HornbyExpertOpinDrugDiscov2015

1. Introduction
2. Etiology of IBS
3. Target pathways for IBS from a
gut ‘outside-in’ perspective
4. Biomarkers for IBS
5. Recent and pending approvals
for local gut-targeted IBS
therapeutics
6. Conclusion
7. Expert opinion
Review
Drug discovery approaches to
irritable bowel syndrome
Pamela J Hornby
Janssen Research & Development, Cardiovascular and Metabolic Disease, Janssen Pharmaceutical
Companies of Johnson and Johnson, Spring House, PA, USA
Introduction: Irritable bowel syndrome (IBS) is defined by symptoms of
abdominal pain and altered bowel habits without detectable organic disease.
Antidepressants and serotonin receptor modulators are used to treat IBS, but
rare serious adverse events highlight the safety hurdle. Newer drugs with
secretory and motility effects via local gut mechanisms have been successfully
approved for IBS, often by registering first in a related, non-IBS condition to
optimize dosing, formulation and therapeutic window.
Areas covered: This review looks at approaches for novel IBS drug discovery.
The underlying pathologies can be tackled locally from the ‘outside-in’ (intes-
tinal lumen, mucosa and neuromuscular) to identify therapeutic targets. The
article discusses the mechanisms associated with bile acid malabsorption,
microbial dysbiosis, decreased intestinal barrier function, immune dysregula-
tion, motility and visceral hypersensitivity.
Expert opinion: Challenges for new drug discovery are the unknown mecha-
nisms underlying IBS, making it difficult to predict clinically efficacious molec-
ular targets, limited options for translational research and disease progression
biomarkers. Drugs acting locally via multiple targets (e.g., eluxadoline [The
U.S. Food and Drug Administration approved Viberzi (eluxadoline) for IBS-D
on May 27th 2015], crofelemer) to validated mechanisms are proving success-
ful with tolerable safety margins. Novel mechanisms, identified and
optimized based on the emerging role of nutrient signaling, probiotics or
microbial products, are promising. Therapeutic treatment earlier in disease
progression may improve response and have longer term benefits.
Keywords: barrier function, crofelemer, dorsal root ganglion, eluxadoline, endotoxemia, enteric
nervous system, gastrointestinal secretion, linaclotide, microbiome, motility, secretomotor reflex,
vagus nerve, visceral pain
Expert Opin. Drug Discov. (2015) 10(8):809-824
1. Introduction
Irritable bowel syndrome (IBS) is the most clinically recognized and studied of the
functional gastrointestinal (GI) disorders and affects up to 20% of the US popula-
tion. Although the formal diagnostic criteria for IBS is usually in the domain of gas-
troenterologists, most primary care physicians would recognize the key IBS
symptoms of intermittent or chronic abdominal pain associated with altered bowel
habits [1]. IBS is a diagnosis of exclusion currently defined by symptom criteria in
the absence of detectable organic disease. This can be challenging since available
methods, including the current gold standard, the Rome III criteria, perform only
moderately well. The symptom classification of IBS is for diarrhea-predominant
(IBS-D), constipation-predominant (IBS-C) and alternating IBS of which about
one-third patients fall into each category with associated pain/discomfort that can
be present to varying degrees across all groups.
There is a tremendous need for new therapies that are effective, well tolerated and
have a positive impact on quality of life in IBS sufferers as well as to reduce disease
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burden, as reviewed recently by several groups [2-4]. Treatments
are sought for improvements both in stool frequency and con-
sistency, and pain/discomfort and bloating in IBS. Typically,
attention to diet is advised, and treatments include fiber, laxa-
tives, antidiarrheals, antispasmodics, serotonin receptor modu-
lators and psychotropic agents. For IBS-D, the serotonin
5-HT3 receptor antagonist alosetron (restricted prescription
only) directly or indirectly reduces discomfort [2], and tricyclic
antidepressants, in addition to acting centrally, can also act
directly on the intestine to alter motility and secretion [5]. For
IBS-C, the serotonin 5-HT4 receptor agonist tegaserod
(2007 approval withdrawn in US as restricted use for women
as an investigational new drug), lubiprostone (approved for
the treatment of C-IBS in women only) and the GCC agonist
linaclotide (approved for the treatment of C-IBS and chronic
idiopathic constipation) have shown efficacy in improving
abdominal pain and constipation [3]. The development of the
newer therapeutics target signaling pathways locally in the
gut, where mechanisms involved in motility and secretion
can be modulated ‘proximally’, whereas more ‘distal’ targeting
to mechanisms in the CNS and immune systems await a
clearer understanding of the relationships between neural, met-
abolic, immune and intestinal functions.
Furthermore, alosetron and tegaserod were withdrawn
because of rare complications, ischemic colitis and thrombotic
episodes, respectively. The newer therapeutics for IBS can
normalize transit or are prosecretory, and these drugs also
have low systemic bioavailability, for example, linaclotide,
lubiprostone and rifaximin. Their efficacy is based on a local
site of action in the gut and they are associated with a lower
incidence of adverse effects [6]. That said, lubiprostone is asso-
ciated with a higher incidence of nausea [7] and includes a
Pregnancy Category C Risk warning. IBS is largely a chronic
cyclical disease with a female gender bias (including during
childbearing years) with high co-morbidities and low mortal-
ity. Any new effective medication has to be associated with an
extremely low incidence and severity of adverse effects. This
underscores the safety profile requirement for any new
approaches to therapies in IBS.
