Intravenous and oral propafenone for treatment of tachycardia in infants and children: pharmacokinetics and clinical response.

To elucidate contribution of an active metabolite to overall clinical responses to propafenone, steady-state disposition of propafenone and its active metabolite and the clinical responses to treatment were examined in pediatric patients receiving intravenous or oral propafenone. There were more than ten-fold interindividual differences in apparent clearance, resulting in a wide range of the steady-state trough plasma concentrations of propafenone. The active metabolite, 5-hydroxypropafenone, was detected in four of the six patients receiving oral propafenone; however, two neonates receiving oral propafenone and all eight receiving intravenous propafenone had no detectable levels of 5-hydroxypropafenone in plasma. In nine patients for whom electrocardiographic (ECG) data were available, the PQ interval was significantly increased, whereas the QRS duration and the QTc interval were not. There was no close relationship between plasma concentrations of propafenone or 5-hydroxypropafenone and ECG parameters. Lack of good correlation between serum concentrations and clinical response precludes using a serum-concentration targeting strategy with propafenone therapy.

Estimation of transplacental and nonplacental diphenhydramine clearances in the fetal lamb: the impact of fetal first-pass hepatic drug uptake.

Previous estimates of maternal and fetal placental and nonplacental clearances in pregnant sheep using a two-compartment open model have revealed higher values of fetal placental clearance (CLfm) compared to the maternal placental clearance (CLmf) for most drugs. This includes the antihistamine diphenhydramine (DPHM), which also has the highest weight-corrected fetal nonplacental clearance (CLfo) among the drugs studied. This study was designed to determine the reasons for this CLfm - CLmf difference and to identify the sites of high CLfo for DPHM. DPHM and a stable isotope-labeled analog, [2H(10)]DPHM, were simultaneously infused to steady state to the mother and fetus, respectively, in five pregnant sheep. CLmf, CLfm, CLmo and CLfo averaged 50.3 +/- 13.2, 214.4 +/- 30.8, 36.6 +/- 1.9 and 109.8 +/- 22.3 ml/min(-1)/kg(-1), respectively. By measuring diphenylmethoxyacetic acid and [2H(10)]diphenylmethoxyacetic acid levels in samples obtained from our previous study of fetal hepatic first-pass DPHM uptake, the hepatic first-pass extraction ratio of the drug from umbilical venous blood was estimated to be 0.44 +/- 0.05. This can account for virtually all of CLfo. Fetal hepatic first-pass uptake of maternally derived DPHM in the paired infusion study reduces the fetal/maternal plasma DPHM concentration ratio and results in significant underestimation of CLmf. When the CLmf estimate is corrected for this factor and for maternal-fetal DPHM plasma protein binding differences, its value approaches CLfm. Fetal hepatic first-pass uptake may also be a factor in the underestimation of CLmf for most of the other drugs. Conversely, a lower value of CLmf compared with CLfm provides evidence for significant fetal hepatic uptake of these compounds.

Hepatic first-pass uptake of diphenhydramine. A comparative study between fetal and adult sheep.

In this study, two factors that could affect fetal drug exposure were examined: 1) the extent of elimination of drug delivered to the fetal liver from the placenta via the umbilical vein; and 2) the degree to which there is preferential distribution of drug in umbilical venous blood to the fetal upper body, as is the case with oxygen and other endogenous substances. Studies were conducted with the histamine H1 antagonist, diphenhydramine (DPHM), in chronically instrumented nonpregnant and pregnant sheep (115-138 days gestation). Hepatic presystemic DPHM elimination was assessed using simultaneous and separate administration of DPHM and stable isotope labeled DPHM ([2H10]DPHM) via the umbilical vein (test route) and abdominal inferior vena cava (control route) in fetal lambs by either bolus injection (N = 6) or 90-min infusion (N = 5), and in nonpregnant sheep (N = 5), by simultaneous and separate bolus injections of the two forms of the drug via the mesenteric vein (test route) and abdominal inferior vena cava (control route). With the bolus injection protocol, hepatic presystemic elimination was estimated by the ratio of the area under the arterial drug concentration vs. time curve for the test and control routes of drug administration, whereas with the fetal infusion study it was calculated as the difference in fetal DPHM clearance values for the test and control routes of administration. To test for isotope effects in the disposition of [2H10]DPHM in the ewe or fetus, both DPHM and [2H10]DPHM were simultaneously injected via the inferior vena cava; however, no isotope effects were noted. In the ewe, there was extensive (93.2 +/- 1.4%) presystemic elimination of DPHM (F 0.068 +/- 0.014). However, in the fetus, this did not occur with bolus drug injection (F = 1.10 +/- 0.07), nor were there differences in the fetal DPHM clearance values estimated from the tarsal and umbilical venous infusions. During the latter experiments, DPHM levels were higher in the femoral artery than in carotid artery, with both umbilical venous and inferior vena caval drug infusion, and this was opposite of the differences in the concentrations of glucose, lactate, and oxygen between these two vessels. Thus, there is extensive hepatic presystemic elimination of DPHM in adult sheep, but no evidence for this phenomenon in the fetus. Furthermore, the preferential distribution of umbilical venous blood to the upper body of the fetal lamb does not result in higher drug levels in ascending aortic blood.

