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Retosiban, GSK221149A



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Retosiban structure.svg

Retosiban, GSK221149A


MW 494.5827, MF C27 H34 N4 O5

Oxytocin antagonist

Threatened pre-term labour


UNII-GIE06H28OX, GSK 221149A,  820957-38-8,



(3R.6R)-3-(2,3-dihvdro-1 H-inden-2-v0-1 -\( R)-1 -(2-methyl-1 ,3-oxazol-4- yl)-2-(4-morpholinyl)-2-oxoethyll-6-r(1S -1-methylpropyn-2.5- piperazinedione

2,​5-​Piperazinedione, 3-​(2,​3-​dihydro-​1H-​inden-​2-​yl)​-​1-​[(1R)​-​1-​(2-​methyl-​4-​oxazolyl)​-​2-​(4-​morpholinyl)​-​2-​oxoethyl]​-​6-​[(1S)​-​1-​methylpropyl]​-​, (3R,​6R)​-

Morpholine, 4-[(2R)-[(3R,6R)-3-(2,3-dihydro-1H-inden-2-yl)-6-[(1S)-1-methylpropyl]-2,5-dioxo-1-piperazinyl](2-methyl-4-oxazolyl)acetyl]-

Retosiban (GSK-221,149-A)[1][2] is an oral drug which acts as a selective, sub-nanomolar (Ki = 0.65 nM) oxytocin receptor antagonist with >1400-fold selectivity[3] over the related vasopressin receptors and is being developed by GlaxoSmithKline for the treatment of preterm labour.[4][5]

Retosibanis an oxytocin (OT) antagonist in phase III clinical trials at GlaxoSmithKline for the prevention of preterm labor. OT antagonism is widely known to inhibit spontaneous uterine contractions.

Retosiban is a diketopiperazine nonpeptide compound with high potency and selectivity for the OT receptor over vasopressin receptors.

This  candidate has been shown to block oxytocin-induced uterine contractions when administered intravenously and to exhibit oral activity

Preterm labor is a major clinical problem leading to death and disability in newborns and accounts for 10% of all births and causes 70% of all infant mortality and morbidity.(Goldenberg, R. L.; Rouse, D.Prevention of premature birth N. Engl. J. Med. 1998, 339, 313)
Oxytocin (OT) is a potent stimulant of uterine contractions and is responsible for the initiation of labor via the interaction with the OT receptors in the mammalian uterus. OT antagonists have been shown to inhibit uterine contractions and delay preterm delivery. So there is increasing interest in OT antagonists because of their potential application in the prevention of preterm labor.
Although several tocolytics have already been approved in clinical practice, they have harmful maternal or fetal side effects.(Enkin, M.; Kierse, M.; Neilson, J.; Preterm Labour: A Guide to Effective Care in Pregnancy and Childbirth, 3rd ed.; Oxford University Press: Oxford, UK, 2000; pp 211225. )
The first clinically tested OT antagonist atosiban has a much more tolerable side effect profile and has recently been approved for use in Europe.
Atosiban SW.svgATOSIBAN

However, atosiban is a peptide and a mixed OT/vasopressin V1a receptor antagonist that has to be given by iv infusion and is not suitable for long-term maintenance treatment, as it is not orally bioavailable.((a) Bossmar, T.Treatment of preterm labor with the oxytocin and vasopressin antagonist atosiban J. Perinat. Med. 1998, 26, 458– 465

See also,(b) Coomarasamy, A.; Knox, E. M.; Gee, H.; Khan, K. S.Oxytocin antagonists for tocolysis in preterm labour—a systematic review Med. Sci. Monit. 2002, 8, RA268RA273)

