New Therapies for IBD: From the Bench to the Bedside
New Therapies for IBD: From the Bench to the Bedside
Extensive studies into T-cell polarisation in IBD have attempted to ascribe an easily addressed pathological mechanism to both CD and UC. Research shows that the patchy transmural inflammation of the gastrointestinal tract characteristic of CD is associated with the activation of types 1 and 17 T-helper (Th) cells in response to the production of interleukin (IL)-12, IL-18, IL-23 and transforming growth factor beta by antigen-presenting cells (APC) and macrophages. In turn Th1 and Th17 cells increase secretion of the pro-inflammatory cytokines IL-2, IL-17, interferon (IFN)-γ and TNFα. These cytokines feed into a self-sustaining cycle whereby they stimulate APC, macrophages, fibroblasts and endothelial cells to produce TNFα, IL-1, IL-6, IL-8, IL-12 and IL-18.
In contrast, the uniform mucosal inflammation of the colon found in patients with UC is suggested to be associated with a Th2 immune response mediated by IL-4, IL-5 and IL-13 that results in an increase in levels of IL-13. In both scenarios T cells are also activated by direct contact with antigens presented on the surface of APC and can be regulated by anti-inflammatory cytokines such as IL-10. Therefore, the majority of new biomolecules for IBD aim to reduce pathogenic T-cell activation and its effects by inhibiting the actions of pro-inflammatory cytokines, increasing anti-inflammatory cytokines, blocking T-cell co-stimulation, or inducing T-cell apoptosis.
The development of anti-TNFα therapies such as infliximab, adalimumab and certolizumab pegol have made a significant difference to the health-related quality of life of many patients with IBD, but among primary responders only a third will maintain remission after 1 year, and the risk of later surgery is still high in patients receiving anti-TNF therapy. The development of anti-TNFα therapies continues with novel TNF inhibitors such as golimumab, dersalazine, HMPL-004 and ozoralizumab (ATN-103) currently in various phases of the clinical trial process, but researchers are also exploring other avenues. One such closely related avenue is the development of a vaccine against TNFα consisting of the TNFα derivative TNFα kinoid (Debio-01512; Novacs, Paris, France), a complex of inactivated human TNFα and the carrier protein keyhole limpet haemocyanin. Having shown promise in animal models of chronic inflammation and rheumatoid arthritis (RA), TNFα kinoid was moved into phase I/II clinical trials in patients with moderate to severe CD. A recent report of the results of the trial indicates that TNFα kinoid was well tolerated with no serious adverse effects and with 76% of patients showing a clinical response and 43% achieving remission.
In addition to developing alternative ways to target TNFα, researchers have also turned their attention towards targeting other pro-inflammatory cytokines such as IL-12/IL-23, the α-chain (CD25) of the IL-2 receptor (IL-2R), and IFN-γ. Apilimod mesylate, ustekinumab, briakinumab (ABT-874) and SCH-900222 all act to inhibit the effects of IL-12/IL-23 and have shown limited results in early phase clinical trials. Apilimod is a small molecule inhibitor of the transcription of IL-12 and IL-23, ustekinumab and briakinumab both target the p40 subunit common to both IL-12 and IL-23, whereas SCH-900222 targets the p19 subunit that is specific to IL-23. An initial open-label dose-escalating trial with apilimod mesylate in patients with active CD indicated that the drug demonstrated clinical activity and was well tolerated, with the most common adverse events being dizziness, nausea, headache and fatigue. However, a subsequent randomised controlled trial showed that apilimod did not have a significantly greater efficacy than placebo treatment.
Ustekinumab was also subjected to phase II trials in patients with moderate to severe CD with slightly more promising results. Following 6 weeks of treatment, clinical response rates were significantly higher for the patients given ustekinumab at 53% compared with 30% for placebo. However, at week 8 the response rate was not significantly different, at 49% for ustekinumab and 40% for placebo. The long-term response to the drug is unknown but it is interesting that a subgroup of patients who had previously responded to infliximab had a greater response to ustekinumab. This effect was examined in a further trial in which 59% of patients who had previously been treated with infliximab responded to ustekinumab in comparison with 26% of patients who received placebo. The same trial also indicated that patients with higher baseline C-reactive protein (CRP) levels tend to have a greater response to ustekinumab. A more recent report of a multicentre, larger phase IIB trial in patients with moderate to severe CD who were unresponsive to infliximab indicated that at week 6 39.7% of patients treated with ustekinumab showed a clinical response compared with 23.5% of patients administered placebo (p<0.05), but the rates of clinical remission were similar. Those patients who responded to ustekinumab by week 6 were given maintenance therapy and at week 22 41.7% of patients achieved remission compared with 27.4% of patients given placebo. In addition, 69.4% of patients treated with ustekinumab had a clinical response at week 22 compared with 42.5% administered placebo, indicating that a significant number of patients who respond to ustekinumab remain in remission. In all three trials no serious adverse events were reported. Overall, given the results of those trials, it seems that ustekinumab may be particularly useful in patients who have previously failed to respond to anti-TNF therapy.