Simultaneous analysis of diphenylmethoxyacetic acid, a metabolite of diphenhydramine, and its deuterium-labeled stable isotope analog in ovine plasma and urine.

Diphenylmethoxyacetic acid (DPMA) is a major metabolite of diphenhydramine in monkeys, dogs, and humans. The metabolic fate of diphenhydramine (DPHM) in sheep is not yet well understood; however, preliminary studies have demonstrated the presence of DPMA in the plasma and urine of sheep following an intravenous bolus of DPHM. Our current studies employ the simultaneous intravenous co-administration of DPHM and the stable isotope analog of DPHM to investigate the pharmacokinetics of DPHM in sheep. In these studies, in order to investigate the pharmacokinetics of the DPMA metabolite, measurement of both unlabeled and stable-isotope labeled DPMA is required. Thus, a stable isotope analog of DPMA ([2H10]DPMA) was synthesized, characterized, and purified for use as an analytical standard. The quantitative method for the gas chromatography-electron-impact mass spectrometry (GC-EI-MS) analysis of DPMA and [2H10]DPMA used a single step liquid-liquid extraction procedure using toluene for sample cleanup. The samples were derivatized with N-methyl-N-(tert.-butyldimethylsilyl) trifluoroacetamide. A 1.0-microliter aliquot of the prepared sample was injected into the GC-MS system and quantitated using selected-ion monitoring (SIM). One ion was monitored for each compound, namely, m/z 165 for the internal standard diphenylacetic acid, m/z 183 for DPMA, and m/z 177 for [2H10]DPMA. The ion chromatograms were free from chromatographic peaks co-eluting with the compound of interest. The calibration curve was linear from 2.5 ng/ml (limit of quantitation) to 250.0 ng/ml in both urine and plasma. The intra-day and inter-day variabilities of this assay method were within acceptable limits (below 20% at the limit of quantitation and below 10% at all other concentrations). This method was used to measure the concentration of DPMA and [2H10]DPMA in plasma and urine samples from a ewe in which equimolar amounts of DPHM and [2H10]DPHM were administered by an intravenous bolus dose via the femoral vein. DPMA appeared to persist longer in the plasma and the urine as compared to DPHM. This method is robust and reliable for the quantitation of DPMA and [2H10]DPMA in biological samples obtained from sheep (e.g. plasma and urine).

New approaches for drug and kinetic analysis in the maternal-fetal unit.

The development of more sensitive and selective analytical techniques has greatly improved scientists' ability to study the PK, PD, and metabolism of drugs in pregnancy. This chapter has chronicled the evolution of the study of drug disposition in pregnant sheep in the laboratory. With the advent of more sensitive and selective analytical techniques, researchers have been able to progress significantly beyond measuring the extent of fetal drug exposure by a single-point determination. A number of interesting and significant advances have been made using highly selective and specific analytical techniques (i.e., gas chromatography-electron-capture detection, gas chromatography-nitrogen phosphoros-specific detection, and gas chromatography-mass spectroscopy). Researchers have characterized drug kinetics in multiple fluids from the pregnant sheep (amniotic fluid, fetal and maternal plasma and urine, and fetal tracheal fluid) and have demonstrated extensive accumulation of several basic drugs in the fluid produced by the fetal lung. With the simultaneous administration of labeled and unlabeled drug to the ewe and fetus researchers have, for the first time, examined the assumption that the clearance parameters determined from the model developed by Szeto and coworkers (1982) are not affected by the time interval between maternal and fetal infusions given on separate days. Also using SIL drug, researchers have characterized fetal hepatic first-pass drug clearance in a chronic preparation. With the establishment of a stereoselective assay for labetalol, scientists have begun to examine the fetal exposure to the individual enantiomers of racemic drugs in pregnancy. The availability of newer analytical tools and techniques (i.e., HPLC interfaced with tandem mass spectrometers) will further expand the study of drugs in pregnancy and challenge the creativity of future investigators in this field.

