The American Journal of Medicine VOL. 36 FEBRUARY 1964

Editorial

Analgesic Nep hro toxici ty

A Pharmacological Analysis

W ITHIN the past decade there has been an expanding literature suggesting that com-

aspirin but sometimes antipyrine or one of its derivatives.

mon analgesic drugs, when taken daily in large amounts over many years, may exert a nephro- toxic action. Between 1953 and 1961 all reports of this relationship appeared in foreign journals. However, within the last two years, there has been a growing awareness in the United States of the possible dangers of analgesic abuse, and publications and editorials relating to the problem are appearing with increasing fre- quency [I-7].

The problem of analgesic nephrotoxicity is complicated by the fact that mixtures of drugs are involved rather than a single therapeutic agent. These mixtures vary in composition from country to country. Because all contain acetophenetidin (phenacetin), this particular analgesic has been implicated as the offending agent. With no intent to be facetious, this type of reasoning is reminiscent of the scientist who, having become intoxicated on successive occa- sions from drinking scotch and soda, bourbon and soda and rye and soda, concluded that soda was inebriating.

Not all investigator-a are convinced that the history of analgesic abuse obtained from a number of patients with chronic renal disease necessarily represents a cause and effect relation- ship. (This controversial and vexing question has been reviewed in a previor_ts editorial [I].) For the purpose of the present discussion, such a relationship will be assumed tent.atively. There- fore it becomes important to analyze the pharmacologic actions of the drugs involved in order to arrive at an explanation ior a possible nephrotoxic action. Analgesic anti-inflamma- tory drugs are often prescribed in large amounts over long periods of time; the problem relates to a much larger population than the% who indulge in self-medication and grossly abuse over-the-counter drug mixtures.

Preparations which have been associated with nephrotoxicity have other features in common. Almost all contain caffeine, which may account for their popularity and abuse. This is particu- larly true of Australian and European mixtures in which the amount of caffeine per tablet or powder ranges between 50 and 150 mg. Of greater significance is that all preparations contain active anti-inflammatory agents, usually

Of all the organs in the body which are susceptible to injury by chemical agents, the kidney is by far the most vulnerable. The reasons are obvious. The kidneys receive 25 per cent of the cardiac output and large amounts of a drug can diffuse from peritubular capillaries into renal interstitial fluid. Unique for kidney cells is the dual exposure to drugs from inter- stitial fluid and tubular urine. As glomerular filtrate is reabsorbed, concentration gradients are established which permit the diffusion of most drugs from tubular fluid through the tubular epithelium into peritubular fluid. The back-diffusion of many drugs is markedly affected by the changes in pH which normally

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occur. A drug such as aspirin, an acid with a pKa of 3.49, will he reabsorbed minimally in the proximal tubule in which it is present largely in its ionized form. Moreover, the CCE centration of ionized drug will markedly increase as glomerular fluid is reabsbrbed. When the acidified urine courses through the distal and collecting tubule, the concentration of undissociated drug will be considerably higher than that in interstitial fluid and the undissociated acid will back-diffuse through the tubular epithelium. Thus, the concentration of a drug may be much higher in tubular cells than in any other cells in the I-?.ody. The kidney is also unique in that the action of the counter- current sodium pump in the loops of Henle causes hypertonicity of fluids in the renal medulla. The greatest osrnolarity occurs at the tip of the papillae where (osmotic pressures may be three to four times greater than isotonicity. Therefore, the concentration of drugs which back-diffuse from collecting tubules is very high in the inner medullary and papillary regions of the kidney. It shoulr,i also be noted that the blood cells which course through the vasa recta are subject to the osmotic stress of hypertonicity. The unique characteristics of medullary and papillary tissue are particularly pertinent to the present discussion because the lesions most commonly associated with analgesic abuse are pyelonephriti3 and papillary necrosis.

The kidney also has unique metabolic char- acteristics. Blood flow to the cortex is very high. As a result,, oxygen tension remains high despite a high rate of oxygen consumption. The medulla receives only a small fraction of the renal blood flr;,?i. -Moreover, due to the countercurrent nature of the blood flow through the vasa recta, oxygen tension in the inner medulla and papillae is low [S]. In contrast to cortical tissue, medullary tissue has a high rate of aerobic glycolysis. It may, therefore, derive a con- siderable amount of its energy from this source as well as from the Krebs’ cycle.