For the above reasons -- the unclear relationship within the
gut/brain/immune systems, as well as safety concerns -- my
opinions on IBS drug discovery are pragmatic. They are biased
toward finding new therapies from clinically validated targets in
related GI disorders, and in terms of safety by locally targeting
the mucosa without systemic exposure to the drug. Consistent
with this approach, the comprehensive studies by Hoffman
and colleagues demonstrated that the location of 5-HT4 recep-
tor on the colonic mucosa and the effect of 5-HT4 receptor
agonists on the colonic mucosa after intraluminal delivery
were sufficient to promote motility and reduce visceral pain [8].
2. Etiology of IBS
Despite decades of clinical research, the etiology of IBS is not
understood, but is thought to be the result of interactions
between psychosocial factors and brain--gut--immune dysregu-
lation. The first challenge in target identification and valida-
tion for pathological mechanisms interfering with these
interactions is the lack of predictive biomarkers for the syn-
drome. Efforts to stratify patients even in terms of dominance
of symptoms, such as pain/discomfort, diarrhea, constipation
or alternating, have proven challenging. Without predictive
biomarkers of the presence, or progression, of disease, it
becomes very difficult to establish causality with a large num-
ber of potential underlying pathologies. A short list of these
includes genetic predisposition [9], food intolerance/diet
[10,11], bile acid malabsorption [12], microbial dysbiosis [13],
decreased intestinal barrier function [14,15], infectious and
immune dysregulation [16,17], altered intrinsic and extrinsic
control of motility, visceral hypersensitivity [18] and maladap-
tive stress response [19]. Even increased body mass, which
could involve metabolic changes, has been associated with
increased prevalence of abdominal pain and diarrhea [20].
An enormous number of pharmacological targets are asso-
ciated with each of these mechanisms that could be evaluated
in preclinical models. However, this leads to the second
challenge in drug discovery approaches in IBS -- the ability
to translate mechanisms of bowel dysfunction that are assessed
in vitro or in vivo in preclinical models to drug efficacy and
safety in the clinic. IBS is a complex set of behaviors in
Article highlights.
. The neural, metabolic, immune and intestine
mechanisms, which drive IBS pathology and result in
pain and altered bowel function, are not fully
understood and until these emerge, developing new
drugs against novel targets is not feasible.
. The currently successful drug discovery approaches for
IBS have focused on mechanisms governing motility and
secretion within the intestine, sometimes with proof-of-
concept in related non-IBS gastrointestinal disorder
clinical trials before registration for an IBS indication.
. For targets related to intestinal motility and secretion,
preclinical research in human and rodent tissue is more
likely to be translational, and drugs that can be optimized
for low oral bioavailability will be less likely to be
associated with adverse effects due to systemic exposure.
. Targeting multiple receptors or subtypes in the intestine
may provide greater efficacy and emerging mechanisms
related to bile acid pool and the microbiome could
provide novelty over the known serotonin and µopioid
receptor actions.
. Identification of biomarkers predictive of IBS patient
(sub)populations and target engagement will enable
drug discovery when biomarkers are present in
preclinical models, are predictive of disease progression
and could identify where treatment earlier in disease is
beneficial.
. Novel drug discovery and development will benefit from
cooperative clinical and basic research between industry
and academia.
This box summarizes key points contained in the article.
P. J. Hornby
810 Expert Opin. Drug Discov. (2015) 10(8)
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humans that animal models cannot replicate, including
chronic discomfort and pain, emotional or social events. It
is a syndrome that involves subjective human reports of expe-
riences and symptoms (e.g., sense of urgency, bloating). This
means that the animal models focus on one or two objective
measures (e.g., motility, secretion) that stand in the place of
a subjective constellation of dysfunctional systems in humans.
Some of these can be approximated by animal models better
than others, for example, altered motility after resolution of
overt inflammation in rodents may have translational compo-
nents to post-infectious IBS occurring subsequent to a bout of
enteritis. With the difficulties and limitations of translation
using preclinical models, investigators are increasingly utiliz-
ing human ex vivo mucosal biopsies [21] and tissue [22] to
understand pathophysiology. However, the overarching prob-
lem underlying drug discovery in IBS is the lack of clarity on
the inter-relationships between mechanisms (immune, neural,
metabolic and intestinal) and how these transcend to symp-
toms in IBS (Figure 1). Therefore, although research has iden-
tified multiple processes and mechanisms governing GI
physiology using drugs in in vitro, ex vivo and in vivo models,
only changes in motility, secretion and bile acids may be con-
sidered translational and can be viewed as biomarkers that
associate with IBS symptoms. Other pathophysiological
mechanisms (e.g., visceral hypersensitivity, immune function,
barrier function), while they can be measured in both clinical
and preclinical research, have not been predictive of drug effi-
cacy in IBS. For example, visceral hyperalgesia can be induced
by intracolonic zymosan A, followed by colorectal distention
and quantitation of the pseudoaffective (or nociceptive)
response. This enables testing of targets, such as 5-HT4 recep-
tor and k opioid receptor (OR) agonists, and the effects in
preclinical models can be compared with colorectal distention
in healthy volunteers and IBS patients. However, sensitivity to
distention of a sensitized colon does not readily translate to a
sensation of bloating and discomfort reported by patients in
clinical studies. Thus, developing drugs to target mechanisms
of nociceptive signaling in colorectal distention-induced
visceral pain may only be partially successful in clinical trials.
