TGN1412

TGN1412 (also known as CD28-SuperMAB) is the working name of an immunomodulatory drug which was withdrawn from development, originally intended for the treatment of B cell chronic lymphocytic leukemia (B-CLL) and rheumatoid arthritis.^[1] It is a humanised monoclonal antibody that not only binds, but is a strong agonist for the CD28 receptor of the immune system's T cells.^[2]

In its first human clinical trials, in March 2006, it caused catastrophic systemic failure in the subjects, despite being administered at a supposed sub-clinical dose of 0.1 mg per kg, some 500 times lower than the dose found safe in animals,^[3] resulting in the hospitalization of six volunteers on 13 March 2006. At least four of these suffered multiple organ dysfunction, and one trial volunteer is said to be showing signs of developing cancer. The developing company, TeGenero Immuno Therapeutics, entered into insolvency proceedings later in 2006. Tentative opinions from an as-yet uncompleted inquiry suggest that the problems arose due to "unforeseen biological action in humans", rather than breach of trial protocols, and the case therefore has had important ramifications for future trials of potentially powerful clinical agents.

The drug, which was designated as an orphan medical product by the European Medicines Agency in March 2005, was developed by TeGenero Immuno Therapeutics, tested by Parexel and manufactured by Boehringer-Ingelheim.^[4][5] TeGenero announced the first elucidation of the molecular structure of CD28 almost exactly one year prior to commencement of the TGN1412 phase I clinical trial.

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Description of the drug

Mice of the inbred strain BALB/c were immunized with recombinant human CD28-Fc fusion proteins and boosted with a B lymphoma cell line transfected to express human CD28. Hybridomas were obtained by fusing B cells with the hybridoma partner X63Ag8.653 and screened for reactivity with human CD28 and TCR-independent mitogenic activity. Two monoclonals called 5.11A1 and 9D7 were identified. The more active of the two, 5.11A1, is a mouse IgG1 immunoglobulin. The Complementarity determining regions of 5.11A1 were cloned into the framework of human IgG and combined with IgG1 (TGN1112) or IgG4 (TGN1412) constant regions. According to the company's Investigator Brochure, "TGN1412 is a humanised monoclonal antibody directed against the human CD28 antigen. The molecule was genetically engineered by transfer of the complementarity determining regions (CDRs) from heavy and light chain variable region sequences of a monoclonal mouse anti-humanC28 [sic] antibody (5.11A1, Luhder et al., 2003) into human heavy and light chain variable frameworks. Humanised variable regions were subsequently recombined with a human gene coding for the IgG4 gamma chain and with a human gene coding for a human kappa chain, respectively."^[6] The recombinant genes were transfected into Chinese hamster ovary cells and the recombinant antibody harvested from culture supernatant.

Mechanism of action

  Naive T-cells normally require both signal 1 (the antigen receptor) and signal 2 (co-stimulation) to become fully activated. Studies of monoclonal antibodies specific for mouse, rat or human CD28 identified a so-called "superagonistic" antibodies that could stimulate T-cells without concurrent antigen-receptor stimulation. Whether this activity represents merely a "stronger" activity or a different activity is uncertain. Two antibodies specific for human CD28 were identified. The more active of the two, TGN1112 (originally called 5.11A1) belonged to the IgG1 class of immunoglobulins. TGN1412 (clone 9D7) belonged to the IgG4 class. The TCR-independent agonism of these antibodies involved binding to a specific part of the CD28 molecule called the C"D loop.^[7] Once the investigators found an antibody with this property they initially wondered if it could be therapeutically useful in stimulating the immune system in immunosuppressed patients. However, in vitro and in vivo data from animal studies later suggested that administration would lead to preferential activation of regulatory T cells, leading to a net effect of T cell downregulation. 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 humans (see below) indicate that this may not always be the case.

A new explanation for the trial mishap was suggested by the findings of a recent paper in Clinical immunology. Pillai et al. found that all T cells that get activated using conventional TCR-mediated stimulation become transiently regulatory and express FOXP3. However, eventually most of these cells downregulate their regulatory capabilities and become effector cells. Hence attempts to induce FOXP3+ T cells might also induce effector cells capable of causing tissue damage.^[8]

Other cells activated by CD28 ligation in humans are eosinophil granulocytes. They can release IFN-γ as well as IL-2,IL-4 and IL-13.^[9][10] However most in vitro experiments are limited to the use of purified peripheral blood mononuclear cells (PBMN's) that do not contain those cells.

