Scientists from the Department of Immunology at Imperial College London and the Babraham Institute, showed that using TGN1412 to stimulate white blood cells, which mediate the immune response, can have an adverse effect if the immune cells have been activated or altered by infection in the past.
The research finally sheds light on the Northwick puzzle, which scientists have been at a loss to explain until now, and the information could be used to prevent further clinical trial tragedies in the future. The severe negative effects observed in the six human volunteers were especially unexpected because no adverse were seen in previous studies on monkeys even though they were given a dose 500 times greater than the human volunteers.
Instead of subtly 're-tuning' the immune system, as developer TeGenero hoped, TGN1412 induced a so called 'cytokine storm'; the immune system was sent into overdrive and attacked healthy organs with tragic results. However, key questions remained: what started the storm and why wasn't it observed in preclinical research?
The scientists discovered that when they stimulated CD28 on these previously activated 'memory' T cells, they migrated from the blood stream into organs where there was no infection and caused significant tissue damage.
"Our research suggests that this is because the human subjects' memory T-cells lost their sense of direction and started migrating into several areas of the body where they were not supposed to go, and caused damage," said Dr Frederica Marelli-Berg from Imperial College London.
Around 50 per cent of adult human T-cells are memory cells, having been activated by illness or injury during a person's life. However, animal models, such as those used to test TGN1412 before human trials, do not have memory T cells because they are deliberately kept in an aseptic environment where they are protected from infection.
The scientists reached their conclusions by activating CD28 in memory T-cells and then injecting them into mice. The cells immediately migrated from the blood into many organs including the kidney, heart and gut, where they are not normally found unless there is an infection.
It was originally hoped that TGN1412 could be used to treat chronic inflammatory conditions, such as rheumatoid arthritis and multiple sclerosis, which are caused by the immune system attacking itself, as well as haematological malignancies such as leukaemia. The drug was designed to stimulate the CD28 molecule in white blood cells - specifically T lymphocyte cells - to produce cytokines. This family of proteins then signals the T-cells to travel to the site of infection and causes the cells to produce more cytokines.
However, if uncontrolled, this vicious circle can cause cytokines to build up to dangerous levels - a cytokine storm.
The 'first-in-man' clinical trial of TGN1412, a monoclonal antibody, began last March. It was run by Parexel at their dedicated clinical research facility at Northwick Park Hospital, London, under contract from TeGenero.
The explanation of why the trial went so badly wrong has taken far longer than expected. At the time, the Medicines and Healthcare products Regulatory Agency (MHRA) suspended the trial and began investigating the data.
"We are not sure how long it will take to get to the bottom of this, but we expect it to be within the next few weeks, if not days," Sarah Coakley, MHRA spokesperson, told sister-site Outsourcing-Pharma.com last March.
"Something like this has never happened before," she said.
Although the design of the clinical trial was criticised in an interim report from the MHRA a month later no evidence of wrongdoing on the part of Parexel or TeGenero was found. For example, the report said that the drug should have been given to volunteers one by one with an appropriate period of observation in between instead of giving the drug to all six volunteers at once. Despite this, TeGenero filed for insolvency in July 2006 stating the disastrous trial made it impossible to attract further investment in the company.
The final report, headed by Professor Gordon Duff was published in December 2006. It made 22 recommendations on clinical trial design. However, a second phase of investigation into why the trial failed, conducted by scientists at the National Institute for Biological Standards and Control (NISBC), could offer no further reasons into the cause of the adverse effects.
Blood tests of TGN1412 did not stimulate the release of a cytokine storm or stimulate lymphocyte proliferation. The report stated: "This suggests that the dramatic adverse effects experienced by the volunteers at Northwick Park cannot be explained by a rapid induction of cytokines resulting from simple ligation of CD28."