Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis

Abstract

Our understanding of the etiology and pathogenesis of IBD has improved extensively over the past years. At the center of the pathogenesis seems to be an excessive pro-inflammatory immune reaction towards normal intestinal flora. The different factors involved in this concept will form the focus of this review. The initial phase of antigen processing and presentation can be influenced by either modulation of the intestinal flora via antibiotics or probiotics or by direct stimulation of macrophages through GM-CSF treatment. Antigen recognition and activation of T-cells can be down-regulated by immunosuppressives such as azathioprine, CsA or methotrexate thus building the basis for current treatment in IBD. The pro-inflammatory character of the immune reaction is defined by the predominance of certain T-cell subpopulations. By targeting cytokines the disbalance of these subpopulation should be reconstituted. Here we will focus first on preliminary clinical as well as experimental data for the pro-inflammatory mediators IL-12 and IL-18 as well as for the anti-inflammatory cytokine IL-10. Second, the clinical data for the TNFalpha antibody that has been proven to be efficacious in Crohn's disease and the associated risks will be discussed. Last, recent clinical and experimental data on targeting cell adhesion as well as intracellular signaling pathways will be presented. In summary, with regard to this review, treatments, which intervene as early as possible in the initiation of the pathological immune reaction and simultaneously have a favorable side-effect profile, must be the focus of future research.

Full text

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Siegmund B, Zeitz M
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Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis
Abstract
Our understanding of the etiology and pathogenesis of
IBD has improved extensively over the past years. At the
center of the pathogenesis seems to be an excessive pro-
inflammatory immune reaction towards normal intestinal
flora. The different factors involved in this concept will form
the focus of this review. The initial phase of antigen process-
ing and presentation can be influenced by either modulation
of the intestinal flora via antibiotics or probiotics or by direct
stimulation of macrophages through GM-CSF treatment.
Antigen recognition and activation of T-cells can be down-
regulated by immunosuppressives such as azathioprine,
CsA or methotrexate thus building the basis for current
treatment in IBD. The pro-inflammatory character of the
immune reaction is defined by the predominance of certain
T-cell subpopulations. By targeting cytokines the disbalance
of these subpopulation should be reconstituted. Here we will
focus first on preliminary clinical as well as experimental
data for the pro-inflammatory mediators IL- 12 and IL-18 as
well as for the anti-inflammatory cytokine IL-10. Second,
the clinical data for the TNF antibody that has been proven
to be efficacious in Crohn’s disease and the associated risks
will be discussed. Last, recent clinical and experimental data
on targeting cell adhesion as well as intracellular signaling
pathways will be presented. In summary, with regard to this
review, treatments, which intervene as early as possible
in the initiation of the pathological immune reaction and
simultaneously have a favorable side-effect profile, must be
the focus of future research.
Key words: inflammatory bowel disease, mucosa, therapy.
Introduction
The entire etiology and pathogenesis of inflammatory bowel
diseases (IBD) is still unresolved, however, the understanding
has improved extensively over the past years. In light of the
diversity of substances and bacteria within the intestinal lumen,
it is remarkable that the gut is not perpetually inflamed. The
presence of low-level physiologic inflammation within the
healthy intestinal mucosa represents a state of preparedness
to deal with potentially harmful agents, but a more vigorous
response would be not appropriate if directed toward the
innocuous commensal flora of the gut. Inflammation is kept
in check through an active process of immune tolerance. The
dysregulation of the process of tolerance accompanied by an
excessive pro-inflammatory immune reaction towards normal
intestinal flora builds the current basis for the understanding of
the pathogenesis of IBD, and furthermore, this concept creates
the basis for distinct immunomodulatory approaches.
The present review will provide a systematical overview of
the immunomodulatory approaches in IBD. As illustrated in
Fig. 1, we will first focus on antigen processing, presentation and
activation of macrophages and underline the therapeutic impact
of probiotics and antibiotics. In addition, the potential role of
granulocyte macrophage-colony stimulating factor (GM- CSF)
therapy will be discussed. Activation of macrophages results
in T-cell activation thus in a second step antigen recognition
and activation of CD4
+
T-cells will build the focus. The
unspecific suppression of the T-cell response by the classic
immunosuppressive drugs azathioprine, cyclosporine A,
methotrexate and cyclophosphamide and their efficacy in IBD
Therapeutic approaches in inflammatory bowel
disease based on the immunopathogenesis
Siegmund B, Zeitz M
Department of Medicine I, Charité Universitätsmedizin Berlin, Campus Benjamin Franklin, Germany
ADDRESS FOR CORRESPONDENCE:
Martin Zeitz, M.D.
