Effect of Cimetidine and other antihistaminics on the elimination of aminopyrine, phenacetin and caffein

Life Sciences, Vol. 26, pp. 1261-1268 Printed in the U.S.A.

Pergamon Press

EFFECT OF CIMETIDINEAND OTHERANTIHISTAMINICS ON THE ELIMINATION OF AMINOPYRINE, PHENACETINAND CAFFEINE P.V. Desmond, R. Patwardhan, R. Parker, S. Schenker, and K.V. Speeg, Jr. Department of Medicine Vanderbilt University School of Medicine and Nashville V.A. Medical Center Nashville, Tennessee 37203 U.S.A. (Received in final form February 12, 1980)

Summary Cimetidine is widely prescribed for the treatment of peptic ulcer disease and has recently been shown to i n h i b i t the metabolism of warfarin, antipyrine and diazepam. To further examine this phenomenon we investigated the effect of various doses of cimetidine and other related drugs on 14C-aminopyrine, 14C-phenacetin and 14Ccaffeine breath tests. Cimetidine caused a dose-related inhibition of the metabolism of aminopyrine and caffeine but had no effect on the phenacetin breath test. Metiamide, H1-antihistomines, phenothiazines and local anesthetics also inhibited the ominopyrine breath test. Cyproheptadinehad no effect on either phenacetin or caffeine elimination. This study demonstrates a complex drug-drug interaction which may have widespread clinical implications. Cimetidine, an antagonist of histamine H2-receptors, is effective in healing peptic ulcers and is widely prescribed. I n i t i a l c l i n i c a l t r i a l s demonstrated cimetidine to be relatively free of significant adverse effects. Recently there have been case reports of bone marrow depression associated with cimetidine (1,2), a decreased sperm count in men after nine weeks of treatment with cimetidine (3) and reports of mental depression in elderly subjects, those on high doses of the drug, or patients with renal and/or l i v e r disease (4). The most recent adverse effect of cimetidine relates to a possible drug-drug interaction. There were i n i t i a l l y two brief reports of potentiation by cimetidine of the anti-coagulation effect of warfarin (5,6). Subsequently Puurunen and Pelkonen (7) demonstrated prolongation of hexobarbital sleeping time and inhibition of aminopyrine breath test in rats pretreated with cimetidine. Serlin et al. (8) then demonstrated that the clearance of warfarin and antipyrine in man is inhibited by therapeutic doses of cimetidine. This study was designed to further investigate the interaction of cimetidine and drug metabolism. Using the 14C-aminopyrine breath test as a marker of drug metabolism we investigated l ) whether there is a dose-related inhibition of drug metabolism by cimetidine and 2) whether other histamine antagonists, ~henothiazines and/or local anesthetics impair drug metabolism. In addition to 4C-aminopyrine which is demethylated and metabolized via cytochrome P-450 (9), we studied inhibition of other routes of drug metabolism using 14C-phenacetin, a drug which is de-ethylated and potentially an indicator of cytochrome P-448 a c t i v i t y (lO). The phenacetin breath test has also been shown to be a sensitive indicator of hepatic function ( l l ) . Further, we used a breath test with 14C-caffeine, a drug which is demethylated, which has recently been described also to be an indicator of cytochrome P-448 a c t i v i t y (12). 0024-3205/80/151261-08502.00/0 Copyright (c) 1980 Pergamon Press Lid

