Influence of Age-Dependent Pharmacokinetics and Metabolism on Acetaminophen Hepatotoxicity

March 1992 Volume 81, Number 3

JOURNAL OF PHARMACEUTICAL SCIENCES A publication of the American Pharmaceutical Association

RE VIEW A RTlCLE

Influence of Age-Dependent Pharmacokinetics and Metabolism on Acetaminophen Hepatotoxicity MARTHA M. RUM ORE'^ AND ROBERTG.BlAIKLOcK* Received Au ust 24, 1990, from the ‘Department of Pharmacy Administration, and the *De artment of Pharmacology, Arnold & Marie SchwaN College of P armacy and Health Sciences, Long island University, University Plaza, Brook&, NY 11201, Accepted for publication May 16, 1991.

f

Over the past 20 years, a major shift has occurred in analgesic and antipyretic use in children. (The American Academy of Pediatrics defines a neonate as <1month old, an infant as between 1 month and 12 months, and a child as between 12 months and 12 years old; for the purpose of this paper the term “children under six” will be used unless the specific neonate or infant population is being discussed.)l Today, most pediatricians prefer acetaminophen (APAP)over aspirin. APAP, in the proper dosage, is as effective as aspirin against fever, has fewer side effects, and is not associated with Reye’s syndrome. Seven concentrations of APAP are currently available, either alone or in combination, in >300 pharmaceutical products in the United States.2 With chronic high doses of APAP, the main site of organ injury is the liver. Hepatotoxicity may be caused by the accumulation of a metabolic product.- Children under 6 years of age show less hepatotoxicity, even when the drug reaches plasma levels clearly within the toxic range on the APAP nomogram. Reasons for this reduced toxic response are unclear but may involve differences in the metabolic pathways for APAP in this age group. This last phenomenon will be the focus of more extensive discussion in this review.

Toxic Effects in Adults In adults, chronic APAP doses >5 g/day usually result in liver toxicity. For individuals taking liver enzyme-inducing drugs or who are chronic alcohol abusers, the dose causing liver toxicity can be lower. Signs of APAP-induced centrilobular hepatic necrosis include elevated levels of hepatocellular enzymes in serum (aspartate aminotransferase [ASTI and alanine aminotransferase [ALTI, 1000 IU/L), hypoprothrombinemia, histological evidence, and abnormal liver function tests. Signs of hepatic encephalopathy, such as lethargy, disorientation, hypotension, hypothennia, hypoglycemia,and coma, may also be present. In severe cases, renal tubular necrosis and death may occur.6 Microcirculatory changes that occur with APAP overdose may contribute to hepatotoxicity. Plugging of hepatic miOO~-3~9/92/0300-0203$02.50/0 0 1992, American Pharmaceutical Association

crovasculature occurs by neutrophil accumulation secondary to the initial hepatic injury. Ischemic infarction may expand the region of necrosis.’ APAP is absorbed in the small intestine, with peak concentrations in serum occurring within 30 to 60 min of ingestion. It is rapidly distributed (Vd = 0.8-1.36 Llkg) into most body tissues with minimal protein binding. APAP is metabolized in the liver, and the metabolic products are excreted by the kidney. In normal adults, the elimination half-life (tl,2) ranges from 1.25 to 4 h.3-4 Patients with hepatotoxicity have increased values of APAP tl,z and hepatic damage is uniformly probable if this figure is >4 h. Acute APAP toxicity in adults usually occurs at doses >15 g.