Determination of metoclopramide and two of its metabolites using a sensitive and selective gas chromatographic-mass spectrometric assay.

A modified gas chromatographic-mass spectrometric (GC-MS) assay has been developed to quantitate metoclopramide (MCP) and two of its metabolites [monodeethylated-MCP (mdMCP), dideethylated-MCP (ddMCP)] in the plasma, bile and urine of sheep. The heptafluorobutyryl derivatives of the compounds were formed and quantitated using electron-impact ionization in the selected-ion monitoring mode (MCP, m/z 86, 380; mdMCP, m/z 380 and ddMCP, m/z 380). No interference was observed from endogenous compounds following the extraction of various biological fluids obtained from non-pregnant sheep. Sample preparation has been simplified and the method is more selective and sensitive (2 fold) than our previous assay using electron-capture detection. The limit of quantitation for MCP, mdMCP and ddMCP was 1 ng/ml in plasma, urine and bile, requiring 0.5 ml of sample. This represents 2.5 pg of the analytes at the detector. The standard curves were linear over a working range of 1-40 ng/ml. Absolute recoveries in plasma ranged from 76.5-94.7%, 79.2-96.8%, 80.3-102.2% for MCP, mdMCP and ddMCP, respectively. In urine, recoveries ranged from 56.5-87.8%, 61.5-87.5%, 62.6-90.2% for MCP, mdMCP and ddMCP, respectively. Recoveries in bile ranged from 83.5-100.9%, 78.5-90.5%, 66.9-79.2% for MCP, mdMCP and ddMCP, respectively. Overall intra-day precision ranged from 2.9% for MCP in plasma to 12.6% for mdMCP in bile. Overall inter-day precision ranged from 5.9% for MCP in urine to 14.9% for ddMCP in bile. Bias was the greatest at the 1 ng/ml concentration in all biological fluids ranging from a low of 2.4% for mdMCP in plasma to a high of 11.9% for ddMCP in urine. Applicability of the assay for pharmacokinetic studies of MCP, mdMCP and ddMCP in the plasma and urine of a non-pregnant ewe is demonstrated.

Simultaneous analysis of diphenhydramine and a stable isotope analog (2H10)diphenhydramine using capillary gas chromatography with mass selective detection in biological fluids from chronically instrumented pregnant ewes.

This report describes both the synthesis of a stable isotope analog of the H1 receptor antagonist diphenhydramine (DPHM), and the simultaneous quantitation of DPHM and a deuterated stable isotope analog of DPHM, viz. (2H10)DPHM in biological fluids from the chronically instrumented pregnant ewe. (2H10)DPHM was synthesized and purified, and both its structure and purity were verified. Biological samples were prepared for analysis using liquid-liquid extraction prior to capillary gas chromatography/mass spectrometry. The method employed electron impact ionization with selective ion monitoring of ions with m/z 165 for DPHM and m/z 173 for (2H10)DPHM. The minimal quantifiable concentration of DPHM and (2H10)DPHM from a 1.0 ml sample was 2.0 ng ml-1 in fetal and maternal plasma, fetal tracheal fluid and amniotic fluid. The method was validated from 2.0 ng ml-1 to 200.0 ng ml-1 for both DPHM and (2H10)DPHM in plasma, fetal tracheal fluid and amniotic fluid. Differences in the disposition between DPHM and (2H10)DPHM were not apparent during a control experiment in which both labeled and unlabeled DPHM were administered to a chronically instrumented fetal lamb. This method provides the required sensitivity and selectivity for the simultaneous quantitation of unlabeled and labeled DPHM during pharmacokinetic experiments conducted in near-term pregnant sheep.