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Methods Male Sprague-Dawley rats weighing 175-225 gm were used in all experiments and animals were fed Purina rat chow and water ad l i b . Breath tests were begun one hour after an intraperitoneal injection of saline (controls) or one of the test drugs. 0.5 ml of 14C-aminopyrine l ~Ci/19.3 ~g/ml (Amersham Searle Corp.), 0.3 ml of 14C-phenacetin l uCi/O.2 mg/ml (labeled on ethyl group) (kindly donated by Dr. fan Calder, University of Melbourne, Australia), 0.6 ml of l methyl-14C-caffeine 2.5 ~Ci/mmole/ml (ICN Pharmaceuticals) or 0.25 ml of 14Cformate 2.5 uCi/O.04 umole/ml (New England Nuclear) was injected into a t a i l vein of unanesthetized rats. Rats were housed in individual a i r t i g h t restraining cages; exhaled 14C02 was drawn through concentrated sulfuric acid to remove water and then through a s c i n t i l l a t i o n vial containing lO ml of a 2:1 (v/v) methanol:ethanolamine mixture to trap all expired C02. Previous experiments with two vials placed in series demonstrated that all expired 14C02 was trapped in the f i r s t vial. All C02 expired was collected during ten consecutive 15minute periods starting immediately after the test drug was administered. Trapped radioactivity was determined after adding lO ml AcsTM (Amersham, Arlington Heights, l l . ) and counting in a liquid s c i n t i l l a t i o n counter, using the automatic external standardization procedure to correct for quenching. The exhaled 14C02, as measured by the total DPM per collection period, peaked and then decreased exponentially. The elimination rate constant (Kel) was calculated by a least square regression analysis of the logarithm of the amount of 14C02 produced with respect to time after peak. The elimination h a l f - l i f e was calculated by dividing 0.693 by the elimination rate constant (13,14). The number of counts at peak level was expressed as a percentage of the dose administered by dividing the highest DPM for a 15-minute collection period by the number of DPM administered. Drugs tested included cimetidine (TagametR HCl injection, SK & F Lab Co, Carolina P.R.); metiamide (generously provided by Smith, Kline and French Laboratories); pyrilamine maleate, diphenhydramineHCI, and histamine dihydrochloride (Sigma Chemical Co.); dimenhydrinate (DramamineR injection, 50 mg/ml, G. D. Searle & Co.); betazole hydrochloride ( L i l l y Co.); chlorpromazine (ThorazineR injection, 25 mg/ml, Smith, Kline and French Laboratories); lidocaine (2% lidocaine HCI injection, Elkins-Sinn, Inc.); benzyl alcohol (Fisher Scientific Co.); and cyproheptadine HCI (a generous g i f t of Merck, Sharp and Dohme Research Laboratories). The doses used are listed in Table I and there were at least five animals in each group. Statistics S t a t i s t i c a l evaluations were performed with the two-tailed unpaired Student's t test, and linear regression analysis was performed to compare dose, elimination rate constant, and peak level. In all cases P < 0.05 was taken to be the minimal level of s t a t i s t i c a l significance. Results Aminop~rine breath test In control animals the peak 14CO2 level occurred between 15-30 minutes following administration of the labeled substrate, after which there was a mono-exponential decline in the rate of 14C02 exhaled (Fig. l ) . The h a l f - l i f e of the decline of expired 14C02 was 34.0 + 4.6 min. In rats pretreated with 20 mg/kg cimetidine, the peak level was lower and the h a l f - l i f e was s i g n i f i cantly prolonged by 31 percent. There was l i t t l e difference between 20 and 40 mg/kg cimetidine on the breath test but 80 and 120 mg/kg caused progressively lower peaks and longer half-lives of aminopyrine (Fig. l and Table I). There was a linear dose-response inhibition of aminopyrine metabolism when elimination rate constant was compared to dose of cimetidine (r = -0.89, P < O.OOl)..

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TABLE I Effect of Various Drugs on the Aminopyrine Breath Test (Mean ~ S.D.) Dru~ CONTROL:

Elimination Rate Constant (Kel) (min-I) 0.021 + 0.003

Half-Life (T~) {min)

Peak (% o--f-Dose)

34.0 + 4.6

8.8 + 0.4

HISTAMINE-H2 ANTAGONISTS: Cimetidine 20 mg/kg Cimetidine

0.016 + 0.002* -

44.5 + 7.2* -

7.5 + 1.7 -

40 mg/kg Cimetidine

0.016 + 0.003* -

43.6 + 6 . 5 * -

7.3 + 1 . 4 " -

80 mg/kg Cimetidine 120 mg/kg Metiamide

0.009 + O.OOS* 0.005 + 0.004* -

84.2 ~ 22.9*

3.2 + 1.5"

134.1 + 120.2" -

2.3 + 0.3* -

BO mg/kg Metiamide 120 mg/kg

0.014 ~ 0.002*

49.2 + 5 . 6 * 66.8 + 17.2"

6.1 + 0 . 8 * 4.6 + 0 . 8 *

0.019 ~ 0.003

37.0 ~ 8.2

8.7 ~ 0.3

0.017 ~ 0.003*

42.4 + 6.9* -

7.0 + I . I * -

0.009 + 0 . 0 0 1 "

73.3 + 11.9"

4.5 + 1 . 5 "

0.011 + 0.003*

HISTAMINE-HI ANTAGONISTS: Pyrilamine 55 mg/kg Pyrilamine 80 mg/kg Dimenhydrinate 100 mg/kg