Translational research integrates data derived from molecu-
lar, cellular, tissue and systems approaches in a way that is
more likely to predict the clinical response to therapeutics.
For example, targeting control of motility through µ OR
signaling in preclinical studies appears to be translatable to
IBS patients, based on experience with loperamide. Recent
efforts (see the following section) to identify predictive bio-
markers of IBS have identified colonic transit, GI secretion
and bile acid malabsorption and these biomarkers could be
incorporated into preclinical models to determine drug
effects. On the other hand, clinical research demonstrates effi-
cacy in terms of patient response to the therapy but some of
these responses cannot be replicated in preclinical models
such as defecation urgency, bloating and hyper-attentiveness.
While specific animal models are useful for testing mecha-
nisms and endpoints where there is precedence, there are no
truly acceptable animal models of IBS. This is an issue for
new targets and mechanisms emerging from basic sciences
and clinical associations, because the data generated in rodent
models are not reliably translated from rodent into humans
for drug discovery purposes. Novel targets that could impact
IBS at the molecular level are numerous, encompassing neu-
ronal, anti-nociceptive and anti-inflammatory approaches.
However, despite many attempts in different pharmaceutical
companies, discovery of drugs for novel mechanisms, such
as mast cell-mediated inflammation, or anti-hyperalgesia
beyond the validated opioids has a low probability of success
to move forward clinically, despite the evidence in these
mechanistic animal models (Figure 1).
Bloating
Symptoms
Motility
Bile acids
SecretionVisceral sensation
Hyper-attentiveness
Microbiome
Intestinal barrier
Immune function
Enteritis
Epigenetics
Urgency
Biomarkers
Physiology
Nutrient signals
Discomfort/Pain
Stool consistency
Stool frequency
Translation
Figure 1. A perspective on the unclear relationship between
(patho)physiological mechanisms measured in research and
their translation to clinical irritable bowel syndrome (IBS)
symptoms. Multiple processes and mechanisms govern
gastrointestinal (GI) physiology and can be studied in
preclinical in vitro, ex vivo and in vivo models using tools
to understand mechanism of action of drugs. However, only
the changes pertaining to motility, secretion and bile acids
may be considered translational biomarkers that associate
with IBS symptoms. These biomarkers can be predictors of
drug-target engagement or disease progression and provide
an important link between preclinical and clinical research.
Measures of colonic transit, GI secretion and bile acid
malabsorption are relatively validated as pathophysiological
changes that correspond to IBS and these can be incorpo-
rated into preclinical models. Other pathophysiological
mechanisms (e.g., visceral hypersensitivity, immune function,
barrier function), while they can be measured in both clinical
and preclinical research, have not been predictive of drug
efficacy in IBS. Conversely, symptoms such as defecation
urgency, discomfort and bloating cannot be back-translated
into preclinical models. It is this lack of clarity on the
relationship between mechanisms and symptoms that
challenges new target drug discovery and research in IBS.
Drug discovery approaches to IBS
Expert Opin. Drug Discov. (2015) 10(8) 811
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The underlying mechanisms of IBS have been divided
into two main camps -- from the ‘bottom up’, e.g., a trigger-
ing event locally in the gut such as infectious
diarrhea [23] -- or from the ‘top down’, e.g., CNS hyper-
attentiveness to gut function or a stress-induced influence
on perception and interpretation of gut sensations. In the
former, local non-absorbed antibiotic treatment may be use-
ful to prevent long-term effects of enteritis, and in the latter,
a tricyclic antidepressant. In addition, to a bottom-up frame-
work, it may be insightful for discovering new therapeutics
to take an ‘inside-out’ or ‘outside-in’ approach for IBS
(Figure 2). Although the contents of the alimentary canal
are thought of as inside the body, ingested substances are
actually ‘outside’ and excluded from the systemic circulation
by intestinal barrier function. Therefore, the first layer of
target therapeutics from an ‘outside-in’ approach is attention
to nutrition, diet and the omission of certain foods because
of suspected allergies. The importance of diet is becoming
increasingly important as a relationship of certain foods
and exacerbation of symptoms can often be identified [24].
Patients with IBS can benefit from a diet low in fermentable
oligosaccharides, disaccharides, monosaccharides and polyols
Blood
vessel
1
7
10
2
6
CNS reflex pathways
Enterocytes
3
5
Extrinsic afferent nerves
TPH1; SST; OR (δ)
CCK
GLP1
4
8
IBAT; NHE-3;
CIC-2; cGMP/GC-C;
TGR5; FXR
5-HT1A; 5-HT3; 5-HT7; NK;
OR (k- δ- μ) ; CB1R
Zonulin;
β-catenin
5-HT2B; CCKR CB1R; mAChR; NO; VIP
5-HT
CB2R
Cytokines
Nerve growth factors
Defensins
Th17
mucins
FODMAP; FOSDCA; CDCA
SCFA; hydrolase
NO
NFkB
LPS TLRs
9
Figure 2. Outside-in diagram indicating key components of IBS pathology spanning the most proximal environmental triggers
in the intestinal lumen (nutrition, microbiome) through the host tissue from the lamina propria, muscularis externa to the distal
extrinsic neural elements. A selected number of targets are indicated at each level that have been shown to, or that potentially,
alter IBS pathology. 1. Nutrition, diet and the omission of certain foods (FODMAP) or supplementation with prebiotics (FOS).
