Evaluation of 1-deamino-[D-Tyr(Oethyl)2, Thr4, Orn8] vasotocin, an oxytocin antagonist, in animal models of uterine contractility and preterm labor: a new tocolytic agent

Arrest of mouse preterm labor until term delivery by combination therapy with atosiban and mundulone, a natural product with tocolytic efficacy

There is a lack of FDA-approved tocolytics for the management of preterm labor (PL). In prior drug discovery efforts, we identified mundulone and mundulone acetate (MA) as inhibitors of in vitro intracellular Ca2+-regulated myometrial contractility. In this study, we probed the tocolytic potential of these compounds using human myometrial samples and a mouse model of preterm birth. In a phenotypic assay, mundulone displayed greater efficacy, while MA showed greater potency and uterine-selectivity in the inhibition of intracellular-Ca2+ mobilization. Cell viability assays revealed that MA was significantly less cytotoxic. Organ bath and vessel myography studies showed that only mundulone exerted inhibition of myometrial contractions and that neither compounds affected vasoreactivity of ductus arteriosus. A high-throughput combination screen identified that mundulone exhibits synergism with two clinical-tocolytics (atosiban and nifedipine), and MA displayed synergistic efficacy with nifedipine. Of these combinations, mundulone+atosiban demonstrated a significant improvement in the in vitro therapeutic index compared to mundulone alone. The ex vivo and in vivo synergism of mundulone+atosiban was substantiated, yielding greater tocolytic efficacy and potency on myometrial tissue and reduced preterm birth rates in a mouse model of PL compared to each single agent. Treatment with mundulone after mifepristone administration dose-dependently delayed the timing of delivery. Importantly, mundulone+atosiban permitted long-term management of PL, allowing 71% dams to deliver viable pups at term (>day 19, 4-5 days post-mifepristone exposure) without visible maternal and fetal consequences. Collectively, these studies provide a strong foundation for the development of mundulone as a single or combination tocolytic for management of PL.

Uterine contractions in rodent models and humans

Aberrant uterine contractions can lead to preterm birth and other labour complications and are a significant cause of maternal morbidity and mortality. To investigate the mechanisms underlying dysfunctional uterine contractions, researchers have used experimentally tractable small animal models. However, biological differences between humans and rodents change how researchers select their animal model and interpret their results. Here, we provide a general review of studies of uterine excitation and contractions in mice, rats, guinea pigs, and humans, in an effort to introduce new researchers to the field and help in the design and interpretation of experiments in rodent models.

Contractility Measurements of Human Uterine Smooth Muscle to Aid Drug Development

Discovery and characterization of novel pharmaceutical compounds or biochemical probes rely on robust and physiologically relevant assay systems. We describe methods to measure ex vivo myometrium contractility. This assay can be used to investigate factors and molecules involved in the modulation of myometrial contraction and to determine their excitatory or inhibitory actions, and hence their therapeutic potential in vivo. Biopsies are obtained from women undergoing cesarean section delivery with informed consent. Fine strips of myometrium are dissected, clipped and attached to a force transducer within 1 mL organ baths superfused with physiological saline solution at 37 °C. Strips develop spontaneous contractions within 2-3 h under set tension and remain stable for many hours (>6 h). Strips can also be stimulated to contract such as by the endogenous hormones, oxytocin and vasopressin, which cause concentration-dependent modulation of contraction frequency, force and duration, to more closely resemble contractions in labor. Hence, the effect of known and novel drug leads can be tested on spontaneous and agonist-induced contractions. This protocol specifically details how this assay can be used to determine the potency of known and novel agents by measuring their effects on various parameters of human myometrial contraction. We use the oxytocin- and V1a receptor antagonists, atosiban and SR49059 as examples of known compounds which inhibit oxytocin- and vasopressin-induced contractions, and demonstrate how this method can be used to complement and validate pharmacological data obtained from cell-based assays to aid drug development. The effects of novel agonists in comparison to oxytocin and vasopressin can also be characterized. Whilst we use the example of the oxytocin/ vasopressin system, this method can also be used to study other receptors and ion channels that play a role in uterine contraction and relaxation to advance the understanding of human uterine physiology and pathophysiology.