A phase I study of the response to briakinumab in patients with active CD showed that by week 8 the drug produced a response in 75% of patients compared with 25% given placebo. By week 18, however, the difference in response rates was no longer significant. A more recent study in patients with moderate to severe CD also concluded that briakinumab was not effective for the induction or maintenance of remission. In both trials the rates of serious adverse events were comparable between briakinumab and placebo. In the more recent study one patient treated with briakinumab died of respiratory failure due to pancreatitis, although this was thought to be unrelated to treatment. The final compound targeting the IL-12/IL-23 pathway, SCH-900222, is very early in development with the only trial in progress being a phase I/II dose-finding study in psoriasis.
IL-2 is a crucial cytokine for T-cell activation and proliferation, and has been investigated as a very attractive target for therapeutic intervention. Inhibitors of IL-2R include basiliximab and daclizumab, which are both monoclonal antibodies against CD25. An initial study with basiliximab in 10 patients with steroid-resistant UC indicated that nine out of 10 patients achieved clinical remission in 8 weeks. Eight of the nine responders then relapsed but remission was re-achieved with corticosteroids and azathioprine. At 24 weeks, seven patients were in full remission and tests indicated that patients were rendered steroid sensitive by the presence of basiliximab. A further open-label trial, in which patients with steroid-resistant UC were given a single dose of basiliximab in combination with steroids, resulted in 65% of the patients achieving clinical remission at week 24. Adverse events noted in the two trials included two cases of herpes zoster and two cases of fever that were resolved with treatment, along with other minor adverse events that did not require treatment. As there was no placebo control group it was not possible to determine if the events were attributable to the administration of basiliximab. In contrast to the results with basiliximab, although an initial open-label study of 10 patients with refractory UC indicated that daclizumab produced some clinical benefit, a larger randomised controlled trial in patients with moderate UC showed that those treated with daclizumab were not more likely to be in remission after 8 weeks of treatment compared with patients given placebo. Adverse events were few with daclizumab, with the most common being nausea in patients simultaneously treated with azathioprine as well as nasopharyngitis and fever.
Fontolizumab is the only biological therapy targeted against IFN-γ currently being tested in patients with IBD. An initial study in 45 patients with moderate to severe CD indicated that fontolizumab appeared to have a biological effect and was reasonably well tolerated, although there were slightly more reports of chills, flu-like symptoms and asthenia compared with placebo as well as two cases of worsening of CD symptoms. In a further study in 133 patients with CD, fontolizumab produced a clinical response in patients, but only after two doses given a month apart, with an increased response rate (69%) seen at day 56. Analysis of baseline CRP levels also indicated that elevated CRP was associated with pronounced clinical benefits. Fontolizumab was also well tolerated in this study and there were no evident patterns of adverse events attributable to fontolizaumab. A more recent study, in which the primary endpoint was clinical response on day 29, failed to show any significant clinical benefits in patients with CD treated with fontolizumab compared with placebo. However, at time points beyond 29 days patients treated with fontolizumab showed a significantly greater improvement in the CD activity index score and CRP levels, suggesting that the actions of fontalizumab are more gradual.
In addition to targeting IL-12/23, IL-2R and IFN-γ, more recent approaches have expanded to address almost all the pro-inflammatory cytokines shown to be elevated in IBD. Biological agents now entering phase I trials include monoclonal antibodies against IL-6, IL-6R, IL-13, IL-17, IL-18 and IL-21. As a pleiotropic cytokine IL-6 contributes to Th17 differentiation, and increased levels of IL-6 and soluble IL-6R are associated with increased disease severity in IBD. A polymorphism within the IL-6 gene has also been linked with early-onset CD, and persistent activation of the IL-6 signalling pathway plays a role in the development of colon cancer. Biological therapies targeting IL-6 include C326, currently undergoing a phase I trial in patients with CD; sirukumab (CNT0136), which has just been shown to be well tolerated in healthy subjects; CDP6038, currently being tested in patients with RA; and PF-04236921, which is going to be used in a phase I/II trial that is currently recruiting patients with CD who are unresponsive to anti-TNFα. In contrast, tocilizumab is a monoclonal antibody against IL-6R that has been through extensive trials in patients with RA, and has been approved as a second line monotherapy for patients with RA who have failed other approved therapies. Trials of tocilizumab in patients with IBD have been limited. The only trial completed to date was a placebo-controlled phase I study in 36 patients with active CD. The results of the study indicated that 80% of the patients given biweekly intravenous infusions of tocilizumab for 12 weeks had a clinical response compared with 31% of the placebo-treated patients, but only 20% of the patients given tocilizumab went into remission. Although the rates of adverse events were similar between tocilizumab and placebo in the study in patients with CD, the larger studies in patients with RA indicate that tocilizumab is associated with an increase in the incidence of reversible grade 3 neutropenia, elevated lipids, abnormal liver function and an increased risk of infection. These adverse events are generally mild and can be resolved with treatment but the longer term effects of increased lipids and abnormal liver function in response to tocilizumab need to be investigated.
The cytokine IL-13 is produced by naive T cells and activates natural killer T (NK-T) cells, which then also produce IL-13. The IL-13 produced by NK-T cells has been shown to be pathogenic in UC as it impairs the function of the epithelial barrier and also causes apoptosis of epithelial cells. Inhibition of IL-13 production with the administration of IFN-β1a also results in the suppression of inflammation in UC, which makes IL-13 an appropriate target for biological therapy in IBD. Some researchers have argued that enhancing IL-13 activity would also result in a therapeutic response in IBD as IL-13 suppresses the activity of the pro-inflammatory cytokines IFN-γ and IL-17 in murine models. Given the weight of the evidence that IL-13 plays a significant role in the pathogenesis of UC, it is unlikely that enhancing the levels of IL-13 will have a therapeutic effect in UC. Monoclonal antibodies against IL-13 currently in clinical trials for IBD include anrukinzumab, currently in a phase II trial in patients with UC, and QAX567, currently being used in phase I/II trials in patients with CD.
