hepatitis resulting from isoniazid been accompanied by clinical features commonly associated with a hypersen- sitivity reaction.'1 14 On the other hand, ...
Oxidation of hydrazine metabolites formed from isoniazid Studies in rats indicate that the metabolic activation of acetylhydrazine, a metabolite of isoniazid, is a critical determinant of the hepatotoxicity of isoniazid. As demonstrated in that model, the formation of 14C-2. after the administration of 14C-acetylisoniazid reflects the activity of the toxic pathway. A similar approach in man should make it possible to demonstrate the presence and to assess the quantitative importance of this toxifying pathway, and thus to evaluate its role in the pathogenesis of isoniazid hepatitis. We gave 300 mg isoniazid together with 10 ;Xi "C-acetylisoniazid (12 mg) to 17 healthy subjects and determined the time course of the plasma concentrations of isoniazid, acetylisoniazid, acetylhydrazine, and diacetylhydrazine and of the exhalation of "CO2. The time course of '4CO3 in breath closely paralleled the plasma concentration-time curve of acetylhydrazine but not those of acetylisoniazid or diacetylhydrazine, indicating that the "CO, originated directly from the metabolism of acetylhydrazine. The cumulative exhalation of "CO2 increased with decreasing rate of acetylation of isoniazid, such that slow acetylators generated more 14CO2 than rapid acetylators. Simulation studies demonstrated that even if the data are corrected for the different formation of acetylisoniazid from isoniazid in slow and rapid acetylators, the slow acetylators still generated more "CO2. The data therefore indicate that a substantial fraction of the acetylhydrazine formed from isoniazid passes through a pathway that has been shown in animals to generate highly reactive and hepatotoxic intermediates. The observation that slow acetylators metabolize more isoniazid via the toxic pathway, together with recent data showing an exceptionally high incidence of hepatitis in slow acetylators receiving large therapeutic doses of isoniazid, further support the hypothesis that the metabolic activation of acetylhydrazine from isoniazid is an important determinant of the hepatoundcity of isoniazid in man. (CLIN PHARMACOL THER 38:566-571, 1985.)
Bernhard H. Lauterburg, M.D., Charles V. Smith, Ph.D., Elizabeth L. Todd, Ph.D., and Jerry R. Mitchell, M.D., Ph.D. Houston, Tex.
The treatment of tuberculosis with isoniazid is associated with a high incidence of hepatotoxicity, 1,3,7,8,14 but the pathogenesis of isoniazid-induced liver injury in man is uncertain. In only a few patients has the hepatitis resulting from isoniazid been accompanied by clinical features commonly associated with a hypersensitivity reaction.'1 14 On the other hand, evidence for a direct toxic reaction as suggested by experiments in animals is also not conclusive. Based on earlier animal studies, a critical determinant of isoniazid toxicity appears to be the fraction of isoniazid converted to acetylhydrazine, which is then activated in the liver by the cytochrome P-450 monooxygenase system to a chemically reactive hepatotoxic metabolite. 1417 Acetylhydrazine is a metabolite of isoniazid in man From the Department of Internal Medicine and Institute for Lipid Research, Baylor College of Medicine. Received for publication Nov. 19, 1984; accepted July 16, 1985. Reprint requests to: Bernhard H. Lauterburg, M.D., Baylor College of Medicine, One Baylor Plaza, Room 826E, Houston, TX 77030.
566
of acetylhydrazine by hepatic monooxygenases occurs with human liver microsomes.' However, a relationship between exposure to acetylhydrazine and liver injury has not been established, and it is not known whether metabolic activation of acetylhydrazine accounts for isoniazid hepatotoxicity. In animal models the toxic pathway can be assessed by following the covalent binding of the toxic radical metabolite to hepatic macromolecules and correlating covalent binding with hepatocellular necrosis.' Such an approach is not feasible in man. However, the toxic metabolite also yields acetate, which is further metabolized to CO2.21 After the administration of "C-acetylhydrazine or "C-acetylisoniazid labeled in the acetyl moiety, "CO2 will appear in breath and the amount of "CO2 exhaled will reflect the fraction of the dose passing through the toxic pathway.2I A similar approach should make it possible to demonstrate the presence and to assess the quantitative importance of this toxifying pathway in man. We have also,22 and metabolic activation
VOLUME 38 NUMBER 5
tested this hypothesis by administering a tracer dose of 'C-acetylisoniazid together with 300 mg isoniazid to normal healthy subjects. Our data demonstrate that a substantial fraction of an acetylisoniazid dose is metabolized to CO, and that very slow acetylators, who may be at a high risk of incurring liver injury from isoniazid,' metabolize a larger fraction of this metabolite to CO, than do rapid acetylators.
