* Department of Pediatrics and
Department of Physiological Sciences, Colleges of Medicine and Veterinary Medicine, University of Florida, Gainesville, Florida
Received April 7, 2001; accepted June 20, 2001
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ABSTRACT |
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Key Words: cocaine; repetitive; pregnancy; clearance; fetus.
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INTRODUCTION |
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Through animal research using pregnant sheep, we now know much about the physiological effects of a single dose of cocaine in both the mother and her fetus (Burchfield, 1995; Burchfield and Abrams, 1993
; Burchfield et al., 1990
, 1991
; Moore et al., 1986
; Woods et al., 1987
). Although the effects of a single dose of cocaine in pregnancy has been well studied, epidemiological studies have revealed that human cocaine use is typically repetitive, not an isolated single exposure.
The rate of clearance of cocaine and its metabolites in the chronically exposed fetus has not been well studied. A recent study of the effect of chronic cocaine infusion in adult rats on urine cocaine and metabolite concentrations showed that cocaine, ecgonine methylester, and benzoylecgonine concentrations varied with the dose administered and the duration of administration (Mets et al., 2000). Specifically, urinary concentration of these compounds decreased over the 13 days of infusion. These changes in metabolite concentrations could not be explained by altered cocaine metabolism, but may reflect accumulation in the animal. Given that the fetus has limited metabolic pathways compared to adults, chronic exposure to cocaine in the fetus could result in greater accumulation of the drug and a corresponding change in drug clearance. Alternatively, repetitive exposure to cocaine in the fetus could lead to dispositional tolerance manifested by an increase in the rate of clearance. To test these possibilities, we examined the clearance of cocaine and benzoylecognine in maternal and fetal sheep that had been chronically exposed to cocaine, and compared the results with sheep receiving the drug for the first time.
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MATERIALS AND METHODS |
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After placement of the maternal venous catheter, 8 ewes (randomly assigned) received daily cocaine HCl 2.0 mg/kg (free base) intravenously over 30 s, followed by a continuous infusion at 0.2 mg/kg per min. The 2 mg/kg bolus dose to the ewe gave plasma concentrations that approximate that reported in humans (Barnett et al., 1981; DeVane et al., 1991
; London et al., 1986
) and an infusion of 0.2 mg/ml per min provides a steady-state concentration of approximately 400 ng/ml (Burchfield et al., 1990
). Through the use of a programmable infusion pump, the infusion was steadily decreased over the next 2 h to simulate cocaine clearance as seen in humans with a presumed t1/2 of 45 min. The cocaine was dissolved in 50 ml of saline immediately prior to infusion, to prevent decomposition. Eight other ewes received an equivalent volume of saline as a bolus, followed by a similarly decreasing infusion rate. Infusions of cocaine or saline continued daily from day 75 through day 128.
Surgical procedures to instrument fetal sheep at 125 days gestation were performed, using methods that have been used in this laboratory for the last 9 years. A hysterotomy was carried out under aseptic conditions and the fetal head and forelegs were delivered. Through a skin incision, catheters (1.07 mm ID) were placed in both brachial arteries and the cephalic vein. Sodium ampicillin, 500 mg, was placed in the amniotic fluid before closing the uterus and an additional 500 mg placed in the peritoneal cavity of the ewe. Catheters (1.65 mm ID) were placed surgically in a femoral artery and vein of the ewe. All catheters were tunneled subcutaneously and exteriorized at the ewe's flank, where they were retained in a pack. Animal care conformed to the policy of the American Physiological Society and was approved by the University of Florida Committee for Care and Use of Animals.
Three days following fetal surgery, ewes were brought into the laboratory and maternal and fetal blood pressure and heart rate were continuously recorded for 2 h. Arterial blood (0.3 ml) was removed from the fetus for measurement of blood gases and glucose. Following a 2-h observation period, cocaine, 2.0 mg/kg as free base dissolved in 30 ml saline, was administered intravenously over 30 s to both cocaine-exposed and control ewes. Maternal and fetal blood samples (1.0 ml) were obtained in heparinized syringes before cocaine administration and at 0.5, 2, 4, 8, 12, 16, 20, and 30 min later. Samples were immediately transferred to conical tubes containing 10 µl of 6 N HCl and centrifuged for 5 min. Plasma was separated and stored at 70°C until analyzed. Animals were sacrificed following the last sample, using an overdose of barbiturate followed by saturated KCl.
