S-Adenosyl-L-Methionine (SAMe) for the Treatment of Acetaminophen Toxicity in a Dog
An 8-month-old, spayed female Shetland sheepdog presented 48 hours after ingesting acetaminophen (1 gm/kg body weight). On presentation, the dog was laterally recumbent and hypovolemic. The dog had brown mucous membranes, severe Heinz-body hemolytic anemia, bleeding tendencies, and a red blood cell (RBC) glutathione (GSH) concentration that was 10% of reference values, despite a regenerative erythroid response. Treatment with s-adenosyl-l-methionine (SAMe) as a GSH donor successfully rescued this dog, despite the animal’s late presentation after drug ingestion. A loading dose (40 mg/kg body weight) of a stable SAMe salt per os was followed by a maintenance dose (20 mg/kg body weight) sid for 7 days. Additional therapeutic interventions included an intravenous (IV) infusion of one unit of packed RBCs (on admission), IV fluid support (3 days), and famotidine (7 days) to reduce gastric acidity. Sequential assessment of RBC GSH concentrations and RBC morphology documented response to antidote administration within 72 hours. This case suggests that SAMe may provide a therapeutic option for treatment of acetaminophen toxicosis in dogs capable of retaining an orally administered antidote and maintaining adequate hepatic function for metabolism of SAMe to its thiol substrates.
Introduction
Acetaminophen is one of the most commonly used over-the-counter analgesic and antipyretic medications in the world, being preferred in humans when aspirin is contraindicated, in pediatric patients infected with influenza, and in the circumstance of compromised renal function. Since acetaminophen has a narrow therapeutic dosage window, toxicity in humans is not uncommon. In fact, acetaminophen is the most common cause of drug-induced, fulminant hepatic failure in humans in North America.1–5 Although acetaminophen is only occasionally used as an oral analgesic in dogs, it has been associated with life-threatening toxicities in companion animals because of its widespread availability and accidental ingestion.6 The recommended dose of acetaminophen in dogs is 15 mg/kg body weight, per os (PO) tid, whereas it is contraindicated in cats due to their comparatively reduced ability for acetaminophen detoxification.7
Hepatotoxicity due to acetaminophen is characterized by zone-3 hepatocellular necrosis, with extension into submassive or panacinar necrosis in mice, rats, hamsters, humans, and dogs after ingestion of toxic quantities.1689 In dogs, toxicity may also include methemoglobinemia and Heinz-body formation, although this effect is better characterized and more commonly seen in cats.610–12 Because of its widespread availability, acetaminophen toxicosis can result from intentional administration by ignorant pet owners or consumption by unsupervised pets. In dogs, toxic effects may develop following ingestion of as little as 200 mg/kg body weight in a single dose.13
Case Report
Approximately 48 hours after ingestion of 10,000 mg (1,020 mg/kg body weight) of acetaminophen (20 extra-strength Tylenol capletsa), an 8-month-old, 9.8-kg, spayed female Shetland sheepdog presented as an emergency to the Cornell Hospital for Animals (CHA). The dog had no prior health problems, was fed a well-balanced commercial diet, and was vaccinated adequately. Immediately following drug ingestion, the dog had been taken to the local veterinarian where induced emesis removed approximately half of the ingested caplets. Therefore, the dog had absorbed a dose of approximately 500 mg/kg body weight. Activated charcoal (dose unknown), metronidazole (dose unknown), and bismuth subsalicylateb (1 teaspoon, PO bid) were administered, and the dog was discharged for at-home observation. When the dog developed hematemesis and hemorrhagic diarrhea the next day, she was presented to the CHA.
On presentation to the CHA, the dog was unresponsive, laterally recumbent, febrile (104.5°F), tachypneic, and had weak femoral pulses. Mucous membranes were pale brown, and the dog was approximately 6% dehydrated. There was discomfort on palpation of the cranial abdomen. No surface bruising was noted, although rectal examination disclosed blood-stained, nonformed, dark stool. A venous blood sample was noted to be subjectively dark, consistent with methemoglobinemia. The dog was hospitalized for critical supportive care and routine diagnostic blood and urine tests.
