Per Melin

Inhibition of uterine contractions of premature labour with an oxytocin analogue. Results from a pilot study

Summary. A competitive inhibitor of the action of oxytocin on the uterus, l‐deamino‐2‐D‐Tyr‐(OEt)‐4‐Thr‐8‐Orn‐oxytocin, was studied for the first time in 13 patients with established, uncomplicated premature labour. Intravenous infusion of 10–100 μg/min of the analogue was given for 1–10 h and the effect was monitored by external cardiotoco‐graphy. In all women an inhibition of uterine activity was observed, and in the majority of patients infused with 25 μg/min and a total dose of about 5 mg or more of the drug total inhibition of uterine contractions was achieved. There were no effects on the maternal and fetal pulse rates, nor were there any other side‐effects. The results of this preliminary study support the concept of an increased concentration of uterine oxytocin receptors being aetiologically important in uncomplicated premature labour. They also suggest that the present oxytocin antagonist could be an interesting therapeutic alternative in the condition, primarily because of the marked selectivity of its effect.

Receptors for and myometrial responses to oxytocin and vasopressin in preterm and term human pregnancy : effects of the oxytocin antagonist atosiban

OBJECTIVE Our purpose was to study myometrial oxytocin and type V1 vasopressin receptors, the in vitro contractile effects of these hormones, and the influence of an oxytocin antagonist. STUDY DESIGN Women delivered by cesarean section preterm (n = 51) and at term (n = 71), with and without labor contractions, gave myometrium for the estimation of oxytocin and V1 vasopressin receptors. The in vitro myometrial effects of the peptides and the influence on these of the competitive oxytocin receptor blocking agent 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin were also tested. RESULTS The median concentration of oxytocin receptors was 116 fmol/mg protein (range 15 to 372 fmol/mg protein) in patients delivered preterm not in labor, 134 fmol/mg protein (27 to 1421 fmol/mg protein) in the beginning of labor, and 46 fmol/mg protein (9 to 140 fmol/mg protein) in advanced labor. At term the corresponding concentrations were 172 (25 to 629), 223 (24 to 414), and 70 (21 to 92) fmol/mg protein. The concentration of V1 vasopressin receptors also decreased in advanced labor. In advanced labor after oxytocin infusion a reduction in the concentration of the receptor for this hormone was observed, which appeared to be related to the duration and dose of treatment. Oxytocin receptors did not vary between women with different indications for cesarean section. The oxytocin effects in vitro and the degree of inhibition by the antagonist of oxytocin responses correlated with the concentration of oxytocin receptors but not with that of V1 vasopressin receptors. No correlation was seen between the response to vasopressin and concentrations of oxytocin or V1 vasopressin receptors. CONCLUSIONS The effect of oxytocin on the myometrium in pregnancy is mediated by an oxytocin receptor, whereas vasopressin acts on both oxytocin and vasopressin receptors. The initiation of labor both preterm and at term may be primarily related to increased release of oxytocin, which is locally produced in the uterus and not detectable in the plasma, but oxytocin and vasopressin receptors may play a role in the regulation of labor. The analog 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin, which blocks both the oxytocin and the V1 vasopressin receptor, should inhibit labor both preterm and at term, the former confirming results of recent clinical studies in Sweden and the United States.

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.

Uterine blood flow and myometrial activity at menstruation, and the action of vasopressin and a synthetic antagonist

Summary. Local endometrial blood flow was measured by a thermistor technique and myometrial activity by intrauterine pressure recording in 10 women before and during menstruation. The effect of lysine vasopressin infusion (1 pmol/kg body‐weight per min) and of bolus injection of a synthetic oxytocin analogue, l‐deamino‐2‐d‐Tyr(OEt)‐4‐Thr‐8‐Orn‐oxytocin (10 nmol/kg body‐weight), were studied. Spontaneous variations in blood flow were seen synchronous with clearly demarcated uterine contractions, the myometrial activity being significantly increased in early (day – 1 to day +2) compared with late (day +3 to day +5) menstrual phase. The vasopressin infusion decreased blood flow, stimulated uterine activity and caused slight to moderate dysmenorrhoea‐like pain. These effects were completely inhibited by the injection of the oxytocin analogue. In‐vitro studies on uterine arteries confirmed that the analogue also inhibited the vasopressin‐induced constriction of the uterine arteries. This antagonist was more effective than two other analogues, l‐deamino‐2‐d‐Tyr(OEt)‐4‐Val‐8‐Orn‐oxytocin and l‐deamino‐2‐Tyr(OEt)‐oxytocin. The counteracting effect of l‐deamino‐2‐D‐Tyr(OEt)‐4‐Thr‐8‐Orn‐oxytocin on the vasopressin‐induced decrease of blood flow and increase of contractions supports the therapeutic value of the drug in primary dysmenorrhoea and preterm labour.

