The drug that left six men seriously ill after its first tests on humans had caused the glands of two monkeys to swell in previous experiments, the company behind it said today.
However, TeGenero said the monkeys' symptoms were completely different to those suffered by the men being treated at Northwick Park hospital, in north-west London.
The company said two of the 20 monkeys used in pre-clinical tests of the TGN1412 drug - designed to treat rheumatoid arthritis, leukaemia and multiple sclerosis - had experienced a "transient increase in size of lymph nodes".
In a statement, it said information that there was a small chance of temporary swelling as the drug worked on the immune system had been included in details submitted to UK regulators. The information was also on the consent form signed by the volunteers before the trial began.
"We are encouraged by the progress being made by the volunteers, but remain deeply concerned for all of them as they continue their treatment," Thomas Hanke, the TeGenero chief scientific officer, said.
"Once again we would like to thank the excellent work by the doctors at Northwick Park hospital. We are continuing to do all we can to ensure that the investigations into what went wrong proceed as quickly as possible."
The condition of four of the men is continuing to improve after almost a week in intensive care, their doctors said yesterday.
Three of the four had now been removed from organ support, Ganesh Suntharalingam, the clinical director of intensive care at the hospital, added.
However, he said two other patients remained in a critical condition and said it was still too early to comment on their prognosis despite some early signs of improvement.