Similar to IL-6 and IL-13, monoclonal antibodies have also been developed to target IL-17, IL-18 and IL-21, which have all been shown to be present at elevated levels in the inflamed intestinal mucosa of patients with IBD. AMG827 and secukinumab (AIN457) both target IL-17, GSK1070806 targets IL-18, and ATR-107 (PF-05230900) targets IL-21, and all are currently in phase I/II clinical trials either in patients with CD or healthy volunteers. Of these therapies initial data are only available for secukinumab. In a double-blind, placebo controlled, proof-of-concept study in 59 patients with CD patients given secukinumab had disease worsening compared with placebo. In addition, there were some data to suggest that the inhibition of IL-17A exacerbated CD in a subset of patients and increased infection risk. The reason for such unexpected and opposite clinical effects are obscure because in psoriasis and RA blockade of IL-17A has been beneficial in proof-of-concept studies, but probably an effect of IL-17A in protecting barrier integrity and regulatory T-cell function is crucial in IBD compared with other inflammatory diseases. On the contrary, vidofludimus (4SC-101/SC12267), a small molecule inhibitor of the release of IL-17, has shown promise. Results from the single-arm, open-label ENTRANCE study indicate that vidofludimus is safe, well tolerated, with only mild to moderate drug-related adverse events reported, and may be useful in maintaining clinical remission. Following the administration of vidofludimus for 12 weeks in 26 steroid-dependent patients in remission with CD or UC 53.9% of patients remained in steroid-free remission, 34.6% were in remission at a lower corticosteroid dose than their baseline dose, and 11.5% showed no response. This suggests that attempts to modulate the function of IL-17 in IBD deserve more investigation.
In addition to inhibiting either cytokines or cytokine receptors an alternative means of reducing the inflammatory response in IBD is to block the downstream signalling pathways mediated by cytokines. As signalling molecules that interact with cytokine receptors the Janus kinases (JAK), JAK1, JAK2 and JAK3, play a crucial role in cell growth, survival, development and differentiation of immune cells. JAK1 and JAK2 are ubiquitously expressed but JAK3 is found only in haematopoietic cells and is part of the signalling pathways activated by IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. Tofacitinib (CP-690,550) is a first-in-class small molecule inhibitor of JAK3 currently in phase II clinical trials for various immune-related disorders including CD and UC. In-vitro studies of the activity of the drug indicate that it interferes with Th2 and Th17 cell differentiation, blocks the production of IL-17 and IL-22 and offers the possibility of effective suppression of the pathological immune response present in IBD. Recent results from a multicentre, double-blind, placebo controlled study in patients with moderate to severe active CD show that although tofacitinib is well tolerated it produced no clinically significant response following 4 weeks of treatment compared with placebo. Tofacitinib was, however, associated with a significant decrease in CRP and fecal calprotectin levels. In contrast, a study in patients with moderate to severe UC showed that 8 weeks of treatment with tofacitinib was associated with a dose-dependent improvement in both clinical response and remission rates. In trials both in CD and UC the overall incidences of adverse events were similar between patients administered tofacitinib and those given placebo. However, as has been seen to a lesser degree with tocilizumab, there was a dose-dependent increase in low-density lipoprotein cholesterol observed in patients treated with tofacitinib, which will require further investigation to determine the long-term effects.
An alternative and logical approach to reducing the inflammation present in IBD would be to restore the balance between pro-inflammatory and anti-inflammatory cytokines by increasing the amount of anti-inflammatory cytokines present in the body. Anti-inflammatory cytokines that have been developed as biological therapies for IBD include IL-10, IL-11 and IFN-β. Unfortunately the results from clinical trials administering these compounds have not been very useful.
IL-10 first came to light as an anti-inflammatory cytokine when it was shown that IL-10-deficient mice spontaneously develop colitis. Since then it has been shown that IL-10 decreases intestinal inflammation and that variants of the IL-10 gene are linked to susceptibility to developing UC. This led to the development of recombinant human IL-10 that could be administered subcutaneously. An initial trial in 46 patients with steroid-resistant CD showed that 50% of patients administered IL-10 achieved remission compared with 23% in the placebo group. Unfortunately, further larger clinical trials showed that although the therapy was well tolerated and there was a tendency towards clinical improvement, remission was not achieved in any patients included in the studies. The only notable adverse effects observed in the two trials were dose-dependent, asymptomatic and reversible anaemia and thrombocytopaenia. IL-10 therapy was also tested in patients with CD who had undergone ileal or ileocolonic resection, but treatment did not prevent the recurrence of inflammation when compared with placebo, even when IL-10 was given daily. One possibility for the failure of the treatment may be due to the method of drug delivery, with systemic administration of IL-10 resulting in very little of it reaching the site of inflammation. A novel way of bypassing the systemic circulation has been developed using oral administration of genetically modified IL-10 producing Lactococcus lactis to target the cytokine directly to the gut mucosa. This approach has been shown to be safe, and has gone into phase II clinical trials, which showed lack of clinical efficacy. Similar to IL-10, recombinant human IL-11 has also been developed for subcutaneous administration. Although initial trials showed evidence that the approach was well tolerated and produced a response in more than 33% of patients with CD, when it was compared with prednisolone, IL-11 produced a significantly inferior response rate and rate of short-term remission. The most significant adverse effect noted in the three trials of IL-11 was a dose-dependent increase in platelet count, along with mild adverse events such as fever, rash, nausea/vomiting and headache.
