Comparison of Oxytocin and Oxytocin Antagonist Metabolism in the Plasma of Pregnant Humans and Baboons

Design of Oxytocin Analogs

The neurohypophyseal hormone oxytocin (OT) and related modulators of the oxytocin receptor (OTR) have been the subject of intensive research for nearly seven decades. Despite having rather poor drug-like properties, OT is used as a treatment for labor induction, postpartum hemorrhage, and lactation support. The potential use of OT in the treatment of central nervous system (CNS)-related diseases has recently renewed interest in the pharmacology of OT. Oxytocin is one of the most extensively studied cyclic peptides and since the elucidation of its structure in 1953 thousands of peptidic OT analogs with antagonistic and agonistic properties have been synthesized and biologically evaluated. Among them are atosiban, a mixed oxytocin receptor (OTR)/vasopressin 1a receptor (V_1aR) antagonist used as a tocolytic agent approved (in certain countries), and carbetocin, a longer acting OTR agonist on the market for the treatment of postpartum hemorrhage. Many other OT analogs with improved pharmacological properties (e.g., barusiban, Antag III) have been identified. These peptides have been tested in clinical trials and/or used as pharmacological tools. In this chapter, the modifications of the OT molecule that led to the discovery of these compounds are reviewed.

Barusiban, an effective long-term treatment of oxytocin-induced preterm labor in nonhuman primates

Preterm labor (PTL) affects up to 25% of human pregnancies in developing countries, but there are few therapeutic options. Based on the key role of oxytocin (OXT) in labor and parturition, OXT antagonists are a potentially useful class of drugs for PTL. Barusiban is a new selective, potent, and long-acting OXT receptor antagonist. In this study barusiban was given by continuous i.v. infusion to monkeys during the last 3 wk of pregnancy; the monkeys were also given daily doses of OXT to induce uterine contractions and simulate PTL. Barusiban effectively suppressed OXT-induced PTL-like contractions and prevented early delivery. In contrast, fenoterol (a beta2-adrenoceptor [beta2-AR] agonist used as a comparative control) did not inhibit uterine contractions in this model. Barusiban was particularly effective in maintaining low intrauterine pressure (IUP) near the end of pregnancy, which is when IUP in both OXT controls and fenoterol-treated females increased substantially. Although barusiban delayed the onset of labor, it did not prevent normal delivery. These data demonstrate the safety and efficacy of barusiban in reducing uterine contractility in response to repeated OXT challenge, and suggest that barusiban may be therapeutically effective in long-term treatment of PTL.

Telemetric uterine contraction model in preterm cynomolgus monkeys

A telemetric-based model is presented for evaluation of uterine contractions and preterm labor (PTL) in pregnant cynomolgus monkeys. The model allows continuous monitoring of electromyography (EMG) and intrauterine pressure (IUP) as indicators of uterine activity. A pressure sensor was implanted into the amnion of pregnant monkeys on gestational day (GD) 120 +/- 3 and biopotential sensors were attached to the uterus. A telemetry transmitter was placed in a subcuticular pocket located in the flank. Venous catheters were tethered to the next room for dosing and blood sampling without disturbing the conscious animals. EMG and/or IUP were monitored continuously post-operatively. IUP is a reliable parameter for monitoring intrauterine activity, as demonstrated by a close relationship between bursts of activity in the EMG and increases in IUP. Animals close to term showed a basal level of uterine activity during the daytime, with irregular contractions of <10 mmHg. In the night, spontaneous contractions (10-40 mmHg; maximum between 18:00 and 01:00 h) appeared every 3-6 min. Artificial contractions of 15-40 mmHg that mimicked preterm labor were induced at any time of the day by infusion of 5-60 mU oxytocin (OT) per kilogram per hour. These contractions showed a dose-dependent response to OT, and were stable for up to 14 h of constant infusion of OT. Following withdrawal of oxytocin, contractions returned to baseline within 1h. No desensitization of oxytocin-induced contractions was observed when oxytocin was administered daily for up to several weeks. This telemetric model characterizes uterine contractions in non-human primates and provides an excellent method to evaluate pharmacological characteristics of drug candidates intended to treat PTL.