fields of medicine. In collaboration with Minkowski in 188g6 he showed that pancreatectomy produces diabetes in dogs. In collabora- tion with Emil Fischer in 1903? he did the first pharmacological study of barbi- turic acid derivatives and introduced barbital into medicine.

Von Mering and M U S C ~ ~ U S * isolated the metabolite of chloral hydrate from human urine in 1875, when von Mering was 26 years old. Eight years later von Meringg announced the identification of the substance as the conjugation prod- uct of trichloroethanol with glucuronic acid. Not only was this difficult isolation

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accomplished, but the product was obtained in large amounts and in a high state of purity. The modem worker prides himself in isolating and identifying some- thing in microgram quantities. His predecessor of a former generation took pride in obtaining drug metabolites in kilogram quantities.

Drug metabolism was one of Schmiedeberg's principal interests, but most of his work was done in laboratory animals rather than in man. It was a time when moving from the bedside to the laboratory to attack clinical problems was the direction of scientific advance. The use of laboratory animals rather than man permitted Schmiedeberg to study drug metabolism in ways not possible in man. One of his notable achievements was the demonstration in 1876," by perfusing the isolated kidney of the dog, that hippuric acid synthesis occurs in the kidney.

The other aspect of drug metabolism, the study of the distribution of drugs and the kinetics of their entry into the elimination from the body, also had attracted interest as early as the middle of the last century, although it must be said that there were few triumphs in this field until well into the 20th century. Buchheim recognized that a theoretical understanding of the mechanism of action of a drug often can be arrived at only through quantitative knowledge of the concentrations in the various parts of the body, and much of his efforts were toward the application of the chemistry of his day to pharmacological problems. Several of his students worked on quantitative analytical methods for determi- nation of alcohol in blood. The dissertation that Schmiedeberg submitted to the University of Dorpat in 1866 in fulfillment of the requirements for the degree of Doctor of Medicine concerned an analytical method for determination of chloro- form in blood.

Until recent years analytical chemistry advanced slowly and lacked the methodology needed for measurement of the low concentrations in which most drugs occur in the body. As recently as 40 years ago there was little knowledge of the concentrations in blood or tissues of drugs other than some of the inhalation anesthetics and alcohol. These are drugs that can occur in the body in such high concentrations that analytical methods of relatively low sensitivity and specific- ity suffice for their determination. The revolutionary developments in instru- mentation and analytical methodology that have taken place in the past three decades have progressively increased the potentialities for analytical determina- tion of drugs in the concentrations in which they occur in the body,and advances in this field of study of drug metabolism have been at a pace that could not have been conceived of in the last century.

I think that the quantitative work with the sulfonamides that Marshall and his colleagues began in 1937 did more than anything else to bring about a reali- zation of the amount of information that can be gained from quantitative mea- surement of the concentration of a drug, even when that determination is restricted to blood (the only tissue generally available for sampling in man), and of what great practical value this information can be in guiding the use of the drug so as to attain effective levels most efficiently and safely. The lesson had been well learned by the time the World War I1 antimalarial program was initi- ated. Much of the effort in that program was devoted to the development of an- alytical methods for determination of drugs in plasma. This effort was amply rewarding in that it permitted the design of optimal dosage schedules before the drugs were ready for extensive clinical trial. These examples did much t o bring a general acceptance of the view that the availability of an analytical method for determination of the drug in plasma is desirable when any new drug is first submitted to clinical trial.

When we speak of drug metabolism, we are likely to be thinking of chemical

506 Annals New York Academy of Sciences

reactions that convert drugs usually to pharmacologically inactive products, occasionally to pharmacologically active products-but reactions that of them- selves are of no particular consequence to the organism. Yet there are numerous known examples of drugs or their metabolites reacting irreversibly with cellu- lar constituents and thereby eliciting pharmacological responses. There are irreversible enzyme inhibitors. Cytotoxic alkylating agents can react with a number of cellular constituents. The homolytic fission of the carbon-halogen bond in carbon tetrachloride and the more reactive positive halogen vesicant agents produces free radicals that can form covalent bonds with a number of compounds of vital importance to the cell. Thus, 5-fluorouracil is chemically incorporated into RNA and 5-bromouracil into DNA.

