A loading model for ritodrine administration in preterm labour

OBJECTIVE

To develop a ritodrine infusion scheme for preterm labour that avoids plasma levels above those needed for tocolysis, requires only one rate adjustment, and is easy to apply in practice.

DESIGN

Prospective study of tocolytic effect and plasma ritodrine concentrations during application of the infusion scheme.

SETTING

High risk labour ward.

SUBJECTS

Consecutive series of 31 women in labour at less than 36 weeks' gestation.

INTERVENTION

Loading dose ritodrine infusion followed, as soon as tocolysis is reached, by a decrease in the infusion rate calculated on the basis of the interval between start of treatment and tocolysis.

RESULTS

Overall, steady state ritodrine levels were nearly identical to those at the time of tocolysis and correlated well with levels anticipated on the basis of our calculation (n = 30; r = 0.91; P < 0.001). Adjustments during steady state were made in 12 women (40%), but in only two of them within 12 h after tocolysis had been reached. Delivery was postponed for more than 48 h in 29 women (93.5%) and beyond 37 weeks' gestation in 19 (61.3%).

CONCLUSION

The loading model is easy to apply, avoids relative overdoses, requires few adjustments, is well tolerated, uses smaller quantities of ritodrine, and results in lower plasma ritodrine concentrations than the conventional infusion scheme.

Currently recommended oral regimens for ritodrine tocolysis result in extremely low plasma levels

OBJECTIVE

Our aim was to compare plasma drug levels in patients receiving ritodrine intravenously with those in patients receiving ritodrine orally at recommended dosages.

STUDY DESIGN

Plasma samples from 20 pregnant patients treated with intravenous ritodrine (50 to 300 micrograms/min), 9 patients treated with oral ritodrine only (60 to 120 mg per 24 hours), and 9 patients treated first with intravenous and subsequently with oral ritodrine were analyzed for ritodrine concentration with the use of high-performance liquid chromatography.

RESULTS

Average plasma ritodrine levels of patients receiving different intravenous dosages ranged from 27.8 +/- 3.5 to 113.3 +/- 38.8 ng/ml. Levels during oral therapy ranged between 9.8 +/- 3.2 and 13.8 +/- 4.4 ng/ml. In both modes of drug delivery, concentrations were significantly correlated with doses. In patients treated first with intravenous ritodrine and subsequently with the oral form, plasma concentrations during oral therapy averaged 27.7% +/- 18.8% of those obtained during intravenous infusion.

CONCLUSION

Subtherapeutic plasma concentrations might be responsible for the failure to demonstrate clinical benefits of oral ritodrine in prevention of recurrent preterm labor. A twofold to threefold increase in the maximum recommended oral dosage of ritodrine should be considered, especially for patients who had previously required relatively high intravenous infusion rates (> 100 micrograms/min).

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.

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.

Comparison of pulsatile and continuous ritodrine administration: effects on uterine contractility and beta-adrenergic receptor cascade

In this study we compare the uterine contractility and beta-adrenergic receptor effects of identical doses of ritodrine administered intermittently or continuously for 24 hours in pregnant sheep. Ritodrine was administered intravenously to five animals as a pulse, 16 micrograms/kg every 1.5 hours, whereas five other animals received ritodrine as a continuous infusion of 0.18 microgram/kg/min. Ritodrine plasma concentrations at steady state were comparable in both groups and averaged 18 ng/ml. Animals receiving ritodrine pulses demonstrated no alteration of myometrial beta-adrenergic receptors or adenylyl cyclase activity, and ritodrine inhibited oxytocin-induced contractility comparably at 4 and 24 hours. Animals receiving ritodrine continuously had a significant decrease in myometrial beta-adrenergic receptors and adenylyl cyclase activity, yet ritodrine inhibition of oxytocin-induced uterine contractility was sustained for 24 hours. Oxytocin receptors were not affected by ritodrine administration and were similar in both groups. At the dose studied, oxytocin-induced contractions are comparably inhibited by ritodrine for 24 hours whether the drug is given continuously or in a pulsatile fashion.

