An experimental drug that has left one person brain dead and five others seriously ill in France was given to 90 people – and there is no antidote, it emerged today.

Prosecutors have opened an investigation into what Health Minister Marisol Touraine called 'an accident of exceptional gravity... without precedence' in France at the Biotrial lab in Rennes.

The drug trial, which was testing a new painkiller, involved 90 volunteers who were given the drug in varying doses, she told reporters today at a news conference in Rennes.

Biotrial had been carrying out the drugs trial for Portuguese pharmaceutical company Bial.

All six hospitalised men were between 28 and 49 and were healthy when the trial began on January 7, she said, adding that one man now classified as brain dead was admitted to hospital on Sunday.

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French health Minister Marisol Touraine (pictured) said one person had been left brain-dead and five others hospitalised after a 'serious accident' during a drugs trial in France

The prosecutor's office has opened an investigation into what the French health minister called a 'serious accident during a clinical test' at the Biotrial laboratory in Rennes

Research company Biotrial had been carrying out the drugs trial for Portuguese pharmaceutical company Bial

Three others were suffering a 'handicap that could be irreversible', said Professor Gilles Edan, the chief neuroscientist at the hospital in Rennes.

She said there is no known antidote for the drug.

I t's rare for volunteers to fall seriously ill when testing new drugs. Researchers generally start with the lowest possible dose for humans after extensive drug tests in animals.

The French ministry statement said those who fell ill had taken an oral medication in the first phase of testing, which was studying safe usage, tolerance and other measures on healthy volunteers.

Touraine said the drug contained neither cannabis nor any substance derived from cannabis, dismissing reports from a source close to the case that it had contained cannabinoids, an active ingredient found in cannabis plants.

The minister said the drug was meant to act on the body's endocannabinoid system, which deals with pain.

The six patients had been taking part in a 'trial of an oral medication being developed by a European laboratory' in the northwest French town of Rennes (file picture, above, of the Town Hall)

Biotrial, with headquarters in Rennes and offices in London and Newark, New Jersey, says it has over 25 years of experience in clinical trials and uses 'state-of-the-art facilities.'

In France, adults volunteering for Biotrial tests can earn between 100 euros and 4,500 euros ($110 to $4,922).

Clinical trials typically have three phases to assess a new drug or medical innovation for safety and effectiveness.

Human participation in such trials and scrutiny by outside watchdogs are essential for obtaining market authorisation.

Phase I entails a small group of volunteers, and focuses only on safety.

Phase II and Phase III are progressively larger trials, typically involving hundreds or thousands of volunteers, to assess the drug's effectiveness, although safety remains paramount.

Touraine said the drug contained neither cannabis nor any substance derived from cannabis, dismissing reports from a source close to the case that it had contained cannabinoids, an active ingredient found in cannabis plants (like the one above)

The company conducts its Phase I trials at a 150-bed facility in Rennes and also in Newark, New Jersey, from where it carries out 'a large variety of early clinical studies,' according to its website.

Biotrial says it is able to fast-track early patient studies by 'combining the favourable regulatory environment in Western Europe with fast and efficient patient recruitment in Eastern Europe.'

In 2006, Britain saw a similar incident, when six previously healthy men were treated for organ failure only hours after being given an experimental drug targeting the immune system.

That prompted a review of procedures and resulted in the U.K. regulatory agency imposing new testing standards, including recommendations to use the lowest possible dose and to test new drugs only in one person at a time.

The six men in Britain now apparently have a higher risk of cancer and autoimmune diseases tied to their exposure to the experimental drug.

In gene therapy, setbacks have included the death of an 18-year-old US volunteer, Jesse Gelsinger, in 1999, and the development of cancer in two French children treated for 'bubble baby' syndrome, a chronic lack of immune defences.