2. Bile acid malabsorption diarrhea and specific bile acids affects on motility and secretion. 4. Epithelial cell lineages (enterocyte,
and associated immune cells have numerous targets related to innate immunity and host defense mechanisms, secretion,
absorption. 5. Intestinal permeability including tight junction functionality is perturbed associated with pathogeneisis of
metabolic and inflammatory disorders. 6. T-cell populations are altered in mucosal biopsies of IBS patients suggesting adaptive
immune changes. 7. Submucosal secretomotor neuronal functions coordinate local decisions that ultimately contribute to
global GI symptomatology. 8. Sub-epthelial myofibroblast perform multiple supporting functions for the epithelium, nerves and
other cell types, and are also immune-like supporting as Th-17 pathways. 9. Myenteric coordination of peristaltic motility and
contractility in circular and longitudinal smooth muscle have been targeted for IBS at a variety of receptors. 10. Extrinsic nerves
involving sensory reflexes at the level of the dorsal root ganglion contribute to peripheral nociception and have been targeted
by peripheral anti-hyperalegics mechanisms. Vagal afferents and vago-vagal reflexes have targets for perception of fullness,
nausea and early satiety associated with upper GI functional bowel disorders but their influence wanes in the large intestine.
5-HT: Serotonin receptors; IBAT: Ileal bile acid transporter; CB1R: Cannabinoid 1 receptor; CCK: Cholecystokinin; CCKR Cholecystokinin receptor; CDCA: Chenodeoxycholic
acid; CIC-2: Type 2 chloride channel; DCA: Deoxycholic acid; EC: Enterochromaffin cell; EE: Enteroendocrine; FODMAP: Fermentable oligosaccharides, disaccharides,
monosaccharides and polyol; FOS: Fructo-oligosaccharides; FXR: Farnesoid X receptor; GC-C: Guanylate Cyclase C; IEL: Intraepithelial cell; LPS: Lippopolysaccharide;
mAChR: Muscarinic acetylcholine receptors; NHE: Apical Na+ /H+ exchanger; NK: Neurokinin; NO: Nitric oxide; OR: Opioid receptor; SST: Somatostatin; SCFA: Short chain
fatty acids; TGR5: G protein-coupled receptor 5; TLRs: Toll-like receptors; TPH1: Tryptophan hydroxylase 1; VIP: Vasoactive intestinal peptide.
P. J. Hornby
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(FODMAPs), which has been shown in a controlled, cross-
over study to reduce functional GI symptoms [25]. However,
nutraceutical and diet approaches are beyond the scope of
this article.
After food is ingested, bile acids are released into the small
intestine and emerging mechanisms involved in the etiology
of IBS will be reviewed, which go beyond simply the forma-
tion of micelles and absorption of lipids. Members of the
intestinal microbiome (e.g., lactobacillus), critical for decon-
jugation and hydrolase reactions of bile acids, if perturbed,
also cause downstream changes in bile acid signaling to intes-
tinal nuclear receptors and circulating molecules [26]. Manip-
ulation of the microbiome through probiotic mixtures or
fecal microbial transfer will be considered for their utility in
mining for IBS therapeutic targets. Furthermore, the loss of
intestinal barrier function (low-grade endotoxemia) appears
to be a risk factor for pathogenesis of IBS, as well as for
inflammatory bowel (IBD) and metabolic diseases, where
treatments improving barrier function may also be assessed
clinically in IBS. The luminal contents are continuously sam-
pled by circulating and resident immune cells as well as being
recognized by innate receptors. Clinical evidence suggests that
IBS is associated with innate and adaptive immune changes,
although these do not rise to the level of IBD and targeting
these could be challenging. All these mechanisms could be
considered part of the physiological response to the contents
of the intestines while still ‘outside’ the body and are the focus
of more recent work. In addition, mechanisms involving
motility and secretion in the gut wall itself and the local trans-
mission of visceral nociception (enteric and extrinsic afferents)
are being exploited by newer therapeutics.
Ultimately, this information is conveyed to the CNS where
there is clinical evidence for cortical hyper-attentiveness which
may be useful in stratifying patients, but would be difficult to
target pharmaceutically. The alternative medical practices of
meditation, acupuncture and biofeedback may be effective
complementary and alternative approaches [27] but these are
beyond the scope of this review.