An evaluation of central penetration from a peripherally administered oxytocin receptor selective antagonist in nonhuman primates

The physiology of the oxytocin receptor has increasingly become a focus of scientific investigation due to its connection with social behavior and psychiatric disorders with impairments in social funciton. Experimental utilization of small molecule and peptide antagonists for the oxytocin receptor has played a role in deciphering these biological and social behavior connections in rodents. Described herein is the evaluation of a potent and selective oxytocin receptor antagonist, ALS-I-41, and details to consider for its use in nonhuman primate behavioral pharmacology experiments utilizing intranasal or intramuscular administration. The central nervous system penetration and rate of metabolism of ALS-I-41 was investigated via mass spectroscopy analysis of cerebrospinal fluid and plasma in the rhesus macaque after intranasal and intramuscular administration. Positron emission tomography was also utilized with [¹⁸F] ALS-I-41 in a macaque to verify observed central nervous system (CNS) penetration and to further evaluate the effects of administration rate on CNS penetration of Sprague-Dawley rats in comparison to previous studies.

Noncompetitive nature of oxytocin antagonists with general structure Mpa(1)Xxx(2)Sar(7)Arg(8)

Eight oxytocin (OT) antagonists with general structure Mpa(1)Sar(7)Arg(8), substituted at position 2 with conformationally constrained and bulky amino acids, were synthesized and pharmacologically tested. Binding affinities and selectivities of compounds for OT, and vasopressin receptor subtypes were investigated. In vitro effects of antagonists were evaluated via inhibition of OT-induced contractions of isolated guinea-pig uterus. The abilities of OT antagonists to inhibit spontaneous contractility in 24 h postpartum rat uterus were investigated. These peptides exhibited pseudoirreversible pharmacological properties, and comprise a novel group of OT antagonists for potential clinical use. Their noncompetitive pharmacological nature can be of therapeutic benefit through a sustained effect on myometrium.

Peptides as receptor ligand drugs and their relationship to G-coupled signal transduction

Peptides act as effector agents that regulate and/or mediate physiological processes, serving as hormones, neurotransmitters and signal transducing factors. The low molecular weight peptides affect receptor-mediated events, which influence cardiovascular, gastrointestinal and neurocranial systems. While some peptides have been marketed as drugs, many have served as leads or templates for the development of non-peptide drugs that mimic peptide actions. This review presents the advantages and disadvantages of using peptides as drugs that bind as ligands to cell-surface receptors and considers their applications in such events. The value of both the peptides and their mimics is based on their participation in the biomodulation of physiological processes, which frequently employ scaffolding proteins acting in a cascading sequence of protein-to-protein interactions. The peptides bind to G-coupled surface receptors to initiate a signal that is transduced to the interior of the cell through multiple layers of phosphorylating enzymes and binding proteins. Peptides have been further employed to identify the molecular targets of signal transduction, the uncoupling of which might provide a means for various disease therapies. The exploitation of such peptide-mediated signal pathways, which are of primary importance to tumour cells, may provide an attractive strategy for anticancer therapy in the future.

Oxytocin-induced stimulation and inhibition of bladder activity in normal, conscious rats--influence of nitric oxide synthase inhibition

The role of the oxytocin-containing projections to the autonomic nuclei of the spinal cord for lower urinary tract function has not been clarified. The hypothesis was tested that oxytocin acts as a mediator of bladder contraction at the spinal cord level. In conscious female rats undergoing continuous cystometry, intrathecal oxytocin (30 ng approximately 30 pmoles) significantly increased micturition pressure (P<0.001), and decreased bladder capacity (P<0.01) and micturition volume (P<0.01). Residual volume increased (P<0.05), and so did the amplitude and frequency of non-voiding contractions (P<0.01). Immediately after administration of oxytocin, the animals showed frequent stretching movements and yawning, and they licked their tails. The effects of oxytocin were dose-dependent; high concentrations (100 ng) were ineffective. Intra-arterial injection of oxytocin (30 ng) near the bladder had no effect. In isolated detrusor strips, oxytocin caused a concentration-dependent contraction; the concentration response curve was concentration-dependently shifted to the right by the oxytocin antagonist, 1-deamino, 2-D-Tyr(OEt), 4-Thr, 8-Orn-OT. Intrathecal injection of the antagonist (500 ng), had per se no effect on micturition. However, when the antagonist was given intrathecally 4-5 min prior to intrathecal oxytocin (30 ng), the effects of oxytocin were reduced or completely prevented. When given after intrathecal administration of the nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine methyl ester, intrathecal oxytocin (30 ng) abolished micturition within 5-7 min; all animals developed overflow incontinence, and paralysis of the hindlimbs. These results suggests that in the rat, oxytocin, released from descending pathways, may act as a modulator of the micturition reflex at the spinal level, and that it may interact with nitric oxide. The physiological implications of the findings remain to be established.