Oxidation of hydrazine metabolites
567
CO2 tl(co KINICKINI4
Isoniazid
ICINH
Acetyl-
KhAcINH
isoniazid
K.AcINH
Acetylhydrazine
--TCAcHz
Citellz
Diacetylhydrazine
17--(DiAcHz
METHODS
Fig. 1. Kinetic model describing the formation of CO, from isoniazid.
Seventeen subjects (seven women and 10 men) between 19 and 28 years old gave informed consent to participate in the study, which had been approved by the Baylor Institutional Review Board for Human Research. Subjects had normal hepatic and renal function as judged from their levels of serum transaminase, alkaline phosphatase, bilirubin, albumin, and creatinine and BUN. They had not taken any other medication during the 10 days before the study and none of the subjects was overweight. After an overnight fast they took 300 mg isoniazid dissolved in water together with 10 pCi "C-acetylisoniazid (12 mg). The labeled compound (specific activity 0.15 mCi/mmol) was synthesized as described previously,' and its purity exceeded 98% as verified by TLC on silica gel with propanol : methanol (70:30) as the mobile phase and zonal scanning. Blood was drawn from an antecubital vein at intervals for 4 hours for the determination of the rate of disappearance of isoniazid. In six subjects blood was drawn at intervals for 36 hours for the determination of the time course of the plasma concentrations of isoniazid, acetylisoniazid, acetylhydrazine, and diacetylhydrazine. Expired CO, was collected by blowing exhaled air through a glass tube into a scintillation vial containing 2 ml IN Hyamine 10X (Rohm & Haas Co.), a base, and 2 ml ethanol plus one drop phenolphthalein until neutralization was achieved by the trapping of 2 mmol CO, (indicated by the loss of color of the indicator). Subjects remained bedridden for the duration of the study to minimize variations in the endogenous production of CO,. They received their first meal 3 hours after drug dosing and then followed a regular diet. After this initial study, six subjects took phenobarbital, 2 mg/kg/day, in two divided doses for 5 days. Thereafter, they underwent a repeat study of isoniazid metabolism 24 hours after the last dose of phenobarbital. The specific activity of the exhaled CO, was calculated from the known amount of CO, trapped in each vial (2 mmol) and the radioactivity contained in the 4 ml of neutralized trapping solution. Radioactivity was measured by liquid scintillation spectrometry with the
channels ratio technique for quench correction. The cumulative expiration of labeled CO, was estimated from these intermittently measured specific activities as the product of the AUC of "CO, in breath and endogenous CO, production, which was assumed to be 9 mmol/kg hr.' Plasma concentrations of isoniazid, acetylisoniazid, acetylhydrazine, and diacetylhydrazine were measured by GC-MS with deuterated analogs as internal standards.' The fractional rate of disappearance of isoniazid from the circulation and of "CO, from breath was calculated by least-squares regression analysis after logarithmic transformation of the data. The data points following the peak value were used in the calculations. The acetyl moiety of acetylhydrazine giving rise to the exhaled "CO, is not present in the parent compound isoniazid, which is why labeled acetylisoniazid and not labeled isoniazid was used. Our experimental data therefore do not take into account the influence of the first metabolic step, i.e. , the acetylation of isoniazid to acetylisoniazid, on the fraction of a dose of isoniazid passing through the toxic pathway. To study the effect of this first metabolic step on the generation of CO2 from isoniazid, we simulated the production of CO, from isoniazid for a slow and a rapid acetylator using the model shown in Fig. I. The two simulations differ only in the rates of acetylation of isoniazid (k," = 0.55 in the rapid and 0.1 in the slow acetylator) and of acetylhydrazine which was assumed to be half the rate of isoniazid acetylation,4 i.e., 0.275 for the rapid and 0.05 for the slow acetylator. The rates of urinary excretion of isoniazid (k'INH = 0.025) and the rate of hydrolysis of acetylisoniazid to acetylhydrazine (knAciNn = 0.09) are assumed to be identical in the slow and rapid acetylators, and the parameter estimates are based on published data.4" The rate of formation of CO, from acetylhydrazine (kc02) was assumed to be 0.02 in both the rapid and the slow acetylators. For the simulation, the system of differential equations describing the model was integrated numerically by the computer program DIFFEQ of PROPHET,' which uses the algorithm of MLAB.
