Source: Health Products Regulatory Authority (IE) Revision Year: 2022 Publisher: AS Grindeks, Krustpils Iela 53, Riga, 1057, Latvia
Pharmacotherapeutic group: Oxytocin and analogues
ATC code: H01BB02
Oxytocin stimulates contractions (frequency and strength) during labour, accelerates the involution of the uterus and pulls together the myoepithelial cells of the mammary gland, thereby making the emptying process easier. Being synthetic, Ofost does not contain vasopressin and is therefore not blood pressure-raising with the recommended doses, and may therefore be used at pre-eclampsia.
Plasma levels of oxytocin following intravenous infusion at 4 milliunits per minute in pregnant women at term were 2 to 5 microunits/mL.
When intravenous infusion is used the effect occurs gradually, with steady state usually after 20‑40 min.
After intravenous or intramuscular (i.m.) injection, Ofost acts rapidly; in approx. 1 min after i.v. injection and 2‑4 min after i.m. injection. The effect remains for 30‑60 min after i.m. injection and probably for a somewhat shorter period after i.v. injection.
At steady state the distribution volume is approx. 12,2 L or 170 ml/kg in men. Plasma protein binding is low. Oxytocin crosses the placenta in both directions. Oxytocin may be found in small quantities in mother’s breast milk.
The enzyme oxytocinase, a glycoprotein aminopeptidase, is produced during pregnancy. The enzyme is found in plasma and it is capable of degrading oxytocin. The enzymatic activity increases gradually until labour begins it then increases rapidly, and decreases again after delivery. The enzyme activity is also high in the placental and uterine tissues during this period. Liver and kidneys play an important role in metabolising and clearing oxytocin from the plasma. Thus, liver, kidney and systemic circulation contribute to the biotransformation of oxytocin.
Plasma half‑life of oxytocin ranges from 3 to 20 min.. The metabolites are excreted in urine whereas less than 1 % of the given dose is excreted unchanged in the urine. The metabolic clearance is ~17 mL/kg/min in pregnant women. Metabolic clearance is approx. 20 ml/kg/min in both men and non-pregnant women.
No studies have been performed in renal impaired patients. However, considering the excretion of oxytocin and its reduced urinary excretion because of anti-diuretic properties, the possible accumulation of oxytocin can result in prolonged action.
No studies have been performed in hepatically impaired patients. Pharmacokinetic alteration in patients with impaired hepatic function is unlikely since metabolising enzyme, oxytocinase, is not confined to liver alone and the oxytocinase levels in placenta during the term has significantly increased. Therefore, biotransformation of oxytocin in impaired hepatic function may not result in substantial changes in metabolic clearance of oxytocin.
Non-clinical data reveal no special hazard for humans based on conventional studies of safety pharmacology, repeated dose toxicity, genotoxicity and mutagenicity.
No standard teratogenicity, reproductive performance and carcinogenicity studies with oxytocin are available.
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