TGN1412

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This article documents a current event.
Information may change rapidly as the event progresses.

TGN1412 (also known as CD28-SuperMAB®) is the working name of an immunomodulatory drug intended for the treatment of B cell chronic lymphocytic leukemia (B-CLL) and rheumatoid arthritis.^[1] TGN1412 is a humanised monoclonal antibody which not only binds, but is superagonistic to the CD28 receptor of the immune system's T cells.^[2] TeGenero itself made the announcement of the first elucidation of the molecular structure of CD28 almost exactly one year prior to commencement of the TGN1412 phase I trial. It received designation as an orphan medical product from the European Medicines Agency for the treatment of B cell chronic lymphocytic leukemia in March 2005.^[3] It was developed by TeGenero Immuno Therapeutics and manufactured by Boehringer-Ingelheim.^[4]

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Mechanism of action

T-cells normally require signal 1 (the antigen receptor) and signal 2 (co-stimulation) to become fully activated. The superagonistic properties of this agent means it fully activates T-cells without the need for additional antigen-receptor stimulation. Superagonism of these antibodies was found to be dependent on binding to a specific part of the CD28 molecule called the C"D loop.^[5] Once the investigators found antibodies with these properties they wondered if they could be therapeutically useful in stimulating the immune system in immunosuppressed patients as these antibodies would be expected to crudely activate all T-cells simultaneously. However, in vitro and in vivo data from animal studies later suggested that administration would lead to increased activation of regulatory T cells, leading to a net effect of immunosuppression. On its website, the company writes: "A pronounced T-cell activation and expansion mediated by CD28-SuperMAB® in animal models is accompanied by the expression of anti-inflammatory cytokines, like IL-10, rather than by the toxic cytokine storm of pro-inflammatory mediators induced by other agents that address the TCR complex.".^[1] As it turned out, the results of the first trial in human beings (see below) indicate that this may not always be the case.

Other cells activated by CD28 ligation in humans are eosinophil granulocytes. They can release potent neurotoxins, INF-γ as well as IL-2,IL-4 and IL-13. [1],[2]. However most in vitro experiments are limited to the use of purified peripheral blood mononuclear cells (PBMN's)thus do not contain those cells.

To function as a superagonist TGN1412 needs to be a whole antibody including the constant region (Fc). According to the report by TeGenero [3] the F(ab)2 is not able to generate the required stimulation. Unlike its related clone TGN1112 an IgG1 TGN1412 is of the subclass IgG4. This choice was made as TGN1112 did show Antibody Dependent Cellular Cytotoxicity on CD28+ Jurcat cells. Thus the function of antibody binding via an Fc-gamma receptor seems to be a requirement for the immuneregulation. However, cell opsonisation by antibody leads normally to phagocytosis of the labelled cells as for example seen in the case of HIV [4]

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Clinical trials

Phase I clinical trials conducted by PAREXEL^[6] at Northwick Park Hospital, London, resulted in hospitalization of six volunteers in March 2006, at least four of whom suffered major organ failure^[7] Healthy volunteers were recruited to the study with a £2000 fee, reportedly much higher than the 'few hundred quid' offered for other medical tests in the region ^[8] The most severely affected volunteer, Mohammed Abdalla, a 28-year old who had hoped to set his brother up in business in Egypt, was said to have suffered a ballooned head similar to the "Elephant Man".

The trials were carried out by PAREXEL International, a company that carries out drug trials on behalf of pharmaceutical companies, at their facilities at Northwick Park Hospital, in Harrow in Greater London. Eight young, healthy men participated in the trial, with two receiving a placebo and the remaining six receiving TGN1412. The doses were given by injection with an interval of around two minutes between each patient.^[9] Roughly five minutes after the last participant had received his dose, the participant receiving the first dose started complaining of a severe headache, fever and pain. He took his shirt off, complaining that he felt like he was burning. Shortly after, the remaining participants who received the actual drug became ill as well, vomiting and complaining of severe pain. Within twelve hours all six had collapsed. At least one participant begged the doctors to "put [him] to sleep" because of the suffering.^[10]

As of 31 March 2006, four of the six patients had returned to their homes. One of the other two has been transferred out of Critical Care, and the worst-affected patient "is now fully conscious and we are encouraged by his progress," according to a press release on the North West London Hospitals NHS Trust website [5].

