Rheumatism: An Antibody Taken To The Grave Too Early?

5. August 2015
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About ten years ago the drug TGN1412 almost killed six people during its first clinical test. Now it's celebrating a successful resurgence under a different name. A story about the analysis of a disaster and its consequences.

The name of this leukocyte subgroup alone already suggests its diverse areas of application, whenever the immune system has to once again see to it that the body operates in a clean state. Regulatory T cells (Treg) apparently bring a halt not only to attacks against the enemy in the instance of infection when the danger has already passed, but they also limit the damage that a massive inflammation wreaks in the tissue. These cells reduce the risk of rejection in the post-transplantation period and protect the foetus during pregnancy from the overzealous defence system in the mother. Finally, they help to reduce the consequences of a myocardial infarction or stroke. Mice without regulatory T cells die early over the course of their lives from serious autoimmune reactions.

Immune system depressants activated specifically

No wonder immunologists have for decades been trying to activate these cells using suitable agents, to sharpen them up for clinical use when needed. One of these promising development projects became famous only for tragic reasons, then seemed as good as buried, and is now celebrating its not quite as spectacular rebirth: TGN1412, as the German company TeGenero named this apparent piece of gold in its research pipeline. Already in the 90s researchers took into their studies a surface molecule via which Treg could be especially well activated, whereas other cells such as CD4 effector T cells also reacted but more weakly. This type of antibody to CD28, which initiated a signal sequence going beyond its own action (CD28SA = CD28 super agonist), set regulatory cells off into a preferential proliferation cycle when used in rat experiments; other cell populations only responded weakly. This all happened after the assessment of the researchers at the time, independent of any involvement of the antigen specific T cell receptor.

CD28 is considered the co-receptor in the recognition of antigen via the T cell and initiates the production of cytokines for the proliferation, survival and differentiation of naive T-cells. With effector cells these include first and foremost proinflammatory cytokines such as IL-2 or IFN-γ or TNF-α.

Catastrophic test in London

All preclinical studies indicated a straightforward path for TGN1412 all the way to release: there were successful experiments on rodents, cynomolgus monkeys and in cell cultures with human cells. The clinical test involving eight volunteers in London on 13 March 2006 however ended in a failure – for six of them almost fatally. A “cytokine storm” occurred which began with nausea, headache, backache and a few hours later led to a life-threatening organ failure caused by a massive scale secretion of a wide range of proinflammatory cytokines.

Fortunately, everyone was able to be saved and they recovered in the months thereafter. One of the victims, however, lost some fingers and toes in this mishap and was rewarded with a seven-figure sum. The accident not only appeared to seal the fate of TeGenero, which went bankrupt a few months later, but also that of the antibody TGN1412.

Analysis and conclusions

Immediately after this totally failed phase I study the investigations began. What had gone wrong? First, the amount of antibody administered seemed much too high. Experts criticised that the dose of 0.1 mg / kg body weight as employed was sufficient to cause occupation of most of the CD28 receptors on T-cells. Soon thereafter, the first conclusions were drawn: the authorities now prescribe for the first human tests, which use totally new mechanisms of action, a dose leading to a maximum occupancy of ten percent of the target molecules.

Yet why was this not noticed in the preliminary animal experiments? The scientists involved at the time tried 500 times this dose. In the bodies of investigated cynomolgus-primates involved, the T-effector cells – unlike with people – lose their CD28 molecules over the course of their maturation – which unfortunately is also not what happens in other species of primates [Paywall], all which happened not to be on the list of experimental animals used here. Rats and mice live under controlled and very hygienic conditions: little work for T-effector memory cells, which when activated need the second T-cell receptor signal in order to crank up the inflammatory response. By contrast, regulatory T cells responded well to the impetus given by their co-stimulator CD28, particularly when the dose was kept comparatively low. It was these which also held the few activated T effector cells at bay.

What’s more, in tissue culture, as was demonstrated in work done by the German working group led by Thomas Hünig, a system had been chosen that only inadequately reflected conditions in the body. The team of TGN1412 developers figured out that in peripheral blood involving leukocytes the T-cell receptor’s signal is much too weak to bring the effector cells to the boil, as happened in London. If, however, the T-cells – as it is in the living body – have the opportunity in a very dense cell structure to constantly “sample” their neighbours, they quickly come upon a suspect molecule, perhaps left over from a previous infection.

Luck renewed with new cell culture

The reasons for the failure in the first test on humans were revealed. So how did the antibody story proceed from there? For one Russian investor, the regulatory T cell control project using the “super-agonists” still seemed very appealing. Under the name TAB08 instead of TGN1412, the antibody underwent new tests using a two-day pre-culture with a dense cell layer, and the antibody showed activity that had not been seen before 2006. Put together with a healthy dose of CD28 stimulation, the cytokine storm was now able to be simulated in vitro. At a very low concentration, TAB08 finally worked more on Treg and no longer on the dreaded effector cells. When methlyprednisolone was added to the culture, at higher CD28SA titres the release of proinflammatory cytokines was also inhibited, whereas the T cell controllers were significantly less influenced by the corticosteroid.

Finally,TheraMAB, the new proprietor, dared to undertake a new phase I study. The dose of the antibody used, at maximum 7μg / kg body weight – again more than an order of magnitude lower than that used with TGN1412 – travelled slowly through an infusion over several hours into the circulation system of the subjects. After this first new experiment ended with no side effects, doctors tried the new old drug on nine patients with rheumatoid arthritis. Here too, the company reported impressive results. In these patients the condition of swollen joints improved shortly after treatment, by 20 to sometimes as much as 70 percent. That’s more than any other comparably effective biological pharmaceutic used for rheumatism, such as infliximab. etanercept or adalimumab, can achieve. In addition, the antibody is concentrated in low doses in the immunologically active areas which regulatory T cells deal with.

Versatile use in autoimmune diseases

Although mouse models for the reasons mentioned may not be regarded as truly reflecting conditions in the human body, a whole series of disease models was nonetheless still able to be derived as viable options for the use of CD28 superagonists. With cerebral ischemia the antibody reduced the extent of the neurological damage [Paywall] via the multiplication of regulatory cells. The same applies with EAE mice, the models for multiple sclerosis, colitis, glomerulonephritis or transplantation of kidney, heart, liver or oesophagus in rodents, where the T-cell controllers help to keep the organ active in the body. Thomas Hünig, co-developer of TGN1412 and now advisor to TheraMAB, harbours hopes of having many fields of application for the resurrected former bearer of misfortune. “Everywhere where regulatory T cells do their work, the principle of transient Treg activation can be applied. This allows its potential use against many autoimmune diseases.” Marcel Kenter and Adam Cohen of Leiden in the Netherlands in an editorial in the British Journal of Clinical Pharmacology also reveal that they were impressed. “The exceptional development path of TGN1412 shows that innovative and potentially hazardous substances can be safely tested on people, if only one has enough accurate insight into their mechanisms of action. A further requisite is having good preclinical studies using biomarkers which allow an accurate prediction of the effect in people when using a suitable dose.” In short: those who are written off sometimes live longer.

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