Toxic Effects in Children In children, risk is defined as doses >150 mg/kg. Maximum liver damage occurs 2-4 days after ingestion, and elevated levels of bilirubin in serum and jaundice may appear 2-6 days after ingestion. Symptoms include vomiting, anorexia, and epigastric pain. Hepatic encephalopathy may develop, leading to coma and death, and renal tubular necrosis may also develop, even in relatively mild cases. APAP levels in serum associated with hepatotoxicity are usually >300 pg/mL at 4 h postingestion or >50 pg/mL a t 12 h. Hepatotoxicity is usually not seen a t levels <120 or <50 CLglmL at 4 and 12 h, respectively.3~6.8 The clinical symptomology of acute APAP poisoning in children under 6 years is similar to that in adults. However, hepatotoxicity occurs less otten and with lower severity with children, and these cases are rarely fatal. Adolescents are approximately six times more likely to develop evidence of hepatotoxicity and two times more likely to develop potentially toxic levels than would a child under 6 years.5 Other studies show lower incidenceof APAP toxicity in children under 6years. Of 11195 cases of suspected APAP overdose, only 1.4%of those at high risk for hepatotoxicity were under 5 years 01d.9 One might argue that the low reported rates of hepatotoxicity are a result of low incidence of exposure in children. It Journal of Pharmaceutical Sciences I 203 Vol. 81, No. 3, March 1992

is true that -85% of the cases of toxic APAP overdoses were children over 15 years,10Jl and the adult-to-child ratio for exposure to adult APAP formulations is 1.2 to 1.However, a t doses that should clearly be hepatotoxic and fatal, on the basis of the Rumack-Matthew nornogram,12 toxic consequences among children under 6 years have been few. Fatalities from APAP toxicity are extremely rare among children under 6 years, even in cases where APAP concentrations in serum reached levels that would be hepatotoxic in adults. As of 1981, only one case of APAP-produced hepatotoxicity in a child and one case of overdose have been reported. The first case involved ingestion of -35 325-mg tablets (11.3 g) by a 3-year-old girl. At 50 days after ingestion, the levels of liver enzymes returned to n0rmal.13 This rare report is in contrast to the more frequent hepatotoxic reactions reported for the adolescent age group. The second case involved administration of excessive doses chronically over several days. This case involved APAP liquid, which is more rapidly absorbed than tablet formulations.14 Table I summarizes some of the published cases of APAP overdose in children under 6 years of age.lz-23 Although the AST and ALT levels vary greatly, they are high and indicate hepatotoxicity. However, the elevation of liver enzyme levels is not predictive for mortality in these children. In one study of >300 cases of accidental ingestion of APAP by children 6 years old and younger, few resulted in toxic reactions. APAP levels in plasma and liver function were assessed in 13 of these cases. The APAP doses in these cases ranged from 3.2 to 4 g (250-294 mgkg). Although the APAP levels in serum were high enough to be toxic in adults, liver transaminase levels in serum were not elevated in any case.11 In another study covering the period between September 1976and February 1984,417 cases of APAP overdose with 7.5 g or more in children of ages 14 days to 5 months were evaluated. Of these cases, 55 (13%)had toxic APAP levels in serum. The AST levels were measured in 49 of these cases and only three (6%)had values consistent with hepatotoxicity. In addition, bilirubin levels in patients with toxic levels of APAP, as defined by nomogram, did not differ from those in patients with normal transaminase values.24.~Comparable data from adolescents and adults from a parallel study showed that 23.2% had toxic APAP levels in serum. However, in contrast to the under 6-year-old group, 29% of the adolescent and adult patients developed hepatot0xicity.m If there is a hepatotoxic reaction in a child, it is a transient hyperaminasemia, with little change in hepatic functiop. Table I-Published

Case Reports

Reference

Age

13 14 15 16 17 18

3 years 3.5years 2 months 7 weeks 6 years 6 weeks

18 18 18 19 20 20 22 23 24

1.5 years

1 1 months 22 months

15 months 1.5 years

1 1 months

7 months

1 1 months 2 years

Dose 11.3g 5 ge 4.2g, rectal' 1.4g" 7 g' 30 mg every 4 h + unknown amount for 48 h 148 mgkg x 2 days 5.4g" 6.6ge 150 mgkg' 6.5g, rectal 5.4g, rectal 152 mg/kge 420 mghglday 1 1 g"