568
CLIN PHARMACOL THER NOVEMBER 1985
Lauterburg et al. RAPID
SLOW
20 10
10
-I
20
10
20
5
5
5j
0 0
12
12
214
20
10
20
10
5
10
ace
24
CNI
0 10
5-\ cu
12
12
214
20
10
20
10
5
10
24 10
5
5
0
I
/
12
24
2
12
0.4
0.6
h
1
INH
ts. 0
0.2
24
TIME, h
Fig. 2. Time course of the plasma concentration of acetylhydrazine and "CO, in breath after 300 mg isoniazid and 10 Ci "C-acetylisoniazid by mouth to three slow and three rapid acetylators.
RESULTS After the oral administration of isoniazid and '4Cacetylisoniazid, the specific activity of exhaled 14CO2 rose rapidly and then declined, with an apparent t112 ranging from 3.9 to 14 hours. The rate of disappearance of '4CO2 u from breath was much slower than, but increased with, the increasing rate of disappearance of isoniazid; the latter is determined mainly by the rate of acetylation. Like the apparent plasma t112 of acetylhydrazine , the proposed precursor of the '4CO2, the t112 of 14C-2 v in breath was on the average four times longer than that of isoniazid. Indeed, the time course of '4CO2 in breath paralleled the simultaneously determined plasma concentrations of acetylhydrazine, further supporting the contention that the measured 14CO2 originated from the metabolism of acetylhydrazine (Fig. 2). The estimated cumulative exhalation of "'CO2 ranged from 1.8% to 20.5% of the administered radioactivity and decreased as the rate of disappearance of isoniazid (and thus the rate of acetylation) increased (Fig. 3).
Fig. 3. Cumulative exhalation of 'CO, as a function of the rate of disappearance of isoniazid. Arrow denotes the subject who metabolized much of the '4C-isoniazid to "CO, even before phenobarbital dosing.
The fraction of the administered radioactivity appearing in breath was related closely to the plasma acetylhydrazine AUC as measured in six subjects (r = 0.92; Table I). In contrast, the plasma AUCs of acetylisoniazid and diacetylhydrazine tended to increase as the rate of acetylation increased (Table I). Consequently, there was a negative correlation between the fraction of the administered radioactivity appearing in breath and the plasma AUCs of acetylisoniazid (r = 0.74) and diacetylhydrazine (r = 0.81), making these metabolites unlikely sources for the exhaled 'CO2. The production of "CO2 increased in five of the six subjects after phenobarbital pretreatment (Table II). The subject in whom the exhalation of '4CO2 decreased slightly after phenobarbital metabolized an extraordinarily high percentage of the administered radioactivity to 'CO2 before phenobarbital, suggesting that his drug metabolizing enzyme system might have been induced even before phenobarbital treatment. The rates of disappearance of isoniazid from plasma did not change after phenobarbital. In these studies all subjects received an identical dose of labeled acetylisoniazid from which they generated
VOLUME 38 NUMBER 5
Oxidation of hydrazine metabolites
Table I. Formation of diacetylhydrazine Subject No. 1
2 3
4 5
6
*
(hr-') 0.198 0.218 0.273 0.422 0.545 0.572
14CO2
569
from '4C-acetylisoniazid and plasma AUCs of acetylisoniazid, acetylhydrazine, and
Acetylisoniazid AUG (nmollml hr)
Acetylhydrazine AUG (nmol/ml hr)
Diacetylhydrazine AUG (nmollml hr)
"CO2 (% dose)
328 380 379
25
143
7.4 7.0 5.6 2.5 2.6
248
1.8
34 51
100 100 65 122
31 19
197 150 103
75
*Fractional rate of isoniazid disappearance from circulation.
similar amounts of labeled acetylhydrazine, because the rate of hydrolysis of acetylisoniazid to acetylhydrazine is much the same in all individuals and is independent of the acetylator status. 13 If acetylisoniazid is formed in vivo, however, not all subjects will be exposed to the same amount, because rapid acetylators will form more acetylisoniazid and thus generate more acetylhydrazine than will slow acetylators. To assess the influence of this first acetylation step, which is missing in the experimental data, we therefore simulated the formation of CO2 from isoniazid for a rapid and a slow rate of acetylation. The simulations predict that 9.8% and 3.1% of a dose of acetylisoniazid are metabolized to CO2 if the rates of acetylation are 0.10 and 0.55 hours" ', respectively. These values agree well with the experimentally determined fractions (Fig. 4). The simulations further demonstrate that the fraction of a dose of isoniazid metabolized to CO2 via acetylhydrazine is smaller than that of acetylisoniazid. However, because rapid acetylators metabolize most of the isoniazid to acetylisoniazid, only a minimally smaller fraction (2.9%) appears in breath if the formation of CO2 from isoniazid via acetylisoniazid and acetylhydrazine is simulated (Fig. 4). In contrast, the fraction decreases from 9.8% to 6.0% if the rate of acetylation is slow, because slow acetylators excrete approximately one third of a dose of isoniazid unchanged ( Fig. 5). Nevertheless, the fraction of a dose of isoniazid metabolized to CO2 will still be twice as high in slow acetylators.