Hence there has been considerable interest in overcoming the shortcomings of the peptide OT antagonists by identifying orally active nonpeptide OT antagonists with a higher degree of selectivity toward the vasopressin receptors (V1a, V1b, V2) with good oral bioavailability. Although several templates have been investigated as potential selective OT antagonists, few have achieved the required selectivity for the OT receptor vs the vasopressin receptors combined with the bioavailability and physical chemical properties required for an efficacious oral drug.(Borthwick, A. D.Oral Oxytocin Antagonists J. Med. Chem. 2010, 53, 65256538)
Therefore  the objective was to design a potent, orally active OT antagonist with high levels of selectivity over the vasopressin receptor with good oral bioavailability in humans that would delay labor safely by greater than seven days and with improved infant outcome, as shown by a reduced combined morbidity score.
The most potent of these was the 2,4-difluorophenyl dimethylamide 1, which has good in vitro (pKi = 9.2) and in vivo (IC50 = 227 nM) potency and is 20-fold more potent than atosiban in vitro. Compound 1 also has good pharmacokinetics with bioavailability >50% in both the rat and the dog.
Moreover, it is >500-fold selective over all three human vasopressin receptors (hV1aR, hV2R, and hV1bR) and has an acceptable P450 profile. In addition, it has a satisfactory safety profile in the genotoxicity screens and in the four day oral toxicity test in rats.


However, 1 had poor aqueous solubility and high intrinsic clearance in human and cynomolgus monkey liver microsomes, so a compound was required that retained high antagonist potency and excellent pharmacokinetics in animal species seen with 1 but was more soluble and with improved human intrinsic clearance to decrease the risk of low bioavailability in humans.
first approach was to replace the 7-aryl ring with a five-membered heterocycle, which led to the oxazole Retosiban (106) a clinical candidate.(Borthwick, A. D.; Liddle, J.The design of orally bioavailable 2,5 diketopiperazine oxytocin antagonists: from concept to clinical candidate for premature labour Med. Res. Rev. 2011, 31, 576604)
As a backup to 106, an alternative replacement of the 7-aryl ring with a six-membered heterocycle was considered and in this report we describe how we investigated the modification of the 7-aryl ring to the 7(3′-pyridyl) ring and optimized substitution in this ring as well as modifying the isobutyl group to obtain good potency, lower intrinsic clearance in human microsomes, and good pharmacokinetics in animal species.


L-368,899 structure.pngL-368899

L-371,257 structure.pngL-371257


Pyridyl-2,5-diketopiperazines as potent, selective, and orally bioavailable oxytocin antagonists: Synthesis, pharmacokinetics, and in vivo potency
J Med Chem 2012, 55(2): 783


The discovery of GSK221149A: A potent and selective oxytocin antagonist
Bioorg Med Chem Lett 2008, 18(1): 90

Full-size image (4 K)

Full-size image (30 K)


Reagents and conditions: (a) triethylamine, MeOH; (b) H2, Pd/C, ethanol/acetic acid; (c) carbonyl diimidazole, CH2Cl2 3 h then acetone/2 N HCl; (d) benzotriazol-1-yloxytripyrrolidinophosphonium hexafluorophosphate, dichloromethane 1 h then morpholine.

GSK221149A and other tertiary amides were prepared in four steps via the Ugi reaction as outlined in Scheme . A 2:1 mixture of diastereoisomers 24 was formed with the desirable (R)-diastereoisomer being the minor product. Hydrogenation of crude 24 furnished the cyclised phenol 25, again enriched with the undesirable (S)-diastereoisomer.

Activation of the mixture 25 with carbonyl diimidazole followed by the addition of 2 N HCl promoted epimerisation at the exocyclic position and yielded the acids 26 with the required (R)-diastereoisomer as the major product.

Acid activation with benzotriazol-1-yloxytripyrrolidinophosphonium hexafluorophosphate followed by the addition of morpholine and subsequent column chromatography yielded homo-chiral GSK221149A.



WO 2005000840

Example 3

(3R.6R)-3-(2,3-dihvdro-1 H-inden-2-v0-1 -\( R)-1 -(2-methyl-1 ,3-oxazol-4- yl)-2-(4-morpholinyl)-2-oxoethyll-6-r(1S -1-methylpropyn-2.5- piperazinedione ( 2R)-[(benzyloxycarbonyl)amino](2,3-dihydro-1 H-inden-2-yl)ethanoic acid (35.84g, 0.110mol) in a 500mL round bottomed flask was treated with 2,2,2-trifluoroethanol (165mL) followed by methanol (55ml) and triethylamine (11.13g, 15.33mL, 0.110mmol) the slurry was stirred for 3.5hrs until dissolution was observed. The solution was then added to (D)- allo Isoleucine methyl ester hydrochloride (20g, .110mol) in a separate flask. The slurry was stirred until dissolution was observed. 2-methyl-4- formyloxazole (12.24g, 0.110mmol) was then added followed by 2- benzyloxyphenylisocynanide (23.04g, 0.110mmol). The dark brown reaction mixture was then stirred at 20-25°C for 24hrs. The solution was then concentrated to a volume of ca. 130mL by distillation at reduced pressure.

