Mar 25, 1981 - yr) treatment with human growth hormone (hGH). After short-term treatment the mean volume of distribution of antipyrine (aVd) (also a measure ...
Effects of growth hormone on antipyrine kinetics in children The kinetics of antipyrine, a drug used as a clinical indicator of hepatic drug-metabolizing enzyme activity, were examined in children after short- (5 days to 1 wk) or long-term (6 wk to 1 yr) treatment with human growth hormone (hGH). After short-term treatment the mean volume of distribution of antipyrine (aVd) (also a measure of total body water) increased from 0.49 to 0.58 1/kg (p < 0.005). The mean aVd after long-term treatment did not differ from the mean pretreatment value, but it rose in three of the eight subjects examined. Neither the mean serum half=lik (t1/2) nor the metabolic clearance rate of antipyrine for the group as a whole was altered after short- or long-term treatment with hGH. However, t1/2 rose to 135% to 151% of control value in three of nine children and decreased to 63% of control value in one after short-term treatment, while after long-term treatment it rose to 128% to 176% of control value in four of eight children. The results indicate that hGH can increase total body water and should be used cautiously in children with impaired cardiac, renal, or hepatic function. The data further suggest that hGH may alter antipyrine t1/2 in some children. The variable nature of the changes precludes any uniform prediction about growth hormone effects on drug metabolism, but it may be necessary in some children to modify the dosage of other drugs administered with hGH.
Arleen B. Rifkind, M.D., Paul Saenger, M.D., Lenore S. Levine, M.D., Judy Pareira, R.N., and Maria I. New, M.D. New York, N. Y. Departments of Pharmacology, Medicine, and Pediatrics, Cornell University Medical College, and Department of Pediatrics, Monteflore Hospital and Medical Center
Because of its limited availability, the therapeutic use of growth hormone has been restricted to children with documented human growth hormone (hGH) deficiency, but there is new evidence that hGH can improve growth in some children with short stature and normal hGH levels." Since hGH can now be produced in the laboratory,8 it is likely that it will be used more widely. To assure safe use it is important Supported in part by the National Institutes of Health, Division of Research Facilities and Resources, Pediatric Clinical Research Center Award RR47 and NIH Grant GM-24796 and a grant from the Tudor foundation. Received for publication Aug. 11, 1980. Accepted for publication March 25, 1981. Reprint requests to: Dr. Arleen B. Rifkind, Cornell University Medical College, 1300 York Ave., New York, NY 10021.
to characterize pharmacologic effects as fully as possible.
We examined antipyrine kinetics in children after short- and long-term administration of hGH.* Antipyrine is principally metabolized by the mixed-function oxidase system of the liver, and its clearance has been widely used as a clinical indicator of hepatic drug-metabolizing enzyme activity.° It has well-known advantages as a test drug: it is not bound to plasma proteins, it is distributed in total body water, its clearance rate is independent of hepatic blood flow:23 and there is a large body of published data on its rate of metabolism under varying
0009-9236/81/070127+06$00.60/0 C) 1981 The C. V. Mosby Co.
*A gift of the National Pituitary Agency, Baltimore.
127
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Table I. Clinical data Subject No.
Age (yr)
1
4.5
2
5
Sex
Diagnosis
F M
Idiopathic GH deficiency Idiopathic GH and TSH deficiencies Idiopathic OH deficiency Short stature, idiopathic intrahepatic cholestasis Idiopathic GH deficiency Idiopathic OH deficiency Poland's syndrome with idiopathic GH deficiency Idiopathic growth retardation Idiopathic GH and TSH deficiencies
3
6
4
6
F F
5
6.5 7.5 8.2
M M M
6 7 8
10
9
10.9
M M
Medications other than GH
Hormonal deficiencies
None L-Thyroxine (0.05 mg/day) None None
GH GH, TSH
None None None
OH GH OH
None L-Thyroxine (0.15
None OH, TSH
OH None
mg/day) 10
12
11
12.2 13.3
12
M
F
Idiopathic growth retardation Idiopathic growth retardation Hypothalamic disorder with pathic GH deficiency
None None
idio-None
None None GH
GH = growth hormone; TSH = thyroid stimulating hormone.