A complete blood cell count (CBC) revealed a moderate regenerative, normocytic, hyperchromic anemia [see Table] and erythrocyte morphology characterized by the presence of many Heinz bodies, eccentrocytes, ghost cells, and a few Howell-Jolly bodies [Figure 1A]. There was a neutrophilic leukocytosis (white blood cells [WBC], 21.3 × 103/μL; reference range, 7.5 to 19.9 × 103/μL; neutrophils, 17.0 × 103/μL; reference range, 3.9 to 14.7 × 103/μL) and thrombocytopenia (estimated platelet count, 40 to 60 × 103/μL; reference range, 179 to 510 × 103/μL). Abnormalities disclosed on a serum biochemical profile included mild hyponatremia (139 mEq/L; reference range, 141 to 156 mEq/L); hypokalemia (3.6 mEq/L; reference range, 3.8 to 5.5 mEq/L); low bicarbonate (12 mEq/L; reference range, 16 to 26 mEq/L); increased albumin (5.4 g/dL; reference range, 3.0 to 4.5 g/dL) and increased cholesterol (410 mg/dL; reference range, 124 to 335 mg/dL) concentrations; a ninefold increase in aspartate aminotransferase (AST) activity, and hyperbilirubinemia [see Table]. Centrifuged urine yielded a red supernatant and a dark sediment containing intact erythrocytes. In consideration of the detrimental influence of acetaminophen on red blood cell (RBC) glutathione (GSH) concentrations, the RBC GSH concentration was measured in a sodium-heparinized blood sample; the RBC GSH was markedly decreased (18.9 mg/dL; reference range [established in eight normal dogs], 45 to 131 mg/dL).
Initial Treatment
In consideration of the dog’s dehydration and electrolyte abnormalities, she was treated with intravenous (IV) fluids (0.9% sodium chloride [NaCl]), supplemented with 28 mEq/L potassium chloride and B-complex vitamins.c Considering the mucous membrane pallor, Heinz-body anemia, suspected methemoglobinemia, RBC hemolysis, and apparent bleeding tendencies, a packed RBC (pRBC) transfusion was administered (17.9 mL/kg body weight) and the dog was placed in an oxygen (O2) cage (40% O2). Prior to the transfusion, the dog was given a single injection of diphen-hydramined (1 mg/kg body weight, subcutaneously [SC]) to avert transfusion reaction. In view of the hemorrhagic diarrhea and historical hematemesis, famotidinee (0.5 mg/kg body weight, IV bid) was administered to reduce gastric acid production. Cefazolinf (22 mg/kg body weight, IV tid) was administered to combat potential bacteremia from the disrupted enteric mucosal barrier. A loading dose of s-adenosyl-l-methionineg (SAMe) was administered (40 mg/kg body weight, PO once), followed by a maintenance dose of 20 mg/kg body weight, PO sid for 9 days. The reasons for SAMe being selected as an antidote will be explained in the Discussion. After approximately half of the pRBC transfusion (5 hours after the loading SAMe dose), a second blood sample was collected for determination of RBC GSH concentration.
Approximately 8 hours after admission, the dog remained lethargic and tachypneic, had passed dark red-brown urine, and had maintained pale-brown mucous membranes. An arterial blood gas analysis disclosed a normal partial pressure of O2 (86 mm Hg; reference range, 80 to 100 mm Hg), O2 saturation (SO2, 96%; reference range, 95% to 98%), and blood pH (7.35; reference range, 7.32 to 7.43); mild hypocapnia (28.4 mm Hg; reference range, 29 to 42 mm Hg); and a mildly decreased bicarbonate concentration (16 mEq/L; reference range, 17 to 24 mEq/L). The calculated alveolararterial (A-a) O2 gradient demonstrated moderate impairment of O2 exchange (28.5 mm Hg; reference range, 0 to 25 mm Hg). The blood gas values were consistent with a compensated metabolic acidosis, which could reflect tissue oxidant damage or reduced O2 delivery due to anemia and methemoglobinemia, and reduced tissue perfusion due to hypovolemia at initial presentation. The increased A-a gradient was thought to reflect both a diffusion impairment resulting from vascular instability (i.e., leakiness) associated with acetaminophen oxidant damage (which may reduce functional capillary surface area in the lung) as well as atelectasis derived from the dog’s prolonged lateral recumbency.414 The SO2 reported was obtained using a portable clinical analyzerh that calculates SO2 from measured partial pressure of oxygen (PO2), partial pressure of carbon dioxide (PCO2), and pH, assuming the patient’s functional hemoglobin and hematocrit (HCT) are normal. Information derived from application of a peripheral pulse oximeter using two wavelengths of light to determine percentage of hemoglobin saturation was considered invalid due to the methemoglobinemia and hemolysis. Such units measure a ratio of transmitted red and infrared light intensities and relate these to a table of empirical SO2 values.15 Immediately after the full pRBC transfusion, the HCT was 44%.