Transplacental and nonplacental clearances, metabolism and pharmacodynamics of labetalol in the fetal lamb after direct intravenous administration.

Labetalol has been previously shown to cause significant maternal and fetal metabolic effects in pregnant sheep after maternal administration. To investigate these observations further, the present study describes the pharmacokinetics, metabolism and pharmacodynamics of labetalol in the fetal lamb after direct fetal i.v. bolus (4 mg) administration. The fetal total body clearance of labetalol (50.45 +/- 1.37 ml m-1 kg-1), which was significantly higher than that previously determined in the ewe, was composed of transplacental and nonplacental CLs of 23.4 +/- 8.99 ml m-1 kg-1 and 27.05 +/- 10.36 ml m-1 kg-1, respectively. The maternal to fetal plasma labetalol area under the curve ratio was 0.031 +/- 0.002 and the CLmp and CLmn were 7.27 +/- 2.11 ml m-1 kg-1 and 30.5 +/- 5.94 ml m-1 kg-1, respectively. Labetalol concentrations in fetal tracheal fluid were consistently higher than that in fetal plasma. The glucuronide conjugate of labetalol was found in the amniotic fluid at up to 20 times the free drug concentration but the oxidative metabolite, 3-amino-1-phenyl-butane, was not detected in plasma or amniotic fluid samples. The fetal effect of labetalol was characterized by an acute lactic acidosis. The calculated hind limb arteriovenous lactate flux showed a net output of lactic acid equal to 3.85 +/- 2.05 g from the hind limb over 24 h after labetalol administration. Although the fetal exposure to labetalol in this study was roughly 4 times that after a 100-mg maternal bolus administration, the magnitude of fetal lactic acidosis was not significantly different in these studies. The clinical implications of the observations made in this study remain to be investigated.

Sensitive high-performance liquid chromatographic method for direct separation of labetalol stereoisomers in biological fluids using an alpha 1-acid glycoprotein stationary phase.

A chiral high-performance liquid chromatographic assay for the separation of the four stereoisomers of labetalol, an antihypertensive, in biological fluids has been developed. Baseline separation of the isomers was achieved using an alpha 1-acid glycoprotein stationary phase. No interference from endogenous substances was observed following extraction from various biological fluids obtained from pregnant (ewe and fetus) and non-pregnant sheep. The concentration of the individual isomers of labetalol was determined by first measuring the total concentration of racemic labetalol obtained from an achiral assay followed by reassay of each sample by the chiral method after which, by using the estimate of the percentage of each individual isomer, the individual concentration of each of the four isomers was determined. The mobile phase was 0.02 M phosphate buffer containing 0.015 M tetrabutylammonium phosphate. The pH of the mobile phase was adjusted to 7.10. The detector was set at an excitation wavelength of 230 nm and emission wavelength of 400 nm to monitor the nascent fluorescence intensity of the isomers of labetalol. The limit of detection of the individual isomers was 0.15 ng (0.6 ng of injected racemic labetalol). The assay was linear over the range 0.6-15.0 ng of labetalol (injected) with the intra- and inter-day mean coefficients of variation being less than 9.0 and 6.0%, respectively. Application of the assay in the study of pharmacokinetics of the stereoisomers of labetalol in sheep following administration of racemic labetalol has been demonstrated.

Disposition, metabolism, and pharmacodynamics of labetalol in adult sheep.