Pharmacokinetics of orally administered ritodrine

The oral dosing regimen for ritodrine was based in large part on kinetic data obtained in nonpregnant subjects. There are limited kinetic data after oral administration of ritodrine in pregnancy. The purpose of the present study was to compare ritodrine kinetics in pregnant and nonpregnant women, evaluate the effect of feeding on ritodrine absorption in pregnant women, and determine if the plasma concentration of ritodrine is proportional to the dose administered in nonpregnant women. Plasma concentrations after a single 20 mg dose of ritodrine were significantly greater in fasting nonpregnant women than in fasting pregnant women. The area under the concentration time curve was 1372 +/- 385 and 1001 +/- 257 ng/ml/min, respectively. In pregnant women ingesting 20 mg of ritodrine, plasma concentrations were not significantly different in the fed or fasted state; plasma concentrations peaked at 11 ng/ml and were less than 3 ng/ml within 4 hours. In nonpregnant subjects the concentration of ritodrine in plasma was proportional to the dose. After ingestion of 10, 20, or 30 mg of ritodrine, the area under the curve was 751 +/- 253, 1372 +/- 385, and 2148 +/- 571 ng/ml/min, respectively. These data indicate that ritodrine concentrations in pregnant women after a 20 mg oral dose are low. Increases in dosage will probably result in proportional increases in plasma concentration. The maximal dose of ritodrine recommended for prevention of recurrent preterm labor should be increased.

Intramuscular versus intravenous ritodrine hydrochloride for preterm labor management

A prospective, interinstitutional comparative trial was undertaken to examine the efficacy, safety, and pharmacodynamics of different administration routes of ritodrine hydrochloride for the management of preterm labor. Forty-five subjects between 20 and 36 weeks' gestation received either intravenous (n = 24) or intramuscular (n = 21) therapy. Successful tocolysis occurred in 14 of 21 (67%) patients in the group treated intramuscularly and in 16 of 24 (67%) patients in the group treated intravenously. A greater mean dose (8.6 versus 3.3 mg/hour) and a higher mean serum concentration (38.9 versus 24.7 micrograms/ml) were needed to achieve successful tocolysis in the intravenous group as compared with the intramuscular group. Patients who did not respond to tocolytic therapy in both groups had levels of ritodrine in the blood either equivalent to or greater than those of subjects who were successfully treated. Analysis of ritodrine levels in the successfully treated intramuscular group demonstrated significant differences in blood levels depending on muscle group used. These differences can be at least partially attributed to higher mean doses administered to patients receiving vastus lateralis injections as compared with those receiving gluteal muscle injections. The results suggest that intramuscular administration of ritodrine is an efficacious and safe route of drug delivery. Additional studies are needed to better define dose-response curves for the intramuscular administration of ritodrine hydrochloride.

Effect of pregnancy on ritodrine pharmacokinetics

Dosing regimens for ritodrine are based in large part on pharmacokinetic studies performed with nonpregnant subjects. Pregnancy is characterized by changes in renal blood flow, plasma volume, protein concentration, and hepatic function. These physiologic changes frequently alter drug pharmacokinetics. To define the effect of pregnancy on ritodrine kinetics, we compared ritodrine pharmacokinetics in four pregnant and four nonpregnant rhesus monkeys. Significant differences were demonstrated in the distribution phase half-life (0.40 +/- 0.08 hours in the pregnant monkeys and 0.21 +/- 0.03 hours in the nonpregnant animals), volume of distribution (1.99 +/- 0.94 L/kg in the pregnant monkeys and 4.75 +/- 0.90 L/kg in the nonpregnant animals), plasma clearance (18.8 +/- 7.1 ml/min/kg in the pregnant monkeys and 27.2 +/- 5.0 ml/min/kg in the nonpregnant animals), and disposition half-life (1.8 +/- 0.4 hours in the pregnant monkeys and 3.3 +/- 0.4 hours in the nonpregnant animals). Pregnant animals receiving ritodrine had higher steady-state plasma concentrations than nonpregnant animals (104 versus 53 ng/ml at an infusion rate of 2 micrograms/kg/min). These data indicate that dosing regimens for ritodrine based on studies of nonpregnant subjects may be subject to considerable error.

A loading-dose infusion scheme for intravenous tocolysis with ritodrine: a pilot study

A loading-dose infusion scheme for intravenous ritodrine therapy was tested in twelve patients with preterm labour. We started with a rather high (386 micrograms/min) infusion rate, but the moment tocolysis was reached this infusion rate was reduced to a level needed to maintain the plasma concentration then found. Plasma samples of ritodrine were taken the moment tocolysis was reached and in the steady state, and compared with each other and with expected and calculated plasma concentrations. In the dynamic loading phase we found a half-life for ritodrine of 1 h. This half-life of 1 h can be explained by cumulation of ritodrine in the central compartment and is therefore called cumulation t1/2. In developing an infusion scheme with a loading dose for ritodrine, this cumulation t1/2 of 1 h should be taken into account.