Professor of Medicine
Department of Internal Medicine I
Charité Universitätsmedizin Berlin
Campus Benjamin Franklin
Hindenburgdamm 30
1200 Berlin, Germany
Tel: ++30-8445-2347 Fax: ++30-8445-4481
e-mail: gastro.cbf@charite.de
Received 8.03.2004 Accepted 6.06.2004
Roczniki Akademii Medycznej w Białymstoku · Vol. 49, 2004 · Annales Academiae Medicae Bialostocensis

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Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis
therapy will be evaluated. In particular, the recently described
mechanism of action for azathioprine will be summarized. The
broad T-cell activation is followed by the generation of a T-helper
cell type 1 and 2 response with distinct functions. Targeting of
cytokines has been a promising approach within the last years.
The clinical and experimental data of IL-10, IL-12 and IL-18 will
be described. The anti-TNF therapy with infliximab will serve
as example to outline therapeutic impact as well as difficulties
associated with anti-cytokine therapy, in particular infectious
complications. Cytokines and other mediators activate second
messenger systems which then perpetuate the inflammatory
process. MAPK inhibitors targeting these signaling pathways
are currently in phase II clinical trials for Crohn’s disease. Last,
inflammatory cells require the expression of adhesion molecules
to allow for cell recruitment, migration, and adhesion, therefore
blockade or neutralization of these adhesion molecules seems
to be an attractive approach, clinical data are as yet not that
conclusive.
The overview provided, does not claim to be complete,
however, it may serve to facilitate the understanding of the
complex variety of immunomodulatory approaches in IBD
(Fig. 1).
Immunomodulatory approaches
Antigen processing and presentation
There is evidence that specific microbes in the commensal
gut microflora are more important than sporadic infections in
atopic disease prevention. Gastrointestinal flora is essential
for the maintenance of mucosal tolerance, which is disrupted
in patients with inflammatory bowel disease. Several processes
seem to be involved in this mechanism: T-helper cell type
immunity [1], induction of oral tolerance [2], and IgA production,
an essential component of immune defense [3]. Inflammatory
bowel disease is a relapsing inflammatory disorder of unknown
cause in which genetic, immunological, and environmental
factors may be involved [4]. It has also been postulated that
ulcerative colitis is at least partly caused by infection because
there are common histological features with infectious colitis
and it is difficult to induce colonic inflammation in germ-free
animals [4].
Probiotics
Probiotics are cultures of potentially beneficial bacteria of
healthy gut microflora [5]. Several bacteria have been proven to
Figure 1. Pathogenesis of inflammatory bowel disease and potential therapeutic targets. The different steps from antigens/bacteria cross-
ing the epithelial barrier to phagocytosis and antigen presentation by antigen-presenting cells followed by T-cell activation and subsequent
release of pro- and anti-inflammatory mediators and the migration of inflammatory cells via adhesion molecules is shown. The bold
printed headings in the boxes indicate the different steps that will be discussed in this review
TFNα
(-)
IL-10
IL-18
IL-12
IL-4
IL-2
adapted from Sands 2000
Antigens
Bacteria
Repair and restitution
– ?Heparin
– ?IL
-11
– ?Nicotine
Ag processing & presentation,
activation of macr
ophages:
– Antibiotics, P
robiotics
– GM-CSF
Ag recognition & activation of
CD4+ T cells:
– AZA/6MP
– CyA, Tacrolimus
– Mtx, Cyclophosphamide
Generation of Th1/Th2 response:
– Infliximab
– IL-10
– IL-12, IL-18
Recruiment, migration and adhesion:
– Antisense oligonucleotides to ICAM-1
– Anti-α4 integrin antibody
Inflammation and injury:
– Aminosalicylates
– Corticosteroids
Second messenger systems
:
– MAPK inhibitors
ICAM-1
IFNγ
(-)
IFNγ

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Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis
express beneficial effects in vivo. Kaliomäki and colleagues could
demonstrate that Lactobacillus GG was effective in prevention
of early atopic disease in children at high risk [6]. In addition,
treatment with a non-pathogenic E. coli has an equivalent effect
to mesalazine in maintaining remission of ulcerative colitis.