Those are a few examples of irreversible chemical reactions that produce pharmacological effects, and it seems strange that such reactions are likely not to be thought of as belonging in the realm of drug metabolism. Yet the field in which irreversible reactions might be expected to be of the greatest practical consequence has been little explored. This concerns the reactions with protein that relate to the problem of drug allergy. Landsteiner is not usually thought of as one of the workers in the field of drug metabolism. Yet I think that Landstei- ner should be recognized as one of the great figures in the history of the study of drug metabolism. His work in the 1930s demonstrated that some of the most potent skin-sensitizing agents react chemically with protein and that the altered protein so produced is antigenic. This work clearly pointed to a mechanism that could account for those allergic reactions to drugs that so closely resemble pro- tein allergies. The small amount of altered protein sufficient to elicit an allergic reaction is an obstacle to the establishment of its actual existence. The reaction of a few drugs with protein has been demonstrated. Penicillin, one of the most frequent producers of allergic reactions of the protein-hypersensitivity type, undoubtedly does react with protein," although the mechanism of the reaction and the immunological role of the product are not well understood. Recently it has been reported that acetylsalicylic acid acetylates plasma albumin in uiuo. l2 The acetylation of albumin or other proteins could conceivably play a role in hypersensitivity reactions to acetylsalicylic acid. There has been little explora- tion of the whole field of reaction of drugs and drug metabolites with protein. The study of these reactions, which may occur only to such a small extent that they would be difficult to detect, could be of greater theoretical and practical interest than the study of major reactions that lead to inactive products.

Now that we are called upon to justify all our research in terms solely of its social value, the question must be asked to what extent the study of drug metab- olism has been of practical value. It might seem that study of chemical reactions that convert drugs to inactive products would be of minimal practical interest. This monograph should amply demonstrate the value to practical therapeutics of research on drug metabolism. Nearly every report contains lessons that can be directly applied to the use of drugs in safer or more effective ways.

There has long been philosophical argument as to the purpose of scientific research. One view has been that the pursuit of truth for the sake of truth is a laudable occupation and that the beauty of the intellectual process of discover- ing and contemplating truth calls for no justification other than its beauty. The opposite view, which has been espoused by persons of such disparate antece- dents as Francis Bacon, Karl Marx, and certain administrators of the National Institutes of Health, is that the only true object of research is the material bene- fit of mankind. It can be said of Bacon and Marx, at least, that they had little direct working knowledge of the scientific mind. Most scientists do not waste

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time philosophizing about their motivations. Those who do think that they ardently believe in one or the other of these supposedly irreconcilable views of the purpose of research are continually finding themselves in inconsistencies. The advocate of truth for its own sake can be found defending his pursuit of truth as the laying of foundations for future practical applications. One holding the most doctrinaire Marxist view of the purpose of science may actually be found engaged in the most esoteric and impractical line of research, arguing that through some inexplicable contingencies it should lead to the material better- ment of man.

In medical research, the word “medical” itself implies the objective of bene- fiting mankind. The basic medical sciences grew up as part of medicine. Their departments exist today in medical schools because they are relevant to medi- cine, and their members should not forget it. Some of us whose research has yielded results of clear therapeutic import and who have been disappointed to see our work fail to influence the practice of medicine in the way we thought it should may perhaps be excused if we are unreceptive to lectures from our patrons on the need to do socially significant research.

Even if a bleakly ugly Marxist view of science may seem to prevail in high places, it is to be hoped that the pharmacologists of today, no less than their predecessors of a century ago, can find intellectual gratification in applying advances in the physical and biological sciences to the improvement of thera- peutics.

References

1. LEWIN, L. 1924,1964. Phantastica; Narcotic and Stimulating Drugs. 1st edit. (Ger.) Stilke. Berlin, Germany. Dutton. New York, N.Y. 1924.

2. SCHMIEDEBERG, 0.1911. Arch. exp. Path. Pharmakol. 67: 1. 3. BUTLER, T. C. 1970. Bull. Hist. Med. 44: 168. 4. DOUGALL, J. 1870. Glasgow Med. J. 4th Ser. 12: 495. 5. BUTLER, T. C. 1948. J. Pharmacol. Ekp. Ther. 94 401. 6. VON MERING, J. & 0. MINKOWSKI. 1889. Arch. exp. Path. Pharmakol. 28: 371. 7. FISCHER, E. & J. VONMERING. 1903. Ther. Gegenw. 44: 97. 8. VON MERING, J. & MUSCULUS. 1975. Ber. dutsch. chem. Ges. 8: 662. 9. VON MERING, J. 1882. Z. physiol. Chem. 8: 480. 10. BUNGE, G. & 0. SCHMIEDEBERG. 1876. Arch. exp. Path. Pharmakol. 6 233. 11. SCHNEIDER, C. H. & A. L. DE WECK. 1968. Biochim. Biophys. Acta 168: 27. 12. HAWKINS, D., R. N. PINCKARD, I. P. C R A W F O ~ & R. S. FARR. 1969. J. Clin. Invest.

48: 536.