conditions with which experimental results can be compared. Materials and methods
The subjects were 12 children with growth problems of varying etiology. Table I shows each child's age and sex, diagnosed condition, identified hormone deficiency, and the other drugs each received. All but one had normal liver function; subject 4 showed evidence of intrahepatic cholestasis (idiopathic) on liver biopsy. Abnormal liver function test results in that subject at the time of growth hormone evaluation and antipyrine clearance tests included elevated serum glutamic-oxalic acid (194 mU/ ml), and pyruvicglutamic acid transaminases (166 mU/m1), alkaline phosphatase (514 mU/ ml), and bilirubin (2.9 mg/100 m1). Serum albumin/globulin ratio, prothrombin time, and thromboplastin generation time were normal. The only subjects who received other drugs were subjects 2 and 9, who were on replacement doses of L-thyroxine. They were euthyroid on the replacement doses. The effects of short-term hGH treatment on antipyrine clearance (studied in subjects 2, 3, 4, 6, 7, 8, 10, 11, and 12) were examined under controlled environmental conditions while the children underwent endocrine evaluation at the
pediatric clinical research center. During the study period each child was on a diet with known sodium, calcium, potassium, phosphorous, and nitrogen intakes. Each child's caloric intake was determined during an initial test period by the amount of food the child would reliably consume completely; this ensured constant intake. Caloric intake under these hospital conditions was therefore less for each subject than the amount reported by the National Academy of Sciences' 8 to be necessary for normal weight gain in children (range 58% to 90% of recommended caloric intake, mean 64%). Antipyrine clearance tests were performed under baseline conditions after at least 5 days of equilibration to the diet and again after administration of U hGH/day intramuscularly for 5 to 7 days. The effects of long-term hGH treatment were examined in subjects 1, 2, 3, 5, 6, 9, 11, and 12 after receiving 2 U hGH intramuscularly three times a wk for 6 wk to 12 mo on an outpatient basis. In four of those subjects (2, 3, 6, and 11), antipyrine clearance was determined after both short- and long-term treatment, while in three (subjects 1, 5, and 9) it was determined only before hGH and after the longer period of treatment. Subject 12 had a second baseline antipyrine clearance test before starting long-term hGH administration because the treatment was 1
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hGH effects on antipyrine kinetics in children
129
Table II. Effects of short-term administration of growth hormone on antipyrine kinetics Serum antipyrine t1/2 (hr)
Antipyrine MCR (mIlminIkg)
Before treatment
After treatment
Before treatment
treatment
2 3
9.5
6.0 6.9
4
9.6 5.3 4.9 5.7
0.47 0.55 0.55 0.57 0.44 0.57 0.50 0.45 0.29 0.49 ± 0.03
0.48 0.62 0.74 0.62 0.46 0.78 0.66 0.52 0.30 0.58 -1- 0.05*
Subject No.
5.1
6 7 8 10 11
12
Mean
Antipyrine aVd (1/kg)
SE
10.4 10.8 3.1 7.2 -1- 1.0
14.4
5.2 7.4 4.9 10.2 10.1 3.1
7.6
-±
1.2
After
Growth hormone was administered at a dose of 1 U/day for 5 days to subjects 2, 3, and 6; for dose of 5 U/day for 7 days to subject 8. *p < 0.05 with respect to ''before treatment."