Furthermore, Gionchetti and colleagues demonstrated that
oral administration of a probiotic preparation is effective in
preventing flar-ups of chronic pouchitis [7].
Antibiotics
Several studies indicate an efficacy of a combinational
therapy of metronidazole plus ciprofloxacin with or without
steroids in the treatment of recurrent or refractory pouchitis
[8- 10]. In addition, patients with Crohn’s disease and
involvement of the colon might benefit from this combined
antibiotic therapy [11].
Granulocyte-macrophage colony
stimulating factor
Clinical data
Granulocyte-macrophage colony stimulating factor (GM-
CSF) is a growth factor that has been in clinical practice for years
in particular in oncology in order to reconstitute the immune
system after chemotherapy or bone marrow transplantation. It
appears to be a provocative approach to introduce GM-CSF in
the treatment of IBD where immunosuppressive strategies have
been dominating for decades. In a pilot study performed at the
University of Washington, St. Louis, USA, eleven patients with
Crohn’s disease were treated with GM-CSF. Nine out of 11
treated patients showed a significant decrease in the Crohn’s
disease activity index (CDAI) [12]. Six out of 11 patients went
into remission while treatment with GM-CSF. In one patient
a fistula closure was observed. After the GM-CSF treatment
period, 2 patients presented with a relapse, however responded
again after readministration of GM-CSF [12]. However, these
are preliminary data, in order to evaluate efficacy and to be
able to compare GM-CSF treatment with standard therapy
the results of the planned double-blind multi center trial will
provide conclusive data.
Potential way of mechanism
GM-CSF is targeting one of the first steps by supporting
potentially weak cell populations in IBD patients, the
macrophages and granulocytes [13]. Both are responsible for
the unspecific immune defense against invading bacteria. Is
this unspecific defense failing, the specific immune response
is stimulated and followed by an exacerbation of inflammation.
GM-CSF is expressing anti-inflammatory potency by supporting
the unspecific immune system and therefore inhibition of the
specific immune response. Preliminary data show promising
results in patients with IBD, but the results of the phase II trial
will have to proof this approach.
T-cell recognition and activation
of CD4
+
T-cells
The early steps of T-cell activation in the mucosa can be
down-regulated in by immunosuppressives such as azathioprine,
cyclosporine A, methotrexate or cyclophosphamide.
Azathioprine
Clinical data
The thioprine analogues azathioprine and 6-mercaptopurine
have been considered as treatments for Crohn’s disease since
the initial report of Brooke and colleagues describing healing
of fistulas with azathioprine [14]. A decade later the efficacy
of this drug was proved in a randomized controlled trial by
Present and colleagues [15]. A meta-analysis of studies of
azathioprine and 6-mercaptopurine in Crohn’s disease has
provided the best summary of the effects of these drugs. For
active disease, treatment produced an odds ratio of response
of 3.09 compared with placebo, with improved response when
treatment was continued for at least 17 weeks [16]. Convincing
evidence of benefit was also seen in maintenance of remission
(odds ratio over placebo 2.27) [16]; glucocorticoid sparing [17,
18], and improvement in fistulas [16]. In addition, a recent study
provided important information to the question whether or not
one has to discontinue ongoing immunosuppressive therapy
with azathioprine/6-mercaptopurine before elective surgery?
The study by Aberra and colleagues demonstrated convincingly
that azathioprine/6-mercaptopurine alone and the addition
of azathioprine/6-mercaptopurine to corticosteroid therapy
did not result in a significant increased risk of postoperative
infectious complications [19]. Overall, approximately one half
to two thirds of patients may respond to therapy. In contrast
to glucocorticoids, mucosal healing is frequently seen with
adequate dosing of thioprine agents [20].