3. Target pathways for IBS from a gut
‘outside-in’ perspective
3.1 Bile acids, receptors and transporters
Bile acids have multiple functions beyond absorption of
dietary fat through the formation of micelles, and are now
viewed as key modulators of metabolism [28]. Bile from
various species have been used for digestive health and cheno-
deoxycholic acid (CDCA), or later urodeoxycholic acid, was
dosed to patients for gallstone dissolution before the develop-
ment in 1990s of laparoscopic cholecystectomy. Therefore, it
is not surprising that interest in bile acids to promote colonic
secretion for constipation (i.e., based on the occurrence of bile
acid diarrhea) is being actively pursued for treatment of
chronic idiopathic constipation first and may be extended to
the treatment of IBS-C once a better understanding of the
therapeutic window is gained. This is discussed in more detail
for elobixibat trials in chronic constipation (below), which
provided a dose range for efficacy while minimizing the
likelihood of adverse effects (abdominal pain and diarrhea).
Primary bile acids are synthesized from cholesterol in the
liver, where the rate-limiting step is the enzyme cholesterol
7 a-hydroxylase (CYP7A1), and then secreted with the bile
into the small intestine. Bile acids are conjugated with glycine
or taurine, which reduces pKa and favors the deprotonated
form in the duodenum for fat emulsification where they can
be referred to as bile salts. Secondary bile acids are deconju-
gated by bacterial enzymes (the role of the microbiome is
covered in the following section) and in the ileum, bile acids
are actively transported into the enterocytes by a sodium-
dependent bile acid transporter. The ileal and liver transport-
ers coordinate enterohepatic recycling of bile acids in the
small intestine through the circulation and resulting in their
reuptake in liver [29].
Bile acids activate nuclear receptors (farnesoid X receptor,
pregnane X receptor, vitamin D receptor) and are ligands
for TGR5, the G-protein-coupled bile acid receptor 1. In
addition to regulating glucose, lipid and energy metabo-
lism [28], the farnesoid X receptor also maintains intestinal
barrier integrity, prevents bacterial overgrowth through innate
immunity [30] and modulates drug metabolism [29]. Farnesoid
X receptor and TGR5 activation increase release of ileal
Fibroblast Growth Factor-19 (FGF-19 or FGF-15 in rodents)
into circulation. After a meal, serum levels of FGF-19 are
increased by CDCA and decreased by compounds that
sequester bile acid in the gut [31]. This is because increasing
bile acid concentration in ileal enterocytes increases the circu-
lating level of FGF-19, which drives negative feedback for
hepatic bile acid synthesis through inhibition of the expres-
sion of CYP7A1 and thus reducing the production of bile
acids in the liver [32].
This highly regulated system results in ~ 600 mg of bile
acids excreted in the feces daily, and excessive amount of
bile acids in the colon leads to diarrhea. Deoxycholic acid
and chenodeoxycholic increase intestinal motility and
secretion, respectively [33], and, at least in rodent studies,
deoxycholic acid activates TGR5 on enteric neurons [34].
A positive homocholic acid-taurine test is a frequent finding
in patients with chronic diarrhea [35] and up to 30%
diarrhea-predominant IBS patients may have bile acid
malabsorption [36-38]. Diagnosis of bile acid overproduction
is often based on a patient’s response to bile acid sequestrants
(e.g., cholestyramine, sevelamer colestipol or colesevelam) and
there is an association of TGR5 SNP variation with GI transit
differences [39]. In addition to being beneficial for dyslipi-
demic patients, colesevelam could be beneficial for at least a
subset of patients with IBS-D identified by SNPs [40] and
colestipol was beneficial in an open label study of IBS patients
with a positive bile acid test, lower FGF-19 levels and acceler-
ated transit [41].
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The future treatments of IBS-C may include
TGR5 agonists that are in preclinical development for meta-
bolic disorders, as well as farnesoid X receptor agonists to
increase circulating FGF-19 [42]. Farnesoid X receptor activa-
tion in small number of patients with Crohn’s colitis treated
with CDCA for 8 days resulted in increased FGF-19
circulating levels and increased gall bladder volumes [43].
However, treatments aimed at activation of farnesoid X recep-
tor agonists to increase ileal production of FGF-19 are chal-
lenging. Farnesoid X receptor agonists are reported to have
low selectivity relative to other nuclear receptors and may
have adverse effects on liver farnesoid X receptor [44]. A gut-
restricted agonist fexaramine robustly increased rodent
FGF-15 [45], but whether the effect in humans to increase
FGF-19 would be beneficial in IBS is unknown, as well as if
there is a sufficient safety profile in these patients.
An approach aimed at reducing constipation is pharmaco-
logical inhibition of ileal bile acid reabsorption an reduced
circulating levels of FGF-19 to increase bile acid production.
Increased colonic bile acid would enhance colonic transit
and improve stool consistency. Elobixibat (formerly A3309)
is a minimally absorbed inhibitor of ileal bile acid transport
inhibitor for the treatment of chronic idiopathic constipation.
There were no serious adverse events in the single-center pilot
study [35] although a dose-finding Phase IIb study of elobixi-
bat in chronic constipation reported dose-related abdominal
pain and diarrhea, which occurred most commonly in
patients receiving the 15-mg dose [46]. Based on this, a
10-mg dose was considered to provide the best balance of effi-
cacy and safety.
The examples illustrate the potential of targets within bile
acid homeostasis for IBS therapies, with the caveat that a del-
icate balance is required for positive therapeutic effects with
minimal safety concerns, even when avoiding systemic
exposure.
