A researcher of international acclaim has reassured Bradford patients taking part in clinical trials their safety is of paramount importance.

It comes in the wake of the dramatic health problems suffered by six men who agreed to test a new drug designed to treat conditions such as rheumatoid arthritis and leukaemia.

Within hours of taking it the six young volunteers had to be admitted to intensive care in Northwick Park Hospital in north-west London. Two are in a critical condition, while four others are showing signs of improvement.

Professor Chris Twelves, a consultant oncologist at Bradford Teaching Hospitals and a professor at the University of Bradford, has played a lead role in many trials to test new cancer drugs.

He said: "Everyone's reaction both in and outside the research world is sympathy.

"In the longer term, obviously, we hope this will not affect our own patients' attitudes towards clinical trials. It is almost unheard of to see this sort of side effect."

He said the trials conducted in Bradford and in Leeds, but involving Bradford patients, were quite different as they involved people already suffering from cancer, rather than healthy volunteers.

"We would assure people that the mechanisms we use are very detailed," he said. "We remain keen that research continues.

"In our own area we have never had anything of this nature while the benefits we have seen have been very important.

"For example, the drug herceptin used to treat early stage breast cancer, which has been developed in the last ten years, is already extending the lives of many breast cancer patients across Yorkshire."

He said patients taking part in clinical trials in the district worked with a specialist team of top doctors and nurses and it was important patients still came forward to volunteer to test new drugs.

"Phase 1 clinical trials are an important first step," he said. "Without them we would never get to the later stage trials that tell us if this is a drug that can prolong the lives of cancer sufferers or even cure cancer.

"The people who take part in our trials are people with cancer who have a genuine desire to help future generations be free of the threat of cancer and we are very grateful for that.

"What I hope it will do is raise awareness of clinical trials and people will understand that a series of trials is the only way that we can have drugs that quite clearly reduce the numbers of people whose lives are blighted by cancer."

Many people from Bradford are among the 1,000-plus healthy volunteers in clinical trials at The Covance Clinical Research Unit, in Leeds, every year.

The unit has been in operation more than 20 years and has seen more than 20,000 people through its doors.

Dr Chris Springall, vice-president, said: "We have never seen anything remotely approaching the events in London.

"This is a highly regulated activity and I would say people have nothing to fear. We have not seen an impact on the behaviour of our volunteers or levels of inquiries."