The third approach using anti-inflammatory cytokines is that of the administration of IFN-β, which has been tested both in patients with CD and UC. IFN-β was first administered subcutaneously to patients with steroid-refractory UC, in which it produced an 88% remission rate with a mean length of remission of 13 months. Subsequent open-label and double-blind, placebo controlled studies in patients with UC produced a variety of results, with some studies indicating that IFN-β did have a therapeutic effect while others reported that IFN-β did not produce a significant therapeutic effect compared with placebo. It is interesting to note that the positive reports of an effect of IFN-β were from open-label trials, whereas the negative results were from randomised controlled trials. In randomised controlled trials higher rates of remission were always observed with IFN-β compared with placebo, but rates of remission in patients given placebo were also high enough to confound the results. A more recent trial with IFN-β in patients with CD also came to the conclusion that there was no difference between the effects of the administration of IFN-β and placebo. There is some suggestion that response to IFN-β is dependent on the levels of circulating IL-17 and IL-6, with increased levels of the cytokines being associated with no response to IFN-β. An additional complication of IFN-β therapy is that there have been reports that the administration of the cytokine in patients with multiple sclerosis was associated with the development of IBD. Furthermore, recent investigations in a mouse model of colitis indicated that the administration of IFN-β exacerbated colitis. Data from all the clinical trials conducted so far in patients with IBD do not confirm this suggestion, with the most common adverse effect being flu-like symptoms and no exacerbation of IBD symptoms reported.
During the process of physiological inflammation the population of activated T cells that responds to pathogens is constantly balanced by the occurrence of an equal amount of proliferation and apoptosis. During chronic pathological inflammation proliferation of T cells far outweighs apoptosis of T cells. One approach to controlling pathological inflammation would be to increase the T-cell apoptosis that occurs in response to a lack of antigen-specific and costimulatory signals from APC. Visilizumab is a monoclonal antibody against the CD3 chain of the T-cell antigen receptor (TCR), blockade of which leads to T-cell apoptosis. An initial phase I study in patients with severe steroid-resistant UC reported that all the patients achieved remission with the administration of visilizumab. This was followed by a further study in which 84% of patients with steroid-refractory UC achieved a clinical response. However, although a phase I/II dose escalation trial of visilizumab in severe UC showed evidence of clinical response, it also resulted in 100% of patients reporting adverse events including abdominal abscess, atrial fibrillation, cytomegalovirus infection and herpes zoster. Following on from that study a placebo controlled trial indicated that visilizumab was not effective in patients with thromboembolic adverse events. A trial in patients with CD also indicated that visilizumab is associated with cytokine release syndrome that results in transient liver injury. Therefore, although biologically attractive, excessive T-cell targeting often leads to serious adverse events.
As an alternative to targeting the proliferation of T cells, rituximab targets the CD20 component of the TCR on B cells with the aim of inducing apoptosis in circulating B cells. A phase II randomised controlled trial of rituximab in 24 patients with steroid-refractory moderate UC showed that although there was some clinical response with rituximab the rate of induction of remission was no different from placebo. In addition, any observed response was short term and was only maintained to week 12 following the administration of rituximab. Similar to the use of visilizumab, some severe adverse reactions have been reported with rituximab although they have generally been seen in other disease states. In isolated cases, including a patient with follicular lymphoma, a patient with Grave's disease and a child with nephrotic syndrome, the administration of rituximab was associated with the development of UC. In one patient with refractory UC, rituximab caused an exacerbation of symptoms that was associated with the depletion of IL-10-producing B cells. Although few adverse reactions were reported in the trial of Leiper et al, with the most notable being one chest infection and three mild infusion reactions, it remains to be seen if, during more prolonged larger studies, targeting TCR on B cells leads to the same severe reactions as it does with T cells.
In addition to TCR–antigen interaction T cells require separate costimulatory signals for full activation. CD28, which is expressed on T cells, interacts with its ligands CD80 and CD86 on APC, but the cytotoxic T-lymphocyte-associated antigen 4 can interrupt this interaction to induce T-cell anergy and apoptosis. Abatacept is a soluble recombinant fusion protein containing cytotoxic T-lymphocyte-associated antigen 4 and IgG1 that is approved for use in RA and is currently being trialled for IBD. However, trials both in patients with CD and UC at doses superior to those approved for RA indicated that although abatacept was well tolerated with no significant difference in adverse effects compared with placebo, the rate of remission with abatacept was not significantly different either.
These disappointing results including the many failures in the attempt at blockade of T-cell function in IBD are difficult to interpret. However, one could speculate that general blockade of T cells is not key or could even be detrimental if it includes T-regulatory cell blockade, and perhaps specific blockade of the pathogenic T cells secreting inflammatory cytokines should be aimed at.