Oxytocin receptor binding and uterotonic activity of carbetocin and its metabolites following enzymatic degradation

Metabolites of the analogue 1-deamino-1-carba-2-tyrosine(O-methyl)-oxytocin (carbetocin) following incubation with a rat kidney homogenate were isolated and their pharmacodynamic properties investigated. Apart from the parent compound two metabolites were identified namely desGlyNH2-carbetocin (carbetocin metabolite I) and desLeuGlyNH2-carbetocin (carbetocin metabolite II). Both carbetocin, carbetocin metabolite I and carbetocin metabolite II displayed binding affinities to the myometrial oxytocin receptor of a similar magnitude as oxytocin. Carbetocin was found to have agonistic properties on isolated myometrial strips and it was found to exert this effect through generation of inositol phosphates, as is the case for oxytocin. However, maximal contractile effect of carbetocin was approximately 50% lower than that of oxytocin (2.70 +/- 0.12 g compared to 5.22 +/- 0.26 g) and EC50 was approximately ten times higher (48.0 +/- 8.20 nM compared to 5.62 +/- 1.22 nM). Neither carbetocin metabolite I nor carbetocin metabolite II were able to contract isolated myometrial tissue. All three compounds displayed antagonistic properties against oxytocin in vitro, with carbetocin being the strongest inhibitor (pA2 = 8.21) and carbetocin metabolite II (pA2 = 8.01) being stronger than carbetocin metabolite I (pA2 = 7.81). These results indicate that carbetocin is a partial agonist/antagonist to the oxytocin receptor while the two metabolites carbetocin metabolite I and carbetocin metabolite II are pure antagonists. All three analogues bound to the myometrial vasopressin V1 receptor, albeit with much lower affinities than to the oxytocin receptor. Carbetocin metabolite II showed the weakest binding affinity of 33.7 +/- 7.34 nM compared to 7.24 +/- 0.29 nM for carbetocin and 9.89 + 2.80 nM for carbetocin metabolite I. Only carbetocin bound to the renal vasopressin V2 receptor though the binding affinity was very low (61.3 +/- 14.6 nM).

Effects of a Vasopressin Antagonist in Women with Dysmenorrhea

We compared menstrual pain, uterine contractility and blood circulation, and plasma concentrations of vasopressin and prostaglandin F2α metabolite in women with versus without primary dysmenorrhea, and determined the effects of a vasopressin antagonist, 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin (Atosiban), on these parameters. Our results do not support the contention that vasopressin is involved in the etiology of dysmenorrhea, plasma concentrations of vasopressin being similar in dysmenorrheic women and controls, and the vasopressin antagonist Atosiban having no effect on menstrual pain, intrauterine pressure or uterine artery pulsatility index in dysmenorrheic women.

