Atosiban prevents the myometrium from releasing IP3 through oxytocin. Both the intracellular calcium released from myometrial cells' sacroplasmic reticulum and the external Ca2+ influx through voltage-gated channels are decreased. Furthermore, atosiban can prevent decidua from releasing PGE and PGF when oxytocin is present [1].

Atoposiban influences the physiological effects of arginine vasopressin on the fetal-maternal cardiovascular and renal systems. The posterior pituitary hormones oxytocin and arginine vasopressin differ in structure by just two amino acids. A trial using atosiban for one hour did not result in any changes to the cardiovascular systems of the mother or the fetus in late-gestation sheep [1]. In the parabrachial nucleus of mice, atosiban inhibits the activation of neurons that express the oxytocin receptor [2].

Absorption, Distribution and Excretion
In women receiving 300 μg/min by infusion for 6-12 h, average steady state concentrations of 442 ng/mL were reached within 1 h. Steady state concentrations increase proportionally to dosage.
Small amounts of atosiban are found in the urine with 50 times the amount appearing as the large fragment metabolite (des-(Orn⁸, Gly⁹-NH2)-[Mpa¹, D-Tyr(Et)², Thr⁴]-oxytocin. The amount of drug excreted in the feces is not known.
Atosiban has a mean volume of distribution of 41.8 L. Atosiban crosses the placenta and, at a dose of 300 μg/min, was found to have a 0.12 maternal/fetal concentration ratio.
Atosiban has a mean clearance rate of 41.8 L/h.

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Metabolism / Metabolites
There are two metabolites of atosiban created through the cleavage of the peptide bond between ornithine and proline which is thought to be facilitated by prior cleavage of the disulfide bridge. The larger fragment remains active as an antagonist of oxytocin receptors but is 10 times less potent than the parent molecule. At a dosage of 300 μg/min the ratio of parent molecule to the main metabolite was observed to be 1.4 at the second hour and 2.8 at the end of infusion.

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Biological Half-Life
Atosiban does not conform to either 1-compartment or 2-compartment kinetics. It has been determined to have an initial half life (tα) of 0.21 h and a terminal half life (tβ) of 1.7 h.

Protein Binding
Atosiban is 46-48% bound to plasma proteins in pregnant women. It is not known to partition into red blood cells. Differences in the free fraction of drug between maternal and fetal compartments are unknown.

[1]. Sanu O, et al. Critical appraisal and clinical utility of atosiban in the management of preterm labor. Ther Clin Risk Manag. 2010 Apr 26;6:191-9.
[2]. Philip J Ryan, et al. Oxytocin-receptor-expressing Neurons in the Parabrachial Nucleus Regulate Fluid Intake. Nat Neurosci. 2017 Dec;20(12):1722-1733.

Atosiban is an oligopeptide.
Atosiban is an inhibitor of the hormones oxytocin and vasopressin. It is used intravenously to halt premature labor. Although initial studies suggested it could be used as a nasal spray and hence would not require hospital admission, it is not used in that form. Atobisan was developed by the Swedish company Ferring Pharmaceuticals. It was first reported in the literature in 1985. Atosiban is licensed in proprietary and generic forms for the delay of imminent pre-term birth in pregnant adult women.

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Drug Indication
Atosiban is indicated for use in delaying imminent pre-term birth in pregnant adult women with: - regular uterine contractions of at least 30 s duration at a rate of at least 4 per 30 min - a cervical dilation of 1-3cm (0-3cm for nulliparas) and effacement of at least 50% - a gestational age of 24-33 weeks - a normal fetal heart rate
Tractotile is indicated to delay imminent pre-term birth in pregnant adult women with: regular uterine contractions of at least 30 seconds duration at a rate of ≥ 4 per 30 minutes; a cervical dilation of 1 to 3 cm (0-3 for nulliparas) and effacement of ≥ 50%; a gestational age from 24 until 33 completed weeks; a normal foetal heart rate.
Atosiban is indicated to delay imminent pre-term birth in pregnant adult women with: regular uterine contractions of at least 30 seconds' duration at a rate of ≥ 4 per 30 minutes; a cervical dilation of 1 to 3 cm (0-3 for nulliparas) and effacement of ≥ 50%; a gestational age from 24 until 33 completed weeks; a normal foetal heart rate.