The solution was the diluted with dichloromethane (200mL) and washed with water (2 x 200mL). The organic phase was then diluted with N-methyl pyrrolidinone (460mL) was and the dichloromethane removed by stirring at 40°C under vacuum for 2hrs. Acetic acid 46mL) was then added followed by palladium on carbon catalyst (69. Og of 10% Pd wt, 57% water, Johnson Matthey type 87L) and the mixture hydrogenated under balloon pressure of hydrogen with rapid stirring for 2hrs. The reaction mixture was then filtered, washed through with ethyl acetate (960mL) and washed with 3%w/v aq sodium chloride solution (960mL). The biphasic mixture was filtered and the organic phase separated and washed with 3%w/v aq sodium chloride solution (2 x 960mL). The organic solution was then diluted with ethyl acetate (200mL) and concentrated by distillation at atmospheric pressure by distilling out 385mL of solvent. The concentrated solution at 20-25°C was treated with 1 ,1′-carbonyldiimidazoIe (21.46g, 0.132mol) and stirred at 20-25°C for 1 hr then treated with water (290mL) and stirred rapidly at 20-25°C for 24hr. The mixture was allowed to settle and the ethyl acetate layer separated and discarded. The aqueous phase was washed with ethyl acetate (290mL) and the mixture allowed to settle and the aqueous phase was separated and acidified to pH 1-2 by the addition of concentrated hydrochloric acid (18mL).

The aqueous phase was then extracted into ethyl acetate (290mL and then 145mL). The combined ethyl acetate solution was then concentrated by distillation at atmospheric pressure to a volume of ca. 93mL. This solution was then diluted with tetrahydrofuran (62mL) and treated with triethylamine (11.02g, 15.20mL, 0.109mol) and cooled to -78°C. The solution was then treated with trimethylacetyl chloride (4.81 g, 4.92mL, 39.90mmol) and stirred at – 78°C for 7hr. The reaction mixture was then treated with a solution of morpholine (15.82g, 15.83mL, 0.181 mol) in tetrahydrofuran (23mL) and stirred at -78°C for 1hr 20mins before being allowed to warm to 20-25°C. The solution was then diluted with ethyl acetate (76mL) and washed with saturated aqueous sodium bicarbonate solution (2 x 153mL) followed by water (153mL). The organic solution was then diluted with ethyl acetate (54mL) and distilled down to a volume of 69mL at atmospheric pressure. The solution was then cooled to 20-25°C at which point crystallisation of the title compound occurred. The slurry of was then cooled further to 0°C before the title compound was isolated by filtration and sucked dry. Yield 8.92g.



  • 1  Liddle J, Allen MJ, Borthwick AD, Brooks DP, Davies DE, Edwards RM, Exall AM, Hamlett C, Irving WR, Mason, AM, McCafferty GP, Nerozzi F, Peace S, Philp J, Pollard D, Pullen MA, Shabbir SS, Sollis SL, Westfall TD, Woollard PM, Wu C, Hickey DM (January 2008). “The discovery of GSK221149A: A potent and selective oxytocin antagonist”. Bioorganic & Medicinal Chemistry Letters 18 (1): 90–94. doi:10.1016/j.bmcl.2007.11.008. PMID 18032036.
  • 2
  • Borthwick, A. D.; Liddle, J. (January 2013). “Retosiban and Epelsiban: Potent and Selective Orally available Oxytocin Antagonists”. In Domling, A. Methods and Principles in Medicinal Chemistry: Protein-Protein Interactions in Drug Discovery. Weinheim: Wiley-VCH. pp. 225–256. ISBN 978-3-527-33107-9.
  • 3
  • McCafferty GP, Pullen MA, Wu C, Edwards RM, Allen M.J, Woollard PM, Borthwick AD, Liddle J, Hickey DM, Brooks DP, Westfall TD (March 2007). “Use of a novel and highly selective oxytocin receptor antagonist to characterize uterine contractions in the rat”. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology 293: R299–R305. doi:10.1152/ajpregu.00057.2007. PMID 17395790.
  • 4
  • USAN Council (2007). “Statement on a Nonproprietary Name Adopted by the USAN Council” (PDF).
  • 5  Borthwick AD, Liddle J (July 2011). “The Design of Orally Bioavailable 2,5-Diketopiperazine Oxytocin Antagonists: From Concept to Clinical Candidate for Premature Labour”. Medicinal Research Reviews 31 (4): 576–604. doi:10.1002/med.20193. PMID 20027670.