Overdose often does not even result in toxic APAP levels in children. In a study in two poison control centers with 2787 overdosed children under 6 years old, none had toxic APAP levels in plasma or elevated levels of liver enzymes.6 During 1984,2231cases of exposure in children 12 years of age or less were reported to the National Data Collection System of the American Association of Poison Control Centers. These data showed that -96% of children experienced no more than minor effects after exposure, with no deaths reported. AB a group, adults had considerably more serious consequences, including 10 deaths.20

Metabolism of APAP The metabolism of APAP plays an important role in hepatotoxicity. Metabolic biotransformation of APAP involves capacity-limitedcoqjugation pathways of APAP glumnidation and APAP sulfation (Scheme 11.27-29 The formation rate constants for these metabolites are smaller than the elimination rate constants, and the appearance of these products in the urine depends on the rate of formation. Together, these liver metab olites account for -82% and 68% of APAP eliminated in urine for adults and children, respectively (Figure 1).In the urine, 25%of APAP is unchanged parent drug eliminated by firstorder kinetics.3 In adulta, a small percentage of a therapeutic dose is metabolized by cytochrome P-450 to N-acetyl-pbenzoquinoneimine. This reactive metabolite is subsequently conjugated with glutathione and excreted in the urine as the cysteine conjugate or the mercapturic acid conjugate. These metabolites account for -&lo% of APAP excreted in the urine. The bile is also an important route of elimination for the cysteine conjugate, and further study is needed to characterize APAF' biliary excretion in humans." When APAP doses are increased to toxic levels, metabolism and excretion are altered. In adults, ingestion of APAP as a single dose of 15 g or more causes the glucuronide and sulfate pathways to become saturated. A larger fraction of the dose is then metabolized to toxic N-acetyl-p-benzoquinoneimine, which, in turn, is conjugated with glutathione and excreted as the mercapturic acid or cysteine conjugate. This conjugation is necessary to detoxify the product. When the levels of glutathione fall to <70% of normal, the toxic N-acetyl-p benzoquinoneimine also conjugates (binds covalently and irreversibly) with hepatoproteins, thereby producing centrilobular hepatic necrosis.27 Conflicting evidence for the involvement of peroxidation or oxidation mechanisms also has

APAP Level.

ALT Levelb (IU/L)

AST Level" (IUIL)

Toxic Toxic

1 3 30f

-

20 376 22 OOO

Low Toxic Toxic

480 5 140 825

1180 loo00 2350

Low Toxic Low

7 680

13640

10 274 7360 8 980 6600 880 10 OOO

10 230 6 120 13 640 7 120 1170 26 840

-

>24 hg

Low Low LOW

Toxic >24 hu

-

-

-

-

-

NACd

Outcome

N

Recovered Fatal Fatal Recovered Fatal Recovered

N N

Y

N Y N Y

N N -

-

Recovered Recovered Recovered Recovered Recovered Recovered Recovered Recovered Fatal

a Based on Rumack-Matthew nomogram (<120p@mL at 4 h or <50 p@mL at 12 h). Alanine aminotransferase. " Aspartate aminotransferase. NAcetylcysteine administered: (N) no; (v) yes. Chronic ingestion or administration. '-, information not provided. Level taken >24 h postingestion.

204 I Journal of Pharmaceutical Sciences Vol. 81, No. 3, March 1992

< /

APAP

I

P

a,---

APAP Sulfate APAP Qlucuronida

6y%lll

I Acatyl-p-bonaoquinonaimina

+

Glucuronlde

.-

Hepatoprotain Conjugation

Cpsteina conjugate

Unchanrp3

Sunale

Qlutathione

izyeen

/sZVYears

__

1 c Mums _ _

Flgure 2-Urinary excretion patterns of APAP (unchanged, sulfate, giucuronide). (Adapted with permission from ref 32. Copyright 1976 Mosby-Year Book, Inc.)

Heroapturic Acid conjugate

Scheme I-Proposed pathways for APAP metabolism (with glutathione depletion, the metabolite combines with the protein of hepatocytes, thereby causing necrosis).