In view of these considerations, it is easy to understand why a wide variety of drugs and chemical agents are nephrotoxic. Fortunately, the behavior of chemical agents in the kidneys of man and of experimental animals is similar and, almost invariably, the nephrotoxic poten- tial of drugs can be predicted accurately from the results of careful studies in animals. Conse- quently, only rarely is the renal toxicity of a drug first discovered in man provided that

adequate studies of toxicity are performed in animals. Exceptions to this statement are drugs that cause renal damage as a result of hyper- sensitivity reactions.

Although the number of drugs which can damage the kidneys is impressive, little is known of the basic mechanisms of nephrotoxic action. The kidney is very susceptible to agents that can oxidize or bind sulfhydryl groups. Metals such as mercury, cadmium, bismuth and arsenic are believed to be nephrotoxic because they form mercaptides. Similarly, compounds with an oxidation-reduction potential capable of oxidizing sulfhydryl groups are potent nephrotoxic agents. For example, tetrathionate is reduced in the body to thiosulfate by the oxidation of sulfhydryl-containing compounds. An infusion of this anion can destroy the kidney without causing *discernible damage to other organs [9]. Similarly, a variety of other oxidizing agents are nephrotoxic.

With these facts in mind, let us turn to the evidence that analgesics are nephrotoxic and to a consideration of those pharmacologic proper- ties of analgesic drugs which could, on theo- retical grounds, lead to renal injury. It is not possible to produce significant renal damage consistently in any animal species thus far studied by administering large amounts of acetophenetidin over long periods of time. In fact, nephrotoxicity has been observed in only one study [lo]. This cannot be dismissed lightly with the casual statement that many drug reactions in man cannot be reproduced in animals. The metabolic fate of acetophenetidin has been studied carefully in man and in many species of mammals, and there is no reason to believe that a unique metabolite forms in man. On the basis of this evidence it has been argued there there must be other factors involved in the etiology of the renal lesion as it occurs clinically.

In the early reports of acetophenetidin nephrotoxicity, the lesion was described as an interstitial nephritis with a relatively high incidence of papillary necrosis. However, as the literature expanded, it was found that the vast majority of alleged cases of analgesic-induced renal disease consist of a rather typical bacterial pyelonephritis characterized by an unusually high incidence of papillary necrosis. This can be interpreted in several ways: (1) Analgesics may cause primary injury to the kidney with secondary bacterial invasion; (2) analgesics may render the kidney less resistant to infection,

Editorial 169

and analgesic nephrotoxicity may be an accelerated bacterial pyelonephritis; and (3) not to be dismissed is the possibility that in many cases pre-existing renal disease is the reason for analgesic abuse, which could account for the wide variety of lesions that have been reported.

To some extent results of animal studies support the view that analgesic drugs can render the kidney more sensitive to bacterial injury. In some experiments, renal injury in animals pretreated for long periods with various analgesics was more severe following intravenous injections of cultures of E. coli or other bacteria than in their controls [70-731, but this has not been a consistent finding [ 741.. In experiments in which positive results were obtained, no distinction could be made between the effects of different agents (aspirin, acetophenetidin, N-acetyl-p-aminophenol). It would seem that almost any drug, when given in sufficiently large dosage, can produce enough stress to impair defense mechanisms to bacterial inva- sion. It would be of great interest to repeat experiments of this kind employing ascending infections of the urinary tract.

Let us now consider the known pharmaco- logic properties of the two types of analgesic agent present in drug mixtures allegedly asso- ciated with nephrotoxicity in an attempt to relate these properties to a potential nephrotoxic action. The two types are acetophenetidin on the one hand, and the anti-inflammatory anal- gesics, aspirin and antipyrine derivatives on the other.

The most undesirable side-action of aceto- phenetidin is its augmentation of methemoglobin formation. In view of the susceptibility of the kidney to oxidizing agents, it is tempting to think that this provides the answer to the problem. However, within recent years a great deal has been learned of the mechanism of hemoglobin oxidation, and the reactions which occur are unique to the red cell. Acetophenetidin is not capable of oxidizing hemoglobin. How- ever, metabolic products which appear in very small amounts (such as para-aminophenol) can be oxidized to intermediates (including free radicals) which accelerate electron transfer from ferrous heme groups and from sulfhydryl groups of hemoglobin and glutathione to molecular oxygen. These partially oxidized intermediates are produced by the catalytic action of hemoglobin and trace metals, and themselves act in minute amounts in the manner

VOI.. 36, FEBRUARY 1964

of catalysts to provide a redox system in the red cell [75].