Comparison of in vitro and in vivo inhibitory effects of peptide and nonpeptide oxytocin antagonists on radioligand binding and uterine contractility of rats during pregnancy

OBJECTIVE

Our purpose was to compare the effects of peptidyl and nonpeptidyl oxytocin antagonists on the pregnant rat uterus in relation to the progress of gestation.

STUDY DESIGN

Pregnant rats with gestational ages of 17 and 21 days were used. A saturation binding of tritiated oxytocin to myometrial membrane preparation and its displacement by unlabeled oxytocin and the oxytocin antagonists were examined. The inhibitory effects of peptidyl and nonpeptidyl oxytocin on spontaneous, oxytocin-induced, and prostaglandin F2 alpha-induced uterine contractions were also evaluated in vitro in vivo.

RESULTS

The number of tritiated oxytocin binding sites in myometrial membranes of pregnant rats increased markedly at day 21 of gestation compared with day 17 of gestation, whereas the dissociation constants for tritiated oxytocin did not differ significantly. As for the binding affinities to oxytocin receptors of myometrial membranes, the inhibition constant values of nonpeptidyl oxytocin were 79 and 351 times larger than those of peptidyl oxytocin at pregnancy days 17 and 21, respectively. Both drugs remarkably inhibited oxytocin-induced uterine contractions in a dose-dependent manner. However, peptidyl oxytocin did not effect spontaneous and prostaglandin F2 alpha-induced contractions except for spontaneous ones of rats of pregnancy day 21 in vivo. On the other hand, nonpeptidyl oxytocin suppressed spontaneous and prostaglandin F2 alpha-induced contractions of the uterus both in vivo (pregnancy day 17) and in vitro (pregnancy day 21).

CONCLUSION

These results suggest that peptidyl oxytocin may inhibit uterine contractions by selectively antagonizing the oxytocin action at the receptor site, whereas nonpeptidyl oxytocin at high concentrations may have the additional effect of directly suppressing the contractions. This effect of nonpeptidyl oxytocin may become therapeutically advantageous in clinical application for preterm labor.

The pharmacokinetics of the oxytocin antagonist atosiban in pregnant women with preterm uterine contractions

OBJECTIVE

Our purpose was to evaluate the pharmacokinetics of atosiban, an oxytocin antagonist, during and after intravenous infusion in pregnant patients having at least six contractions per hour. The relationship between atosiban infusion and uterine activity was also assessed.

STUDY DESIGN

Plasma samples from eight pregnant patients treated with intravenous atosiban (300 micrograms/min for 6 to 12 hours) were analyzed for atosiban concentration by a specific radioimmunoassay procedure. Contraction rate data were obtained by external tocodynamometry for 1 hour before the infusion and during the subsequent infusion.

RESULTS

The average steady-state plasma concentrations of patients receiving intravenous atosiban were 442 +/- 73 ng/ml (mean +/- SD), with steady state achieved by 1 hour after the start of the infusion. After the completion of the infusion, plasma concentrations declined rapidly in a biexponential manner with initial and terminal half-life estimates of 13 +/- 3 and 102 +/- 18 minutes, respectively. The effective half-life was 18 +/- 3 minutes. The plasma clearance of atosiban was relatively high (42 L/hr) and the volume of distribution (approximately 18 L) was consistent with distribution into extracellular fluid. Of the seven patients evaluated for uterine activity, the mean contraction rate decreased by 75% during the third hour of treatment and remained low until treatment termination.