THE EFFECT ON THE HUMAN UTERUS OF TWO NEWLY DEVELOPED COMPETITIVE INHIBITORS OF OXYTOCIN AND VASOPRESSIN

Abstract. in order to develop inhibitors of vasopressin (VP) and oxytocin (OXY) action on uterine activity, 1‐deaminat‐ed vasotocin derivatives with modifications at positions 2, 4 and 8 were developed. Two of the most effective analogues in the rat, l‐deamino‐2‐D‐Tyr(OEt)‐4‐Val‐8‐Orn‐vasotocin (dE‐VVT) and l‐deamino‐2‐D‐Tyr(OEt)‐4‐Thr‐8‐Orn‐vaso‐tocin (dE‐TVT) were now tested on human nonpregnant myometrium obtained at hysterectomy in fertile age and on pregnant myometrial tissue obtained at elective cesarean section. the effect was compared with that of a previously synthesized analogue l‐deamino‐Tyr(OEt)‐oxytocin (dE‐OXY) which has already been tested in nonpregnant and pregnant women in vivo. Both of the new analogues competitively inhibited the action of the posterior pituitary hormones. on the nonpregnant uterus dE‐VVT was about five times and dE‐TVT almost twenty‐five times more potent than dE‐OXY in inhibiting the effects of VP. on pregnant myometrium, dE‐TVT inhibited oxytocin action about as effectively as a five‐fold stronger concentration of dE‐OXY, and dE‐VVT slightly less. A moderate agonistic effect of dE‐OXY on pregnant myometrium was found, whereas it was minimal with dE‐VVT and not detectable at all with dE‐TVT. It appears that these two analogues, particularly dE‐TVT, would be interesting for clinical testing both in dysmenorrhea, where increased VP secretion could be of etiological importance, and in premature labor where an increased myometrial concentration of OXY receptors has been demonstrated.

Vasotocin analogues which competitively inhibit vasopressin stimulated uterine activity in healthy women

Summary. Three analogues of posterior pituitary hormones, 1 ‐ deamino ‐ 2 ‐ D ‐ Tyr(OEt) ‐ 4 ‐ Val ‐ 8 ‐ Om ‐ vasotocin(dE ‐ VVT), l‐deamino‐2‐D‐Tyr(OEt)‐4‐Thr‐8‐Orn‐vasotocin(dE‐TVT) and 1‐deamino‐2‐D‐Tyr(OEt)‐oxytocin(dE‐OXY) were compared for their inhibitory effects on vasopressin (VP)‐induced uterine activity in healthy women. At menstruation, during recording of intrauterine pressure (18 recording sessions in 11 women), intravenous infusion of lysine vasopressin (LVP, 1 ng/min/kg/body weight) induced an increase of the uterine activity and dysmenorrhoea‐like symptoms. Intravenous injections of all analogues (10 μg/kg body weight) caused relief of symptoms and inhibition of uterine activity, dE‐TVT was the most effective and dE‐OXY was least active. With dE‐TVT almost complete inhibition of contractions was seen during the first 10 min after injection. The duration of effect was also greatest with that analogue (40–50 min). Only dE‐OXY had an agonist effect on spontaneous uterine activity. Pharmacokinetic studies of intravenous dE‐TVT (10 ng/kg body weight) showed that the plasma half‐life was approximately 16 min and the clearance 30 1/h. The bioavailability of 100 ng/kg given intra‐nasally was about 5·5%. Further studies are recommended.

Pharmacokinetic properties of the tocolytic agent [Mpa1d‐Tyr(Et)2, Thr4, Orn8]‐oxytocin (antocin) in healthy volunteers

OBJECTIVE The aim of this study was to study the pharmacokinetics of antocin, the tocolytic oxytocin antagonist [Mpa1d‐Tyr2(Et), Thr4, Orn8]‐oxytocin.

Vasopressin effects on isolated non‐pregnant myometrium and uterine arteries and their inhibition by deamino‐ethyl‐lysine‐vasopressin and deamino‐ethyl‐oxytocin

Summary. The contractile effects of lysine‐ (L) and arginine‐ (A) vasopressin (VP) on isolated non‐pregnant myometrium and uterine arteries and the inhibition of these actions by two analogues of posterior pituitary hormones, deamino‐ethyl‐LVP (dE‐LVP) and deamino‐ethyl‐oxytocin (dE‐OXY) were investigated. Both AVP and LVP effectively stimulated the smooth muscle preparations. The threshold dose for both agonists was about 2 ng/ml of bath fluid and a maximal response was obtained with approximately 75 ng/ml. No distinguishable effect was produced by dE‐LVP and dE‐OXY alone, but when given before the agonists in concentrations of 150 or 300 ng/ml, dose‐dependent inhibition of the contractions was seen. The inhibition of the uterine artery responses was always greater than the inhibition of the effects on myometrial activity and dE‐LVP appeared to have stronger antagonistic effects than dE‐OXY on the myometrial responses to the agonists. The inhibition of the myometrial and uterine artery responses to AVP could explain the therapeutic effect of dE‐OXY recently found in primary dysmenorrhoea, where increased VP secretion seems to be of aetiological importance.