The following morning, although the dog appeared brighter and the mucous membranes were now pink, she had continued to pass red urine containing both a red supernatant and intact erythrocytes. Blood was collected for a CBC, a serum biochemical profile, and RBC GSH concentration. The HCT had decreased to 32%, consistent with systemic dispersal of the transfused packed cells and continued enteric and urinary tract bleeding, hemolysis, or both. The number of Heinz bodies observed on a freshly made blood smear later that day was markedly reduced compared to the initial blood sample [Figure 1B]. The anemia remained regenerative (reticulocytes, 3.2%). Alkaline phosphatase activity was mildly increased; the total bilirubin concentration and AST activity had substantially decreased, but they remained above reference range values [see Table].
Over the following 72 hours, the HCT stabilized, pigmenturia resolved, and the dog began eating and drinking. Examination of RBC morphology 72 hours after admission revealed rare, basophilic, stippled RBCsp, occasional eccentrocytes, moderate polychromasia, a few spherocytes, and very few Heinz bodies [Figure 1C]. Sequential blood samples were collected to monitor changes in the RBC GSH concentrations. Liver enzyme activities and total bilirubin concentration were normal on day 5 when the dog was released from the hospital [see Table]. Famotidine (0.5 mg/kg body weight, PO, sid for 7 days) and SAMe (20 mg/kg body weight, PO, sid for 3 days) were prescribed.
When the dog was reevaluated 5 days later (day 10), there had been no vomiting or diarrhea, and physical examination was within normal limits. A CBC revealed a mild, regenerative anemia (HCT, 37%), and a serum biochemical analysis revealed only mild hypercholesterolemia (341 mg/dL). A blood sample for RBC GSH was submitted. On reevaluation 14 days later (day 24), the dog continued to do well. The anemia had resolved, and the urinalysis and serum biochemical analyses were within reference ranges. A final RBC GSH determination was made.
The RBC GSH concentrations were determined using the method of Prins and Lo, which measures reduced GSH.16 Control samples from healthy dogs and GSH standards were evaluated concurrent with patient samples. During hospitalization, the first three RBC GSH values were collected while the dog was totally anorectic. Thereafter, samples for RBC GSH were collected after an overnight fast. Samples were collected in sodium heparin, transported on ice, and immediately analyzed. Results are reported as reduced GSH per 100 mL of RBC mass on the basis of the HCT of the sample collected in sodium heparin. Prior to antidote administration and packed cell infusion, the GSH concentration was 18.9 mg/dL RBC, and it increased to 30.5 mg/dL RBC following a half-unit of pRBC and the 40 mg/kg body weight loading dose of SAMe [Figure 2]. The GSH concentration in the pRBC was 48 mg/dL. After the administration of the remaining half-unit of pRBC (24 hours of SAMe therapy) and continued regenerative erythroid response, the dog’s GSH increased to 74.2 mg/dL RBC (392% of entry value). Subsequent RBC GSH concentrations were 59.5 mg/dL, 68.2 mg/dL, 45.0 mg/dL, and 59.4 mg/dL on days 2, 5, 10, and 24, respectively.