To function as an agonist, it has been suggested that TGN1412 needs to be a whole antibody including the constant (Fc) region. According to a report by TeGenero the F(ab)2 is not able to generate the required stimulation.^[11] Unlike a related clone, TGN1112, an IgG1, TGN1412 is of the subclass IgG4. This choice was made as TGN1112 showed antibody-dependent cellular cytotoxicity on CD28+ Jurkat cells. Thus the function of antibody binding via an Fcγ 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.^[12]

Clinical trials

Phase I clinical trials were conducted by PAREXEL at independent clinical trials unit in leased space on the premises of Northwick Park and St. Mark's Hospital, London, on 13 March 2006.^[13][14] Parexel is a company that carries out drug trials on behalf of pharmaceutical and biotechnology companies. Healthy volunteers were recruited to the study with a £2,000 fee, reportedly much higher than the 'few hundred quid' offered for other medical tests in the region.^[15] The trial resulted in hospitalization of all six volunteers administered the drug, at least four of whom suffered multiple organ disfunction.^[16] The payment was seized upon by newspapers and reported in a sensationalist fashion. Good Clincal Practice (GCP) prohibits payments being made to Phase I trial volunteers on the basis of risk, and specifies that payments must be based upon the amount of time given up and the number of invasive procedures (e.g. blood sampling). For most trials, payments are in the range of around £1,000 per week - but the 'few hundred quid' trials mentioned above are those which the general public are most familiar with (since they only last a day or two, do not require working individuals to take time off work, and hence are more common). The trial was a double-blind, randomized, placebo-controlled study, with two of the eight subjects receiving a placebo, and six receiving 1/500th of the highest dose used in previous experiments with cynomolgus macaques. All six of the trial subjects who received the drug were male, aged 19 to 34 (median 29.5); none had a notable medical history, and all were well in the 2 weeks before the trial.^[14] The drug was given by intravenous infusion, starting at 8am, with an interval of around 10 minutes between patients, and each infusion lasting from 3 to 6 minutes.^[14] Roughly five minutes after the last participant had received his dose, the participant who had received the first dose complained of headache, and soon afterwards 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 also became ill, vomiting and complaining of severe pain. The first patient was transferred to the Northwick Park hospital's intensive care unit 12 hours after infusion, with the others following within the next 4 hours.^[14] A severely affected volunteer, Mohammed Abdalla, a 28-year old who said he had hoped to set his brother up in business in Egypt, was described as having suffered a ballooned head. This led to his description as being similar to the "Elephant Man".

All of the men were reported to have experienced cytokine release syndrome resulting in angioedema, swelling of skin and mucous membranes, akin to the effects of the complement cascade in severe allergic reaction. The patients were treated with corticosteroids to reduce inflammation, and plasma-exchange to attempt to remove TGN1412 from their circulation. The treating doctors confirmed in August 2006 that all six men had suffered from a cytokine storm, and that, paradoxically, the men's white blood cells had vanished almost completely several hours after administration of TGN1412.^[3]

According to a press release from 5 July 2006 on the North West London Hospitals NHS Trust website, where the men were treated, the patients continued to improve and "five of them went home within a month of the incident, while one patient remained in hospital until 26 June, when he also went home."^[17] However, 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.^[15]

An article by The Sunday Times on July 30 2006 reported lawyers' claims that the long-term damage to the patients may be worse than originally thought. Medical assessment by immunologist Professor Richard Powell were said to have revealed that the blood of the patients contained a low number of regulatory T-cells, below one percent compared to three to five percent for healthy male adults - although the clinical significance of any such finding are unknown. Powell also reportedly claimed that one of the patients has "definite early signs that a lymphoid malignancy is developing". Some of the men involved in the trial are said to have been told that they face "a lifetime of contracting cancers and all the various auto-immune diseases from lupus to MS, from rheumatoid arthritis to ME."^[18]

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."^[19]

  TeGenero has apologized to the families involved, insists that these effects were completely unexpected, and said that all protocols have been followed. An investigation by the UK drug regulator reported that the reaction was not due to contamination of the dose, or an incorrect dose being administered, but suggested that the problem was due to "on target" effects of 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.^[20] 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.^[21] Alternative explanations of the outcome include accidental erroneous dosing, aggregation of the monoclonal antibody, or contamination of the formulation with bacterial endotoxin during manufacture.

The document "tgn1412 clinical trial: protocol" states that 50 mg/kg (maximum dose applied in primates) was considered the "no observed adverse effect level" (NOAEL) and thus a 1 in 500 reduction in dose was considered a safe starting level for treatment.^[22] However, for antibodies of average affinity this 'safe' dose (0.1 mg/kg) is sufficient to label all cd28 receptors available in the human body as can be easily deducted from simple antibody titration experiments in flow cytometry, where doses of 0.03 mg/kg show full labelling of the target receptor.^[23]

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 statement 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.”^[24]

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.^[25] 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.”^[26] TeGenero say this was transient and was evidence of the extra T cells that the drug produces.^[27] Experiments with another drug affecting the CD28 receptor (but to a lesser extent than TGN1412) had also shown side effects in human trials.^[28] There have been criticisms that the risks taken and the design of the protocol were insufficiently justified by proper statistical evidence.^[29]

The MHRA's views

On 5 April 2006, the Medicines and Healthcare Products Regulatory Agency issued an interim report on the TGN1412 trial.^[22] 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.

See also

References