Mechanisms of action
The molecular mechanism of azathioprine action however
has been unresolved. Recent data suggest that azathioprine
and its metabolites 6-mercaptopurine as well as 6-thioguanine
induce apoptosis in primary intestinal lymphocytes [21].
Additional studies revealed a suppression of NF- B and the
MEK kinase phosphorylation after azathioprine treatment. The
authors explain this by binding of the azathioprine-generated
6- Thio-GTP to GTPase Rac1 instead of GTP. This is resulting in
an inhibition of the Rac/MEKK/NF- B pathway and a following
caspase-9-dependent apoptosis. The affinity of the 6-Thio-GTP
Rac binding is 20-times lower than that of regular GTP which
is explaining the delay of therapeutic efficacy well known for
azathioprine. In summary, these data suggest that azathioprine
is inducing a selective apoptosis of activated T-lymphocytes in
the lamina propria and therefore expressing anti-inflammatory
potency.

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Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis
Cyclosporine A
Cyclosporine is being used increasingly in severe ulcerative
colitis. Favorable results have been reported for intravenous use
(4 mg/kg) and have been confirmed in a placebo-controlled trial
[22]. From 50 to 80% of patients with severe attacks who fail
to respond to intravenous glucocorticoids may avoid colectomy
during the attack. Anecdotally, patients appear to respond
well but tend to relapse soon after treatment is stopped. In
contrast, there appears to be no significant role for cyclosporine
in Crohn’s disease. A series of uncontrolled and randomized
controlled trials has shown high doses of cyclosporine to be
efficacious in treating inflammatory disease and fistulas but an
unacceptable high cost in adverse effects [23].
Methotrexate
Clinical data
A promising open-labeled study of methotrexate in IBD
[24] led to a randomized controlled trial in Crohn’s disease.
Patients with chronic active Crohn’s disease despite at least 3
months of prednisone 12.5 mg/day or more with at least one
failed attempt to taper off treatment were enrolled [25]. Overall,
39.4% of patients assigned to methotrexate achieved remission
off prednisone compared with 19.1% of placebo-treated
patients [25]. Methotrexate is also beneficial in maintenance of
remission. A follow-up study randomized patients who achieved
remission on methotrexate 25 mg once weekly i.m. to receive
either placebo injections or 15 mg methotrexate i.m. At week
40, 65% of patients treated with methotrexate were still in
remission compared with 39% of placebo-treated patients [26].
Treatment was well tolerated.
Experimental data
Although methotrexate is a folate antagonist, the drug is
often given with folic acid to prevent nausea and stomatitis.
Therefore, other modes of action are likely responsible for its
efficacy. The drugs possess a variety of immune-modulating
and anti-inflammatory effects, including inhibition of IL- 1,
IL-2, IL-6 and induction of adenosine, which has direct
immunosuppressive properties [27].
Methotrexate may be considered as an alternative to the
thiopurine analogs, particularly among patients who do not
tolerate these drugs. Some patients who fail to respond to
6- mercaptopurine may respond to methotrexate [28].
Cyclophosphamide
One major problem in the management of steroid
refractory attacks of patients with IBD is the establishment
of a rapidly acting immunosuppressive regimen. Previous
studies investigating the efficacy of cyclophosphamide in
Crohn’s disease are either single case reports or low dose oral
cyclophosphamide instead of intravenous pulse therapy was
administered [29,30]. Intravenous cyclophosphamide pulse
therapy is well known for its efficacy in vasculitis. Based on these
experiences seven steroid-refractory patients, five with Crohn’s
disease, two with indeterminate colitis, received 4 to 6 cycles of
monthly treatment of intravenous cyclophosphamide (750 mg)
in a prospective, uncontrolled pilot study [31]. All patients
improved after two intravenous pulses of cyclophosphamide and
six of seven patients achieved complete remission. Tapering to
low dose steroids was possible in all responders. Remission was
maintained in all patients for 18 months but required a second
course of cyclophosphamide in one patient. The drug was
well tolerated, except for two episodes of candida esophagitis.
These preliminary results are promising and merit evaluation in
a controlled trial [31].
Generation of a Th1/Th2 response
With the targeted use of cytokines attempts have been made
to influence the balance between the T-cell subpopulations.