Discussion
Acetaminophen is rapidly and nearly completely absorbed from the stomach and small intestine following oral administration. Since it is only slightly bound to plasma proteins, it has wide systemic distribution. It is biotransformed by three competing pathways in hepatocytes, including glucuronidation, sulfation, and cytochrome P-450-mediated oxidation. In humans, 85% to 95% of acetaminophen is metabolized in the liver via glucuronidation and sulfate conjugation, and it is excreted in the urine as pharmacologically inactive GSH and sulfate conjugates. Most of the remaining drug undergoes oxidative metabolism via cytochrome P-450 isoenzymes (CYP) 2E1 and 1A2 and then conjugation with glucuronide, cysteine, or mercapturic acid. A small amount (<5%) is excreted unchanged in urine. Although many drugs undergoing phase I reactions (i.e., oxidation, reduction, or hydrolysis) are detoxified, the CYP2E1-catalyzed metabolite of acetaminophen, N-acetyl benzoquinoneimine (NAPQI), is an oxidizing and arylating agent that can covalently bind to GSH or nucleophiles such as hepatocellular proteins, disrupting their availability and function and producing tissue necrosis. Under normal conditions, electrophilic toxins such as NAPQI are deactivated by conjugation with GSH before they can react with cellular macromolecules. However, in acetaminophen toxicity, GSH depletion permits irreversible binding of NAPQI to cellular macromolecules. In the liver, this results in panlobular zone 3 necrosis.
In dogs, glucuronide conjugates predominate as the metabolite excreted in urine.14 While the glucuronidation and sulfation pathways produce nontoxic metabolites of acetaminophen, both are capacity-limited in dogs.11 Therefore, the elimination of acetaminophen through these pathways declines as the dose increases, resulting in prolonged high blood concentrations of the parent drug and a greater chance for its biotransformation to NAPQI. The biotransformation of acetaminophen in cats is also dose dependent; however, cats are much more sensitive to systemic acetaminophen toxicity because of their deficiency of hepatic acetaminophen-directed uridine 5’-diphosphate (UDP) -glucuronosyltransferase activity and, presumably, their limited sulfation capabilities.61718
Glutathione plays an essential role as an antioxidant and in detoxification reactions, in the maintenance of reduced thiol status in certain proteins and other molecules, and as a storage mechanism for tissue and plasma cysteine.19–21 It detoxifies reactive molecules either by spontaneous conjugation or by a reaction catalyzed by GSH-S-transferase. Glutathione’s activity as an intracellular antioxidant involves oxidation to its disulfide form (GSSG) in a reaction catalyzed by GSH peroxidase. Reduction back to GSH is catalyzed by GSSG reductase. This reduction is rapidly accomplished in a closed-system “redox” cycle; otherwise, GSSG is exported from the cell. The liver is the major source of GSH for the body; hepatic GSH production provides 90% of the plasma GSH.21 Systemic transport of GSH occurs in plasma and erythrocytes as the intact molecule or substrate constituents. While plasma GSH undergoes dynamic fluctuation as a result of variations in hepatic GSH synthesis and efflux, systemic tissue GSH utilization, oxidant challenge, as well as nutritional and metabolic variables, the concentration of GSH within the erythrocyte remains relatively stable.21 Glutathione is an essential antioxidant in mammalian erythrocytes, necessary for the protection of hemoglobin sulfhydryl bonds and as a mechanism for attenuating circulating oxidant challenge. In fact, erythrocytes normally undergo rapid GSH turnover, as they can both synthesize GSH and export the oxidized form (i.e., GSSG) into the plasma compartment.21 Erythrocyte GSH declines with increasing cell age; but overall, the dynamics of the RBC mass maintain a relatively constant range unless a systemic oxidant challenge is encountered, as described in this case.21–23 Since young erythrocytes in the dog have been estimated to contain up to threefold more reduced GSH than aged cells (due to a greater ability to synthesize GSH from precursor substrates), higher-than-normal RBC GSH concentrations would be expected in the circumstance of a regenerative anemia.23 Since reticulocytes were increased on initial presentation [see Table] and increased during the next 48 hours, it is likely that the GSH values determined on presentation underestimated the extent of erythrocyte GSH depletion. The influence of the transfused RBC on the measured GSH concentrations in this patient can be estimated by considering the approximate quantity of GSH-“endowed” erythrocytes that were administered. The pRBC transfusion of 9 mL/kg body weight would have provided 90 mL of pRBC into a blood volume estimated as 800 mL (80 mL/kg body weight). This would permit a maximal increase in RBC mass of only 10%, yet the increase in RBC GSH was 160%. The abrupt increase in RBC GSH could reflect the metabolism of SAMe providing GSH substrates, an increased metabolic activity of transfused cells in comparison to the patient’s RBCs, or the relatively young population of patient erythrocytes continuing to accumulate.23 At 48 hours, the depletion of RBC GSH was likely ongoing, since some of the acetaminophen metabolites presumably remained in the system.