Cocaine (C17H21NO4) and its major metabolite in sheep, benzolecognine (C16H19NO4), were measured by a gas chromatographic-mass spectroscopy technique as previously described in detail (Phillips et al., 1996). This technique, with a level of detection of 1 ng/ml for benzolecognine and 2 ng/ml for cocaine, is capable of detecting other metabolites as well, if present in quantities above 5 ng/ml. Concentrations were plotted and half-life was determined, assuming linearity. Plasma half-life was compared by 2-sided Student's t-test. Differences were considered significant if p
0.05. Data are presented as means ± SD.
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RESULTS |
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Half-life for cocaine was not different between the saline and the experimental groups in either the ewe or the fetus (Fig. 1). Cocaine-exposed ewes eliminated cocaine with a t1/2 2.00 ± 0.50 min compared to 1.92 ± 0.33 min in saline-treated controls (not significant). Likewise, cocaine-exposed fetuses eliminated cocaine with t1/2 2.38 ± 0.96 min vs. 2.33 ± 0.97 min in controls (not significant).
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DISCUSSION |
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Following a single intravenous dose of cocaine in pregnant ewes, placental transfer occurs quickly, with cocaine appearing in the fetus by 30 s (DeVane et al., 1991). Fetal-placental clearance of cocaine is a rapid, first-order pharmacokinetic process, with plasma benzoylecgonine concentrations accumulating significantly with prolonged cocaine exposure (Downs et al., 1996
). In this study we have demonstrated that, following chronic cocaine administration in pregnant sheep, there are no differences in clearance or metabolism of a single dose in the mother or fetus. These findings are similar to previous studies which have shown no effect of cocaine pretreatment on the pharmacokinetics of a single dose of cocaine in rats (Mets et al., 2000
; Pan and Hedaya 1999
). The significance of these findings is that the bioavailability of a single dose of cocaine is likely unaltered, whether the subject had previously experienced multiple exposures or not. Thus, tolerance or sensitization to cocaine is likely to be unaffected by changes in metabolism or clearance following chronic use.
Limited metabolism of cocaine to benzoylecgonine by the fetus and human newborns has been reported and is considered to be associated with an immature esterase system (Browne et al., 1992; Dusick et al., 1993
). Chronic maternal use of cocaine could therefore result in an accumulation of cocaine in the fetus, resulting in elevated drug levels and a greater potential for neurotoxicity. Evidence of inhibited metabolism and elimination of cocaine or benzoylecgonine was, however, not apparent in this sheep study.
Several possible mechanisms for neurotoxicity induced by cocaine or its active metabolites have been suggested: alterations of sodium channel and monoamine transporter development, release of epinephrine from the adrenal medulla with subsequent hyperglycemia, vasoconstriction with subsequent hypoxia, and decrease of nutrient supply, calcium ion chelation, superoxide formation, or infarction following repeated ischemia and reperfusion, enzyme inhibition, reduced neurotrophic activity, altered gene expression, and plasma membrane changes (Olsen, 1995). We hypothesized that cocaine administered to sheep during the last half of pregnancy would up regulate drug clearance mechanisms, increasing the rate of elimination. The data from these experiments clearly do not support this hypothesis. Therefore, it is likely that following a single dose to the ewe, the concentration of cocaine in the central nervous system is consistent between animals chronically exposed versus animals receiving their first exposure. This suggests that each dose of cocaine administered to the mother would have a similar toxicological effect on the fetal nervous system.