Labetalol causes significant maternal and fetal metabolic effects in pregnant sheep (Yeleswaram et al., J. Pharmacol. Exp. Ther. 262, 683-691 (1992)). This study was undertaken to investigate the contribution of skeletal muscles in the development of metabolic acidosis induced by labetalol and to explore the involvement of active metabolite(s) using conscious, chronically instrumented adult nonpregnant ewes. Following a 100 mg iv bolus, the disposition of labetalol was similar to that observed in pregnant sheep. The effects of labetalol included hypotension, reflex tachycardia, a significant increase in femoral blood flow, hyperglycemia, lactic acidosis, and increased hind limb oxygen consumption. The arteriovenous flux of labetalol, glucose, lactate, and oxygen across the hindlimb was calculated using the Fick principle. The net output of lactate from the hindquarter over 12 hr following drug administration was calculated to be 6.25 +/- 1.35 g (0.07 +/- 0.015 mol). Glucuronidation, sulfation, and oxidative metabolism of labetalol were studied using urine and bile samples. The cumulative urinary excretion of labetalol as unchanged drug, glucuronide and sulfate was found to be 1.61 +/- 0.3, 11.46 +/- 2.83, and 1.47 +/- 0.74% of the dose, respectively. Using GC-mass selective detection, the presence of 3-amino-1-phenylbutane (3-APB), a close congener of amphetamine, in urine and bile samples was established. The cumulative excretion of 3-APB in urine represents 0.044 +/- 0.016% of the dose. Pharmacokinetic analysis shows the apparent elimination half-life of the metabolite to be 13.5 +/- 3.8 min. Conjugates of 3-APB were also found in the bile and urine.(ABSTRACT TRUNCATED AT 250 WORDS)

Transplacental and nonplacental clearances of diphenhydramine in the chronically instrumented pregnant sheep.

Pharmacokinetic studies of the histamine H1-receptor antagonist diphenhydramine were conducted in eight chronically instrumented pregnant sheep at 126-138 days of gestation. Diphenhydramine was administered by simultaneous intravenous bolus injection and infusion to steady state given 48 h apart, to the ewe and the fetus on separate occasions. Average steady-state drug concentration in plasma after maternal infusion was 212.1 +/- 67.8 ng/mL in the mother and 36.3 +/- 14.4 ng/mL in the fetus, resulting in a fetal-to-maternal concentration ratio of 0.19 +/- 0.10. Following fetal infusions, maternal and fetal steady-state drug concentrations were 31.1 +/- 11.6 and 447.6 +/- 185.2 ng/mL, respectively. The free fraction of diphenhydramine determined in the fetus (0.277 +/- 0.087) was significantly greater than that in the mother (0.141 +/- 0.079). Transplacental and nonplacental clearances were calculated at steady state according to a general two-compartment open model, with drug elimination occurring from both compartments. The total fetal clearance (472.7 +/- 215.7 mL/min) was relatively small compared with the total maternal clearance (3426.1 +/- 905.8 mL/min). The transplacental clearance from fetus to mother (264.4 +/- 138.7 mL/min) was approximately threefold higher than that from mother to fetus (82.4 +/- 40.5 mL/min). Maternal nonplacental clearance (3343.8 +/- 890.7 mL/min) accounted for 97.8 +/- 1.1% of the maternal total clearance, whereas fetal nonplacental clearance (208.4 +/- 80.4 mL/min) accounted for 45.1 +/- 4.7% of the fetal total clearance. It is concluded that in the fetus both the transplacental and nonplacental pathways are important for drug elimination.

Identification and quantitation of an oxidative metabolite of labetalol in sheep: pharmacokinetic and metabolic implications.

A sensitive and selective assay has been developed for the identification and quantitation of 3-amino-1-phenyl butane (3-APB), a metabolite of labetalol, in biological fluids using electron impact gas chromatography/mass-selective detection. Samples were extracted with n-hexane, derivatized with heptafluorobutyric anhydride and chromatographed on a cross-linked fused-silica capillary column. A positive EI spectrum was obtained using a mass-selective detector. Identification of the metabolite was accomplished using an authentic standard; quantitation was performed in the selected ion monitoring mode using ions m/z 345 (M+) and 132. The assay was linear over the calibration range of 0.5-1000 ng of the analyte and the intra-sample coefficients of variation were less than 12% in all cases. The absolute recovery of 3-APB following extraction from urine and bile was found to be 102.9 +/- 4.9% and 98.3 +/- 1.45% (mean +/- SEM) respectively. The minimum quantitation limit of the assay was 0.5 ng ml-1 (approximately 2 pg injected). Application of the assay in a pharmacokinetic-pharmacodynamic study of labetalol in sheep is demonstrated. The metabolite was detected in urine and bile samples obtained from adult non-pregnant sheep following labetalol administration. The cumulative amount of 3-APB excreted in urine over 24 h was found to be 71.55 micrograms in one animal following a 100 mg dose of labetalol. Evidence for biliary excretion, glucuronidation and sulfation of 3-APB was also found.