Methemoglobinemia (MetHb) has been reported in both dogs and cats as a consequence of acetaminophen toxicity.61417182425 This is a nonfunctional complex formed from hemoglobin by the oxidation of hemoglobin iron from the ferrous (+2) to the ferric (+3) state. Methemoglobinemia can accumulate either as a result of increased hemoglobin oxidation (i.e., NAPQI exposure) or decreased reduction of methemoglobin due to impaired activity of methemoglobin reductase within the RBC.26 Increased MetHb impairs RBC O2-carrying capacity, causing tissue hypoxia and a characteristic muddy-brown mucous membrane color as observed in this case. A conventional blood gas analysis will not detect this reduced O2-carrying capacity, since this analytic procedure measures the partial pressure of gas dissolved in blood. Formation of methemoglobin also will not be detected by examining the hemoglobin saturation on a blood gas report, as this is a calculated value based on the arterial dissolved gas concentrations and the reported blood hemoglobin concentration. Quantification of MetHb requires analysis of this specific hemoglobin moiety.27 Unfortunately, measurements of methemoglobin were not done in this case, and its presence was inferred based on the brown coloration of the mucous membranes and dark color of the dog’s blood noted during venipuncture at the time of initial hospitalization.
The goals of therapy for acetaminophen toxicity are to decrease the absorption of acetaminophen from the gastrointestinal tract, to hasten its elimination, to limit the formation of the toxic electrophile NAPQI, to supplement GSH substrates in an effort to protect membranes and cells and to promote elimination of NAPQI, and to provide supportive therapy aimed at correcting hydration, acid base, electrolyte, and O2 delivery abnormalities. Since NAPQI has a very short half-life and is rapidly conjugated with GSH, under normal conditions it is rapidly eliminated. However, in the circumstance of excess NAPQI, GSH stores are rapidly depleted, leaving the electrophile free to permanently covalently bind to cell membranes. Antidote treatment is directed at decreasing the toxic metabolite through conjugation with hepatocellular GSH. Administration of GSH itself is ineffective, because it is not readily available to intracellular sites and it is commercially unavailable. Consequently, the administration of substrate thiol donors, such as cysteamine hydrochloride, L-methionine, acetylcysteine, or SAMe, has alternatively been explored as a mechanism to promote hepatic synthesis of GSH.1128–33 Although GSH is directly formed from cysteine, use of cysteine as a direct GSH substrate is complicated by its rapid deterioration in plasma. Because of the side effects of cysteamine hydrochloride (severe nausea, profuse vomiting, drowsiness) and L-methionine (gastrointestinal distress, Heinz-body hemolytic anemia in cats), up until now N-acetylcysteinei has been the only reasonable antidote. N-acetylcysteine is usually administered IV as an initial bolus followed by either constant-rate infusion or intermittent repeat bolus injections or chronic intermittent oral administration.30 Unfortunately, IV administration of N-acetylcysteine is not without risk of side effects; in humans, hypersensitivity reactions include rash, angioedema, and shock. The therapeutic window for reliable rescue from acetaminophen lethal toxicity in humans is between 12 and 16 hours. Within this time frame, treatment nearly abolishes the toxic hepatocellular effects.1428303334 After 16 hours, thiol donation is less efficacious, because the oxidation of acetaminophen to NAPQI is nearly complete. Nevertheless, beneficial effects may still be acquired from N-acetylcysteine due to its stabilizing influence on endothelium.4 Endothelial toxicity is thought to be a cause of some of the clinical signs of acetaminophen toxicity, including bruising and bleeding (as observed in this case) or swelling of extremities and face (as observed in the cat), and, possibly, the pulmonary dysfunction recognized in this dog.1014 Cimetidine is currently recommended as adjunctive therapy in acetaminophen toxicity for its inhibitory influence on the cytochrome P-450 pathway, which decreases NAPQI formation.3536 However, cimetidine must be administered within the first 16 hours of intoxication to provide an effective metabolic blockade of NAPQI formation.
