Carbetocin

Carbetocin differs from oxytocin by deamination at position 1, replacement of the disulfide bridge with a carba bond, and O-methylation of tyrosine at position 2. Removing the disulfide makes it resistant to the disulfide-reducing and aminopeptidase degradation that clears oxytocin in minutes, so a single injection produces roughly an hour of sustained uterotonic effect rather than requiring a continuous drip. It binds the same Gq-coupled oxytocin receptor on myometrium, raising intracellular calcium and producing the tetanic then rhythmic contractions that compress the placental bed and stop bleeding. The heat-stable formulation developed with WHO stays effective at 30 degrees C for years, which matters enormously in settings with no reliable cold chain.

Targets: Oxytocin receptor, Uterine myometrium

• · 100 mcg intramuscularly, or 100 mcg intravenously over 1 minute after caesarean delivery. A single dose only; there is no repeat dosing.

Work out your exact syringe units →

• commonNausea and vomiting
• commonAbdominal pain and uterine cramping— Expected pharmacology.
• commonFlushing, headache and tremor
• commonHypotension— More pronounced with rapid intravenous administration; give over a full minute.
• rareWater retention and hyponatraemia— Far less likely than with a prolonged oxytocin infusion because it is a single dose.

• Use before delivery of the infant, which is absolutely contraindicated
• Induction or augmentation of labour
• Known hypersensitivity to oxytocin or carbetocin
• Serious cardiovascular disease
• Epilepsy, where caution is advised
• Severe hepatic or renal impairment

• redundantoxytocin-obstetric — Same receptor and indication; carbetocin replaces the oxytocin infusion rather than supplementing it.
• conflictatosiban — Direct pharmacological antagonists at the oxytocin receptor.

• · Uterine tone and blood loss after delivery
• · Blood pressure and pulse
• · Fluid balance

Prescription obstetric drug in the EU, UK, Canada, Australia and many other countries; not FDA approved in the United States.

• Widmer et al. 2018 NEJM, CHAMPION trial of heat-stable carbetocin versus oxytocin for prevention of postpartum haemorrhage (trial)
• WHO recommendations on uterotonics for the prevention of postpartum haemorrhage (guideline)

Carbetocin is a case study in how much clinical difference three small chemical changes make without touching the pharmacophore. It binds the same Gq-coupled oxytocin receptor on myometrium, activating phospholipase C, generating inositol trisphosphate and releasing calcium from the sarcoplasmic reticulum, with diacylglycerol and PKC sensitising the contractile apparatus. The receptor-level behaviour is oxytocin's. What changed is survival: replacing the disulfide with a carba bond removes the substrate for disulfide oxido-reductase, and deamination removes the aminopeptidase site, so a single injection sustains a plateau for roughly an hour instead of the two to four minute half-life that forces oxytocin into a continuous infusion. Physiologically the effect is biphasic, a tetanic contraction lasting a couple of minutes followed by rhythmic contractions for around an hour, and it is the sustained rhythmic phase that mechanically compresses the spiral arteries of the placental bed. That compression, not clotting, is the actual haemostatic mechanism of the third stage of labour, which is why uterine tone matters more than any coagulation parameter in the first hour after delivery. Myometrial oxytocin receptor density rises enormously across gestation and peaks in labour, so responsiveness is a property of gestational stage rather than of dose, and giving more drug to an under-receptored uterus achieves nothing. The heat-stable formulation matters for a reason that has nothing to do with pharmacology: oxytocin degrades in tropical heat, so in settings without a reliable cold chain a substantial fraction of administered oxytocin is sub-potent, and a molecule that survives 30 degrees C for years fixes a supply-chain problem rather than a receptor problem.

The absolute rule is that carbetocin is never given before the baby is delivered. Giving a sustained uterotonic to a uterus with a fetus still in it causes tetanic contraction and fetal compromise, and this is a contraindication rather than a caution. Second, treating it as a repeatable drug: it is a single dose, the receptor is already occupied for an hour, and a second dose adds nothing while delaying escalation to an agent that would work. Third, rapid intravenous push causing avoidable hypotension in a patient who has just had a spinal. Fourth, using it for induction or augmentation of labour, which it is not licensed for and is not suited to because it cannot be titrated or switched off. Fifth, a supply-chain point that matters in practice: the conventional formulation still needs refrigeration and only the heat-stable version tolerates 30 degrees C, so assuming any carbetocin vial is heat-stable is a mistake. Sixth, it is not FDA approved in the United States at all, so American readers will not encounter it.

1. 100 mcgImmediately after delivery of the infant — 100 mcg intramuscularly as part of active management of the third stage, or 100 mcg intravenously over a full minute after caesarean delivery. A single dose only; there is no repeat dose and no titration. Persistent bleeding is managed by escalating to a different uterotonic class.

