The only drug that has been licensed for the treatment of preterm labour in the US is ritodrine, a b-2 adrenoceptor agonist. b-2 Adrenoceptors are present on the myometrium and on activation can stimulate relaxation of the muscle. The effectiveness, however, is limited to extending pregnancy by approximately 48 h and there are significant maternal side effects such as tremor, tachycardia, arrhythmias and, in very rare cases, death. A delay in delivery of 48 h is useful as it allows the administration of steroids to the mother, which accelerates lung development of the foetus. In addition to ritodrine, other b-2 agonists have been used off-label in both the US and Europe, the most commonly prescribed being terbutaline. During recent years, the use of b-2 agonists has declined following cautions on the side effects and ritodrine has now been withdrawn in the US.
Various non-licensed drugs are used in both the US and Europe. In the US magnesium sulphate is commonly used. Though there is significant doubt of clinical efficacy , magnesium sulphate is safe if used carefully. The calcium channel blocker nifedipine is used in Europe, though as yet there is very limited data on clinical efficacy and safety. Non-steroidal antiinflammatory drugs have also been shown to have clear efficacy. For example, indomethacin has been demonstrated to extend gestation by >1 week. Use of these drugs, however, is limited by significant foetal side effects such as constriction of the ductus arteriosus, renal impairment and intraventricular haemorrhage .
More recently, a novel form of therapy in the form of oxytocin antagonists has become available. Though currently only an intravenous formulation for acute therapy is available in Europe (but not in the US), this class of drugs offers hope for a more effective treatment for preterm labour.
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