A new administration form of ritodrine. Ritodrine plasma levels with sustained-release capsules in a steady state

A sustained-release administration form of oral ritodrine was introduced to cope with the problem of the short dosage interval inherent to conventional oral ritodrine administration. The bioavailability of this application form was assessed in a clinical study including 11 patients. With a dosage frequency of only 3 times daily and a dosage form index of 1.8, plasma levels of 14.6-26.5 ng/ml ritodrine (equivalent to an infusion rate of approximately 50 micrograms/min) were obtained without serious cardiovascular or metabolic side effects. It was concluded that with this new formulation clinical applicability of orally administered ritodrine in tocolytic therapy has increased.

Ritodrine pharmacokinetics

The purpose of this study was to add to the limited information available regarding the pharmacology of ritodrine in the peripartum period when treatment fails and labor is not inhibited. Plasma or urine samples from eight parturients and 13 infants were studied; in addition plasma samples at delivery were obtained from a total of 26 mothers and infants. All the mothers received ritodrine in the 24 hours before delivery. Plasma and urine ritodrine (free and conjugated) were determined with HPLC by electrochemical detection. In maternal plasma, an apparent rapid distribution phase with a t1/2 of 32 +/- 21 minutes was followed by a prolonged equilibrium phase with a t1/2 of 17 +/- 10 hours. Seventy-six percent of the ritodrine excreted by the mother was in the form of a conjugate. Ninety percent of the ritodrine excreted by the neonate was also excreted in the form of a conjugate.

Concentrations of ritodrine hydrochloride in maternal and fetal serum and amniotic fluid following intravenous administration in late pregnancy

Seven healthy pregnant volunteers undergoing elective cesarean section at 39-40 weeks of gestation were studied for transplacental passage of ritodrine hydrochloride, which was administered by intravenous infusion at the rate of 72-149 micrograms/min for 161-335 min. The concentrations of ritodrine in the collected maternal and fetal blood and the amniotic fluid were radioimmunoassayed. The levels of ritodrine in maternal serum were between 22.5 and 51.0 ng/ml one hour after the initiation of infusion, 33.7-66.4 ng/ml after 2 h, 18.2-73.6 ng/ml after 4 h and 45.7-189.6 ng/ml at delivery, respectively. The umbilical blood and amniotic fluid concentrations of ritodrine at delivery were between 15.6 and 35.1 ng/ml in arterial blood, 12.5-29.6 ng/ml in venous blood and 10.0-49.1 ng/ml in amniotic fluid. The ratio of umbilical venous blood concentrations to maternal venous concentrations (CV/MV) ranged from 0.066 to 0.544 with the mean of 0.263 +/- 0.063 (M +/- S.E.). The results obtained substantiate the rapid and appreciable transfer of ritodrine to the fetus and amniotic fluid.

Evaluation of the pharmacodynamics and pharmacokinetics of ritodrine when administered as a loading dose. On establishing a potentially useful drug administration regimen in cases of fetal distress

Inhibition of labor during the intrapartum period has been suggested as a method of managing acute fetal distress. In such cases, rapid tocolysis is desirable but, in high doses, beta-adrenergic-receptor agonists, such as ritodrine, may cause severe maternal hypotension that could aggravate the existing fetal distress. We undertook the present study to establish a safe infusion protocol for ritodrine that achieves high plasma concentration rapidly. Twelve nonpregnant female volunteers received, on separate days, three infusions of ritodrine, that is, 1, 2, and 3 mg, during a 2-minute period. The peak plasma concentration measured by high-performance liquid chromatography with electrochemical detection averaged 37, 74, and 100 ng/ml after the 1, 2, and 3 mg doses, respectively. Ritodrine concentrations decreased rapidly and with the 3 mg dose the ritodrine concentration was only 14 ng/ml after 15 minutes. The elimination phase half-life of ritodrine averaged 6.11 hours. None of the doses significantly affected systolic blood pressure but ritodrine increased heart rate and the plasma glucose level and decreased diastolic blood pressure and the plasma potassium concentration. Even at the highest infusion rate, the maximal changes in cardiovascular and metabolic variables were short-lived and clinically modest; heart rate increased 29 bpm, diastolic blood pressure decreased 8 mm Hg, glucose level increased 26 mg/dl, and potassium concentration decreased 0.6 mEq/L. These data indicate that high plasma concentrations of ritodrine can be achieved rapidly without serious side effects.