begun more than a year after the initial study. Antipyrine clearance was measured as described.'5 An oral dose of antipyrine in syrup (18 mg/kg) was given in the morning and 3 ml blood was drawn at 2, 4, 6, 8, and 24 hr after receiving the drug. Our experience12' and the experience of others'6' 20 indicates that the absorption of an oral dose of antipyrine in syrup is complete within 2 hr. Antipyrine was measured (in triplicate using 0.4 ml serum for each determination) by the method of Brodie et al.2 scaled down to accommodate the smaller amount of serum used in our procedure. A standard curve constructed from a series of known dilutions of antipyrine in serum was included in each experiment. The half-life (t1/2) antipyrine was calculated by nonlinear regression analysis; theoretical initial plasma concentration of antipyrine was determined by extending the regression line to its y axis intercept, and apparent volume of distribution (aVd) was calculated according to the equation: aVd =
Antipyrine dose Concentration
and is given in units of //kg body weight. The metabolic clearance rate (MCR) of antipyrine was calculated according to the equation: MCR
-
aVd x 0.693 t1/2
7
Before treatment
treatment
0.57
0.92
After
1.24
1.04
0.66
0.59
1.24 1.04 1.16
0.72
0.55 0.48
0.75 0.59
1.38
1.84
1.08
0.89 ± 0.11
1.12
0.99
Li
0.14
days to subjects 4, 7, 10, 11, and 12; and at a
where 0.693/t1/2 equals the first-order elimination rate constant. The statistical significance of differences in antipyrine t1/2s for subjects was determined by Student's t test. Significance of differences in mean values for antipyrine t1/2s, aVd, and MCRs for the groups were determined by the paired t test. Results
After hGH for a week or less (short-term treatment, Table II), antipyrine aVd rose in all subjects from 2% to 37%. Mean aVd after treatment (0.58 //kg) was greater (mean increase 18%) than before treatment (0.49 Ilkg) (p < 0.05). Since antipyrine is distributed in a volume equivalent to that of total body water,'7 these data suggest that after treatment there was a mean 18% rise in total body water. On constant sodium intake there was a mean decrease of 23% (p < 0.05) in urine sodium excretion after treatment, indicating that sodium retention accompanied the changes in body water. The sodium retention occurred in 7 of the 9 subjects but did not correlate significantly with individual changes in the aVd of antipyrine. Serum sodium levels were normal, indicating that preferential retention of water over sodium had not occurred. There was no significant weight change after short-term treatment with hGH although there were small increases in weight (1% to 4%) in five subjects.
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Rifkind et al.
Table III. Effects of long-term administration of growth hormone on antipyrine kinetics
Duration
of adminSubject
istration
1
2 3 5
6 9 11
12
Mean ± SE
6 wk 12 8 6 8 6 5 8
mo mo wk mo wk mo wk
Serum antipyrine t1/2 (hr)
Before treatment
Antipyrine aVd (1/kg) Before treatment
After
treatment
8.6 9.5 5.1
11.0
4.6
8.1
7.6 6.8
5.3
5.4
9.4
14.1
10.8 5.3
11.7
4.4 7.3 ± 0.9 8.6 ±
1
1
After treatment
0.47 0.55 0.47 0.73 0.55 0.54 0.53 0.51 0.57 0.53 0.50 0.53 0.67 0.45 0.47 0.47 0.50 ± 0.02 0.57 ± 0.03
The mean antipyrine t1/2 for the group as a whole after short-term hGH treatment was 6% longer than in controls (p = NS). The antipyrine t1/2 increased to 135% to 151% of pretreatment values in three children (subjects 3, 4, and 7) and fell to 63% of the values in subject 2. The mean MCR after short-term treatment increased by 11% over mean pretreatment value, but that increase was not statistically significant. In summary, short-term hGH administration increased mean antipyrine aVd but did not alter mean t1/2 or MCR for the group as a whole. Individual differences in the effects of hGH on a Vd, t1/2, or MCR of antipyrine did not correlate with age or sex, clinical evidence (or lack thereof) of hGH deficiency, or caloric intake. After hGH administration for periods of 6 wk to 12 mo (Table III), mean a Vd was no longer increased, although in four of the children the posttreatment aVd exceeded pretreatment values (subject 1, 2, 9, and 11). Mean t1/2 after long-term treatment was 18% longer than before treatment, but the increase was not statistically significant. In four of the children, however, antipyrine t1/2s rose to 128% to 176% of control values (subjects 1, 3, 5, and 9). Mean MCR fell to about 4% less than pretreatment values (p = NS).