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NOTES |
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REFERENCES |
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Browne, S. P., Tebbett, I. R., Moore, C. M., Dusick, A., Covert, R., and Yee, G. T. (1992). Analysis of meconium for cocaine in neonates. J. Chromatogr. 575, 158161.[Medline]
Burchfield, D. J. (1995). Effects of cocaine on fetal brain metabolism and behavioral state in the sheep model. NIDA Res. Monogr. 158, 5866.[Medline]
Burchfield, D. J., and Abrams, R. M. (1993). Cocaine depresses cerebral glucose utilization in fetal sheep. Brain Res. Dev. Brain Res. 73, 283288.[ISI][Medline]
Burchfield, D. J., Abrams, R. M., Miller, R., and DeVane, C. L. (1991). Disposition of cocaine in pregnant sheep: II. Physiological responses. Dev. Pharmacol. Ther. 16, 130138.[ISI][Medline]
Burchfield, D. J., Graham, E. M., Abrams, R. M., and Gerhardt, K. J. (1990). Cocaine alters behavioral states in fetal sheep. Brain Res. Dev. Brain Res. 56, 4145.[ISI][Medline]
DeVane, C. L., Burchfield, D. J., Abrams, R. M., Miller, R. L., and Braun, S. B. (1991). Disposition of cocaine in pregnant sheep: I. Pharmacokinetics. Dev. Pharmacol. Ther. 16, 123129.[ISI][Medline]
Downs, T., Padbury, J., Blount, L., Kashiwai, K., and Chan, K. (1996). Ovine fetal-placental cocaine pharmacokinetics during continuous cocaine infusion. J. Soc. Gynecol. Investig. 3, 185190.[ISI][Medline]
Dusick, A. M., Covert, R. F., Schreiber, M. D., Yee, G. T., Browne, S. P., Moore, C. M., and Tebbett, I. R. (1993). Risk of intracranial hemorrhage and other adverse outcomes after cocaine exposure in a cohort of 323 very-low-birth-weight infants. J. Pediatr. 122, 438445.[ISI][Medline]
Frank, D. A., Zuckerman, B. S., Amaro, H., Aboagye, K., Bauchner, H., Cabral, H., Fried, L., Gingson, R., Kayne, H., Levenson, S. M., et al. (1988). Cocaine use during pregnancy: Prevalence and correlates. Pediatrics 82, 888895.[Abstract]
Graham, K., and Koren, G. (1991). Characteristics of pregnant women exposed to cocaine in Toronto between 1985 and 1990. CMAJ 144, 563568.[Abstract]
London, E. D., Wilkerson, G., Goldberg, S. R., and Risner, M. E. (1986). Effects of L-cocaine on local cerebral glucose utilization in the rat. Neurosci. Lett. 68, 7378.[ISI][Medline]
Mets, B., Soo, E., Diaz, J., Pantuck, C., Singh, G., and Blair, I. A. (2000). Chronic continuous cocaine infusion in rats: Effect on urine cocaine, ecgonine methylester, and benzoylecgonine concentrations and bolus-dose cocaine pharmacokinetics. J. Pharm. Pharmacol. 52, 389395.[ISI][Medline]
Moore, T. R., Sorg, J., Miller, L., Key, T. C., and Resnik, R. (1986). Hemodynamic effects of intravenous cocaine on the pregnant ewe and fetus. Am. J. Obstet. Gynecol. 155, 883888.[ISI][Medline]
Olsen, G. D. (1995). Potential mechanisms of cocaine-induced developmental neurotoxicity: A minireview. Neurotoxicology 16, 159167.[ISI][Medline]
Pan, W. J., and Hedaya, M. A. (1999). Cocaine and alcohol interactions in the rat: Effect of cocaine and alcohol pretreatments on cocaine pharmacokinetics and pharmacodynamics. J. Pharm. Sci. 88, 12661274.[ISI][Medline]
Phillips, D. L., Tebbett, I. R., and Bertholf, R. L. (1996). Comparison of HPLC and GC-MS for measurement of cocaine and metabolites in human urine. J. Anal. Toxicol. 20, 305308.[ISI][Medline]
Woods, J. R., Jr., Plessinger, M. A., and Clark, K. E. (1987). Effect of cocaine on uterine blood flow and fetal oxygenation. JAMA 257, 957961.[Abstract]