Table ICRatlor of APAP Glucuronlde to APAP Sulfate by Age'

Reference

Neonates ~

32 33 42

0.34 5 0.08 0.27

-

3-9

Years ~

0.75 f 0.10

-

12 Years

Adults

~

1.61 2 0.21

-

1.80 5 0.32

-

2.1

?

0.14

'Values are expressed as mean 2 SE; -, information not provided.

-

u&-m

O l v c M n l d S m 18%

6%

UaWllJk Add 4%

krn 4%

Flgure 1-Urinary excretion patterns of APAP in neonates and adults.

been reported. The oxidation mechanisms are presumed to arise from the generation of reactive oxygen species either through increased turnover of cytochrome P-450 or by redox cycling of APAP itself.31 Pharmacokinetic studies have shown that the urinary excretion patterns of APAP for adults, neonates, and children under 9 years old differ (Figure 1).32,33In neonates and children <9 years old, a greater proportion of urinary APAP is excreted as the sulfate conjugate. The average value of the rate constant for APAP sulfate formation in neonates is higher than the highest value reported for adults. A larger percentage of the dose was excreted as APAP sulfate by neonates and young children than by older children and adults. There is, then, a clear age-related ability to metabolize APAP via the sulfate pathway. Likewise, the percentage excreted as the glucuronide increases with age and is, therefore, highest for adults. The APAP glucuronide excretion by 3-9-year-old children was significantly higher than that by neonates and infants but was significantly lower than that by 12-year-oldchildren or adults. In fact, excretion by geriatric patients is even higher. By 9-12 years of age, the metabolic pattern of APAP in children resembles that in adults (Figure 21.32 Less than 4% is excreted as unchanged drug at any age, and there are significant age-related differences in the total amount of drug recovered in urine as conjugates.32 A relatively large percentage of APAP excreted in urine is uncharacterized in neonates. Reported ratios of APAP glucuronide to APAP sulfate are listed in Table 11. The differencesin these ratios may be related to the relative amount of enzymes and cofactors available in the differentage groups. Glucuronidation involves the transfer of glucuronic acid from uridine 5'-diphosphat~+glucuronicacid (UDP-GA) to an aglycone, such as APAP, with the enzyme UDPglucuronyl transferase. Availability of the cofactor UDP-GA or activity of the enzyme may regulate the overall process.32

Sulfation is mediated by sulfotransferases, which are heterogeneous cytosolic enzymes that participate in elimination of a number of xenobiotics.34Cofactors of sulfotransferases, such as 3'-phosphoadenosine 5'-phosphosulfate, may affect the overall rates of product formation.27 Investigations have suggested that sulfation may be enhanced in children under 9 years old, although glucuronidation in children is less than in adults. Neonates are deficient in hepatic UDP-glucuronyl transferase. They have low glucuronide conjugation of bilirubin and drugs such as chloramphenicol andp-aminobenzoic acid. However, by 3 years of age, the glucuronide capacity of children approaches that of adults. The pZ adrenergic agonist ritodrine is inactivated by sulfate and glucuronide conjugation. A recent study showed that the sulfate conjugate accounts for 45% of excretion by adults and 66%of excretion by neonates.35.36 Also, neonates excrete sulfate conjugates of steroids that are not found in adult urine.37 In another study, morphine was almost exclusively eliminated as the glucuronide conjugate by adults. In infants < 3 months old, sulfation (36%) and renal clearance (26%)predominated, with glucuronide conjugation reaching only 37%. In infants 3 months to 1.4 years of age, glucuronidation reached 50%, with sulfation and renal clearance dropping to 17% each.38 The fact that there are no other reports of relatively low rates of glucuronidation in children over 3 years poses an intriguing question: Why should this phenomenon occur with APAP up to the age of 12 years? Two pharmacokinetic differences would reduce APAP toxicity. A reduced rate of hepatic metabolism (P-450 activity)would reduce the rate of production of the toxic metabolite, as seen with the coingestion of alcohol. Cimetidine, another inhibitor of hepatic microsoma1 enzymes, reduces the toxicity of APAP in animals. This reduced toxicity was not observed in humans, where the coqjugation reactions were not affected by cimetidine.39 On the other hand, a n increase in glucuronidationwould prevent the production of the toxic arylating intermediate metabolite and reduce damaging hepatoprotein conjugation. The reduced toxicity of APAP in children may not be related to reduced hepatic metabolism.The cytmhmme P-450 system is well developed by 1 year of age.40 Furthermore, for drugs metabolized by the P-450 system (e.g., phenytoin), values oft!, in plasma are shorter in young children compared with those m adults.41.42 "his difference suggests that a more active P-450 Journal of Pharmaceutical Sciences I 205 Vol. 81, No. 3, March 1992