CONCLUSION

On the basis of earlier published reports, the pharmacokinetics of atosiban in pregnant patients are similar to those in nonpregnant women. Although the patient population was small, a consistent reduction in uterine activity was observed during atosiban infusion.

Effect of oxytocin antagonists on the activation of human myometrium in vitro: atosiban prevents oxytocin-induced desensitization

OBJECTIVE

Our purpose was to investigate whether the sensitivity of myometrial cells to oxytocin is affected by prolonged exposure to oxytocin antagonists.

STUDY DESIGN

Tissue slices or cultured myometrial cells were exposed to peptides in vitro. Myometrial activation was studied by measuring the formation of inositol phosphates and the changes in intracellular calcium. Oxytocin binding was measured by saturation analysis.

RESULTS

Atosiban and related peptides inhibited oxytocin-induced myometrial activation as pure antagonists (inhibition constant 10 nmol/L) but had no effect on prostaglandin E2-induced activation. Long-term (> or = 24 hours) exposure to atosiban had no residual effect on oxytocin sensitivity. However, long-term exposure to oxytocin resulted in homologous desensitization and loss of oxytocin receptors. Oxytocin-induced desensitization was prevented by coincubation with atosiban.

CONCLUSIONS

Atosiban is a pure oxytocin antagonist and has a specific, reversible effect on myometrial cells in vitro. Its potential use for the management or even prevention of idiopathic preterm labor or to reverse uterine hypertony during oxytocin-induced labor should be tested in controlled clinical trials.

Oxytocin antagonist inhibitory effect on the rat and baboon uterus may be overcome by prostaglandins

OBJECTIVE

A potent, long-acting oxytocin antagonist produced in our laboratory (ANTAG-III) can inhibit uterine response to oxytocin in the rat and baboon for hours and even days. The purpose of this study was to evaluate uterine response to prostaglandins subsequent to the administration of ANTAG-III.

STUDY DESIGN

For the rat study one cannula was inserted in the jugular vein, and another cannula to measure uterine activity was inserted in the uterus. In study 1 saline solution or 5 micrograms of ANTAG-III was administered to five rats each, followed by 100 mU of oxytocin at 0.1, 1, and 2 hours. In study 2 six rats each were infused with saline solution of 5 micrograms of ANTAG-III, followed 1 hour later by 5 micrograms of 15-methyl-prostaglandin F2 alpha and uterine activity monitored. After baseline activity returned to normal 100 mU of oxytocin was infused and the uterine response reassessed. For the baboon study ANTAG-III was administered into the aorta of tethered pregnant baboons (n = 2). An oxytocin challenge test was performed starting with 10 mU/min and going up to 400 mU/min. After a significant uterine contractile response was established and activity returned to baseline, a 15-methyl-prostaglandin F2 alpha challenge test was performed.

RESULTS

During the period in which the response to oxytocin was inhibited the uterine response to 15-methyl-prostaglandin F2 alpha of the estrous rat and pregnant baboon was maintained.

CONCLUSIONS

The inhibition of the estrous rat and pregnant baboon uterus to oxytocin caused by ANTAG-III may be prolonged. During this period uterine response to prostaglandins is not altered.

Oxytocin receptor blockade and prostaglandin release in late pregnant sheep

The oxytocin receptor inhibitor 1-deamino-2-D-Tyr-(oET)-4-Thr-8-orn-oxytocin (CAP) was infused into late pregnant sheep. Basal and oxytocin-induced prostaglandin (PG) concentrations in maternal and fetal plasma were determined. CAP had no significant effect on maternal PGFM or PGE2 or fetal PGF2 alpha, PGFM or PGE2 concentrations during late pregnancy or at term. PGF2 alpha was not detectable in maternal peripheral plasma. CAP infusion did not affect fetal well-being. Oxytocin injection to the mother caused a significant, dose-dependent, increase in maternal plasma PGFM concentrations but did not alter maternal PGE2 concentrations or fetal PGF2 alpha and PGE2 concentrations. The increase in maternal PGFM concentrations brought about by oxytocin injection was decreased during intrauterine infusion of CAP over the range of 12.5-100 micrograms/min. A rationale for the use of oxytocin receptor blockade for the prevention of premature labor is thus provided.