• Oxytocin receptor on uterine myometrium — Comparable to oxytocin; numeric affinity not resolved this session.

  Gq-phospholipase C signalling, inositol trisphosphate-mediated calcium release and PKC sensitisation, producing a tetanic contraction within 2 minutes followed by rhythmic contractions for roughly an hour from a single dose.

• Oxytocin receptor on breast myoepithelium — Present

  Milk ejection, as with oxytocin. Not the clinical purpose but not absent either.

• Vasopressin V2 receptor — Weak cross-reactivity, as with oxytocin

  A theoretical water-retention effect, far less relevant than with oxytocin because this is a single dose rather than a multi-hour large-volume infusion.

• Vascular smooth muscle

  Transient vasodilation and hypotension, dose-rate dependent, which is why the intravenous dose must be given over a full minute.

• CHAMPION Phase 3, randomised, double-blind, non-inferiority · 2018

  Prevention of blood loss of 500 mL or more, or use of additional uterotonics, after vaginal birth. Heat-stable carbetocin was non-inferior to oxytocin for the 500 mL endpoint. Non-inferiority was not established for the more severe 1000 mL endpoint, which is a nuance that often gets dropped in summaries.

Uterine contraction begins within about 2 minutes intravenously and 2 to 5 minutes intramuscularly. The tetanic phase lasts a couple of minutes and is followed by rhythmic contractions for roughly an hour, which is the therapeutic window. The plasma half-life of about 40 minutes means the effect has substantially faded by 90 minutes, and the uterus must have achieved durable tone by then. There is no delayed effect and nothing accumulates.

Carbetocin replaces the prophylactic oxytocin dose rather than supplementing it, and giving both is redundant at best. If bleeding continues, the escalation is to a different mechanism, not a second oxytocin-receptor agonist: ergometrine acting on alpha-adrenergic and serotonergic receptors, misoprostol as a prostaglandin E1 analogue, or carboprost as a prostaglandin F2-alpha analogue, each with their own contraindications, ergometrine in hypertension and carboprost in asthma. Tranexamic acid is a genuinely additive partner and the WOMAN trial supports giving it early in established postpartum haemorrhage. Atosiban is a direct oxytocin receptor antagonist and is pharmacologically opposed. In caesarean delivery under spinal anaesthesia, the hypotension of the anaesthetic and of a rapid carbetocin push compound, which is an argument for the slow intravenous administration rather than for a lower dose.

Against oxytocin: one intramuscular injection versus a bolus plus a multi-hour infusion, no pump required, and no cold chain for the heat-stable version. Those are large practical advantages in a busy labour ward and enormous ones in a rural clinic without a reliable fridge. Against that, oxytocin is titratable and switchable-off, which carbetocin is not, and CHAMPION established non-inferiority at the 500 mL threshold but not at 1000 mL. Against ergometrine: ergometrine is cheaper and effective but causes far more nausea and vomiting and is contraindicated in hypertension and pre-eclampsia, which excludes a lot of the population most at risk. Against misoprostol: misoprostol needs no cold chain at all and is oral, which is why it dominates in the most resource-limited settings, but it is less effective than injectable uterotonics and causes fever and shivering. Carbetocin's position is essentially oxytocin's efficacy with misoprostol's logistics.

Not a monthly drug. Priced per single-dose ampoule and used once per delivery. Heat-stable carbetocin was developed under a WHO and Ferring agreement with commitments to affordable pricing in low-income countries; I did not source figures this session.

• ?The three-letter structure is the standard published description of carbetocin, but I did not resolve it from a primary source this session, so verified is false.
• ?Intramuscular bioavailability of around 80 percent is a commonly quoted figure that I did not source in this session; treat it as approximate.
• ?Volume of distribution, clearance and protein binding are not characterised in any source I resolved.
• ?CHAMPION's enrolment, in the region of 30,000 women, was not confirmed against the paper, so participants is null.
• ?The WHO uterotonics recommendation is cited as further reading; I did not confirm its identifiers.
• ?No cost figures were sourced.

• Heat-Stable Carbetocin versus Oxytocin to Prevent Hemorrhage after Vaginal Birth Widmer M, et al., New England Journal of Medicine, 2018 · PMID 29949473

  The CHAMPION trial, a very large WHO-led study that established heat-stable carbetocin as a genuine alternative to oxytocin where the cold chain is unreliable.

• WHO recommendations: uterotonics for the prevention of postpartum haemorrhage World Health Organization, WHO Guidelines, 2018

  Further reading, not verified this session. The guideline that placed heat-stable carbetocin alongside oxytocin as a recommended option in settings where oxytocin quality cannot be assured.