capacity may be operating in young children. Zero-order elimination occurs in children receiving acute overdoses. This fact might mean that liver enzymes are saturated and metabolism is limited. Alternatively, the flow of blood to the liver and, therefore, the access of drug to metabolism, may become rate limiting. For example, hepatic edema leading to diminished hepatic flow could inhibit metabolism without enzyme saturation having occurred.Hepatic microcirculatory changes alluded to earlier may also play a role here. The maximum rate of glutathione synthesis may play an important role in age-related variation in host susceptibility. Children and neonates have a more rapid turnover of hepatic glutathione than adults. Animal data suggest that a higher turnover rate of glutathione in the immature system may increase detoxification of the reactive intermediate metabolites to the mercaptopurate conjugate.- A greater rate of synthesis after depletion of glutathione by diethyl maleate has been observed in younger animals.34 Other studies have shown higher median lethal-dose values in younger animals, a fact indicating that more drug would be required to produce a hepatotoxic reaction.34 Consistent with this conclusion is the fact that hepatotoxic reactions with isoniazid therapy have not been as great a problem with children as they have been with adults. The acetylhydrazine intermediate metabolite could undergo glutathione detoxification. This possibility suggests an efficient glutathione detoxification system in the young child.43.44 We could find no evidence for this theory.

Other Factors Affecting APAP Toxicity Other factors that may be involved include the pediatric rate of MAP absorption and nutritional status.41,46-47 Although only -25% of the drug is metabolized in the first pass through the liver, in one animal study, followingoral adminisbation, -52% of APAP was removed from the blood during a single transit through the liver (extraction ratio, 0.52). This result indicates a mqior contribution of the gastrointestinal tract to the first-pass effect observed after oral administration. Slower absorption of APAP from the gastrointestinal tract of neonates and children might be a possibility.33 Levy et al. observed maximum excretion rates of unmetabolized MAP within 30 to 60 min in adults but only after 3 to 12 h in neonates. They postulated that the slow absorption of APAP by neonates may be the result of prolonged gastric emptying.33 Miller et al. observed this same delay in infanta, as well as in neonates.32 There are also differences in the absorption rate constants of liquid and solid formulations. In acute overdose in children under 6 years old, spontaneousemesis often occurs, and it occurs earlier than in adults. However, this observation fails to explain why those who develop toxic APAP levels in plasma do not develop hepatotoxicity. The nutritional status of children may alter APAP pharmocokinetics. In children with protein calorie malnutrition, the elimination rate constant was slower and t,, in plasma was prolonged. The rate of absorption (K,)was not affected.45 Interindividual drug response may be influenced by the nutritional status of the pediatric patient. In one study, the time of APAP disappearance was 44.7 5 7.1 h for patients with protein calorie malnutrition (PCM)versus 24.6 2 3.3 h for the controls. Area under the curve was significantly increased in cases of PCM.48 Reasons include PCM-induced dilation and fragmentation of rough endoplasmic reticulum of the hepatocytes and decreased domerular filtration rate. Fasting doubles the binding of A P B to the hepatic endoplasmic rethlum.49 The disruption of calcium homeostasis has been proposed as a mechanism involved in the pathogenesis of APAP hepatotoxicity* A recent pharmacolo@cal discovery is that toxic doses of APAP decrease calcium-magnesium ATPase activity in liver plasma membrane VeSiCleS.60This discovery Suggests that APAP has a direct effect on the membrane calcium pump. 206 I Journal of Pharrnacwtiml Sciences Vol. 81, No. 3,March 1992

