BUAV: Animal tests fail to predict drug trial disaster

Thursday, 13, Apr 2006 12:00

The recent drug trial disaster that occurred in March 2006 involving 8 healthy volunteers has raised not only concerns over human clinical trial protocols but more fundamentally the reliability of animal tests. Now that it has been confirmed by the Medicines and Healthcare Products Regulatory Agency (MHRA) that the dreadful effects were not due to contamination of the drug or a mistake with dosing levels (1), it is clear that the fault lies with the animal tests. “…this product showed a pharmacological effect in man which was not seen in preclinical tests in animals at much higher doses”.

So what happened – and what does it mean?

What type of drug was TGN1412?

TGN1412 is a monoclonal antibody (mAb– a relatively new class of drugs that are called ‘biologicals’ because they are derived from biological material, e.g. viruses, antigens and antibodies). Biologicals are increasingly popular drugs, with nearly 25% of human clinical trials in the pipeline involving these. Unfortunately, animals are involved in their production, ingredients as well as their testing. MAbs can turn different kinds of cells in the body on and off and therefore can be used to help people’s own bodies fight diseases. They work by binding to a specific target, usually a protein receptor on the surface of a cell, where they either activate the cell (agonists) or block the receptor (antagonists), preventing other molecules from activating it.

The basic idea of TGN1412 was that it would stimulate specific cells controlling the human immune system and therefore help to treat certain illnesses caused by the immune system being overactive. TGN1412 is an agonist (activating) antibody that binds to the CD28 receptor on the surface of T-cells, which are an important component of the immune system. Normally another receptor is required before the T-cell is stimulated, but TGN1412 is a so-called ‘super agonist’, stimulating the T-cell without the need for the second signal. Studies in animals had suggested that TGN1412 stimulated T-cells to multiply, but specifically the ‘regulatory T-cells’, which control the production of other T-cells and so decrease the immune response. The German company that were testing the drug, TeGenero, assumed that the drug would suppress the immune system by activating these regulatory T cells and would therefore be useful for conditions where the immune system is in overdrive (rheumatoid arthritis, leukaemia). Initial studies on blood samples of human cancer patients and in animal models of arthritis appeared to confirm this.

What happened to the trial volunteers?

It is now thought that, completely in contrast to expectation, the volunteers’ immune systems over-reacted as a consequence of being given the drug and released masses of cytokines (chemicals produced by the body as part of our immune system) producing a huge inflammatory response. This caused the volunteers to experience swelling, pain and organ failure as their immune systems began attacking their own bodies. It is now thought that this may have been because TGN1412 stimulated the production of T-cells overall and not just the regulatory (suppressing) ones.

Why were these effects not picked up by the animal studies?

Both human and animal tests commissioned by TeGenero did suggest that there would be a rise in overall T-cell activity. Rats whose immune system had been completely destroyed were given the drug along with a small dose of normal T cells and their T-cell population was fully restored (4). Highest doses of the drug boosted regulatory T cells from 5% to 20% in healthy mice and rats, but also boosted overall T-cell number 20-fold. A study with human blood cells from cancer patients showed “pronounced expansion and activation of human T-cells”, not just the controlling T-cells but all the T-cells. Other researchers now claim that this over production of all T-cells may have indicated the likely effects on the volunteers. Since some of these studies were conducted on sick animals and humans with weakened immune systems (and not always TGN1412 itself, the huge immune reaction may not have been properly foreseen. Dr. Glover, chief medical officer at Cambridge Antibody Technology said that in hindsight it may have been inappropriate to test the drug on healthy volunteers whose immune systems are already working well, as a further boost might push their systems into overdrive. It now appears that this was what happened.

Nonetheless, the key safety study conducted over 4 weeks on 26, apparently healthy cynomologous monkeys specifically on TGN1412, failed to show evidence of any adverse reactions and also confirmed the company’s suggestion that the drug preferentially boosted the regulatory T-cells. The only side effects were swollen lymph nodes in 2 of the 4 monkeys used to asses dosing levels in the pre-test pilot. This strongly suggests that the animal tests were also falsely reassuring. Why might this be?

Dr Andrew Martin of University College London says that in a study by his team, at least 2 out of the 15 amino acids on the receptor that come into contact with TGN1412 differ in rabbits, mice and rats to humans and that this can ‘easily account for several orders of magnitude in the binding affinity of the antibody for CD28’. In other words, differences between animal and human cells mean that the treatment can potentially have enormously different effects. One of the amino acids that comes into contact with the receptor is also different in monkeys to humans, which could still reduce the effect of the drug on the T-cells and cause a reduced effect. For this reason, Dr Glover also said that “there will be difficulties in testing this drug in different animal species”.

The basic lesson of these studies, as reported, is that they produced data pointing in different directions – with different species and animals in different states of health responding in different ways. This made predicting the human response far more difficult and shows how the use of animals confused, rather than clarified the situation – leading, it appears, to this disastrous event.

What are the implications?

The outcome of this trial raises huge implications for the drug development industry and their reliance on out-dated, unsophisticated animal models for increasingly sophisticated and species-specific drug therapies. It appears that this problem has only been highlighted until now because most mAbs used in human disease treatment are of the antagonist kind (e.g. Herceptin). They work by blocking receptors and reducing the output of their cells. Any differences between animals and humans in amino acids that might affect this blocking action would therefore be more likely to lead to reduced effectiveness of drugs rather than adverse effects – and the most common reason for drugs which pass animal trials to fail human trials is, in fact, that they simply don’t work, rather than that they are dangerous. Perhaps for this reason, these subtle species differences have given less cause for concern, until now.

An expert group has been set up to look into how this class of drugs can be more effectively screened for safety, but there are concerns that they will fail to adequately address the fact that species differences will play a large role. With the hugely increased attention being paid to the development of ‘biologics’ like TGN1412 by researchers and drug companies, the risk of further such problems arising in future human trials appears very real.