Diphenhydramine 53 mg/kg Cyproheptadine 22 mg/kg

-

-

-

0.012 + 0.001" 0.014 ~ 0.002*

59.6 + 4 . 6 * 49.5 ~ 7.7*

5.2 + 0 . 7 * 6.1 ~ 0 . 7 *

0.015 + 0.003*

48.1 + 8 . 2 *

6.3 + 0 . 9 *

0.013 + 0.001"

54.1 ~ 3 . 9 "

5.0~0.5"

0.014 + 0.004*

53.8+ 18.2"

7.2 + 1.9

PHENOTHIAZINE: Chlorpromazine 45 mg/kg LOCAL ANESTHETICS: Lidocaine 69 mg/kg Benzyl Alcohol 75 ~I/animal

* P < 0.05 (different from control) Metiamide also caused a dose-related impairment of aminopyrine breath test, whether h a l f - l i f e or peak level are considered (Table I). Hl°antihista mines, pyrilamine, dimenhydrinate, diphenhydramine and cyproheptadine also caused inhibition of the aminopyrine breath test (Table I) as did the phenothiazine chlorpromazine and the local anesthetics lidocaine and benzyl alcohol (Table I). Histamine dihydrochloride (20 mg/kg) and betazole hydrochloride (20 mg/kg) were without effect upon the aminopyrine breath test ~ata not shown).

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CIMET1DINE

! . IO

i

i

i

i

i

i

i

i

CIMETIDINE

i

i

i

i

i

i

CIMI~TIOINE { i20 mO/k9)

Io

F

~ ;o ~o ,~ ,~o ,~o

~ ~o ~ = ,~o ,~o ,~o

TIM( (MINI

TIM((kilN)

FIG. 1 Effect of increasing dose of cimetidine on the rate of 14C02 expired during the aminopyrine breath test (mean + S.E.) o

TABLE II Effect of Cimetidine and Cyproheptadine on the Phenacetin Breath Test (Mean ~S.D.) Elimination Rate Constant (Kel) (min-])

Half-Life IT½) (min)

Peak (% o--¢--~ose)

Control

0.025 + 0.002

28.2 + 2.6

20.5 + 2.6

Cimetidine 120 mg/kg

0.022 ~ 0.002

31.2 ~ 2.3

17.8 ~ 3.8

Cyproheptadine 22 mg/kg

0.025 ~ 0.002

28.4 ~ 2.7

21.2 ~ 4.4

Phenacetin breath test

In control animals, the peak expired 14C02 was greater following 14Cphenacetin (20.5% of dose) than following 14C-aminopyrine (8.8% of dose) but like aminopyrine the peak also occurred at 15-30 minutes after administration of the labeled phenacetin. The normal elimination h a l f - l i f e at 28.2 minutes

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Cimetidine Impairs Drug Elimination

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was shorter than for aminopyrine (Table I ) . Neither cimetidine (120 mg/kg) nor cyproheptadine (25 mg/kg) had any significant effect on the elimination of phenacetin (Table I f ) . Caffeine breath test /

The elimination h a l f - l i f e for caffeine was longer in the controls (52.7 + 4.8 minutes) than for the aminopyrine or the phenacetin breath test and also the peak expired 14CO2 was lower (I.9 ~ 0.4%). Cimetidine caused a doserelated impairment of caffeine elimination. After 60 mg/kg cimetidine pretreatment the h a l f - l i f e was more than doubled while after 120 mg/kg cimetidine pretreatment the peak level was delayed and there was l i t t l e decay during the five hours of the study (Fig. 2, Table I l l ) . Cyproheptadine (25 mg/kg) had no effect on the caffeine breath test (Table I l l ) . As hypothermia may impair drug metabolism, rectal temperatures were f o l lowed for 2 hours following IP injection of cimetidine (120 mg/kg), dimenhydrinate (lO0 mg/kg), chlorpromazine (44.9 mg/kg), benzyl alcohol (205 pl/kg) and cyproheptadine (22.2 mg/kg). Chlorpromazine caused hypothermia (34.5°C at 2 hours) whereas the other drugs had no effect on rectal temperature. Discussion The data herein reported make several points. First, cimetidine in rats inhibits aminopyrine metabolism in a dose-related manner and a number of other histamine antagonists, phenothiazines and local anesthetics also i n h i b i t aminopyrine metabolism. Second, phenacetin metabolism is not impaired by either cimetidine or cyproheptadine, and third, cimetidine, but not cyproheptadine, impairs the elimination of caffeine. TABLE I l l Effect of Cimetidine and Cyproheptadine on the Caffeine Breath Test (Mean + S.D.) Elimination Rate Constant (Kel) (min-i)