Maternal and fetal cardiovascular effects and placental transfer of the oxytocin antagonist atosiban in late-gestation pregnant sheep

OBJECTIVES

Atosiban is a synthetic oxytocin antagonist that is currently undergoing dose-ranging clinical trials. To date, no data are available on the cardiovascular effects of combined oxytocin and vasopressin blockade during late pregnancy. Our aims were (1) to determine the effects of atosiban infusion on the maternal and fetal cardiovascular system and on uterine blood flow and (2) to determine the maternal pharmacokinetics and the rate of placental transfer of atosiban.

STUDY DESIGN

Five chronically catheterized pregnant sheep were treated with a 2-hour infusion of atosiban (300 micrograms.min-1) at 116 to 126 days' gestation. Maternal and fetal blood pressure and heart rate and uterine blood flow were measured before and during the infusion. Maternal and fetal arterial blood samples were obtained throughout the experiment for measurement of plasma atosiban levels and blood gas values.

RESULTS

No significant change in maternal cardiovascular parameters or uterine blood flow were observed. Similarly, no changes in fetal blood pressure and arterial blood gases were present during the infusion of the atosiban. Maternal plasma levels of atosiban reached a maximum of 585.2 +/- 82.2 (ng/ml mean +/- SD) at the end of the infusion and decreased biexponentially with a mean t1/2 alpha of 17 minutes and a mean t1/2 beta of 2.2 hours. Fetal plasma levels of atosiban were at or below the detection limit.

CONCLUSION

Atosiban does not significantly affect maternal or fetal cardiovascular parameters when it is administered for 2 hours in late-pregnant sheep. Although significant levels were measured in maternal blood, negligible transfer to the fetus occurred.

Oxytocin antagonists in preterm labour and delivery

Evidence has been gained that an oxytocin receptor antagonist given by intravenous infusion effectively stops uterine contractions in threatened as well as in actual preterm labour. The findings suggest that the increase of oxytocin receptors is aetiologically important in uncomplicated preterm labour. Oxytocin antagonists could therefore be an attractive alternative to currently used drugs by virtue of their high specificity and lack of serious side-effects. Their use in prophylactic and maintenance therapy may be greater when modified analogues have been developed that allow non-parenteral therapy.

The effect of the oxytocin antagonists, CAP 476 and F327, on calcium mobilisation in single cultured human myometrial cells

OBJECTIVE

To investigate the mechanism of action of the oxytocin (OT) antagonists, CAP 476 and F327.

DESIGN

A prospective descriptional study.

SUBJECTS

Women undergoing caesarean section at term or hysterectomy.

INTERVENTIONS

Myometrial cells were cultured from uterine biopsies.

MAIN OUTCOME MEASURES

Intracellular calcium ([Ca2+]i), determined in single cells.

RESULTS

Application of OT caused a transient increase in [Ca2+]i. CAP 476 abolished and F327 reduced the response to OT but neither reduced the [Ca2+]i transient induced by cell depolarisation with 120 mmol K+. CAP 476 did not reduce transients caused by prostaglandin E2. F327 reduced the frequency of repetitive [Ca2+]i transients occurring during continuous application of OT.

CONCLUSIONS

The results demonstrate that the antagonists reduce the effect of OT and that their action is relatively specific. Their mechanism of action as clinical tocolytic agents is discussed.

Effects of oxytocin antagonist (dTVT) and ritodrine on spontaneous and oxytocics-induced uterine contractions in pregnant rats

The effect of a recently developed oxytocin antagonist dTVT, i.e. deamino-[2-D-tyrosine(OEt)-4-threonine-8-ornithine] oxytocin on uterine contraction of pregnant rats was studied in vitro. The following results were obtained. 1. dTVT treatment did not affect spontaneous PGE2- or PGF2 alpha-stimulated contraction, while it slightly suppressed PGE1 analogue (Gemeprost)-stimulated contraction of the uterus. 2. Following treatment with dTVT (5-50 micrograms/ml), oxytocin-stimulated uterine contraction was gradually and slowly suppressed, resulting in an attenuation curve. Ritodrine treatment, on the other hand, rapidly suppressed spontaneous uterine contraction as well as contraction stimulated by various oxytocics. Suppression of oxytocin-stimulated uterine contraction by dTVT took much longer (14.8 +/- 1.1 min) to take effect than that by ritodrine (less than 1 min).

