The drug interferes with the action of a hormone called oxytocin, which has a key role in the ordinary birth process, but is also implicated in the onset of premature labour.
More than 700 chemists at the Royal Society of Chemistry's annual meeting in Liverpool yesterday were told that Merck, one of the world's largest pharmaceuticals companies, is 'cautiously optimistic' that trials of the drug will lead to an effective treatment.
Roger Freidinger, senior director of medicinal chemistry at Merck in the US, said the company had spent the past five years developing the drug, and trials had begun last year.
'We are in the very beginning phase of this, but our very preliminary results indicate that the drug is safe. When we are satisfied with this we will go on to studies with women experiencing premature labour.'
One reason why prematurity has been so difficult to treat is that its causes are still poorly understood. Premature labour and delivery are still a serious problem, with incidence in industrialised nations remaining stable at between 6 per cent and 8 per cent of all pregnancies that progress beyond 20 weeks. Premature birth remains the leading cause of illness and death in new-born babies, and can contribute to long-term neurological problems such as cerebral palsy, hearing loss and learning disabilities.
Oxytocin is thought to take part in events leading to the earliest stages of labour, including stimulating contractions. In the final stages of pregnancy, the number of receptors for oxytocin in the uterus increases rapidly. Thus, even low levels of the hormone at this time could initiate labour by contracting the 'sensitised' uterus.
Dr Freidinger said Merck believes its new drug clings to oxytocin receptors in the uterus, preventing the hormone from binding and causing contractions.
There are a few drugs that help prevent premature birth, but these are known to have side effects. One other oxytocin-inhibiting drug is under development, but it has to be given by injection. Tablet form has the advantage of allowing women to administer their own dosage.
Dr Freidinger said the new drug was unlikely to be available routinely for about five years.