Abstract Image

A six-stage stereoselective synthesis of indanyl-7-(3′-pyridyl)-(3R,6R,7R)-2,5-diketopiperazines oxytocin antagonists from indene is described. SAR studies involving mono- and disubstitution in the 3′-pyridyl ring and variation of the 3-isobutyl group gave potent compounds (pKi > 9.0) with good aqueous solubility. Evaluation of the pharmacokinetic profile in the rat, dog, and cynomolgus monkey of those derivatives with low cynomolgus monkey and human intrinsic clearance gave 2′,6′-dimethyl-3′-pyridyl Rsec-butyl morpholine amide Epelsiban (69), a highly potent oxytocin antagonist (pKi = 9.9) with >31000-fold selectivity over all three human vasopressin receptors hV1aR, hV2R, and hV1bR, with no significant P450 inhibition. Epelsiban has low levels of intrinsic clearance against the microsomes of four species, good bioavailability (55%) and comparable potency to atosiban in the rat, but is 100-fold more potent than the latter in vitro and was negative in the genotoxicity screens with a satisfactory oral safety profile in female rats.


(3R,6R)-3-(2,3-Dihydro-1H-inden-2-yl)-1-[(1R)-1-(2,6-dimethyl-3-pyridinyl)-2-(4-morpholinyl)-2-oxoethyl]-6-[(1S)-1-methylpropyl]-2,5-piperazinedione (69)

69 as a white solid (2.4 g, 45%). Recystallisation from ethyl acetate/hexane (1:3) gave colorless needles (75%) mp 140 °C. 1H NMR (CDCl3) δ 7.49 (d, J = 7.8 Hz, 1H, pyridyl-4H), 7.26–7.15 (m, 4H, indanyl-arylH), 7.10 (d, J =8.1 Hz, 1H, pyridyl-5H), 6.68 (s, 1H, NCHpyridyl), 6.49 (d, J = 2.8 Hz, 1H, lactam-NH), 4.10 (dd, J = 10.1 Hz, 4.0 Hz, 1H, NCHindanyl), 4.01 (d, J = 4.5 Hz, NCHsec-butyl), 3.75–2.71 (m, 13H, 8× morpholinyl-H, indanyl-3H, –1H, –2H), 2.62 and 2.58 (2s, 6H, pyridyl-2Me,-6Me), 1.64–1.52 (m, 1H, CHHMe), 0.98–0.79 (m, 2H, CHHMe, CHMeCH2), 0.70 (t, J = 7.1 Hz, 3H, CH2Me), 0.45 (d, J = 6.8 Hz, 3H, CHMe). LCMS m/z 519 (MH+) single component, gradient 2 (tR 2.70 min). HRMS calcd for C30H38N4O4 (MH+) 519.29658, found 519.29667. HPLC: 100% (tR 10.388 min).
To a warm solution of 69 (2.66 g, 5.1 mmol) in acetone (40 mL) was added a solution of benzene sulfonic acid (0.81 g, 5.1 mmol) in acetone (40 mL), and the resulting solution was heated to boiling and allowed to cool to room temperature during 48 h. The resulting crystals were filtered off, air-dried on the filter pad to give the besylate (3.214 g, 92.6%) as white crystals of 69B mp 179–183 °C. 1H NMR (CD3OD) δ 8.30 (d, 1H, J = 8.1 Hz, pyridyl-4H), 7.84–7.80 (m, 2H, PhSO3ortho-H), 7.78 (d, J = 8.3 Hz, 1H, pyridyl-5H), 7.45–7.38 (m, 3H, PhSO3meta-H, para-H), 7.23–7.09 (m, 4H, indanyl-arylH), 6.08 (broad s, 1H, NCHpyridyl), 4.00 (d, J = 4.6 Hz, 1H, NCHsec-butyl), 3.92 (d, J = 9.9 Hz, 1H, NCHindanyl), 3.78–3.39 and 3.14–2.80 (m, 13H, 8× morpholinyl-H, indanyl-3H, –1H, –2H)), 2.79 and 2.78 (2s, 6H, pyridyl-2Me, -6Me), 1.85–1.74 (m, 1H, CHHMe), 1.59–1.48 (m, 1H, CHHMe), 1.15–1.01 (m, 1H, CHMeCH2), 0.92 (d, J = 6.3 Hz, 3H, CHMe), 0.85 (t, J = 7.3 Hz, 3H, CH2Me). LCMS m/z 519 MH+ single components, tR 2.72 min; circular dichroism (CH3CN) λmax 225.4 nm, dE −15.70, E15086; λmax 276 nm, dE 3.82, E5172. HRMS calcd for C30H38N4O4 (MH+) 519.2971, found 519.2972. Anal. (C30H38N4O4·C6H6O3S·3.0H2O) C, H, N, S.