Inhibition of oxytocin-induced uterine contractions by an oxytocin antagonist in the pregnant baboon

Uterine contractions were induced with oxytocin in anesthetized pregnant baboons (Papio anubis) at three stages of pregnancy (days 140, 156, and 169; normal gestation length, 184 days). After the contractile activity was greater than two to three contractions every 10 minutes, beta-mercapto-beta, beta-cyclopentamethylenepropionic acid1-[D-Trp2,Phe3,Ile4,Arg8]-oxytocin, a novel oxytocin antagonist produced in our laboratories, was given simultaneously with the oxytocin for 90 minutes. Contractile force (frequency x mean amplitude) was determined for 30 minutes before and for three 30-minute intervals after the oxytocin antagonist was administered. Animals at 140 days' gestation showed a significant (p less than 0.05) decrease in contractile force in the first 30-minute interval after oxytocin antagonist infusion was initiated, whereas those at days 156 and 169 showed decreases (p less than 0.05) at the second 30-minute interval. In addition, in late gestation a higher dose of oxytocin antagonist per unit of oxytocin was required to prevent uterine contractions. In conclusion, these results suggest (1) that an oxytocin antagonist can inhibit oxytocin-induced uterine contractions in the pregnant baboon and (2) that the interval from oxytocin antagonist administration to significant inhibition of uterine contractions appears to increase with advancing gestational age.

A new tocolytic agent: development of an oxytocin antagonist for inhibiting uterine contractions

A potent oxytocin antagonist has been developed and tested on both the rat and human uterus. In the rat the oxytocin antagonist: (1) inhibited in vitro and in vivo uterine contractions in the nonpregnant animal in response to exogenous oxytocin, (2) inhibited milk letdown, and (3) disrupted the progress of labor. In addition, the oxytocin antagonist inhibited the in vitro contractile response to exogenous oxytocin of human myometrial tissue obtained by cesarean section at term. The results of these studies suggest that the oxytocin antagonist can be used to study the role of oxytocin in labor and has the potential of inhibiting preterm labor in humans.

Vasopressin receptors in human pregnant myometrium and decidua: interactions with oxytocin and vasopressin agonists and antagonists

Saturation analysis and competition experiments were performed to identify and characterize [3H]arginine vasopressin binding to human myometrium and decidua in late pregnancy. [3H]Arginine vasopressin bound with affinity similar to that of [3H]oxytocin to both tissues (dissociation constant 1 to 2 nmol/L). The concentration of [3H]arginine vasopressin binding sites was high, particularly in decidua, but in all instances was about 50% to 60% of [3H]oxytocin binding. Analogs with selective oxytocic potency (4-threonine oxytocin, isotocin) had high affinity to both [3H]arginine vasopressin and [3H]oxytocin binding sites, as did analogs with both oxytocic and vasopressor activity (vasotocin). Analogs with selective antidiuretic activity (1-deamino-8-D-arginine vasopressin) showed drastically reduced affinity to [3H]oxytocin binding sites and relatively low but significantly higher affinity to [3H]arginine vasopressin binding sites. A new oxytocin antagonist (RW22164 or [1-deamino-2D-tyrosine-(O-ethyl)-4-threonine-8-ornithine]oxytocin) competitively bound to both binding sites. Its affinity to [3H]oxytocin binding sites was greater than to [3H]arginine vasopressin binding sites whereas the relative affinities of a predominantly vasopressor antagonist [Manning compound) were reversed, suggesting the presence of distinct receptors for oxytocin and arginine vasopressin in pregnant human myometrium and decidua.

Future directions: anti-hormones in reproductive medicine

Anti-hormones are important in reproductive medicine because they are useful tools that teach us about the normal physiological actions of hormones. They also provide effective therapies to control or treat a variety of pathogenic processes. We expect that the future repertoire of anti-hormones will include the paracrine and autocrine regulators of specific cell functions, in addition to the endocrine systems described here. The availability of recombinant DNA expression systems for an ever larger portion of the human genome will surely accelerate the development of novel anti-hormones.