The elevation of cytostolic calcium levels has been postulated to activate cellular proteases and phospholipases and to disrupt other calcium-controlled processes in the cell, eventually leading to cell destruction.61 If the mechanism by which reduced glutathione promotes hepatotoxicity is an increase in cytoplasmic calcium levels, then drugs that interfere with calcium function in the cell should inhibit toxicity. In two separate studies, chlorpromazine, an inhibitor of calmodulin and phospholipase C and protein kinase activation, was found to be a potent inhibitor of APAP hepatotoxicity.a.49 The clinical importance of this finding in pediatric patients is unknown. The importance of liver metabolism in APAP toxicity is apparent, and differences in pharmacokinetic parameters, particularly in P-450 activity, might be related to differences in APAP toxicity. The hepatotoxic metabolite is synthesized by the P-450 system, and changes in the activity of this system will affect the strength of the toxic reaction. Notable in this regard is the effect of ethanol. Chronic ethanol abuse, which increases P-450 activity, increases APAP hepatotoxicity (decreases the toxic dose of APAP).24 On the other hand, acute dosing with ethanol, which competitively inhibits metabolism, is somewhat protective.25 Coingestion of ethanol is hepatoprotective in children. Competition at the P-450 site is suggested as the mechanism involved.24.25 However, it has not been shown that the relatively small amounts of alcohol in APAP elixirs can cause this effect. The need for glutathione to detoxify the toxic product is also critical. This need forms the basis for the administration of N-acetylcysteine, a glutathione precursor, in cases of APAP overdose.9~28 At therapeutic doses, the rates of APAP metabolism in children and adults are similar. Table III compares the pharmacokinetics of APAP in neonates, children, and adults. Another explanation may be that the capacity of the immature renal transport system has been exceeded.32 In some individuals, a two-compartment elimination model has been characterized. Renal excretion of APAP involves glomerular filtration and passive reabsorption; it does not correlate with urine flow or pH.63 The sulfate conjugate is subject to active renal tubular secretion, and clearance is concentration dependent.53 The t,, values for the appearance in the urine of unchanged APAP o r of APAP conjugates do not vary significantly with age, although they are somewhat longer in newborn infants.32.41 It has been suggestedthat, much like the nonlinear kinetics of phenytoin in children, a small K, and a large V, (apparent maximal velocity for binding) for production of conjugated metabolites may allow clearance of larger quantities of APAP.16 Although age-dependent differences in APAP elimination pathways have been suggested as responsible for the relatively mild liver involvement despite toxic APAP concentrations in serum, further studies are needed. Although hepatotoxicity is unlikely in children under 6 years, until further information is obtained, such patients with APAP levels in the toxic range on the Rumack-Matthew nomogram should receive N-acetylcysteine treatment.

References and Notes 1. Avery, M.E.;First, L. R. Pediatric Medicine; Williams and Wilkins: Baitimore, MD, 1989. 2. The Handbook of Non rescription Dru s, Ninth edition; American Pharmaceutical k)ssociation: Wasfington, D.C., 1990. 3. Clisold. S.P. Drum 1986.32.46-59. 4. Rawling, M. D.; Henderhn,'D. B.; Hijab, A. R. Eur. J . Clin. Pharmacol. 1977,11,283-286. 5 . RumJack,B. H. Pediatr. ToxiCol. 1986,339691-701. 6. Miners, J. O.,et al. Eur. J . Clin. Phurmacol. 1988,35,157-160. 7. Mitchell, J. R,N.Engl. J , Med. 1988,319,1601-1602. 8, Adamson, G . M,~ i ~ hphrmacol, e ~ . 1988,37,41834190, 9. Smilkskin, M.J.; Kna C.L,; Kulig, K. W., et a]. N . E,&. J . Bled. 1988,319,1557-fk2.