IL- 10 as well as the blockade of TNF , IL-12 and IL-18 should
be mentioned in this context. The problems which occur when
a specific immune mediator is blocked will be discussed in more
detail with the available data from the infliximab therapy.
Interleukin-10
Clinical data
IL-10 has first been described in 1989 as anti-inflammatory
cytokine and came soon into the focus of interest as potential
new anti-inflammatory mediator [32]. Administration of rhIL-10
to healthy volunteers is well tolerated. In higher concentrations
a non-clinical significant decrease in hemoglobin as well as
platelet count has been described [33]. Because of the promising
in vitro and experimental data as further described below, IL- 10
was tested as potential therapeutic agent for inflammatory
bowel disease, in particular Crohn’s disease. However so far,
multiple placebo-controlled studies have been performed
without proving clinical efficacy. Different subgroups have been
studied, for instance: mild to moderately active Crohn’s disease
(n=95) [34] and therapy refractory Crohn’s disease (n=329)
[35]. In both studies IL-10 was well tolerated and a tendency
towards clinical improvement could be demonstrated but no
remission was observed. An additional study by Colombel and
colleagues showed that treatment for 12 weeks after intestinal
resection was safe and well tolerated, but there was also no
evidence of prevention of endoscopic recurrence of Crohn’s
disease observed [36].
Anti- and pro-inflammatory properties
of Interleukin-10
IL-10 as a T-helper cell type 2 cytokine is directly inhibiting
several pro-inflammatory mediators known to contribute
to the inflammatory process in inflammatory bowel disease
[37]. In the last years IL-10 has in addition been described
as predominant cytokine of so-called regulatory T-cells [38].
However, there are data from the recent literature suggesting
a distinct pro-inflammatory role for IL-10. In particular,
when LPS administered to humans followed by an injection

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Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis
of IL-10, IL-10 enhanced IFN release as well as the release
of the IFN - dependent chemokines IFN -inducible protein- 10
and the monokine induced by IFN [39]. In addition, when
administering IL-10 to Crohn’s disease patients the serum
neopterin concentrations of IL-10-treated patients increased as
well as the IFN release after post treatment in vitro stimulation
[40].
IL-10 and experimental evidence in animal models
Still, experimental models strongly indicate that the local
IL-10 concentration at the site of inflammation might represent
the crucial factor. For instance, Lactococcus lactis secreting
IL-10 administered orally to mice prevented dextran sulphate
sodium-induced colitis [41]. In addition, the CD4 CD45Rb
low
cells, the population known to prevent colitis induction by
CD4 CD45Rb
high
cells, is not mediating this protective effect
when isolated from IL-10 knockout mice [42]. Therefore current
strategies are focusing on technical approaches facilitating an
increased IL-10 synthesis at the site of inflammation.
Interleukin-12
Clinical data
IL-12 represents the predominant cytokine responsible for
the development of Th1 cells, the T-cell population known to
contribute significantly to the inflammatory process in particular
in Crohn’s disease [43-45]. Currently an antibody against the
IL- 12 receptor is under investigation in a phase IIa clinical
trial in ulcerative colitis patients and in addition, an anti-IL-12
antibody is under investigation in a phase II trial in patients with
Crohn’s disease.
Experimental data
Early experimental data suggest that the neutralization
of IL-12 might results in an amelioration of disease. For
instance, administration of monoclonal anti-IL-12 antibodies
to mice suffering from trinitrobenzene sulfonic acid-induced
colitis led to a striking improvement in both the clinical and
histopathological aspects of the disease and frequently abrogated
the established colitis completely [46]. In later studies it could
be demonstrated that the amelioration of disease severity after
anti-IL-12 treatment is associated with an increase in apoptosis
at the site of inflammation [47]. An increase in apoptosis and the
association with disease improvement has been demonstrated
in several experimental models for a variety of mediators [48,
49] and there exists additional evidence in humans that this
mechanism may in fact be of significance [50].
Interleukin-18
Clinical evidence
Several studies provide strong direct and indirect evidence
for a significant role of IL-18 in intestinal inflammation.