Inline image 1

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MOBILE-+91 9323115463
web link
Retosiban structure.svg
Systematic (IUPAC) name
Clinical data
Legal status
  • Non-regulated
CAS number 820957-38-8
ATC code None
PubChem CID 96025669
ChemSpider 23323798
KEGG D08986
Synonyms GSK-221,149-A
Chemical data
Formula C27H34N4O5 
Molecular mass 494.58 g/mol

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DR ANTHONY MELVIN CRASTO, Born in Mumbai in 1964 and graduated from Mumbai University, Completed his Ph.D from ICT, 1991,Matunga, Mumbai, India, in Organic Chemistry, The thesis topic was Synthesis of Novel Pyrethroid Analogues, Currently he is working with AFRICURE PHARMA, ROW2TECH, NIPER-G, Department of Pharmaceuticals, Ministry of Chemicals and Fertilizers, Govt. of India as ADVISOR, earlier assignment was with GLENMARK LIFE SCIENCES LTD, as CONSUlTANT, Retired from GLENMARK in Jan2022 Research Centre as Principal Scientist, Process Research (bulk actives) at Mahape, Navi Mumbai, India. Total Industry exp 32 plus yrs, Prior to joining Glenmark, he has worked with major multinationals like Hoechst Marion Roussel, now Sanofi, Searle India Ltd, now RPG lifesciences, etc. He has worked with notable scientists like Dr K Nagarajan, Dr Ralph Stapel, Prof S Seshadri, etc, He did custom synthesis for major multinationals in his career like BASF, Novartis, Sanofi, etc., He has worked in Discovery, Natural products, Bulk drugs, Generics, Intermediates, Fine chemicals, Neutraceuticals, GMP, Scaleups, etc, he is now helping millions, has 9 million plus hits on Google on all Organic chemistry websites. His friends call him Open superstar worlddrugtracker. His New Drug Approvals, Green Chemistry International, All about drugs, Eurekamoments, Organic spectroscopy international, etc in organic chemistry are some most read blogs He has hands on experience in initiation and developing novel routes for drug molecules and implementation them on commercial scale over a 32 PLUS year tenure till date Feb 2023, Around 35 plus products in his career. He has good knowledge of IPM, GMP, Regulatory aspects, he has several International patents published worldwide . He has good proficiency in Technology transfer, Spectroscopy, Stereochemistry, Synthesis, Polymorphism etc., He suffered a paralytic stroke/ Acute Transverse mylitis in Dec 2007 and is 90 %Paralysed, He is bound to a wheelchair, this seems to have injected feul in him to help chemists all around the world, he is more active than before and is pushing boundaries, He has 100 million plus hits on Google, 2.5 lakh plus connections on all networking sites, 100 Lakh plus views on dozen plus blogs, 227 countries, 7 continents, He makes himself available to all, contact him on +91 9323115463, email, Twitter, @amcrasto , He lives and will die for his family, 90% paralysis cannot kill his soul., Notably he has 38 lakh plus views on New Drug Approvals Blog in 227 countries...... , He appreciates the help he gets from one and all, Friends, Family, Glenmark, Readers, Wellwishers, Doctors, Drug authorities, His Contacts, Physiotherapist, etc He has total of 32 International and Indian awards

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