In vitro protein binding of propafenone and 5-hydroxypropafenone in serum, in solutions of isolated serum proteins, and to red blood cells.

The binding of propafenone (PF) and 5-hydroxypropafenone (5-OH-PF) in serum and in solutions of isolated serum proteins was examined by equilibrium dialysis. Both PF and 5-OH-PF displayed pH-dependent binding in serum and in a solution of alpha-1-acid glycoprotein (AAG). PF displayed extensive binding to AAG (i.e., free fraction of 0.08 +/- 0.02), whereas the binding of 5-OH-PF to AAG was moderate (i.e., free fraction of 0.54 +/- 0.10). The removal of lipoproteins from serum did not alter the free fraction of PF but significantly increased the free fraction of 5-OH-PF compared with that in intact serum. Both PF and 5-OH-PF displayed concentration-dependent binding in a 19.3-mumol AAG solution. Concentration-independent binding was apparent in solutions of human serum albumin, high-density lipoproteins, low-density lipoproteins, and very low density lipoproteins over the PF and 5-OH-PF concentration ranges examined. By use of previously determined binding parameters (affinities and capacities), the binding model of PF provided an estimate of the free fraction in serum that was similar to the observed free fraction, although the free fraction of 5-OH-PF was overestimated. The distribution of PF and 5-OH-PF into red blood cells was extensive when buffer was used as the supernatant; however, when serum was used as supernatant, the amounts of PF and 5-OH-PF that were distributed into red blood cells decreased substantially. PF and 5-OH-PF interacted with all of the proteins examined.

Pharmacokinetics and pharmacodynamics of labetalol in the pregnant sheep.

The maternal-fetal disposition of labetalol, a combined alpha-1 and beta adrenergic blocker, and its pharmacodynamics in pregnancy are not well understood. This study describes the pharmacokinetics, cardiovascular and metabolic effects of labetalol in the mother and in utero fetus after a 100-mg maternal i.v. bolus administration, in the chronically instrumented pregnant sheep. Labetalol shows a triexponential decline in the mother with a total body clearance of 30.8 +/- 3.83 ml/min/kg, an apparent steady-state volume of distribution (nonparametric) of 3.02 +/- 0.18 liters/kg and terminal elimination half-life of 2.79 +/- 0.66 hr. These estimates are similar to the reported values in pregnant women. Labetalol rapidly crosses the sheep placenta. The peak fetal plasma concentration was 33.7 +/- 5.8 ng/ml, the fetal exposure to labetalol as calculated by the fetal to maternal area under the curve ratio was 14.37 +/- 1.54% and the apparent fetal elimination half-life was 3.71 +/- 0.5 hr. Labetalol persists in the amniotic and fetal tracheal fluids up to 24 hr with concentrations reaching 2- to 4 times the fetal plasma concentration. Whereas there were no significant maternal or fetal cardiovascular effects, some very significant metabolic effects were observed, including fetal and maternal lactic acidosis and hyperglycemia. Lactic acid accumulates in the fetal blood and amniotic fluid with peak concentrations (6.0 +/- 0.31 and 5.5 +/- 0.26 mM, respectively) showing a more than 300% increase over control values. The exact mechanism by which labetalol causes these metabolic effects is not clear, but it may involve its partial beta-2 agonist activity.

Metabolic effects of ritodrine in the fetal lamb.