Consistent with an increased Th1 response in Crohn’s disease,
several groups could independently demonstrate a significant
up-regulation of IL-18 expression in the inflamed lesions of
the intestine, mostly localized to macrophages and epithelial
cells [51,52]. This is of particular interest mainly because no up-
regulation of IL-18 was observed in patients with active ulcerative
colitis or healthy controls. While increased IL- 18 expression is
providing first evidence for a possible role in disease, in order
to prove that IL-18 participates in the inflammatory process of
intestinal inflammation it has to be demonstrated that blockade
of IL-18 results in amelioration of disease severity.
Experimental data
In the recent literature, four studies using different animal
models of colitis and different ways of IL-18 neutralization
approach this question. Convincingly, all groups demonstrate
a therapeutic efficacy for all anti-IL-18 strategies employed
independently from the model investigated [53-56]. Results
from these experimental models, in combination with the
descriptive data from patients with Crohn’s disease, suggest
an important function of IL-18. IL-18 requires the cleavage
of the interleukin-1 converting enzyme (ICE) in order to
become activated [57]. Recent studies examined the acute and
chronic model of DSS-induced colitis in ICE knockout mice
[49]. In particular, during chronic administration of DSS, ICE
knockout mice presented with an almost complete absence of
colitis. Several ICE inhibitors are available for experimental
use. For instance, Pralnacasan is currently in phase II trials in
rheumatoid arthritis. No toxic side effects have been observed
[58]. Compared to currently used strategies to suppress specific
cytokines, which mostly implicate antibody therapy, the
possibility of having an orally available drug whose half-life can
be easily controlled is highly intriguing.
Tumor necrosis factor-
Clinical studies
The TNF antibody infliximab has proved to be effective
in certain clinical situations in Crohn’s disease. Targan and
colleagues could first demonstrate in a multicenter, double
blind, placebo-controlled trial over 12 weeks, the efficacy of
infliximab in patients with moderate-to-severe, treatment-
resistant Crohn’s disease [59]. 48% of patients were in
remission 4 weeks after single 5 mg/kg BW infusion compared
to 4% in the placebo groups [59]. Interestingly, administration
of 5 mg/ kg BW was clearly superior to 10 or 20 mg/kg BW
treatment. D’Haens and colleagues subsequently investigated
the endoscopic scores after treatment with 5, 10 or 20 mg/ kg BW
infliximab [60]. The Crohn’s disease endoscopic index of severity
decreased significantly, 62%, 68% and 21% in the 5 mg/kg BW,
10 mg/ kg BW and the placebo group, respectively [60]. The
clinical improvement was accompanied by significant healing
of the endoscopic lesions and a disappearance of the mucosal
inflammatory infiltrates. In the recently published ACCENT I
trial, Hanauer and colleagues evaluated the long-term
efficacy of repeated infliximab administration [61]. Repeated
infusions of either placebo, 5 mg/kg or 10 mg/kg BW, at 2, 4,
6 and then every 8 weeks were administered. 335 out of 580
patients showed a primary response. These results indicate that
patients with Crohn’s disease who respond to an initial dose of

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Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis
infliximab are more likely to be in clinical remission at week 30
and 54, to discontinue corticosteroids, and to maintain their
response for a longer period of time, if infliximab treatment
is maintained every 8 weeks [61]. Repeated infusions have
been demonstrated to be associated with an increased risk
of infusion reaction. A recent study by Baert and colleagues
could confirm these findings and were able to provide evidence
that the development of antibodies against infliximab is in
addition associated with a reduced duration of response to
treatment. In addition, the data from this study are indicating
that concomitant immunosuppressive therapy reduces the
magnitude of the immunogenic response [62].
However, it has become evident at the same time that such
treatments are combined with corresponding risks: for instance,
severe mycobacterial infections were observed in treated
patients [63] and are discussed in more detail below.