Table IICAPAP Pharmacoklnetlcs in Neonates, Chlldmn, and Adults

Parameter'

Neonatesb

3-9 Yearsb

C

-

1.4 42f2L

-

Vd

1 Lax>h

(APAP), h

-

0.5 2.3

-

2.2-5

t,, (APAP sulfate), h

(APAP glucuronide), h

,f

-

2.111 2 0.747

4, h-'

ti,,

Adults

&, h-'

1-3.5

4.33 f 0.52

-

4.9 f 0.9 4.4f 0.4

4.5 ? 0.6 4.3 ? 0.4

5.5 f 0.6 0.146 f 0.015

4.0f 0.2 0.173 0.012

-

-

*

0.170f 0.22

Urinary disappearance time, h

1.4Lkg 1-2 1.3 5 0.8 2 2.4 t 0.14 1.5-3 3.1 f 0.78 3.6 f 0.1 3.9 2 0.2 4.1 2 1.03 3.8 2 0.2 0.183-c 0.010

-

30

30

-

24.6 f 3.3 25.05 2 2.99(10mgIkg)

AUC

17.95 f 1.30(500mg) 33.2f 5.2 (650mg) Protein binding, YO APAP APAP glucuronide APAP sulfate CL, mUmin APAP APAP glucuronide APAP sulfate ~~