Ritodrine infusion to fetal lambs causes numerous metabolic perturbations including hypoxemia. To investigate these changes further and to elucidate a mechanism for the development of hypoxemia, ritodrine was infused at rate of 2.6 micrograms/min into nine chronically catheterized fetal lambs for 8, 12 or 24 hr. Plasma levels of ritodrine (20.0 +/- 2.7 ng/ml) were within the range of those reported in human fetuses exposed to ritodrine tocolysis. Fetal arterial glucose levels nearly doubled (0.72 +/- 0.07 to 1.29 +/- 0.18 mM), whereas lactate levels rose more than 5-fold (1.54 +/- 0.11 to 8.67 +/- 1.12 mM), with the latter change leading to a decline in fetal arterial pH from 7.370 +/- 0.004 to 7.273 +/- 0.033. Fetal oxygen consumption (VO2) rose from 342 +/- 35 to 407 +/- 30 mumol/min.kg via an increase in fetal fractional O2 extraction (32.0 +/- 1.1 to 49.0 +/- 1.7%). The rise in fetal O2 extraction contributed to concurrent declines in fetal arterial PO2 (21.9 +/- 0.6 to 17.0 +/- 0.5 mm Hg) and O2 content (3.7 +/- 0.2 to 2.1 +/- 0.1 mM). Umbilical venous PO2 and O2 content also fell resulting in a decline in fetal O2 delivery (DO2) from 1115 +/- 97 to 838 +/- 68 mumol/min.kg. The rise in fetal VO2 was reflected by a similar rise uterine VO2 (not significant), with the latter being accompanied by a significant increase in uterine O2 extraction and decrease in uterine venous PO2 and O2 content, perhaps contributing to the fall in fetal DO2. In conclusion, fetal hypoxemia during the infusion of ritodrine results from an increase in fetal VO2 that is not compensated for by a similar increase in umbilical or uterine DO2. These metabolic effects may put the fetus at risk, particularly in situations in which fetal DO2 is already reduced, as may occur in compromised pregnancies.

Clearance and disposition of ritodrine in the fluid compartments of the fetal lamb during and after constant rate fetal intravenous infusion.

The disposition of the beta-2 adrenoreceptor agonist, ritodrine, has been examined in the fetal lamb during and after constant rate fetal intravenous infusion (8-24 h). Samples were collected from the fetal femoral artery, umbilical vein, maternal femoral artery, uterine vein, fetal trachea and the amniotic activity for ritodrine quantitation. The amniotic fluid was also analyzed for conjugated metabolites of ritodrine. Ritodrine appears to be cleared across the sheep placenta only to a limited extent (placental clearance 9.2 +/- 2 ml/min/kg; mean +/- S.E.M.). There is, however, significant fetal nonplacental clearance of ritodrine (fetal nonplacental clearance 52.8 +/- 8.0 ml/min/kg). At least part of this clearance appears to be due to fetal drug metabolism, as evidenced by the accumulation of glucuronide conjugates of ritodrine in the amniotic fluid. Ritodrine was also shown to accumulate in the amniotic and fetal tracheal fluids and persist after fetal arterial plasma ritodrine concentrations became undetectable. The accumulation of ritodrine in the tracheal fluid may be of pharmacologic significance, given the well documented, potent effects of beta-2 agonists on fetal lung function and development.

Determination of ritodrine in biological fluids of the pregnant sheep by fused-silica capillary gas chromatography using electron-capture detection.

A sensitive and selective gas chromatographic assay method employing splitless injection, fused-silica capillary columns and electron-capture detection is reported for the quantitation of the tocolytic drug, ritodrine, in a variety of biological fluids obtained from the pregnant ewe and fetus. This method has improved sensitivity and selectivity over previously published assay procedures. A 25 m x 0.31 mm I.D., cross-linked 5% phenylmethylsilicone, fused-silica capillary column was employed for all analyses. Linearity of response was observed over the range 2.5-75 ng of ritodrine base per 0.05-0.5 ml of biological fluid, representing approximately 1-75 pg at the detector. The coefficient of variation was less than 10% over the range 2.5-75 ng of added ritodrine. The minimum quantifiable amount is approximately 2.5 ng from a 0.5-ml biological fluid sample. Applicability of this method to biological fluids, obtained from ovine subjects, is demonstrated by the analysis of samples obtained during the course of ritodrine placental transfer studies.

Sensitive microbore high-performance liquid chromatographic assay for labetalol in the biological fluids of pregnant sheep.

A rapid and sensitive microbore high-performance liquid chromatographic (HPLC) assay is reported for the quantitation of labetalol, an anti-hypertensive agent, in small volumes (250 microliters) of biological fluids (viz., maternal plasma, fetal plasma, amniotic fluid and fetal tracheal fluid) obtained from the chronically instrumented pregnant sheep. Labetalol was extracted from the samples using ethyl acetate and then partitioned into dilute phosphoric acid. Chromatography was performed on a microbore HPLC system using a 2.1 mm I.D. C18 column and detection was accomplished by a low-dispersion fluorescence detector designed for trace analysis. The drug was well separated from endogenous substances in all biological fluids sampled. The calibration curves were linear for all fluids over the range of study with mean coefficients of variation consistently below 5%. Quantitation was possible down to approximately 30 pg of labetalol injected (approximately 1.6 ng/ml in plasma using 250 microliters).