Mystery of etanercept failure in the treatment
of Crohn’s disease
Etanercept is a construct of two identical extracellular
chains of the soluble TNF-RII (also known as the p75 receptor)
linked to the Fc domain of IgG1. It has been demonstrated
to be highly effective in the treatment of rheumatoid arthritis
[64]. Hence, it was first obvious to assume that etanercept
administration would also prove to be effective in Crohn’s
disease patients. Sandborn and colleagues performed an eight-
week placebo-controlled trial in 43 patients with moderate to
severe active Crohn’s disease. Etanercept was injected twice
weekly in a concentration of 25 mg, the concentration which
has been shown to be of therapeutic impact in patients with
rheumatoid arthritis [64]. After 4 weeks 39% of etanercept-
treated versus 45% of placebo-treated patients presented with
a clinical response [65]. The authors concluded at this point that
administration is safe but not effective and that higher doses may
be required to attain a response in patients with active Crohn’s
disease. However, one might speculate whether or not the low
dose or a different mechanism is responsible for the observed
difference between etanercept and infliximab. Van Deventer
suggests in the Editorial to this study that infliximab but not
eternacept can induce apoptosis by binding to membrane-
bound TNF , thereby expressing its ant-inflammatory potency
[66]. Since apoptosis induction seems to be crucial in order to
achieve clinical improvement this might in fact represent the
crucial difference in between the two therapeutic strategies.
Blockade of tumor necrosis factor- and
tuberculosis
Clinical studies
Infliximab is a humanized antibody against TNF that
is used in the treatment of Crohn’s disease and rheumatoid
arthritis. Approximately 147 000 patients throughout the world
have received infliximab [63]. The half-life of infliximab is 10
days [67], and its biologic effect persists for up to 2 months.
Keane and colleagues evaluated in a recent study the clinical
pattern of disease and the interval between the initiation of
infliximab therapy and the onset of disease in 70 reported
cases of patients treated with infliximab [63]. Excess TNF in
association with TBC may cause weight loss and night sweats, yet
in animal models it has a protective role in the host response to
tuberculosis. There is no direct evidence of a protective role of
TNF in patients with TBC. The data by Keane and colleagues,
although passive surveillance data are often insufficient to prove
a causal relation between an adverse event and a drug, strongly
suggest that the association observed is not coincidental.
In particular, the pattern of TBC disease was unusual, the
majority of patients had extrapulmonary TBC, and 24% had
disseminated disease – forms of TBC that are associated with
marked immunosuppression. In contrast, among cases of TBC
that are not associated with HIV infection, approximately 18%
are manifested as extrapulmonary disease, and disseminated
disease accounts for less than 2% [68]. Although there is no
complete information about the status of these patients with
respect to TBC infection before they received infliximab, it
is likely that most patients had a reactivation disease. Given
the key role of TNF in the innate immune response to TBC,
patients receiving infliximab are probably also susceptible to
disease after primary infection and exogenous reinfection with
M. tuberculosis.
Consequences
As a consequence of the clinical and experimental data the
European Agency for the Evaluation of Medicinal Products
(EMEA) provided a public statement on infliximab with an
update on safety concerns. The Committee for Proprietary
Medicinal Products (CPMP) concluded that infliximab
continues to have a positive benefit/risk balance in both
Crohn’s disease and rheumatoid arthritis, provided that specific
changes to the product information restricting the indications
in Crohn’s disease and reinforcing the special precautions and
special warnings have been made. Therefore, because of safety
Table 1. International recommendations for infliximab therapy in Crohn’s disease (CD)
Target patient group
• Efficacy has been established in active CD, we recommend restriction to its use to refractory CD
• Refractory = full and adequate dosed course of glucocorticoids in addition to immunomodulation
• has failed or other drugs are not tolerated or not appropriate and surgery is not indicated
• First-Line use in non-fistulizing, uncomplicated CD is not recommended
• Infliximab should be part of a long-term strategy
Administration
• Maximum of two infusions of infliximab within 4 weeks
• Clinical benefits are of limited duration
• Readministration is warranted in patients who relapse under adequate immunosuppressives

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Therapeutic approaches in inflammatory bowel disease based on the immunopathogenesis
concerns, the indications for treatment of Crohn’s disease have
been restricted as summarized in Tab. 1 [67].
Cell recruitment, migration, and adhesion
Clinical data
In a multicenter, placebo-controlled trial conducted in 75
patients with steroid refractory Crohn’s disease subcutaneous
treatment with ISIS-2302, an antisense oligonucleotide directed
against intercellular adhesion molecule-1 (ICAM-1), did not
prove clinical efficacy based on primary endpoints [69]. Positive
trends were observed in some of the secondary endpoints [69].