Half-Life (T½) (min)

Peak (%o--~---Dose)

52.7 + 4.8

1.9 + 0.4

I18.5 ~ 35*

0.9 ~ 0.4*

Control

.01323 + .OOl

Cimetidine 60 mg/kg

.00636 ~ .00219"

Cimetidine 120 mg/kg

<.OOl*

>150"

0.9 ~ 0.3*

.013 ~ .002

52.7 ~ 4.8

1.7 ~ 0.4

Cyproheptadine 22 mg/kg * P < 0.05

Breath tests were chosen as a means of looking at the interaction of cimetidine and the metabolism of other drugs because they are relatively simple to perform and are very reproducible. AT~o they may be used to look at d i f f e r ent metabolic pathways by using different substrates. Breath tests, however, do have two major potential limitations. First, there are a number of metabolic steps between cleavage of 14-carbon from the parent compounds and the production of 14C02. For aminopyrine and caffeine the 14C-methyl group is con-

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verted to formaldehyde, then to formate and finally to C02. To determine i f this last step may be the site of cimetidine inhibition of drug metabolism, we measured 14C02 production after administration of 14C-formate to rats pretreated with saline (controls) or cimetidine (120 mg/kg). There was no difference between the two groups of animals (half-life 20.3 + 1.3 min (mean + S.D.) in controls vs. 2~.7 + 1.7 min in cimetidine animals, P-> 0.30). This Tndicates that the site of inhibition of the aminopyrine and caffeine breath tests is prior to the conversion of formate to 14C02. Moreover, for 14C-aminopyrine two independent groups of investigators have demonstrated that the rate of appearance of 14C02 in the breath correlates with the disappearance of the parent compound from the blood (9,]5). A second limitation of breath tests is that they measure only a rate of elimination. This may be expressed as the rate constant of elimination (Kel) or mere cor~nly as a reciprocal function, the h a l f - l i f e (T½). Whichever term is used, this parameter is a hybrid function dependent on two physiological variables -- the volume of distribution and clearance. Clearance is the measure of efficiency of drug elimination and for the purpose of this study is the optimal index of the capacity of the liver to metabolize drugs (13). Assuming no change in volume of distribution, changes in clearance will be reflected by linear changes in Kel or reciprocal changes in T½. Therefore when comparing breath tests after various doses of cimetidine i t is better to compare Kel rather than T~. With regard to this pharmacokinetic approach another interesting finding e~rglng from the present study is the relationship between Kel and the peak level of 14CO2 produced. Both Lauterburg and Bircher (15) and Vesell and Hepner (9) have suggested that the peak 14C02 produced is proportional to the h a l f - l i f e of aminopyrine. Whenall 75 aminopyrine studies were thus analyzed there was indeed a significant relationship (r = 0.62, P < O.OOl). However, there was an even better correlation between Kel and the peak 14C02 level (r = 0.92, P < 0.001) (Fig. 3). This confirms previous findings (9,15) but further demonstrates the importance of using Kel rather than elimination h a l f - l i f e in comparing breath tests. CONTROL

CIMETIDINE

CIMETIDINE

6 0 m~l/k~l

120 m Q / k g

10o

o ,,.,, 1O O U ,r o W e,. 0.

o

~

I~

i~

200

~0

o

I~ I~ ~ M [ (MIN)

2SO ~

0

~

I00

I~

200

2~

FIG. 2 Effect of increasing dose of cimetidine on the rate of 14C02 expired during caffeine breath test (mean ~ S.E.) This study demonstrates that the inhibition of aminowrine metabolism is not confined to cimetidine but also occurs after another H2-blocker ~tiamide. Furthermore, a number of HI-blockers and phenothiazinesalso inhibit aminopyrine ~tabolism. The mechanism of this inhibition was not investigated in this study but i t is probably not related to the structure of cimetidine as

30o

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Cimetldine Impairs Drug Ellmlnatlon