Drug disposition and effects in the fetus.

Surveys of drug use in pregnancy demonstrate that a significant proportion of human fetuses are exposed to prescription and non-prescription drugs antenatally or during labor, although recently a decrease in licit drug consumption during pregnancy may have occurred. However, legitimate medical reasons for drug therapy of the mother or fetus remain and there is an increasing problem with illicit drugs. Several approaches are used to examine placental transfer of and fetal exposure to drugs, including studies on pregnant women, the use of chronically catheterized pregnant animals (particularly sheep), acute studies on small animals (e.g. guinea pig and rabbit) and the use of the perfused placenta. Fetal drug exposure is affected by large number of maternal, placental and fetal factors. For long term maternal drug administration, drug binding in maternal and fetal plasma, and fetal drug elimination seem particularly important. Thus, the rate of non-placental clearance of morphine, methadone, acetamionophen, metoclopramide, ritodrine and diphenhydramine in the fetal lamb is comparable to that in the ewe, although the routes of fetal elimination are not fully elucidated. The fetal liver may be of lesser importance in overall drug elimination than in the adult, but conjugated drug metabolites in the fetus may persist because of limited placental transfer. Hence, we have observed that the glucuronide conjugate of ritodrine accumulates in amniotic fluid after intravenous ritodrine administration. Accumulation of intact drugs in amniotic fluid also occurs via fetal renal excretion and perhaps via the fetal membranes. Also, a number of basic amine drugs are also concentrated in fetal tracheal fluid, probably as a consequence of pulmonary uptake, which also takes place in the adult. However, this could result in high fetal lung levels of agents, such as beta 2-adrenergic agonists, that have potent effects on pulmonary function and maturation. When digoxin, metoclopramide and diphenhydramine are injected into amniotic fluid, they are preferentially distributed to the fetus and, for the latter 2 drugs, uptake by the chorioallantoic membranes appears to be important. Thus, there is the possibility of recycling of drugs from amniotic fluid to the fetal circulation and their persistence in the fetus. Many of the therapeutic agents given to pregnant women in late pregnancy have effects on fetal CNS, cardiovascular or metabolic functions. Alterations in fetal behaviour are elicited by prescription sedatives and anesthetics and illicit drugs, and also by the antihistamine, diphenhydramine, present in various nonprescription medications.

Fetal and maternal placental and nonplacental clearances of metoclopramide in chronically instrumented pregnant sheep.

The placental and nonplacental clearances of metoclopramide were studied in nine chronically instrumented, near-term pregnant sheep using a two-compartment open model. Metoclopramide was administered to the ewe and fetus on separate occasions as an initial iv bolus loading dose followed by a constant-rate infusion, with steady-state maternal and fetal plasma concentrations being obtained by 45 min. Following the maternal infusions, metoclopramide reached average steady-state concentrations of 50.0 +/- 20.2 ng/mL in the ewe and 27.1 +/- 8.6 ng/mL in the fetus, with a mean fetal-to-maternal concentration ratio of 0.57 +/- 0.14. The ability of the fetus to eliminate metoclopramide by nonplacental routes appears to be responsible for this ratio being less than unity, rather than differential protein binding and ion-trapping effects. Mean steady-state concentrations were 13.8 +/- 4.5 and 253.7 +/- 92.1 ng/mL in the ewe and fetus, respectively, after fetal drug administration. Metoclopramide was bound significantly less to fetal (39.5 +/- 8.9%) than to maternal (49.5 +/- 7.9%) plasma proteins, with values similar to that reported for humans (approximately 40%). Clearance of metoclopramide across the placenta from the fetus to the ewe (6.2 +/- 2.4 L/h/kg) was significantly greater than that in the reverse direction (4.3 +/- 1.3 L/h/kg) and accounted for approximately 80% of total fetal drug elimination. This may be explained by the higher percentage of fetal cardiac output to the placenta and the flow-limited transfer of this compound.(ABSTRACT TRUNCATED AT 250 WORDS)