3.2 Microbiome
Mining for targets and mechanistic pathways in disease from
human fecal microbial samples would have seemed inconceiv-
able a decade or so ago. Now there is increasingly compelling
evidence that many individual bacterial species provide bene-
ficial effects to the host beyond their known roles in deconju-
gation of bile acids and nutrient supply, such as short chain
fatty acids. There are a number of challenges to identifying
microbial--host interactions in health, disease process and
restoration of health. The translation of microbiota associa-
tions into causality is confounded by the dynamic nature of
the microbiome and an understanding of the timescale of
reversible changes mediated by microbiota [47]. For example,
changes in population dynamic that can provide benefit range
from short term (changes in diet), to longer term (prebiotics,
antibiotics and probiotics) and even more durable fecal
microbial transfer, at least in terms of resolution of Clostridium
difficile infections.
For determination of microbial populations, the field has
relied on a taxonomic approach by amplification of bacterial
16s ribosomal RNA using the n amplicon as a taxonomic
marker, which gives estimates of relative abundance of
microbes in a sample. This is because the genes coding for
16S component has evolved slowly and can be used in recon-
structing phylogenies (or operational taxonomic units,
OTUs). It is generally accepted that microbial diversity is
compromised in disease states of the intestine. The fecal
microbiota of some IBS patients has a 2-fold increased ratio
of firmicutes to bacteroidetes [48] and reduced microbial bio-
diversity [49]. A low FODMAP diet increases the relative
abundance of butyrate-producing Clostridium cluster XIVa
and mucus-associated Akkermansia muciniphila [50]. Recogni-
tion of bacteria by the host immune system and intestinal bar-
rier integrity in IBS highlights the potential importance of
probiotics in IBS treatment [51] but as reviewed recently, larger
studies are needed to better characterize alterations to the
intestinal microbiome ‘dysbiosis’ in large cohorts of well-
phenotyped patients [52].
Increasingly, a metagenomics is used to provide the
sequencing reads from dozens or hundreds of community
members. This provides a fine level of taxonomy of commu-
nities, and the generation of genome, gene and metabolic pro-
filing of genetic variants, (i.e. big data compositional analysis.
It is important to recognize that the level of granularity at the
phylum, family, species or even SNP level can provide sup-
port for claims of diversity (or lack thereof) when describing
fecal microbiota. An advantage of sequencing, where sequen-
ces are suspected to be or are associated with metabolic
pathways, is that more functional profiling of bacterial popu-
lations is possible (taxomonic vs molecular function analysis).
However, the stability of gene cluster addociation with
function vary and, there is that more stability of function of
bacteriodetes versus firmicutes [53] at least based on where
their function is understood.
Another challenge is the selection of functional groups of
bacteria or probiotics and their translation into mechanism
in preclinical models. Several probiotic species may either
directly, or through their metabolites and secreted proteins,
activate visceral afferents, alter intestinal epithelial barrier
function, or mucosal and systemic immune function [54]. In
caco-2 cell monolayers, gut microbial metabolites (including
a metabolite of linoleic acid) suppressed expression of TNF
receptor 2 and suppressed TNF and dextran sulfate sodium-
induced colonic changes in mice at least partially through a
GPR40 pathway [55]. However, there are many more papers
publishing positive associations, with very little negative data
reported. Consensus about which models are most appropri-
ate and agreement on protocols by which isolated human/
rodent tissue responses are combined with intestinal
geographically relevant (often anaerobic) microbiota or their
products would prevent some of the confusion as to which
microbes in the gut are beneficial or detrimental. Finally, in
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terms of IBS, the precise host gene and protein responses to
help predict a clinical response are not well defined.
With these challenges in mind, researchers are taking
GRAS probiotics directly into the clinic. However, a limita-
tion of studies on probiotics, in addition to absence of causal-
ity, is that the results are inconclusive or conflicting. This can
be due to trial design, which often has small numbers of
patients and non-standardized outcome measures, as well as
the strains and combinations of probiotic bacteria used. For
example, a recent randomized placebo-controlled study was
carried out in 42 IBS patients who received the probiotic
VSL#3 or placebo for 6 weeks [56]. The outcomes measured
were improvement in rectal sensitivity and abdominal pain
duration in the probiotic group and an increase in saliva mel-
atonin levels in the morning in probiotic treated males [56].
Larger cohorts of VSL#3 treated of IBS patients are really nec-
essary before concluding that it is beneficial using established
criteria, and the implication of causality through morning
melatonin levels will require further validation. An example
of conflicting data are in two recent placebo-controlled trials
in small numbers of IBS patients where one yielded no bene-
ficial effects on the symptoms of visceral hypersensitivity [57]
but in another trial, the IBS symptom severity score and qual-
ity of life were both improved [58]. As summarized recently
‘optimizing strain, dose and product formulations, including
protective commensal species; matching these formulations
with selectively responsive subpopulations; and identifying
ways to manipulate diet to modify bacterial profiles and
metabolism’ will be critical for using probiotics in IBS [59].