An alternative possibility is that compounds like para-aminophenol are oxidized in the plasma by enzymes such as ceruloplasmin and gain access to all cells in trace amounts, but only in the red blood cell with its large comple- ment of ferrous ions and sulfhydryl groups is the entropy of the system such as to allow a minute amount of an electron acceptor to set up a regenerative redox system [76]. Therefore, the oxidizing potential of the trace amounts of the metabolic derivatives of acetophenetidin capable of forming redox systems is limited to the red cell.

What of the nephrotoxic potentiality of methemoglobinemia? Although it is true that hemoglobin or methemoglobin in solution in plasma can deposit in and damage renal tubules, this can only occur with massive lysis of red cells. Because trace metabolites of acetophenetidin may operate as a regenerating redox system in red blood cells, the ingestion of acetophenetidin can cause intravascular he- molysis in persons with a deficiency of glucose-6- phosphate dehydrogenase [ 77,781. Aspirin also exerts this action, but to a lesser degree [77,78]. No association has been found between the occurrence of analgesic nephrotoxicity and intravascular hemolysis.

The ability of metabolites of acetophenetidin to oxidize sulfhydryl compounds in the red cell and its membrane can shorten the life span of the red cell. In caremlly controlled long-term animal studies it has been shown that large doses of either acetophenetidin or N-acetyl-p- aminophenol (the primary metabolite of aceto- phenetidin which is also employed as an analgesic) can shorten the life span of the rabbit and dog erythrocyte [ 791. There were no accompanying renal lesions. A shortened life span of erythrocytes can also be demonstrated in man. Although this effect is not great, even in abusers of the drug, it can be quite marked in the person with renal insufficiency [X427]. A sensitizing mechanism has been postulated, but the findings are in keeping with the known facts that both uremia and oxidizing agents reduce the glutathione content of red cells. In any event, patients with renal disease associated with analgesic abuse have a hemolytic disorder sufficiently severe to produce intracytoplasmic hemosiderin deposits in the renal tubules [S]. This raises interesting questions. Does analgesic

170 Editorial

abuse alone shorten the life span of red cells sufficiently to cause a diffusion of extravascular hemoglobin into the plasma with deposition of hemosiderin in the nephron? Would this degree of hemosiderosis lead to interstitial nephritis? The answer to both questions is in all probability “no.” Moderate deposition of hemosiderin in the kidneys of persons with hemolytic disorders is not associated with renal disease [.Z?]. Further- more, it is doubtful whether acetophenetidin alone can increase extravascular hemolysis sufficiently to cause a deposition of hemosiderin. This statement would not apply to the per- son with a glucose-6-phosphate dehydrogenase deficiency.

Although it is doubtful that the slight increase in the rate of red cell destruction caused by the abuse of acetophenetidin can lead to renal damage, it probably represents the closest link that can be made between the known side- actions of acetophenetidin and the kidney. The postulate that methemoglobinemia may lead to anoxic injury to the renal medulla because of its low blood flow is untenable. The mild methemoglobinemia produced by acetopheneti- din would not change oxygen tension and would only slightly reduce the oxygen carrying capacity of the blood. Moreover, the oxygen tension in the inner medulla of the kidney is normally very low as is, presumably, the oxygen demand.

Aspirin exerts many pharmacologic actions in addition to its ability to produce analgesia. In this respect it differs quite markedly from acetophenetidin. As is well known, acute aspirin poisoning results in a generalized metabolic lesion which often proves fatal. Renal damage may accompany acute aspirin poisoning in man [23]. Moreover, it has long been recognized that the chronic administration of large doses of aspirin to animals can cause kidney damage [LV]. Experimental nephrotoxicity can be pro- duced much more consistently with aspirin than with acetophenetidin. In provocation experiments in man, aspirin increases the urine sediment count to a greater extent than does acetophenetidin [25]. Furthermore, in man, following the administration of moderately large doses of aspirin, cells appear in the urine which are believed to be of renal origin 1261. The cell count reaches a maximum on the second or third day and later returns to normal even though administration of the drug is con- tinued. No celluria is produced by aceto- phenetidin. The authors of this last cited paper

state, “The evidence for renal damage caused by phenacetin when taken in high doses over long periods is strong; and there is the para- doxical situation of a probably nephrotoxic drug, phenacetin, failing to cause an increase in renal cells during its early administration in therapeutic dosage, while aspirin which is probably innocuous, causes a brisk exfoliation.” The authors fail to mention that the evidence for renal damage from acetophenetidin is based entirely on the effects of drug mixtures, the vast majority of which contain aspirin and all of which contain other analgesics in addition to acetophenetidin.