Targeting the Adaptive Immune System
Extensive studies into T-cell polarisation in IBD have attempted to ascribe an easily addressed pathological mechanism to both CD and UC. Research shows that the patchy transmural inflammation of the gastrointestinal tract characteristic of CD is associated with the activation of types 1 and 17 T-helper (Th) cells in response to the production of interleukin (IL)-12, IL-18, IL-23 and transforming growth factor beta by antigen-presenting cells (APC) and macrophages. In turn Th1 and Th17 cells increase secretion of the pro-inflammatory cytokines IL-2, IL-17, interferon (IFN)-γ and TNFα. These cytokines feed into a self-sustaining cycle whereby they stimulate APC, macrophages, fibroblasts and endothelial cells to produce TNFα, IL-1, IL-6, IL-8, IL-12 and IL-18.
In contrast, the uniform mucosal inflammation of the colon found in patients with UC is suggested to be associated with a Th2 immune response mediated by IL-4, IL-5 and IL-13 that results in an increase in levels of IL-13. In both scenarios T cells are also activated by direct contact with antigens presented on the surface of APC and can be regulated by anti-inflammatory cytokines such as IL-10. Therefore, the majority of new biomolecules for IBD aim to reduce pathogenic T-cell activation and its effects by inhibiting the actions of pro-inflammatory cytokines, increasing anti-inflammatory cytokines, blocking T-cell co-stimulation, or inducing T-cell apoptosis.
Blockade of Pro-inflammatory Cytokines
The development of anti-TNFα therapies such as infliximab, adalimumab and certolizumab pegol have made a significant difference to the health-related quality of life of many patients with IBD, but among primary responders only a third will maintain remission after 1 year, and the risk of later surgery is still high in patients receiving anti-TNF therapy. The development of anti-TNFα therapies continues with novel TNF inhibitors such as golimumab, dersalazine, HMPL-004 and ozoralizumab (ATN-103) currently in various phases of the clinical trial process, but researchers are also exploring other avenues. One such closely related avenue is the development of a vaccine against TNFα consisting of the TNFα derivative TNFα kinoid (Debio-01512; Novacs, Paris, France), a complex of inactivated human TNFα and the carrier protein keyhole limpet haemocyanin. Having shown promise in animal models of chronic inflammation and rheumatoid arthritis (RA), TNFα kinoid was moved into phase I/II clinical trials in patients with moderate to severe CD. A recent report of the results of the trial indicates that TNFα kinoid was well tolerated with no serious adverse effects and with 76% of patients showing a clinical response and 43% achieving remission.
In addition to developing alternative ways to target TNFα, researchers have also turned their attention towards targeting other pro-inflammatory cytokines such as IL-12/IL-23, the α-chain (CD25) of the IL-2 receptor (IL-2R), and IFN-γ. Apilimod mesylate, ustekinumab, briakinumab (ABT-874) and SCH-900222 all act to inhibit the effects of IL-12/IL-23 and have shown limited results in early phase clinical trials. Apilimod is a small molecule inhibitor of the transcription of IL-12 and IL-23, ustekinumab and briakinumab both target the p40 subunit common to both IL-12 and IL-23, whereas SCH-900222 targets the p19 subunit that is specific to IL-23. An initial open-label dose-escalating trial with apilimod mesylate in patients with active CD indicated that the drug demonstrated clinical activity and was well tolerated, with the most common adverse events being dizziness, nausea, headache and fatigue. However, a subsequent randomised controlled trial showed that apilimod did not have a significantly greater efficacy than placebo treatment.
Ustekinumab was also subjected to phase II trials in patients with moderate to severe CD with slightly more promising results. Following 6 weeks of treatment, clinical response rates were significantly higher for the patients given ustekinumab at 53% compared with 30% for placebo. However, at week 8 the response rate was not significantly different, at 49% for ustekinumab and 40% for placebo. The long-term response to the drug is unknown but it is interesting that a subgroup of patients who had previously responded to infliximab had a greater response to ustekinumab. This effect was examined in a further trial in which 59% of patients who had previously been treated with infliximab responded to ustekinumab in comparison with 26% of patients who received placebo. The same trial also indicated that patients with higher baseline C-reactive protein (CRP) levels tend to have a greater response to ustekinumab. A more recent report of a multicentre, larger phase IIB trial in patients with moderate to severe CD who were unresponsive to infliximab indicated that at week 6 39.7% of patients treated with ustekinumab showed a clinical response compared with 23.5% of patients administered placebo (p<0.05), but the rates of clinical remission were similar. Those patients who responded to ustekinumab by week 6 were given maintenance therapy and at week 22 41.7% of patients achieved remission compared with 27.4% of patients given placebo. In addition, 69.4% of patients treated with ustekinumab had a clinical response at week 22 compared with 42.5% administered placebo, indicating that a significant number of patients who respond to ustekinumab remain in remission. In all three trials no serious adverse events were reported. Overall, given the results of those trials, it seems that ustekinumab may be particularly useful in patients who have previously failed to respond to anti-TNF therapy.
A phase I study of the response to briakinumab in patients with active CD showed that by week 8 the drug produced a response in 75% of patients compared with 25% given placebo. By week 18, however, the difference in response rates was no longer significant. A more recent study in patients with moderate to severe CD also concluded that briakinumab was not effective for the induction or maintenance of remission. In both trials the rates of serious adverse events were comparable between briakinumab and placebo. In the more recent study one patient treated with briakinumab died of respiratory failure due to pancreatitis, although this was thought to be unrelated to treatment. The final compound targeting the IL-12/IL-23 pathway, SCH-900222, is very early in development with the only trial in progress being a phase I/II dose-finding study in psoriasis.