3.3 Intestinal permeability and endotoxemia
A subset of IBS-D patients display increased intestinal perme-
ability, and those patients exhibit greater visceral hypersensi-
tivity [60]. A similar association of increased intestinal
permeability and symptoms has been reported in Crohn’s dis-
ease [61], celiac disease [62], diabetes and obesity [63,64]. Thus,
altered intestinal permeability, increased endotoxin systemic
load and microbial translocation are recognized as a compo-
nent of many diseases [65]. However, causality in disease path-
ogenesis, evidence of manifestation during disease onset or
even association are all strongly debated [14]. Potential mecha-
nisms for the positive effects of probiotics on improving bar-
rier function are alteration of tight junction protein
expression [66-68] or binding to intestinal Muc2 and maintain-
ing mucus physical properties [69]. The regulation of tight
junction proteins and how they determine barrier function
has been reviewed recently [70] and larazotide, which is
thought to prevent tight junction opening, was tested in
patients with celiac disease [62] but not in other diseases to
date. Targeted manipulation of intestinal epithelial myosin
light chain kinase genetically in mice alters permeability
resulting in colitis [71] and an inhibitor prevents a cytokine
mRNA expression and colonic epithelial barrier in stressed
mice [72]. Stress-induced increases in colonic paracellular per-
meability are ameliorated by Lactobacillus farciminis [73].
However, to test if increased intestinal permeability is causal
in IBS, then tool compound tight junction modulators, such
as larazotide, which has already been in clinical trials for celiac
disease, may be beneficial.
3.4 Intestinal infection and immune-mediated
changes
It is estimated that 10% of patients with IBS report the start
of symptoms after infectious enteritis, and after an unfortu-
nate enteritis outbreak in Walkerton, Canada, immune
changes and IBS symptoms were reported [74,75]. In post-
infectious IBS patients, Bacteroidetes phylum were increased
while Clostridia were decreased and host gene pathways,
including amino acid synthesis, cell junction integrity and
inflammatory responses, were consistent with impaired epi-
thelial barrier function [76]. Treatment of IBS-D with rifaxi-
min for 2 weeks was associated with significant symptomatic
improvements of which 36% never relapsed over
18 -- 22 weeks of follow-up (see addendum) [77].
This has led to a suggestion that IBS could be a low-grade
inflammation version of IBD [74,78]. A meta-analysis showed
correlations between IBS and cytokine interleukin-10 and
TNF gene polymorphisms [79] and an imbalance of TNF-a
and interleukin-10 cytokines was also demonstrated in IBS
patients [16]. In IBS-D patients, circulatory inflammatory
cytokines (IL-6 and IL-8) and adipokine (resistin and adipo-
nectin) levels were higher compared to healthy controls simi-
lar to what was found in patients with active celiac disease [80].
Recent studies have shown alterations in the mucosal
immune system in IBS. Specifically, T-helper 17 cells, which
are developmentally and functionally distinct from T-helper
1 and 2 cells, interact with colonic subepithelial myofibro-
blasts in both IBS and IBD patients [81]. In IBD, activation
of TH17 pathways supporting subepithelial myofibroblasts
to improve activity and survival of T-cells [82], is a therapeutic
consideration. Other inflammatory mechanisms in IBS such
as monocytic cell receptors, cytokines or mast cell chymase
and proteinase-activated receptors are actively researched but
have not gained traction for clinical testing in IBS.
3.5 Visceral hypersensitivity
Visceral hypersensitivity is recognized clinically as an impor-
tant component of IBS. However, a drug that reduces sensi-
tivity to colonic distention, whether in a rat or human IBS
sufferer, does not necessarily equate to improvements in
bloating, discomfort or urgency reported by patients in clini-
cal studies. Part of the reason for this challenge is because
colonic sensation can arise from relatively few sensory nerves
which can respond to a variety of locations and types of stim-
uli (chemical/mechanical; low/high intensity); therefore, the
ability to discriminate the precise source and stimulus for
pain is poor. Electrophysiological studies have shown that
normally high-threshold colonic afferent nerves are the ones
that contribute to colonic hypersensitivity in animal
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models [83]. This process may underlie the symptoms dis-
played by patients with IBS but then perhaps the question
becomes, which mechanisms can be targeted to prevent this
conversion in patients that may prevent disease progression?
Chronic visceral pain is difficult to mimic in animal models
because it should reflect the natural stimulus in having similar
onset severity and duration and be as non-invasive as possi-
ble [84]. This is not the case for commonly used models,
such as graded or repeated colorectal distention, zymosan A
colonic inflammation or acetic acid/irritation-induced behav-
iors. Furthermore, colorectal distention responses used both
clinically and preclinically have often failed to translate into
drug efficacy in treating pain and discomfort associated with
IBS. For example, fedotozine (k OR agonist) reduced visceral
hyperalgesia [85], but clinical trials were discontinued due to
lack of efficacy. Asimadoline (k OR agonist) does not cross
the blood--brain barrier and reduced pain sensation in healthy
volunteers in response to colorectal distention but had no sig-
nificant effect on GI transit or colonic motility [86]. It reduced
sensation in response to colorectal distension in IBS patients
although there was no improvement of pain score after on-
demand treatment [87]. On the other hand, chronic asimado-
line treatment in IBS-D patients was associated with
improved pain and discomfort scores [88]. These inconsistent
results in human studies underscore the difficulty in measur-
ing a surrogate response acutely for chronic visceral pain. In
spite of this, compounds are evaluated in acute animal models
of visceral hyperalgesia, in part because of the priority placed
by regulatory authorities and Rome committee on the achiev-
ing endpoints using a composite score of improvements in
both stool and pain/discomfort. Therefore, the preclinical
data for advancing to testing in humans will be built on pos-
itive results in these mechanistic models of visceral nocicep-
tion which have a relatively low probability of translation to
the clinic.