The most serious metabolic lesion caused by aspirin is an uncoupling of oxidative phos- phorylation, an action not shared by aceto- phenetidin or antipyrine [27]. This deprives the cells of adenosine triphosphate and results in the dissipation of the energy of oxidative metabolism as heat. Many of the signs and symptoms of acute aspirin poisoning can be related to this severe disturbance in metabolism [27,28]. Salicylates can cause almost 100 per cent uncoupling in preparations of mitochondria from liver or kidney exposed to a concentration of 2 X 1OV M (approximately 40 mg. per 100 ml.). Although it is difficult quantitatively to relate susceptibility of mitochondria to that of intact cells with their protective cell mem- brane, it should be recalled that large thera- peutic doses of salicylates produce plasma levels of this magnitude. The concentration in the kidney would be much higher, especially in the medulla in which the high osmotic pressure and low pH of the urine in the collecting tubule would permit the diffusion of high concentra- tions of free salicylic acid through the collecting tubules into the interstitium of the medulla. Aspirin has a number of other effects on metabolic activity, and these have been the subject of a recent extensive review [28].

One can only speculate on the effect on the kidney of prolonged exposure to an uncoupling agent. One that might be anticipated would be leakage of potassium ion from tubular cells. (A negative potassium ion balance in acute aspirin poisoning in man has been described [29].) Moreover, both functional and morpho- logic changes in the kidney accompany chronic potassium depletion. In a recent review of the effects of chronic potassium depletion on the kidney [30] there is a thought-provoking parallelism between the description therein of

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the renal effects of potassium loss and those that have been ascribed to analgesic abuse. For example, of eighteen clinical cases reviewed, there were twelve instances of chronic pyelo- nephritis and five of interstitial fibrosis. Also, the kidneys of potassium-depleted animals showed increased susceptibility to experimental infections. An early functional change accom- panying potassium depletion is a hyposthenuria that is resistant to therapy with vasopressin [37], a characteristic ascribed to the kidney damaged by analgesics. Both the organic and functional renal changes caused by potassium depletion are reversible with restoration of electrolyte balance, provided the period of depletion has not been too prolonged [30,31]. Improvement has also been claimed in persons in whom renal function has been compromised by abuse of analgesics when drugs are withdrawn. These facts indicate the complexity of the problem of analgesic nephrotoxicity when a variety of analgesic agents is involved.

The anti-inflammatory activity of the sali- cylates is well known. Aminopyrine, antipyrine and their derivatives also possess this pharmaco- logic property. Acetophenetidin, acetanilid and N-acetyl-p-aminophenol exert no anti-inflam- matory effect. Thus, a feature common to all analgesic mixtures that are allegedly associated with nephrotoxicity is the presence of a drug that can alter the inflammatory response of tissues. In view of the fact that analgesic nephro- toxicity has been closely associated with infec- tion, it is surprising that this property of analgesic drug mixtures has not received any attention. Some investigators claim that the pathologic lesions seen in kidneys damaged by analgesic drugs are unique. Is this because of an alteration in tissue response due to the pres- ence of an anti-inflammatory drug? Is the alleged susceptibility of the kidneys to infection and bacterial injury due to suppression of the defense mechanisms of the host with the variety of pathologic lesions depending on the mode of infection, whether ascending or hematogenous? And finally, the most provoca- tive question-Is the high incidence of papillary necrosis due to the presence of an anti-inflamma- tory agent? Strong arguments could be advanced to support this hypothesis. The highest drug concentration would be found in the papillae. Papillary necrosis has been most commonly associated with infection, and interference with defense mechanisms would increase its inci-

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dence. Finally, in describing the lesion in the papillae most pathologists have stressed the paucity or absence of neutrophils along the margins of the necrotic area, a finding that could be explained by the presence of an anti- inflammatory drug.