IL-2 is a crucial cytokine for T-cell activation and proliferation, and has been investigated as a very attractive target for therapeutic intervention. Inhibitors of IL-2R include basiliximab and daclizumab, which are both monoclonal antibodies against CD25. An initial study with basiliximab in 10 patients with steroid-resistant UC indicated that nine out of 10 patients achieved clinical remission in 8 weeks. Eight of the nine responders then relapsed but remission was re-achieved with corticosteroids and azathioprine. At 24 weeks, seven patients were in full remission and tests indicated that patients were rendered steroid sensitive by the presence of basiliximab. A further open-label trial, in which patients with steroid-resistant UC were given a single dose of basiliximab in combination with steroids, resulted in 65% of the patients achieving clinical remission at week 24. Adverse events noted in the two trials included two cases of herpes zoster and two cases of fever that were resolved with treatment, along with other minor adverse events that did not require treatment. As there was no placebo control group it was not possible to determine if the events were attributable to the administration of basiliximab. In contrast to the results with basiliximab, although an initial open-label study of 10 patients with refractory UC indicated that daclizumab produced some clinical benefit, a larger randomised controlled trial in patients with moderate UC showed that those treated with daclizumab were not more likely to be in remission after 8 weeks of treatment compared with patients given placebo. Adverse events were few with daclizumab, with the most common being nausea in patients simultaneously treated with azathioprine as well as nasopharyngitis and fever.
Fontolizumab is the only biological therapy targeted against IFN-γ currently being tested in patients with IBD. An initial study in 45 patients with moderate to severe CD indicated that fontolizumab appeared to have a biological effect and was reasonably well tolerated, although there were slightly more reports of chills, flu-like symptoms and asthenia compared with placebo as well as two cases of worsening of CD symptoms. In a further study in 133 patients with CD, fontolizumab produced a clinical response in patients, but only after two doses given a month apart, with an increased response rate (69%) seen at day 56. Analysis of baseline CRP levels also indicated that elevated CRP was associated with pronounced clinical benefits. Fontolizumab was also well tolerated in this study and there were no evident patterns of adverse events attributable to fontolizaumab. A more recent study, in which the primary endpoint was clinical response on day 29, failed to show any significant clinical benefits in patients with CD treated with fontolizumab compared with placebo. However, at time points beyond 29 days patients treated with fontolizumab showed a significantly greater improvement in the CD activity index score and CRP levels, suggesting that the actions of fontalizumab are more gradual.
In addition to targeting IL-12/23, IL-2R and IFN-γ, more recent approaches have expanded to address almost all the pro-inflammatory cytokines shown to be elevated in IBD. Biological agents now entering phase I trials include monoclonal antibodies against IL-6, IL-6R, IL-13, IL-17, IL-18 and IL-21. As a pleiotropic cytokine IL-6 contributes to Th17 differentiation, and increased levels of IL-6 and soluble IL-6R are associated with increased disease severity in IBD. A polymorphism within the IL-6 gene has also been linked with early-onset CD, and persistent activation of the IL-6 signalling pathway plays a role in the development of colon cancer. Biological therapies targeting IL-6 include C326, currently undergoing a phase I trial in patients with CD; sirukumab (CNT0136), which has just been shown to be well tolerated in healthy subjects; CDP6038, currently being tested in patients with RA; and PF-04236921, which is going to be used in a phase I/II trial that is currently recruiting patients with CD who are unresponsive to anti-TNFα. In contrast, tocilizumab is a monoclonal antibody against IL-6R that has been through extensive trials in patients with RA, and has been approved as a second line monotherapy for patients with RA who have failed other approved therapies. Trials of tocilizumab in patients with IBD have been limited. The only trial completed to date was a placebo-controlled phase I study in 36 patients with active CD. The results of the study indicated that 80% of the patients given biweekly intravenous infusions of tocilizumab for 12 weeks had a clinical response compared with 31% of the placebo-treated patients, but only 20% of the patients given tocilizumab went into remission. Although the rates of adverse events were similar between tocilizumab and placebo in the study in patients with CD, the larger studies in patients with RA indicate that tocilizumab is associated with an increase in the incidence of reversible grade 3 neutropenia, elevated lipids, abnormal liver function and an increased risk of infection. These adverse events are generally mild and can be resolved with treatment but the longer term effects of increased lipids and abnormal liver function in response to tocilizumab need to be investigated.
The cytokine IL-13 is produced by naive T cells and activates natural killer T (NK-T) cells, which then also produce IL-13. The IL-13 produced by NK-T cells has been shown to be pathogenic in UC as it impairs the function of the epithelial barrier and also causes apoptosis of epithelial cells. Inhibition of IL-13 production with the administration of IFN-β1a also results in the suppression of inflammation in UC, which makes IL-13 an appropriate target for biological therapy in IBD. Some researchers have argued that enhancing IL-13 activity would also result in a therapeutic response in IBD as IL-13 suppresses the activity of the pro-inflammatory cytokines IFN-γ and IL-17 in murine models. Given the weight of the evidence that IL-13 plays a significant role in the pathogenesis of UC, it is unlikely that enhancing the levels of IL-13 will have a therapeutic effect in UC. Monoclonal antibodies against IL-13 currently in clinical trials for IBD include anrukinzumab, currently in a phase II trial in patients with UC, and QAX567, currently being used in phase I/II trials in patients with CD.