An alternative therapy for acetaminophen toxicity is the administration of the neutraceutical SAMe, as described here.37–40 Important in intermediary metabolism, SAMe is an endogenous molecule normally produced intracellularly from methionine and adenosine triphosphate (ATP). Within hepatocytes, SAMe has particular importance as a substrate initiating the transmethylation and transsulfuration pathways. Among a number of important functions, the transmethylation pathway generates phospholipids essential for cell membrane function and fluidity.37 Feeding this pathway by SAMe administration may benefit hepatocellular and erythrocyte membranes when toxic adducts of acetaminophen insult their structural and functional integrity. The transsulfuration pathway initiates by donation of a methyl group from SAMe. This pathway yields cysteine, which is essential for hepatocellular GSH synthesis as well as for production of sulfates that also are important in conjugation and elimination of toxic acetaminophen metabolites.37 Both toxic adduct binding to cellular constituents and proteins and oxidant damage contribute to the cellular injury derived from acetaminophen toxicosis.41–43 Experimental work in vivo in animal models and in vitro in human hepatocytes has shown that SAMe can attenuate liver damage induced by hepatotoxins, including acetaminophen, and that exposure to SAMe can increase intracellular GSH in hepatocytes.44
Since depletion of hepatocellular or erythrocyte GSH, or both, is directly related to the injurious effects of acetaminophen, the basis of thiol antidote therapy is through enhancement of the supply of cysteine, the direct substrate for hepatic GSH synthesis. Fortifying hepatic GSH stores permits greater egress of GSH and its substrate components from the liver into the systemic blood, where they are available for erythrocytes and vascular endothelium. Consequently, the authors were acutely interested in the RBC GSH status in the dog of this report, as both a reflection of the severity of the acetaminophen toxicity as well as of the metabolic advantage provided by SAMe administration. The initial RBC GSH value was consistent with the low values documented in dogs experimentally intoxicated with acetaminophen at a dose of 500 mg/kg body weight.14 In that study, dogs ingesting 100 or 200 mg/kg body weight did not undergo a significant decline in RBC GSH concentration, whereas dogs dosed at 500 mg/kg body weight developed a significantly reduced RBC GSH concentration for 24 hours.14 The dose of acetaminophen ingested by this dog seemingly exceeded that studied experimentally, since the RBC GSH values were pathologically reduced 48 hours after drug ingestion. In fact, at the time of presentation, RBC GSH values in this dog were approximately 28% of values normally measured in healthy dogs and were approximately 10% of values measured in dogs with markedly regenerative anemia due to blood loss or immune-mediated mechanisms [Figure 2].j,45
Experimental work with acetaminophen as a method of modeling fulminant hepatic injury in the dog has well documented the vulnerability of the liver as the organ showing greatest toxicity in this species.89 However, work with single-dose acetaminophen toxicity has shown that a 500 mg/kg body-weight dose reliably induces MetHb and RBC GSH depletion. Serial serum biochemical profiles were obtained in this case in consideration of the expected hepatocellular necrosis in dogs intoxicated with very high-dose acetaminophen. Unexpectedly, this dog’s alanine aminotransferase (ALT) never exceeded the reference range, while the ninefold increase in AST activity resolved by the fifth day following drug ingestion. In the absence of an increase in ALT (implicating liver) or creatinine phosphokinase (implicating muscle), the origin of the markedly increased AST activity remains unexplained, although not unprecedented, in acetaminophen toxicity.46 Despite the fact that the dog described in this report ingested a toxic dose of acetaminophen, it presented primarily for the hematological effects of metabolite toxicity, as more often is observed in the acetaminophen-poisoned cat.61217
The acute emergency care provided by the local veterinarian included induction of emesis, which importantly reduced the quantity of acetaminophen actually realized. While the lack of overt evidence of hepatic damage in this dog was unusual, it is not without precedence. The individual variation among dogs in susceptibility to liver damage from acetaminophen is well recognized and has necessitated use of phenobarbital pretreatment (induction of CYP increasing hepatic biotransformation to NAPQI) and administration of buthionine sulfoximine, which irreversibly inhibits GSH synthesis to ensure the induction of hepatic toxicity in experimental models.47 Investigation of humans developing fulminant hepatic failure after acetaminophen ingestion suggests that low hepatic GSH concentration is an important predisposing factor.448 Since dogs have markedly lower hepatic GSH concentrations as compared to most humans, cats, and rodents, this may play an important role in their proclivity for acetaminophen-induced hepatic injury.49 It also remains possible that this dog may have an underlying erythrocyte abnormality which impairs RBC energy production, reduction of oxidized GSH, or reduction of methemoglobin, which could increase her vulnerability to hematological toxicity of acetaminophen.
