A scientist has questioned whether current clinical trial safety tests are sufficient for the type of drug that left six volunteers seriously ill.
The six are being treated at Northwick Park hospital
The men are still being treated in Northwick Park hospital in north-west London but two remain critically ill.
Independent consultant Dr David Glover says the screening met the guidelines.
But he told the BBC tests on animals would not have indicated a problem with the anti-inflammatory drug designed to boost the human immune system.
"It may well be there are extra considerations that will need to be taken in future and that companies and regulatory authorities looking at applications by companies that do such trials may wish to take a more cautious approach," he said.
However, professor Chris Higgins, director of the UK Medical Research Council's clinical sciences centre, said it is too early to draw conclusions.
An inquiry into the trial, carried out by Paraxel, at Northwick Park Hospital in north-west London, is ongoing.
Police and regulators are probing whether problems were caused by errors or were an unpredictable side-effect of TGN1412.
Doctors said two of the volunteers have been taken off organ support while another two are receiving less organ support - all four are fully conscious.
Another two men remain in a critical condition in the high dependency unit but are also continuing to progress.
Four of the men suffered multiple organ failure within hours of taking the drug on Monday.
Staff from Parexel insist the right procedures were followed.
Manufactures TeGenero said the drug had been tested extensively in laboratories and on rabbits and monkeys with no adverse effects and no drug-related deaths.
It was the first time the drug, designed to treat conditions such as rheumatoid arthritis, leukaemia and multiple sclerosis, had been tested on humans.
An organisation that helps find people for clinical trials, called Biotrax Research Volunteer Information Advisory Support Group, said interest from possible volunteers has surged since the news hit the headlines.