Maternal and fetal plasma concentrations of ritodrine

Previous studies using other beta-adrenergic drugs for tocolysis suggest that if treatment fails and the patient delivers shortly after the therapy is discontinued, there is a direct correlation between neonatal drug concentration and major neonatal complications. In the present study, the disposition of ritodrine was studied in 28 maternal-infant pairs in whom intravenous ritodrine had been administered for clinical indications. The fetal to maternal ratio of ritodrine was 1.17 +/- 0.48. The concentration of ritodrine in both maternal and umbilical vein was found to vary inversely with the length of time the drug was discontinued before delivery. A stepwise multilinear regression revealed that the maternal ritodrine dose in the 24 hours before delivery and the drug discontinuance to delivery interval were both independently related to umbilical vein ritodrine concentrations. When combined, the two variables explained 52% of the variance in umbilical vein ritodrine levels. The frequency of respiratory distress syndrome was increased in the neonates in whom umbilical vein ritodrine was greater than 10 ng/mL, compared with the groups with umbilical vein levels ranging from 3.0 to 10.0 ng/mL. However, neonates with the highest ritodrine concentration were also of lower gestational age (29.4 versus 33.5 weeks, P less than .05) and thus, had greater inherent risk of prematurity-related complications.

Ritodrine serum levels: influence of dose and route of administration

Serum levels of ritodrine were measured by radio-immunoassay both after single and during chronic administration. In 50 non-pregnant women, the maximum ritodrine levels found after a single dose of 10 mg p.o. were 7.4 +/- 4.7 (ng/ml, +/- SD, n = 10); after 20 mg p.o.: 23.8 +/- 5; after 10 mg i.m.: 30.7 +/- 8.2; after 9.3 mg i.v.: 34.7 +/- 5.5; and after 18 mg i.v.: 75.1 +/- 20.7. Ritodrine determinations during chronic intravenous infusion in 116 pregnant women, at one of the seven infusion rates, showed that the serum steady-state ritodrine levels were proportional to the infusion rates below 400 micrograms/min; above that rate the ritodrine levels flattened out. In 91 pregnant women the ritodrine serum levels were measured at one of a number of different time intervals during chronic oral administration of 5 mg (n = 52) or 10 mg (n = 39) every 4 h. During the chronic oral administration of 10 mg every 4 h, the mean ritodrine levels ranged between 14.3 ng/ml at the peak and 4.6 ng/ml just prior to the subsequent dose. For rapid and controlled tocolysis, intravenous infusion is the most appropriate method; intramuscular administration has no obvious advantage. In this respect, a daily dose of 60 mg, 6 X 10 mg, administered by oral route is probably too low; moreover, the bioavailability of the drug by this route was quite different in two volunteers. Although the absolute bioavailability of the drug was estimated in only a small group of volunteers, consistent results (around 40%) were obtained with the 20-mg oral formulation.

Analysis of ritodrine in serum by high-performance liquid chromatography with electrochemical detection

A sensitive and specific assay for ritodrine in serum was developed using high-performance liquid chromatography (HPLC) with electrochemical detection. Serum samples were alkalinized to pH 9.4 by the addition of a sodium carbonate buffer and extracted with ethyl acetate. The extracts were evaporated to dryness and the residues were reconstituted in the HPLC mobile phase and chromatographed on a octadecylsilane reverse-phase column. The detection of ritodrine was achieved by an electrochemical detector with a glassy carbon electrode. The sensitivity was 0.2 ng for on-column injection. The extraction efficiency was 80%.

Serum levels of ritodrine during oral maintenance therapy

In order to elucidate some aspects of the efficacy of orally administered ritodrine, serum concentrations of ritodrine were determined during intravenous and oral therapy. Ritodrine serum concentrations during steady-state intravenous therapy (200 micrograms/min) ranged from 91.0 to 123.0 ng/ml. Serum concentrations during maintenance therapy with 120 mg ritodrine per os fluctuate between 3.2 and 30.5 ng/ml, depending on the moment of tablet intake. During oral maintenance therapy a striking correlation was found between the maternal heart rate and the serum concentration of ritodrine. The findings suggest that efficacy of orally administered ritodrine is doubtful on pharmacological grounds.