Sequential packed cell volumes, blood smears, and RBC GSH values were determined to monitor the influence of SAMe on resolution of the hematological toxicity. After the loading dose of SAMe and the administration of approximately 9 mL/kg body weight of pRBCs, the RBC GSH values increased to 160% of admission-day values. This increase reflects not only the provision of transfused RBCs containing a higher GSH concentration than the patient, but also the loading of GSH substrates derived from the administered SAMe in conjunction with the ever-increasing erythroid regenerative response. Since the assay used to monitor GSH can detect other reduced sulfhydryl groups, the RBC GSH values may reflect increased GSH as well as thiol-bearing GSH substrates provided by SAMe (e.g., cysteine). Response to this therapy was dramatic, with an abrupt decline in hemolysis, normalization of vital signs, and resolution of fever and diarrhea within 48 hours of presentation.
The local veterinarian had not offered detoxification of acetaminophen with N-acetylcysteine, because that medication was not available on an emergency basis in his clinic. This was not unusual, since small practices may not stock N-acetylcysteine because of its limited therapeutic use. Because the time lag between ingestion and presentation at the CHA exceeded the window of therapeutic intervention for blocking acetaminophen toxic adduct formation (NAPQI) with cimetidine, and because this dog’s clinical signs primarily suggested RBC oxidation and hemolysis, the authors reasoned that supplementation with an oral GSH donor might be as effective and less costly than use of N-acetylcysteine.1139–41 Although most recommendations for humans suggest that treatment with N-acetylcysteine has benefit up to 16 hours, initiation of treatment up to 53 hours after drug ingestion has yielded important clinical response.43350 At least one author has suggested that a 72-hour oral regimen of N-acetylcysteine is as effective as a 20-hour IV therapeutic protocol in humans; some evidence suggests that this regimen may be especially prudent in individuals presenting later in the course of toxicity.33 Consequently, the authors had interest in using an oral medication that could be conveniently available and affordable for practicing veterinarians, be administered on an outpatient basis, and that may provide therapeutic success comparable to N-acetylcysteine.3940
The authors also considered that treatment with SAMe would provide a broader spectrum of beneficial biological effects as compared to other thiol donors such as N-acetylcysteine, cysteine, or cysteine precursors, because of the important involvement of SAMe in the transmethylation pathway, in addition to its function in initiating the transsulfuration pathway and other metabolic interactions beyond the scope of this discussion.3738 Since this treatment protocol was unproven, informed consent was obtained from the owner prior to treatment with SAMe and blood products in the absence of cimetidine and N-acetylcysteine. The remarkable response to therapy described herein is not unlike that realized in some humans presented for lethal acetaminophen ingestion beyond the conventional therapeutic window.33
In treating acute acetaminophen toxicity, the most important factor limiting the use of orally administered antidotes is vomiting associated with the toxicosis. Administration of an antiemetic such as ondansetron has been highly successful in suppressing emesis in intoxicated humans treated orally with N-acetylcysteine.51 Intractable vomiting would limit therapeutic use of SAMe, because dosing requires oral administration of the stable SAMe salt encased in an enteric-coated tablet.52 In this case, the authors administered an initial high loading dose of SAMe (40 mg/kg body weight), arbitrarily selected but with knowledge that the lethal oral dose in rodents exceeds 4 gm/kg body weight per day.53 The dose of SAMe subsequently administered (20 mg/kg body weight, PO sid for 1 week) was based on knowledge of its pharmacokinetics in dogs and studies of its influence on hepatic and RBC GSH in dogs.455253
Conclusion
Experience with SAMe in this case of high-dose acetaminophen ingestion in a dog suggests that it may provide a convenient therapeutic option for treatment of acetaminophen toxicity in patients capable of retaining an orally administered antidote. This case demonstrates, as has been shown in humans, that oral antidote intervention may prove beneficial beyond the conventional time frame of 18 hours considered effective for successful rescue from this toxin. Consideration of SAMe as an antidote of acetaminophen toxicosis, however, should include scrutiny of the case for signs of fulminant hepatic failure, since impaired hepatic metabolism may limit its efficacy.