Thus far no mention has been made of the possible role of hypersensitivity reactions in the etiology of the renal lesions. It is well known that drugs such as sulfonamides can produce an interstitial nephritis as a result of an allergic reaction. Hypersensitivity reactions to aceto- phenetidin are infrequent, but they do occur. Antipyrine and its derivatives are little used in this country because of the high incidence of hypersensitivity reactions, especially those in- volving the bone marrow. Aspirin also is a sensitizing drug that can produce angioneurotic edema and urticarial skin lesions, and has been implicated in the etiology of periarteritis. The fact that renal toxicity from analgesic drugs appears to be related to dosage, and only appears after long-term overt abuse, makes hypersensitivity an unlikely explanation, but nevertheless one which cannot be summarily dismissed.

The problems posed by the possible nephro- toxicity of analgesic drugs demand answers. Animal investigations have failed to provide definitive information and effort must be expended on carefully conducted epidemiologic studies. The literature abounds with retro- spective studies suggesting that a history of abuse of analgesics can be obtained in a high percentage of persons suffering from interstitial nephritis or more frequently pyelonephritis. Up to the present, all studies relate to drug mixtures; nothing is known of the contribution of individual agents. Proponents of the belief that acetophenetidin is the specific nephrotoxic agent in these drug mixtures argue that a history of the abuse of other analgesics, such as aspirin alone, is seldom obtained from patients with renal disease. This is a powerful argument provided that careful epidemiologic studies of patients with a history of ingestion of aspirin or other analgesics, in amounts equal to that taken in the form of drug mixtures, fail to produce any evidence of nephrotoxicity. Suitable popula- tions should be available in the United States where the anti-inflammatory salicylates are the analgesics of choice in the treatment of rheuma- toid arthritis. Studies of such populations have been made in Denmark, but unfortunately in

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that country analgesics are almost invariably ingested in the form of drug mixtures. Neverthe- less, the results are of interest. In one retrospec- tive study based upon autopsy findings there was a high incidence of papillary necrosis [32]. This was tentatively attributed to ingestion of acetophenetidin on the assumption that patients with rheumatoid arthritis would ingest large amounts of various drug combinations available in Denmark. In another study [33], all patients admitted to the Department of Medicine, Muni- cipal and County Hospital, Vejle, Denmark, over a period of six months were questioned as to drug history. A substantial percentage used analgesics daily, many in excessive amounts. Renal function was evaluated on the basis of concentrating ability. A strongly positive cor- relation was found between the total amount of acetophenetidin that had been consumed and hyposthenuria. No consideration was given to the concomitant ingestion of other analgesic drugs. Finally, another study of a large popula- tion of patients with rheumatoid arthritis and a control group with no history of analgesic abuse failed to reveal any correlation between the ingestion of acetophenetin and the level of renal function as measured by endogenous creatinine clearance [34. Obviously, much more sophisticated epidemiologic studies must be conducted if an answer to this problem is to be obtained.

Many of the statements in this brief review are conjectural. No apology is offered since the entire literature relating to “analgesic” or “phenacetin” nephrotoxicity falls into this catetory. None of the so-called “weak” anal- gesics is devoid of side actions. Those of aspirin are by far the most serious and could lead to nephrotoxicity. The exposure of an organ prone to infection to high concentrations of drugs which alter resistance to infection would fit many of the features of analgesic nephrotoxicity. A shortened life span of red cells and methemo- globinemia could conceivably be detrimental to the kidney. In brief, analgesic nephrotoxicity may not represent a single clinical entity but may merely reflect the susceptibility of the kidney to a variety of chemical agents for reasons that have been discussed. It might be reassuring to some investigators to sweep the problem under the rug by applying the label “phenacetin nephrotoxicity.” There is no pharmacologic basis for such an appellation. Analgesic drugs, in high dosage, are widely

prescribed by physicians and apparently are widely abused by the laity. It is incumbent upon pathologists, urologists, nephrologists and phar- macologists to obtain answers to the many problems that have been raised. The experi- mental approaches will have to be much more sophisticated than those thus far employed.

ALFRED GILMAN, PH.D.

Albert Einstein College of Medicine Yeshiva University Bronx, New York

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