Pharmacodynamics of ritodrine in pregnant women during preterm labor

We evaluated the relationship of ritodrine concentration to several maternal variables and to fetal heart rate in 17 women who received the drug for inhibition of preterm labor. Ritodrine was measured by high-performance liquid chromatography with electrochemical detection. Ritodrine increased maternal and fetal heart rate and decreased serum potassium in a dose-related manner, but wide variability was noted between patients and within individual patients. Tachyphylaxis of the maternal heart rate response to continuing treatment with ritodrine was seen in at least seven women. Maternal blood pressure, serum glucose concentration, and frequency of uterine contractions were changed by ritodrine treatment, but the changes in these variables were not closely correlated to the concentration of ritodrine (r less than or equal to 0.30 in all cases). The maximal infusion rate and the concentration of ritodrine in maternal serum after 4 hours of treatment were significantly (p less than 0.001) correlated with the frequency of uterine contractions prior to treatment. Successful inhibition of labor was achieved with serum concentrations of 15 to 31 ng/ml in 10 of 17 women; in six of the other seven women, labor could not be inhibited in spite of serum concentrations of 90 to 146 ng/ml. Side effects, such as hypotension, vomiting, chest discomfort, and shortness of breath, were most commonly observed when the infusion rate and concentration of ritodrine were increasing.

Effects of maternal ritodrine administration on neonatal renal function

Because of its effects on the cardiovascular and renin-angiotensin systems and on fluid and electrolyte homeostasis, maternal administration of ritodrine to inhibit preterm labor may cause significant alterations in renal function in the newborn infant. We determined inulin clearance, plasma renin activity, urinary arginine vasopressin excretion, and serum and urine electrolyte concentrations and osmolalities at 12 to 36 hours of life and at 6 days of life in 15 infants whose mothers had received ritodrine and in 15 infants whose mothers did not (control infants). At the time of each study, plasma ritodrine concentrations were obtained in the infants whose mothers received ritodrine. The infants whose mothers had received ritodrine had significantly lower inulin clearances and higher plasma renin activity and urinary arginine vasopressin excretion on day 1 but not on day 6. Gestational age was inversely correlated with plasma ritodrine concentration, plasma renin activity, and urinary arginine vasopressin excretion. There were no overt clinical signs of renal failure in any of the infants, and no differences in serum and urine electrolyte values, osmolality, fractional sodium excretion, or urine flow rate were observed between the groups.

Serum level of ritodrine in man

A sensitive radioimmunoassay has been developed for the determination of ritodrine in serum and plasma. Interference by ritodrine metabolites, viz. the sulphate and glucuronide conjugates, was negligible. The sensitivity was 0.3 ng/ml in 0.1 ml plasma or serum. The method was used to determine the serum level of ritodrine in man after oral or parenteral administration. In healthy volunteers intravenous infusion of 9 mg in 1 h resulted in a peak serum level of about 45 ng/ml. In every subject the heart rate varied with time in approximately the same way as the serum level. Intramuscular injection of 10 mg gave a peak level of 20 ng/ml, and after oral administration of 10 mg the maximum peak concentration was 10 ng/ml. In all instances there was a detectable level in serum for up to 25 h after dosing. The bioavailability of the oral formulation was 30% of the parenteral preparations. The effective half-life was 1.3--2 h. Women in pre-term labour were treated by intravenous infusion of ritodrine hydrochloride, and after parturition, ritodrine was determined in serum from the mother and from umbilical cord blood. The results showed that ritodrine crossed the placental barrier and entered the fetal circulation.

Hemodynamic changes in placenta, myometrium and heart after administration of the uterine relaxant ritodrine

The action of the uterine relaxant ritodrine on placental, myometrial and cardiac blood pools has been examined during spontaneous and oxytocin-induced uterine activity. 113mIndium was injected intravenously and the blood pools were measured by recording gamma-activity externally. After i.v. injection of Ritodrine, the typical blood pool changes in labor were inhibited. Thereafter, placental and myometrial blood pools increased more than due to inhibition of labor alone. Simultaneously, there was a corresponding decrease in cardiac blood pool indicating an increase in cardiac output. The rapid inflow of blood in the uteroplacental region after Ritodrine indicates an improved hemodynamic situation. This action could be of benefit in emergency situations in a hyperactive ischaemic uterus during labor.