Extra-strength Tylenol; McNeil Consumer Healthcare, Fort Washington, PA
Pepto-Bismol; Procter & Gamble, Norwich, NY
B-complex vitamins; The Butler Co., Columbus, OH
Benadryl; Parke Davis, Division of Warner-Lambert Co., Morris Plains, NJ
Famotidine Pepcid; Merck & Co., Inc., West Point, PA
Cefazolin; SmithKline Beecham Pharmaceuticals, Philadelphia, PA
Denosyl SD4; Nutramax Laboratories, Edgewood, MD
iSTAT; iSTAT Corporation, Windsor, NJ
N-acetylcysteine, 10% solution USP; Roxane Laboratories, Columbus, OH
Center SA, unpublished information; College of Veterinary Medicine, Cornell University, Ithaca, NY
Citation: Journal of the American Animal Hospital Association 38, 3; 10.5326/0380246
Citation: Journal of the American Animal Hospital Association 38, 3; 10.5326/0380246
Citation: Journal of the American Animal Hospital Association 38, 3; 10.5326/0380246
![Figure 2—. Graphic depiction of red blood cell (RBC) reduced glutathione (GSH) concentrations in an 8-month-old Shetland sheepdog with acetaminophen toxicity at presentation and sequentially thereafter, GSH values for the administered packed RBCs given on day 1, and the reference range for GSH values in eight normal dogs (mean±standard deviation [SD]).](/view/journals/aaha/38/3/p250fig2.jpeg)
![Figure 2—. Graphic depiction of red blood cell (RBC) reduced glutathione (GSH) concentrations in an 8-month-old Shetland sheepdog with acetaminophen toxicity at presentation and sequentially thereafter, GSH values for the administered packed RBCs given on day 1, and the reference range for GSH values in eight normal dogs (mean±standard deviation [SD]).](/view/journals/aaha/38/3/full-p250fig2.jpeg)
![Figure 2—. Graphic depiction of red blood cell (RBC) reduced glutathione (GSH) concentrations in an 8-month-old Shetland sheepdog with acetaminophen toxicity at presentation and sequentially thereafter, GSH values for the administered packed RBCs given on day 1, and the reference range for GSH values in eight normal dogs (mean±standard deviation [SD]).](/view/journals/aaha/38/3/inline-p250fig2.jpeg)
Citation: Journal of the American Animal Hospital Association 38, 3; 10.5326/0380246
Photomicrograph of a peripheral blood smear from an 8-month-old Shetland sheepdog following acetaminophen ingestion. The blood smear is stained with new methylene blue and counter-stained with a modified Wright’s Giemsa stain (1,000×). Smear was prepared on admission of the patient and shows ghost cells (arrows), Heinz bodies (arrowheads), reticulocytes, and eccentrocytes.
Photomicrograph of a peripheral blood smear stained with new methylene blue and counter-stained with a modified Wright’s Giemsa stain from the dog in Figure 1A (1,000×). Smear was prepared 12 hours after admission, following s-adenosyl-l-methionine (SAMe) loading dose and a packed red blood cell transfusion (9 mL/kg body weight); it shows continued appearance of ghost cells (arrows) and Heinz bodies (arrowheads).
Photomicrograph of a peripheral blood smear stained with new methylene blue and counter-stained with a modified Wright’s Giemsa stain from the dog in Figures 1A and 1B (1,000×). Smear was prepared 72 hours after admission and after SAMe (20 mg/kg per os daily); it shows the lack of ghost cells and eccentrocytes and marked reduction in the number of Heinz bodies.
Graphic depiction of red blood cell (RBC) reduced glutathione (GSH) concentrations in an 8-month-old Shetland sheepdog with acetaminophen toxicity at presentation and sequentially thereafter, GSH values for the administered packed RBCs given on day 1, and the reference range for GSH values in eight normal dogs (mean±standard deviation [SD]).