Similar to IL-6 and IL-13, monoclonal antibodies have also been developed to target IL-17, IL-18 and IL-21, which have all been shown to be present at elevated levels in the inflamed intestinal mucosa of patients with IBD. AMG827 and secukinumab (AIN457) both target IL-17, GSK1070806 targets IL-18, and ATR-107 (PF-05230900) targets IL-21, and all are currently in phase I/II clinical trials either in patients with CD or healthy volunteers. Of these therapies initial data are only available for secukinumab. In a double-blind, placebo controlled, proof-of-concept study in 59 patients with CD patients given secukinumab had disease worsening compared with placebo. In addition, there were some data to suggest that the inhibition of IL-17A exacerbated CD in a subset of patients and increased infection risk. The reason for such unexpected and opposite clinical effects are obscure because in psoriasis and RA blockade of IL-17A has been beneficial in proof-of-concept studies, but probably an effect of IL-17A in protecting barrier integrity and regulatory T-cell function is crucial in IBD compared with other inflammatory diseases. On the contrary, vidofludimus (4SC-101/SC12267), a small molecule inhibitor of the release of IL-17, has shown promise. Results from the single-arm, open-label ENTRANCE study indicate that vidofludimus is safe, well tolerated, with only mild to moderate drug-related adverse events reported, and may be useful in maintaining clinical remission. Following the administration of vidofludimus for 12 weeks in 26 steroid-dependent patients in remission with CD or UC 53.9% of patients remained in steroid-free remission, 34.6% were in remission at a lower corticosteroid dose than their baseline dose, and 11.5% showed no response. This suggests that attempts to modulate the function of IL-17 in IBD deserve more investigation.
In addition to inhibiting either cytokines or cytokine receptors an alternative means of reducing the inflammatory response in IBD is to block the downstream signalling pathways mediated by cytokines. As signalling molecules that interact with cytokine receptors the Janus kinases (JAK), JAK1, JAK2 and JAK3, play a crucial role in cell growth, survival, development and differentiation of immune cells. JAK1 and JAK2 are ubiquitously expressed but JAK3 is found only in haematopoietic cells and is part of the signalling pathways activated by IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. Tofacitinib (CP-690,550) is a first-in-class small molecule inhibitor of JAK3 currently in phase II clinical trials for various immune-related disorders including CD and UC. In-vitro studies of the activity of the drug indicate that it interferes with Th2 and Th17 cell differentiation, blocks the production of IL-17 and IL-22 and offers the possibility of effective suppression of the pathological immune response present in IBD. Recent results from a multicentre, double-blind, placebo controlled study in patients with moderate to severe active CD show that although tofacitinib is well tolerated it produced no clinically significant response following 4 weeks of treatment compared with placebo. Tofacitinib was, however, associated with a significant decrease in CRP and fecal calprotectin levels. In contrast, a study in patients with moderate to severe UC showed that 8 weeks of treatment with tofacitinib was associated with a dose-dependent improvement in both clinical response and remission rates. In trials both in CD and UC the overall incidences of adverse events were similar between patients administered tofacitinib and those given placebo. However, as has been seen to a lesser degree with tocilizumab, there was a dose-dependent increase in low-density lipoprotein cholesterol observed in patients treated with tofacitinib, which will require further investigation to determine the long-term effects.
Administration of Anti-inflammatory Cytokines
An alternative and logical approach to reducing the inflammation present in IBD would be to restore the balance between pro-inflammatory and anti-inflammatory cytokines by increasing the amount of anti-inflammatory cytokines present in the body. Anti-inflammatory cytokines that have been developed as biological therapies for IBD include IL-10, IL-11 and IFN-β. Unfortunately the results from clinical trials administering these compounds have not been very useful.
IL-10 first came to light as an anti-inflammatory cytokine when it was shown that IL-10-deficient mice spontaneously develop colitis. Since then it has been shown that IL-10 decreases intestinal inflammation and that variants of the IL-10 gene are linked to susceptibility to developing UC. This led to the development of recombinant human IL-10 that could be administered subcutaneously. An initial trial in 46 patients with steroid-resistant CD showed that 50% of patients administered IL-10 achieved remission compared with 23% in the placebo group. Unfortunately, further larger clinical trials showed that although the therapy was well tolerated and there was a tendency towards clinical improvement, remission was not achieved in any patients included in the studies. The only notable adverse effects observed in the two trials were dose-dependent, asymptomatic and reversible anaemia and thrombocytopaenia. IL-10 therapy was also tested in patients with CD who had undergone ileal or ileocolonic resection, but treatment did not prevent the recurrence of inflammation when compared with placebo, even when IL-10 was given daily. One possibility for the failure of the treatment may be due to the method of drug delivery, with systemic administration of IL-10 resulting in very little of it reaching the site of inflammation. A novel way of bypassing the systemic circulation has been developed using oral administration of genetically modified IL-10 producing Lactococcus lactis to target the cytokine directly to the gut mucosa. This approach has been shown to be safe, and has gone into phase II clinical trials, which showed lack of clinical efficacy. Similar to IL-10, recombinant human IL-11 has also been developed for subcutaneous administration. Although initial trials showed evidence that the approach was well tolerated and produced a response in more than 33% of patients with CD, when it was compared with prednisolone, IL-11 produced a significantly inferior response rate and rate of short-term remission. The most significant adverse effect noted in the three trials of IL-11 was a dose-dependent increase in platelet count, along with mild adverse events such as fever, rash, nausea/vomiting and headache.