DISCUSSION The formation of '4CO2 from "C-acetylisoniazid demonstrates that the acetyl moiety of acetylisoniazid is metabolized in man as it is in experimental animals. In animal models in which the metabolic activation of the acetyl moiety to a toxic metabolite can be correlated with toxicity by comparing covalent binding of the metabolite with the extent of hepatic damage observed histologically, the metabolism of the acetyl moiety to
Table II. Effect of phenobarbital, 2 mg/kg, for days of the formation of '4CO2 from 'C-acetylisoniazid
5
Before
After
phenobarbital
phenobarbital
(% dose)
(% dose)
6
1.8
7 8
6.2 8.6 9.2
4.2 7.2 10.0
Subject No.
9 10 11
11.2 20.5
10.9 13.8 19.2
CO2 reflects the activity of the toxifying pathway.' It is not possible in man to examine for a similar corre-
lation between the formation of CO2 and the hepatotoxicity of isoniazid, but indirect evidence suggests that the exhalation of HCO2 also reflects the toxic pathway in man. First, the time course of the specific activity of 14CO2 in breath closely paralleled the plasma concentration of acetylhydrazine, which is the substrate for the metabolic step leading to the formation of the toxic metabolite."'" The measured 14CO2 could conceivably result from the metabolism of acetate originating from hydrolysis of '4C-acetylisoniazid and '4C-diacetylhydrazine. Because the clearance of acetylisoniazid is similar in slow and rapid acetylators' and more diacetylhydrazine is formed in rapid than in slow acetylators,"22 rapid acetylators would be expected to metabolize at least an equal if not a larger fraction of the administered radioactivity to 14CO2 if such hydrolysis contributed significantly to the exhaled 14CO2. However, rapid acetylators metabolized a much smaller fraction of the ingested dose to "CO, than did slow acetylators. Secondly, hydrolysis of the acetylated hydrazine metabolites of isoniazid would not be expected to increase after pretreatment with phenobarbital. Nonetheless, the exhalation of '4CO2 increased somewhat after enzyme
CLIN PHARMACOL THER NOVEMBER 1985
570 Lauterburg et al.
10
24
16
32
Hours
Fig. 4. Calculated time course of the plasma concentrations of isoniazid (INH), acetylisoniazid (AcINH ), acetylhydrazine (AcHz), and diacetylhydrazine (DiAcHz) and estimated cumulative appearance of CO, in breath generated from isoniazid in a rapid acetylator.
induction in all but one subject. Even in rats, in which enzyme induction can be accomplished under bettercontrolled conditions and with relatively higher doses of inducer, pretreatment with phenobarbital increases the formation of 'CO, from "C-acetylhydrazine by only 24% (from 9.8% to 12.2% of the administered dose in 6 hours).' Finally, our findings are in agreement with recent observations of an increased susceptibility of slow acetylators to the hepatotoxic effects of isoniazid.316 In our studies the rate of acetylation was a major determinant of the fraction of acetylisoniazid metabolized to 'CO,. Even when the initial metabolic step, the acetylation of isoniazid to acetylisoniazid, is taken into account, slow acetylators metabolize a larger fraction of a dose of isoniazid to CO2. Intuitively, slow acetylators (kINH < 0.3)" might be expected to be exposed to less acetylhydrazine than are rapid acetylators. Although slow acetylators do generate less acetylhydrazine from a given dose of isoniazid, the rate of acetylation of acetylhydrazine to the relatively nontoxic diacetylhydrazine is so much slower in slow acetylators that more acetylhydrazine will be available to the toxifying pathway. The balance between formation and further acetylation of acetylhydrazine leads to a marked increase in the metabolism of acetylhydrazine through the cytochrome P-450 pathway, particularly in very slow acetylators who would therefore be expected to be at greater risk of developing liver injury, from isoniazid. In a recent prospective study of 56 slow and 56 rapid acetylators , clinically overt hepatitis occurred in three pa-
Fig. 5. Calculated time course of the plasma concentrations of isoniazid (INH ), acetylisoniazid (AcINH), acetylhydrazine (AcHz), and diacetylhydrazine (DiAcHz) and estimated cumulative appearance of CO, in breath generated from isoniazid in a slow acetylator.
tients who were very slow acetylators (acetylation index
