Retosiban also known as GSK-221,149-A is an oral drug which acts as an

oxytocin receptor The oxytocin receptor, also known as OXTR, is a protein which functions as receptor for the hormone and neurotransmitter oxytocin. In humans, the oxytocin receptor is encoded by the ''OXTR'' gene which has been localized to human chromosome 3p25. ...

antagonist. It is being developed by GlaxoSmithKline for the treatment of

preterm labour Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 weeks, very early preterm birth is between ...

. Retosiban has high affinity for the oxytocin receptor (K_i = 0.65 nM) and has greater than 1400-fold selectivity over the related

vasopressin Human vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the AVP gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It then trave ...

receptors

Mechanism of action

Retosiban is a competitive oxytocin receptor antagonist which blocks the oxytocin-mediated contraction of the uterine smooth muscle in the female uterus that occurs during the initiation of preterm labour. This has been used to prevent preterm labour and

premature birth Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 weeks, very early preterm birth is between 2 ...

Pharmacology

Retosiban has been shown to be an effective

tocolytic Tocolytics (also called anti-contraction medications or labor suppressants) are medications used to suppress premature labor (from Greek τόκος ''tókos'', "childbirth", and λύσις ''lúsis'', "loosening"). Preterm birth accounts for 70% ...

. By intravenous and oral administration it produces a dose-dependent decrease in oxytocin-induced uterine contractions in non-pregnant female rats. In late-term pregnant rats it significantly reduces spontaneous uterine contractions in a dose-dependent manner by intravenous administration. In humans retosiban prolongs pregnancy and reduces preterm birth. Intravenous administration of retosiban in women with spontaneous preterm labour was associated with a greater than 1-week increase in time to delivery compared with placebo, a significant reduction in preterm deliveries, a non-significant increase in uterine quiescence, and a favourable safety profile. The results demonstrate proof-of-concept in the treatment of threatened spontaneous preterm labour

Pharmacokinetics

The oral bioavailability of retosiban is in the order of 100% in the rat with a half life of 1.4 hours. It has low to moderate intrinsic clearance in

microsomes In cell biology, microsomes are heterogeneous vesicle-like artifacts (~20-200 nm diameter) re-formed from pieces of the endoplasmic reticulum (ER) when eukaryotic cells are broken-up in the laboratory; microsomes are not present in healthy, liv ...

from three pre-clinical species (rat, dog, cynomolgus monkey) and low intrinsic clearance in human microsomes. It has a good

cytochrome P450 Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various co ...

(Cyp450) profile with no significant inhibition, with IC50 > 100μM, low protein binding (<80%) and low predicted CNS penetration.

Physical and chemical properties

physiological pH Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

, retosiban exists in an uncharged state. It has good solubility (> 0.22 mg/ml), with a

logd In the physical sciences, a partition coefficient (''P'') or distribution coefficient (''D'') is the ratio of concentrations of a chemical compound, compound in a mixture of two immiscible solvents at partition equilibrium, equilibrium. This rati ...

of 2.2.

Retosiban consists of a central 2,5 diketopiperazine ring with an ''R''- indanyl group at the 3 position and an ''R'' (''S''-secButyl) at the 6 position, both ''cis'' to each other, and with a ''R''-2-methyl

oxazole Oxazole is the parent compound for a vast class of heterocyclic aromatic organic compounds. These are azoles with an oxygen and a nitrogen separated by one carbon. Oxazoles are aromatic compounds but less so than the thiazoles. Oxazole is a weak ...

ring at the 7 position in the acyclic amide attached to the N1-position. Retosiban is the (3''R'', 6''R'', 7''R'')-isomer and is a sub-nanomolar (Ki = 0.65 nM)

antagonist, while the (3''R'', 6''R'', 7''S'')-isomer where the stereochemistry in the amide side-chain at C-7 is inverted, is 10-fold less potent. Typically in this series of 2,5 diketopiperazine oxytocin antagonists the (3''S'', 6''S'', 7''S'') isomer is >500 less active than the (3''R'', 6''R'', 7''R'')-isomer. In addition to the 2,5 diketopiperazine essential core, retosiban also contains several structural characteristics that improve its effectiveness and safety. An indanyl group at position 3 is the best choice in terms of oxytocin receptor antagonist potency, its replacement by phenethyl and benzyl groups led to a progressive weakening of activity. At C-3, a 4-carbon branched alkyl was shown to be preferred with ''R'' (''S''-secButyl) being the best; smaller alkyl groups result in reduced antagonist activity. The 2-methyl oxazole ring at the 7 position gives good aqueous solubility, low protein binding and minimal Cyp450 interaction. This structure–activity relationship (SAR) is supported by the crystal structure of the human oxytocin receptor in complex with retosiban, where the lipophilic indanyl substituent penetrates into a deep, mainly hydrophobic crevice at the bottom of the binding pocket, while the oxazol-morpholine amide moiety is closest to the extracellular surface. The oxazole ring is the most solvent-exposed substituent, and the morpholine ring has no direct interactions with the receptor. The 2,5-diketopiperazine core specifically interacts with the receptor through a polar interaction interface.

Synthesis

Retosiban is a cyclic dipeptide or 2,5-diketopiperazine and these are formed by cyclising the corresponding linear dipeptide. In the short lab-scale and highly stereoselective synthesis of Retosiban 8 the linear peptide 5 is formed by the four-component

Ugi reaction The Ugi reaction is a multi-component reaction in organic chemistry involving a ketone or aldehyde, an amine, an isocyanide and a carboxylic acid to form a bis-amide. The reaction is named after Ivar Karl Ugi, who first reported this reaction in ...

of the

carboxybenzyl Benzyl chloroformate, also known as benzyl chlorocarbonate or Z-chloride, is the benzyl ester of chloroformic acid. It can be also described as the chloride of the benzyloxycarbonyl (Cbz or Z) group. In its pure form it is a water-sensitive oily c ...

(Cbz) protected R-indanylglycine 1, D-alloisoleucine methyl ester hydrochloride 2, 2-methyloxazole-4-carboxaldehyde 3 and 2-benzyloxyphenylisonitrile 4. Hydrogenation to remove the Cbz and benzyl protecting groups, enabled cyclization of the linear peptide 5 to occur to give the phenolic cyclic dipeptide 6. Hydrolysis of the phenolic amide, by reaction with carbonyl diimidazole (CDI), followed addition of aqueous hydrochloric acid gave the acid 7 which was converted to the amide Retosiban 8 by activating the acid with the peptide coupling reagent

PyBOP PyBOP (benzotriazol-1-yloxytripyrrolidinophosphonium hexafluorophosphate) is a peptide coupling reagent used in solid phase peptide synthesis. It is used as a substitute for the BOP reagent - avoiding the formation of the carcinogenic waste product ...

(benzotriazol-1-yloxytripyrrolidinophosphonium hexafluorophosphate) followed by the addition of morpholine. Although the linear peptide 5 and the cyclic dipeptide 6 are a mixture of diastereoisomers (7''RS'') at the exocyclic amide, the hydrochloric acid hydrolysis of the activated phenolic amide caused epimerisation at the exocyclic position and yielded the acid 7 with the required (7''R'')-stereochemistry as the major product. Synthesis of Retosiban

References