TGN1412 had not previously been given to humans; however, the trial was preceded by animal testing, including in non-human primates. The company claims that these did not indicate any safety issues. The US patent application states "it could be shown in a pilot study that an in vitro administration of anti-human CD28-SuperMAB induces in a rhesus monkey in vivo a profound activation of T cells, without clinically visible side effects" and goes on to say "This antibody—in spite of its strong T cell-stimulatory properties—is very well tolerated in vivo, in contrast to all other known T cell activating substances."^[11] Some of the men were reported to have experienced what sounds like angioedema—massive swelling of skin and mucous membranes, akin to the effects of a severe allergic reaction. The patients were treated with anti-inflammatory corticosteroids to reduce inflammation, and plasma-exchange to remove TGN1412 from their circulation as quickly as possible. Head of pharmacology at University College London Trevor Smart has suggested that the men may never fully recover, and may suffer long-term disruption to their immune systems.^[8]

Structure of human CD28.

TeGenero, has apologized to the families involved and insists that these effects were completely unexpected and that all protocols have been followed. An investigation of the case by authorities has now commenced to find out if the reaction seen is due to contamination of the dose, an incorrect dose being administered, or an inherent flaw in the drug. Criticism has been raised that six participants were given the drug in such a short time, which is against the recommendations of standard literature. However, the Medicines and Healthcare products Regulatory Agency has confirmed that they had approved the trial, including the protocol of giving the dose to all men within a short time. However, it appears the MHRA approved a protocol involving the doses being given within 2 hours whereas one of the placebo-receiving participants has explained the doses were given with 2-minute intervals.

The MHRA has further stated that the initial dose of TGN1412 was intended to be the first of a course of injections, with the dosage being ramped up over time. It has been reported that the initial dose was one five-hundredth of that which the animal studies indicated was a maximum safe dose.^[12] Dr. David Glover, an industry consultant, has suggested that because the antibody was raised against human CD28, the safe dosage may have been lower in humans than in animals ^[13] Alternative explanations of the outcome include accidental erroneous dosing, aggregation of the monoclonal antibody, or contamination of the formulation with bacterial endotoxin during manufacture.

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Criticism and controversy

Not much information has been released to the public about the ongoing investigations. At the moment, there appear to be two issues. There is the issue of the trial protocol of giving the drug to six participants within a short time. While this was approved by the MHRA, as mentioned earlier, it seems a two hour protocol was approved, but that the drug was administered to all participants within just twenty minutes, based on the explanations of a study participant. It appears that neither the companies involved nor the authorities have commented on this point yet.

Another issue is whether the company should have known the drug would provoke this reaction in humans. The comments on the company webpage and in the patent application at least indicate the company knew this type of drug could cause a cytokine storm. An immunologist contacted by New Scientist and who wished to be anonymous has commented that "You don’t need to be a rocket scientist to work out what will happen if you non-specifically activate every T cell in the body.”^[14]

While the drug had appeared to be safe in animal models, researchers have noted that there are reasons why these may not be indicative of the response in humans, particularly with respect to this type of drug. ^[15] The BBC reported that "two of 20 monkeys used in earlier tests suffered an increase in the size of lymph nodes," but that "this information was given to the men and submitted to the test regulators." ^[16] TeGenero say this was transient and was evidence of the extra T cells that the drug produces. ^[17] Experiments with another drug affecting the CD28 receptor (but to a lesser extent than TGN1412) had also shown side effects in human trials. ^[18]

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The MHRA's views

On the 5th of April the Medicines and Healthcare Products Regulatory Agency issued an interim report on the TGN1412 trial.^[19] They found no deficiencies in TeGenero’s pre-clinical work; there was no evidence of undisclosed studies. Parexel’s records and processes appeared in order (including dose measurement and administration) and the MHRA felt that their actions did not contribute to the serious adverse events. Nor was there any deficiencies in the animal work; results accurately reflected raw data.

German Regulatory Authorities inspected the production of the material by Boehringer Ingelheim, looking at the manufacture, testing, storage and distribution of the TGN1412. No deficiencies were identified which could have contributed to the serious adverse effects. Although tests are ongoing on the actual material used, the MHRA state that tests are consistent with the TGN1412 being up to specification at the moment.

The MHRA have concluded that the most likely cause of the reaction in trial subjects was an unpredicted biological action of the drug in humans. The interim report recognises that important scientific and medical questions about the risks of testing these agents in human subjects have been raised. To that end, the UK Secretary of State for Health has agreed to establish a group of leading international experts to consider those issues, and to provide a report on the future authorisation of such trials (with an interim report at three months).

Until the expert group has reported all further clinical trial applications involving first-in-humans trials of any monoclonal antibody or other novel molecules targeting the immune system will not be authorised in the UK, without having had additional expert opinion on whether the effects seen in the TGN1412 may be repeated. There will be a fuller report on the TGN1412 trial in future, but the expert group will run concurrently.

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