In agreement with these data, the second double-blind, placebo-
controlled trial in active steroid-dependent Crohn’s disease also
failed to demonstrate efficacy [70].
Experimental data
However, the concept of blocking adhesion of inflammatory
cells thereby preventing intestinal inflammation is intriguing
and there are a variety of animal studies using different
models and technical approaches for inhibition of adhesion
molecules, supporting this target. While the efficacy of
antisense strategies against ICAM-1 could be proven in the
model HLA- B27/ beta2 microglobuline transgenic rat model,
this approach is questionable since no therapeutic benefit
could be observed in human trials [71]. More detailed animal
studies in the SAMP- 1/ Yit adoptive transfer model of Crohn’s
disease in mice could distinguish between an early acute phase
and chronic phase of inflammation. These studies suggest
that blocking of either ICAM-1 or VCAM-1, in this case by
neutralizing antibodies, may have therapeutic benefit for the
acute inflammatory component of Crohn’s disease [72].
Future studies will have to prove whether for distinct
indications this target might be of therapeutic significance.
Second messenger systems
Clinical data
A further possibility of intervention is the down-regulation
of “second-messenger systems” which are significant for
inflammatory immune reactions. Colonic biopsies from patients
with Crohn’s disease displayed enhanced JNK and p38 MAPK
activation [73]. In a pilot study, 12 patients with severe Crohn’s
disease received the MAPK inhibitor (CNI-1493) for a total of
12 days. Treatment resulted in diminished JNK phosphorylation
and tumor necrosis factor production as well as significant
clinical benefit and rapid endoscopic ulcer healing. A clinical
response was seen in 67% at 4 weeks and 58% at 8 weeks.
Therefore, inhibition of MAPKs provides a novel therapeutic
strategy [73]. The efficacy has to be confirmed in the phase II
trials.
Experimental data
Phosphorylation of intracellular molecules by a protein
kinase called mitogen-activated protein kinase (MAPK) with
a molecular weight of 38 kDa plays a major role in the synthesis
and activity of several proinflammatory cytokines, particularly
IL-1 , TNF , and the chemokines. Inhibitors of MAPK are non-
specific anti-inflammatory agents. P38-MAPK is referred to as
a stress kinase because it becomes activated (phosphorylated)
in response to extracellular stresse such hyperthermia,
hyperosmolarity, ultraviolet light, and radiation. Of major
importance is the fact that this kinase, which contributes to the
activity of IL-1 and TNF , is itself activated by IL-1 and TNF.
Therefore, phosphorylation of p38-MAPK can occur both via
highly evolved, specific cytokine receptor signaling pathways
and by the primitive and environmental stress responses
common to all organisms. The mode of action of p38-MAPK
inhibitors appears to be predominantly through suppression
of pro-inflammatory cytokine production. In healthy human
volunteers injected with endotoxin, the signs and symptoms
of this systemic inflammatory response model were assessed
following a single oral dose of a p38-MAPK inhibitor. No fever
was observed, and peak circulating levels of TNF , IL-6, and
IL-8 were reduced by 90% [74].
Conclusions
This review provides an overview of the increasing variety
of therapeutic approaches in IBD based on the pathogenesis.
The advances in the last years in the understanding of mucosal
immunology contributed significantly to the development of
targeted therapies. Primary goals of therapy are to induce
and maintain remission, thus ameliorating symptoms and
improving the patient’s quality of life. Summarizing all the
different therapies discussed above, one has to conclude
that, although specific treatments provide more insight in the
understanding of disease, the general immunosuppressive drugs
such as azathioprine appear to express the most efficacious
balance between achieving the therapeutic goals and reducing
side-effects. Distinct clinical situations may benefit from more
specific therapies, for instance, infliximab treatment in therapy
refractory fistulas. However, these more targeted drugs seem
to be associated with a yet unknown spectrum of side-effects.
Therefore, it is essential to consider the adverse consequences
of therapy, particularly with regard to any durable consequences
of short-term treatment and adverse effects of maintenance
therapy.
Acknowledgment
Fig. 1 was reprinted from Gastroenterology 118 (2
Suppl 1), Sands BE, “Therapy of inflammatory bowel disease”,
68-82, Copyright (2000), with permission from American
Gastroenterological Association.
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