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Mechanism of Action
Atosiban is a synthetic peptide oxytocin antagonist. It resembles oxytocin with has modifications at the 1, 2, 4, and 8 positions. The N-terminus of the cysteine residue is deaminated to form 3-mercaptopropanic acid at position 1, at position 2 L-tyrosine is modified to D-tyrosine with an ethoxy group replacing the phenol , threonine replaces glutamine at postion 4, and ornithine replaces leucine at position 8. It binds to membrane bound oxytocin receptors on the myometrium and prevents oxytocin-stimulated increases in inositol triphosphate production. This ultimately prevents release of stored calcium from the sarcoplasmic reticulum and subsequent opening of voltage gated calcium channels. This shutdown of cytosolic calcium increase prevents contractions of the uterine muscle, reducing the frequency of contractions and inducing uterine quiescence. Atosiban has more recently been found to act as a biased ligand at oxytocin receptors. It acts as an antagonist of Gq coupling, explaining the inhibition of the inositol triphosphate pathway thought to be responsible for the effect on uterine contraction, but acts as an agonist of Gi coupling. This agonism produces a pro-inflammatory effect in the human amnion, activating pro-inflammatory signal tranducer NF-κB. It is thought that this reduces atosiban's effectiveness compared to agents which do not produce inflammation as inflammatory mediators are known to play a role in the induction of labour.

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Pharmacodynamics
Atosiban reduces the frequency of uterine contractions to delay pre-term birth in adult females and induces uterine quiescence.

C43H67N11O12S2

994.19

993.441

90779-69-4

Atosiban acetate;914453-95-5

5311010

Typically exists as solid at room temperature

1.3±0.1 g/cm3

1469.0±65.0 °C at 760 mmHg

842.2±34.3 °C

0.0±0.3 mmHg at 25°C

1.549

-3.41

11

15

18

68

1770

9

CC[C@H](C)[C@H]1C(=O)N[C@H](C(=O)N[C@H](C(=O)N[C@@H](CSSCCC(=O)N[C@@H](C(=O)N1)CC2=CC=C(C=C2)OCC)C(=O)N3CCC[C@H]3C(=O)N[C@@H](CCCN)C(=O)NCC(=O)N)CC(=O)N)[C@@H](C)O

VWXRQYYUEIYXCZ-OBIMUBPZSA-N

InChI=1S/C43H67N11O12S2/c1-5-23(3)35-41(63)53-36(24(4)55)42(64)50-29(20-32(45)56)38(60)51-30(43(65)54-17-8-10-31(54)40(62)49-27(9-7-16-44)37(59)47-21-33(46)57)22-68-67-18-15-34(58)48-28(39(61)52-35)19-25-11-13-26(14-12-25)66-6-2/h11-14,23-24,27-31,35-36,55H,5-10,15-22,44H2,1-4H3,(H2,45,56)(H2,46,57)(H,47,59)(H,48,58)(H,49,62)(H,50,64)(H,51,60)(H,52,61)(H,53,63)/t23-,24+,27-,28+,29-,30-,31-,35-,36-/m0/s1

(2S)-N-[(2S)-5-amino-1-[(2-amino-2-oxoethyl)amino]-1-oxopentan-2-yl]-1-[(4R,7S,10S,13S,16R)-7-(2-amino-2-oxoethyl)-13-[(2S)-butan-2-yl]-16-[(4-ethoxyphenyl)methyl]-10-[(1R)-1-hydroxyethyl]-6,9,12,15,18-pentaoxo-1,2-dithia-5,8,11,14,17-pentazacycloicosane-4-carbonyl]pyrrolidine-2-carboxamide

Atosiban RWJ-22164 RW22164 TractocileRWJ22164 RW-22164 TractocileRWJ 22164

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment, avoid exposure to moisture.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

H2O : ~16.67 mg/mL (~16.77 mM)
DMSO : ≥ 16.67 mg/mL (~16.77 mM)

Solubility in Formulation 1: ≥ 1.67 mg/mL (1.68 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 16.7 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 1.67 mg/mL (1.68 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 16.7 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 1.67 mg/mL (1.68 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 16.7 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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 (Please use freshly prepared in vivo formulations for optimal results.)