1267

drugs of very different structure i n h i b i t the metabolism of aminopyrine (Table I). The most l i k e l y explanation is a competitive inhibition of aminopyrine metabolism at the enz3n~e level. Another interesting speculation is that a number of these inhibitory drugs have membrane stabilizing ( i . e . , local anesthet i c ) a c t i v i t y . We therefore studied the local anesthetics lidocaine and benzyl alcohol, and both caused inhibition of aminopyrine metabolism. The two mechanisms are not mutually exclusive, but further studies in vitro are required to answer these questions. The lack of effect of cimetidine or cyproheptadine on phenacetin metabolism is interesting and i t is tempting to speculate that this is due to the different routes of metabolis~ of these compounds. Phenacetin is metabolized via de-ethylation and its rate of metabolism can be induced by type I I inducers (3-methylcolanthrene) (I0) suggesting that i t s metabolism is cytochrome P-448 mediated. In contrast, aminopyrine is metabolized via two demethylation steps and i t s metabolism can be induced by phenobarbital (9) suggesting that its metabolism is mediated via cytochrome P-450. We thought i t possible, therefore, that cimetidine and the other inhibitory drugs bind tO cytochrome P-450 but not to P-448. However, the 14C-caffeine breath test has been proposed also as a marker of cytochrome P-448 metabolism (12) and this test was markedly affected by cimetidine but not by cyproheptadine. These divergent findings demonstrate that the interaction of these various drugs is a complex phenomenon and probably cannot be explained by one single mechanism. .024

.020 .OIG

S •

c •

.012

• I

~

Oe •

.008 •

r • 0.93 •

0

0

p
I

I

I

I

I

Z

4

G

8

I0

,,!

12

PEAK 14C02 LEVEL AS A PERCENTAGE OF OOSE

FIG. 3 Correlation between the peak 14C02 expired and the elimination rate constant (Kel) of all 75 aminopyrine breath tests. This study may have a number of important c l i n i c a l implications. Although the doses of drugs used in this study are high on a per kilogram basis i f directly related to man, i t has already been demonstrated in patients that cimetidine in therapeutlc doses inhibits the metabolism of warfarin, antipyrine and diazePam (8,16). I t is therefore possible that the commonly used antihistamines and phenothiazines studied here may also i n h i b i t the metabolism of drugs metabolized by the mixed function oxidase system of the l i v e r .

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Acknowledgn~nt Supported by the Medical Research Service of the Veterans Administration and NIH Grant #AA00267. References I. 2.

15.

S.A. KLOTZand B.F. KAY, Ann. Intern. Med. 88579-580 (1978). D.N. POSNETT, R.S. STEIN, S.E. GRABERand S.B. KRANTZ, Arch. Intern. Med. 139584-586 (lg79). D.H. VAN THIEL, J.S. GAVALER, W.I. SMITH JR and G. PAUL, N. Engl. J. Med. 300 I012-I015 (1979). J.J. SCHENTAG, G. CALLERI, J.Q. ROSE, F.B. CERRA, E. DE GLOPPERand H. BERNHARD, Lancet 1177-181 (lg79). B.A. WALLIN, A. JACKNOWITZ and P.C. RAICH, Ann. Intern. Med. 90993 (I97g). B.A. SILVER and W.R. BELL, Ann. Intern. Med. go 348-349 (Ig7g~'. J. PUURUNENand O. PELKONEN, Europ. J. Pharma~l. 55335-336 (Ig79). M.J. SERLIN, R.B. SIBEON, S. MOSSMAN,A.M. BRECKENITrDGE, J.R.B. WILLIAMS, J.L. ATWOODand J.M.T. WILLOUGHBY, Lancet 2317-31g (Ig79). G.W. HEPNERand E.S. VESELL, N. Engl. J. Med. 2911384-1388 (1974). J.F. SCHNEIDER, D.L. HACKEY, B.D. SCHREIDER, A.N. KOTAKE, D.A. SCHOELLER and P.D. KLEIN, Hepatology, Rapid Literature Review lO 69 (1978). K.J. BREEN, P.V. DESMOND, R. BURY, I. CALDERand M.L. MASHFORD, Gastroenterology 7__22I033 (1977). H.D. GOLDMANand A.N. KOTAKE, Pharmacologist 21206 (1979). G.R. WILKINSON and D.G. SHAND, Clin. Pharmacol. Ther. 18377-390 (1975). K.L. MELMONand H.F. MORRELLI, Clinical Pharmacology, p 25, Macmillan Publishing Co., New York (1972). B.H. LAUTERBURGand J. BIRCHER, J. Pharmacol. Exp. Ther. 196501-509

16.

U. KLOTZ, V.-J. ANTTILA and I. REIMANN, Lancet 2699 (197g).

3. 4. 5. 6. 7. 8. g. lO. II. 12. 13. 14.

(1976).