-

-

~

-

-

Reference

45 8 37 15 52 41 46 29 37 41 27 45 32 32 27 32 32 48 32 48 45 52 46

20% <10% >50%

53

6.322 1.93 100 f 13.6 154 t 6 161 230.6 89 f 4

53

~~~

53 37 53 37

~~

Definitions: &, rate of adsorption; L, time to reach maximum concentration in blood; &, elimination rate; and AUC, area under the curve. Data in between columns are for children who are not neonates but are <3 years old. '-, No data reported. a

10. Harnly, A. N.; Douglas, A. P.; James, 0. Postgmd. Med. J . 1978, 54,400404. 11. Petereon. R. G.: Rumack. B. K.Arch.Znt. Med. 1981,141,390393. 12. Henreti ., F.hi.; Selbst; S. M.; Forrest, C., et al. 'Clin.Pediatr. 1989.28 525-528. 13. Arena, J.M.; Rourk, M.H.; Sibrack, C.D. Pediatrics 1978, 61, 66-72. 14. Nogen, A. G.; Bremmer, J. E. J . Pediatr. 1978,92,832-833. 15. Lieh-Lai, M. W.; Sarnaik, A. P.; Newton, J. F., et al. J . Pediatr. 1984,105,126128. 16. VelasquezJones, L. Bol. Med. Hasp. Zr&ntMex. 1983,40,476-479. 17. Blake, K. V.; Bailey, D.; Zientek, G. M., et al. Clin. Phurmacol. 1988,7,391399. 18. Greene, J. W.; Craft, L.; Ghishan, F.Am. J . Dis. Child 1983,137, .-?-R-L ?-R-7 .. .

19. Agran, P. F.;Zenk, K. E.; Ramansky, S. G. Am. J . Dis.Child 1983,137,1107-1114. 20. Veltri. J . C.: Rollins. D. E. Am. J . Emera. Med. 1988.6.104-107. 21. Clark,'J. H.; Ruasell, G. J.; Fitzgerald, fF. J . India& state Med. Aesoc. 1983, 76,832-835. 22. Smith, D.W.; Isakson, G.; Frankel, R., et al. J . Pediatr. Gastroenterol. Nutr. 1986.5,822-825. 23. Henreti , F.M.; Orel, H.; Werners, S., et al. Vet. Hum.Toxicol. 1986,28489. 24. Rurnack, B. H. Am. J . Dis.Child 1984,138,428433, 25. Rumack. B. H. D r w Intell. Clin. Phurm. 1985.19.911-912. 26. Rumack; B. H.; Pe&rson, R. C.; Koch, G. G., et al..Arch. Intern. Med. 1981,141,380385. 27. Slattery, J. T.; Wilson, J. M.; Kalhorn, T. F., et al. Clin. Phurmacol. Ther. 1987,41,413-418. 28. Slatterv. J. T.: Kouu. J. R.:Lew. G. Clin. ToxiCol.1981.111-117. 29. Nelson: E.;Moriokk T. J: Phuym. Sci. 1963,52,864-&8. 30. Jayasinghe, K. A.; Roberta, C. J. C.; Read, E. Br. J . Clin. Pharmcrcol. 1986,22,363-366. 31. Smith. C. V.: Jaeachke. H. Chem.4wl.Zntemct. 1989.70.241-248. 32. Miller; R. P.'; Roberta,R. J.; Fischer, L. J. Clin.Phar&ol. Ther. 1976,19,284-294.

33. Levy, G.; Khanna, N. M.; Soda, D. M., et al. Pediatrics 1975,55, 816-625. 34. Lauterburg, B. H.; Vaishano, Y.; Stillwell, W. G., et al. J . Pharmacol. Exp. Ther. 1980,213,54-58. 35. Brashear, W. T.; Kuhvert, B. R.; Wei, R. Clin. Pharmacol. Ther. 1988,43,634-641. 36. Finnstrom, 0.; Lundquist, P.; Martensson, J., et al. Metabolism 1983,32,732-735. 37. Mitchell, M. C.; Hanew, T.; Meredith, C. G., et al. Clin. Pharmacol. Ther. 1983,34,48-54. 38. McRarie, T. I.; Slattery, J. T: Lynn, A.M., et al. Abstract, American College of Clinical Pharmacy Eleventh Annual Meeting, August 5-8,1990,San Francisco, CA. 39. Hansten. P. D.Drue Interact. Newsletter 1983.3.55-60. 1960.18,9-15. 40. Rane, A.; Thomas, 6. Eur. J . Clin. Phar-oi. 41. Peterson, R.G.; Rumack, B. H. Pediatrics 1978,62,877-879. 42. Lou hnan, P. M.; Greenwald, A.; Purton, W. W., et al. Arch. Dis. C h i h 1977,52,302-309. 43. Beaudry, P. H.; Brickman, N. F.; Wise, M. B., et al. Am. Rev. Resprr. DLS.1974,110,581-584. 44. Baiai, R. C. Indian Pediatr. 1988.25,775-779. 45. M e h i , S.J.; Nais, C. K.; Sharma, B.; et al. Drug Nutr. Interact. 1982,I , 205-211. 46. Hendrix, J.; Wallace, S. M.; Hindmarsh, K. W., et al. Eur. J . Clin. Pharmacol. 1986,30,273-278. 47. Bharnava. V. 0.Bwoharm. Drue. Disms. 1989.10.389-396. 48. MehG, S.fNais, C. K.;Yadav,, :D et ai. Int. J . Clin: Pharmacol. Ther. Toxicol. 1985.23,311315. 49. Saville, J. G.; Davidson, C. P.; DAndrea, G. H., et al. Biochem. Pharnacol. 1988,37,2467-2471. 50. Tskos-Kuhn, J. 0.; Hugher, H.; Smith, C. V., et al. Biochem. Phurnacol. 1988,37,2125-2131. 51. Tirmenstein. M.A.: Nelson. S.D.J . Bwl. Chem. 1989.. 264.. 9814-9819. 52. w a o u i , A.; Demates, F. M.; Raywal, F., et al. Thempie 1984,39, 353-359. 53. Morris, M. E.;Levy, G. J. Pharm. Sci. 1984,73,1038-1041. Journal of Pharmaceutical Sciences I 207 Vol. 81, No. 3, March 1992