Discussion Our study shows that short-term administration of hGH can increase antipyrine aVd in children. Since antipyrine distributes in a vol-
Antipyrine MCR (mIlminIkg) Before treatment
After treatment
0.63 0.57
0.58
1.24 1.33 1.24 0.61
0.92 0.73
1.11
1.13
0.48
0.43 0.66
1.02
1.23
0.89 ± 0.12 0.85 ± 0.10
ume equal to that of the total body water, its aVd can be used as a measure of total body water. 17 Thus the increase of antipyrine aVd reflects an increase in the total body water. After long-term treatment, aVd increased in half of the children. The differences in aVd on long- and short-term treatment may reflect adjustments in fluid volume in some subjects or the slightly lower dose of hGH each week during long-term treatment. Growth hormone has been reported to induce sodium retention in animals and man" 13' 22 and water retention in animals.3' 7 Humans with acromegaly (who have high serum levels of hGH) have increased total body water." Rudman et al.13 observed, as we did, an increase in sodium retention during short-term hGH administration to children and speculated that it might be due to increase in extracellular water. Our study provides evidence that an increase in body water occurs in children being treated with hGH. Although neither adrenocorticotropic or antidiuretic hormone activity were detectable in the highly purified hGH preparation used, it is not possible at the present time to determine whether the water and sodium retention reflect an intrinsic property of the hGH molecule or an action of a minor contaminant. The absence of body weight increase commensurate with increased total body water can be explained by the fact that the diets of the subjects contained lower caloric content than necessary for weight increase. Absence of weight loss may therefore be taken as indirect
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evidence in support of increased body water. (As explained, the low calorie diet was necessary to ensure complete consumption of meals during the study so as to permit accurate metabolic balance studies to be carried out.) It has been reported that the amobarbital t½ increased while there was no change in the amobarbital a Vd in children treated with hGH.1° These observations can be reconciled on the basis of differences in distribution characteristics of the two drugs: antipyrine is distributed in the total body water, while amobarbital is distributed in a substantially larger volume,5 probably through distribution into body fat. Since the total body water is a smaller fraction of amobarbital than antipyrine aVd, a relatively small change in total body water would be unlikely to be detected as a change in amobarbital aVd. Antipyrine t1/2 was not altered by hGH in our group as a whole but in a few of the children there were substantial increases or decreases (in excess of 30%). The individual changes in t1/2 may simply reflect experimental variation, but based on the magnitude of the changes, we think that they probably are due to real changes in antipyrine t1/2 in some of the children. We base this conclusion on our experience in measuring antipyrine clearance in children. In situations in which a drug treatment has failed to induce changes in mean antipyrine t1/2 for a group of children, we have rarely observed individual pre- and posttreatment differences in excess of 25%*.I5 The finding that hGH alters drug metabolism in some children is in agreement with data in the literature that suggests growth hormone can alter the t1/2 of drugs metabolized by hepatic drug-metabolizing enzymes. Thus, hGH has been found to increase the amobarbital t1/2 '° but to decrease theophylline t1/2 .11 Wilson25 first demonstrated in experiments with rats that growth hormone could alter hepatic drug-metabolizing enzyme activity. The effects of growth hormone on drug metabolism in rats and humans are complex; e.g., it decreases the in vivo rate of metabolism of ethylmorphine, aminopyrine, hexobarbital, and aniline in the liver of male rats6 25' 27 but not in the liver of female rats.6' 26 There is evidence that *Rifkind AB, Saenger P: Unpublished observations.
hGH effects on antipyrine kinetics in children
131
growth hormone increased ethylmorphine metabolism and did not alter aniline metabolism in hypophysectomized male rats,6 and conflicting evidence that growth hormone decreased the metabolism of hexobarbital and aniline both in intact and in hypophysectomized male rats.21. 27 Thus far there is no evidence that there are sex differences in the effects of hGH on drug metabolism in man or that the presence or absence of pituitary growth hormone affects the response to hGH. The differences between the effects of hGH on the kinetics of antipyrine, amobarbital, and theophylline cannot be explained by alterations by hGH of liver blood flow since all three drugs have low hepatic extraction ratios5' 9, 24 and their clearance rates are therefore independent of hepatic blood flow.24 In our study, differences in sex, age, and presence of pituitary growth hormone could not explain the individual differences in hGH's effects on antipyrine kinetics. Factors that could contribute to the differences in response are genetic differences or relative differences in the effects of hGH on liver size and specific activity of drug-metabolizing enzymes in individuals. Thus, any explanation for the divergent effects of hGH on amobarbital and theophylline t1/2 and variable effects of the hormone on antipyrine t1/2 can now be only speculative. An understanding of the basis for changes in drug kinetics by hGH requires more information than is now available. Because hGH may have variable effects on the rate of metabolism of different drugs and on the rate of metabolism of the same drug in different individuals, predictive conclusions cannot be drawn about its clinical effect on the rate of drug metabolism. The physician should, however, be aware that in some patients receiving other drugs with hGH it may be necessary to make compensatory adjustment in the doses of those drugs to maintain therapeutic efficacy or to avoid toxicity. He or she should also be aware that hGH has the capacity to increase total body water. In our patients this increase did not present clinical problems, but such problems could develop in children with poor cardiac, hepatic, or renal function. We wish to thank Melody Troeger and Tonia
132
Ryland et al.
Petschke for their excellent technical assistance and Jason Umans for his helpful discussions.
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