The antiviral was manufactured in GM tobacco with a view to using the same technique to slash the cost of other life-saving drugs in the developing world

An antiviral drug synthesised by genetically modified plants is being tested on a small number of women in the UK to establish its safety, bringing closer the possibility of cheap modern medicines for the developing world.

The drug's developers hope it can be used to prevent HIV infection, but the real breakthrough is that the research demonstrates it is possible for similar molecules – known as monoclonal antibodies – to be produced relatively cheaply in plants to the high standards needed for their use in humans.

The human trial has been approved by the UK licensing body, the Medicines and Healthcare products Regulatory Agency (MHRA), and is taking place in Guildford at the clinical research centre of the University of Surrey.

Pharma-Planta is a project launched seven years ago with the objective of using GM plants to slash the cost of drugs that are hard to produce. The scientists' aim is to increase the availability of these modern medicines – which are often highly effective – in the poorest countries of the world.

Access to medicines in the developing world is extremely limited. The World Health Organization estimates that 23 million infants worldwide do not get adequate basic immunisation and 1.7 million children under five die from vaccine-preventable diseases.

"The driver was to produce these medicines economically and at a level that would satisfy global demand," said Professor Julian Ma from St George's University, London, who is the joint co-ordinator of the European Union-funded project.

Many medicines are synthesised at great expense in fermentation vats containing bacteria or mammalian cells. By contrast Pharma-Planta produced the anti-HIV monoclonal antibody in GM tobacco plants grown in soil in greenhouses in Germany. After 45 days, they were harvested, their leaves were shredded and "highly purified antibodies" were extracted.

The researchers say there is little risk of such GM plants spreading or contaminating other crops because they are contained and would not be grown on anything like an agricultural scale.

Ma said it was "a red letter day" when they received the go-ahead from the drugs regulator. "The approval from the MHRA for us to proceed with human trials is an acknowledgement that monoclonal antibodies can be made in plants to the same quality as those made using existing conventional production systems. That is something many people did not believe could be achieved," he said.

Eleven healthy women have volunteered to take part in the trial and two of them have been given the antibody so far, with a third woman having been given a placebo. The trial is designed only to demonstrate the safety of the antibody, called P2G12, at different dosages. Much bigger trials in women at risk of contracting HIV would be necessary to test whether it could prevent infection.

If it does prove effective, the drug would probably have to be used in combination with other monoclonal antibodies to minimise the chance that the virus developed resistance, as it easily does to antivirals.

The process is between 10 and 100 times cheaper than conventional production systems, said Professor Rainer Fischer of the Fraunhofer Institute for Molecular Biology and Applied Ecology in Aachen, Germany, where the plants were grown.

The most useful monoclonal antibodies, such as the anti-cancer drug Herceptin, are still covered by patents owned by major pharmaceutical companies, but once these expire the new technique could offer a way to make cheap versions available in poor countries.