The third approach using anti-inflammatory cytokines is that of the administration of IFN-β, which has been tested both in patients with CD and UC. IFN-β was first administered subcutaneously to patients with steroid-refractory UC, in which it produced an 88% remission rate with a mean length of remission of 13 months. Subsequent open-label and double-blind, placebo controlled studies in patients with UC produced a variety of results, with some studies indicating that IFN-β did have a therapeutic effect while others reported that IFN-β did not produce a significant therapeutic effect compared with placebo. It is interesting to note that the positive reports of an effect of IFN-β were from open-label trials, whereas the negative results were from randomised controlled trials. In randomised controlled trials higher rates of remission were always observed with IFN-β compared with placebo, but rates of remission in patients given placebo were also high enough to confound the results. A more recent trial with IFN-β in patients with CD also came to the conclusion that there was no difference between the effects of the administration of IFN-β and placebo. There is some suggestion that response to IFN-β is dependent on the levels of circulating IL-17 and IL-6, with increased levels of the cytokines being associated with no response to IFN-β. An additional complication of IFN-β therapy is that there have been reports that the administration of the cytokine in patients with multiple sclerosis was associated with the development of IBD. Furthermore, recent investigations in a mouse model of colitis indicated that the administration of IFN-β exacerbated colitis. Data from all the clinical trials conducted so far in patients with IBD do not confirm this suggestion, with the most common adverse effect being flu-like symptoms and no exacerbation of IBD symptoms reported.
Blockade of T-cell Stimulation and Induction of T-cell Apoptosis
During the process of physiological inflammation the population of activated T cells that responds to pathogens is constantly balanced by the occurrence of an equal amount of proliferation and apoptosis. During chronic pathological inflammation proliferation of T cells far outweighs apoptosis of T cells. One approach to controlling pathological inflammation would be to increase the T-cell apoptosis that occurs in response to a lack of antigen-specific and costimulatory signals from APC. Visilizumab is a monoclonal antibody against the CD3 chain of the T-cell antigen receptor (TCR), blockade of which leads to T-cell apoptosis. An initial phase I study in patients with severe steroid-resistant UC reported that all the patients achieved remission with the administration of visilizumab. This was followed by a further study in which 84% of patients with steroid-refractory UC achieved a clinical response. However, although a phase I/II dose escalation trial of visilizumab in severe UC showed evidence of clinical response, it also resulted in 100% of patients reporting adverse events including abdominal abscess, atrial fibrillation, cytomegalovirus infection and herpes zoster. Following on from that study a placebo controlled trial indicated that visilizumab was not effective in patients with thromboembolic adverse events. A trial in patients with CD also indicated that visilizumab is associated with cytokine release syndrome that results in transient liver injury. Therefore, although biologically attractive, excessive T-cell targeting often leads to serious adverse events.
As an alternative to targeting the proliferation of T cells, rituximab targets the CD20 component of the TCR on B cells with the aim of inducing apoptosis in circulating B cells. A phase II randomised controlled trial of rituximab in 24 patients with steroid-refractory moderate UC showed that although there was some clinical response with rituximab the rate of induction of remission was no different from placebo. In addition, any observed response was short term and was only maintained to week 12 following the administration of rituximab. Similar to the use of visilizumab, some severe adverse reactions have been reported with rituximab although they have generally been seen in other disease states. In isolated cases, including a patient with follicular lymphoma, a patient with Grave's disease and a child with nephrotic syndrome, the administration of rituximab was associated with the development of UC. In one patient with refractory UC, rituximab caused an exacerbation of symptoms that was associated with the depletion of IL-10-producing B cells. Although few adverse reactions were reported in the trial of Leiper et al, with the most notable being one chest infection and three mild infusion reactions, it remains to be seen if, during more prolonged larger studies, targeting TCR on B cells leads to the same severe reactions as it does with T cells.
In addition to TCR–antigen interaction T cells require separate costimulatory signals for full activation. CD28, which is expressed on T cells, interacts with its ligands CD80 and CD86 on APC, but the cytotoxic T-lymphocyte-associated antigen 4 can interrupt this interaction to induce T-cell anergy and apoptosis. Abatacept is a soluble recombinant fusion protein containing cytotoxic T-lymphocyte-associated antigen 4 and IgG1 that is approved for use in RA and is currently being trialled for IBD. However, trials both in patients with CD and UC at doses superior to those approved for RA indicated that although abatacept was well tolerated with no significant difference in adverse effects compared with placebo, the rate of remission with abatacept was not significantly different either.
These disappointing results including the many failures in the attempt at blockade of T-cell function in IBD are difficult to interpret. However, one could speculate that general blockade of T cells is not key or could even be detrimental if it includes T-regulatory cell blockade, and perhaps specific blockade of the pathogenic T cells secreting inflammatory cytokines should be aimed at.
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