3.6 Intestinal motility and secretion
In contrast to models of IBS pain and discomfort, models of
transit and secretion are relatively translational to human clin-
ical responses to therapeutics, especially when the pathophys-
iological mechanism is measurable in non-perturbed models.
Secretion can be measured by short circuit current changes
(reflecting Cl- movement resulting in increased luminal secre-
tion) in ex vivo isolated tissues from both rodents [89,90] and
humans [91]. This, as well as immortalized epithelial cell lines
(Caco-2 cells) which form polarized monolayers in transwell
cultures, has been useful in determining mechanism of action
of agents (e.g., linaclotide). Contractility can be measured
ex vivo in longitudinal muscle-myenteric plexus strips [92]
and circular smooth muscle strips, which is a better indicator
of intestinal motility. The effects on colonic propulsive motil-
ity can be measured by transit time of artificial fecal pellets [93].
The latter has been used to characterize prokinetic actions
of 5-HT4 receptor agonists such as prucalopride and
mosapride [94-96].
When moving to models of perturbed GI transit, their
translational ability is probably related to the target therapeu-
tic molecule. Acute castor oil-induced diarrhea is a popular
model of enhanced GI transit [97-99] and was used to character-
ize morphine and loperamide anti-diarrheal effects. Croton
oil is an inflammatory irritant when given intracolonically
and the resulting acute increased transit is also ameliorated
by OR agonists [100] and cannabinoid receptor agonists [101].
In an attempt to develop an animal model of altered motil-
ity in IBS, subsequent to nerve stimulation or inflammatory
irritation, oil of mustard (allyl isothiocyanate) was adminis-
tered intracolonically and animals were characterized up to
4 weeks later. When administered acutely intracolonically,
this resulted in visceral hyperalgesia and severe, transient coli-
tis that peaked at Day 3 and thereafter subsided until no overt
inflammation was detectable by Day 14. Three and 4 weeks
after intracolonic oil of mustard, mice had significantly accel-
erated upper GI transit rates compared to control mice [102].
This post-inflammatory IBS-like model has been used to
characterize endocannabinoid involvement in accelerated GI
transit [103] which appears to be dysregulated with increased
intestinal anandamide levels [104]. In this model of IBS-like
enhanced transit, mudelta (eluxadoline, see addendum)
reversed the increased upper intestinal transit in mice com-
pared to control [105].
4. Biomarkers for IBS
A big challenge to finding new efficacious treatments is the
heterogeneity of the condition itself and a lack of validated
diagnostic criteria and biomarkers [106]. By investigating dif-
ferent pathophysiological mechanisms in a small number of
patients, a promising approach to biomarkers for IBS is a
combination of bile acid in feces, circulating FGF-19 and
intestinal permeability and altered colonic transit that could
discriminate IBS from healthy volunteers [107]. Other bio-
markers have been proposed [108] and combinations of up to
34 markers may discriminate IBS from healthy volunteers
though this was from a small subset [109].
Disease biomarkers or potential targets for therapeutic
interventions have been identified by ‘omics’ approaches for
transcriptional, protein and metabolite signals, as well as
DNA methylation and epigenetic influences. Beyond this
molecular level, psychomarkers of IBS have been pro-
posed [110] to capture emotional or social cues. However, large
numbers of well-characterized subjects would be required due
to the complex pathogenesis of IBS and although there is an
abundance of biomarker studies in IBS patients, the numbers
of subjects are typically low.
Some target engagement biomarkers in clinical trials are
being used clinically to identify patients who may or may
not respond to the drug. For example, tryptophan hydroxy-
lase 1 gene polymorphisms, which correlate with 5-HT sig-
naling, may be a means to identify those who are less likely
to respond to ramosetron in IBS-D patients [111]. Maybe
P. J. Hornby
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somewhat disturbingly for target engagement biomarker
progress, there are even potential molecular signatures of a
non-drug response in IBS with patient self-reported
outcomes [112].
5. Recent and pending approvals for local
gut-targeted IBS therapeutics
Targeting proximal mechanisms in the mucosa locally (non-
orally bioavailable) for IBS has been an effective strategy
recently. This approach is illustrated by three selected thera-
peutics: eluxadoline (US NDA filed in 2014), linaclotide
(US FDA approval in 2012) and crofelemer (US FDA
approval in 2013).
5.1 Dual OR modulators
Eluxadoline is a high affinity µ OR agonist and d OR antag-
onist [105] that, based on preclinical data and clinical results in
IBS-D patients, the FDA has accepted for NDA filing (Acta-
vis) in September 2014. The synthesis and structure of eluxa-
doline [113] was the culmination of chemical efforts to discover
mixed OR subtype modulators that act locally in the intes-
tine [114]. Pharmacokinetic studies in rats, mice and primates
demonstrated that eluxadoline has low systemic exposure after
oral administration, which is consistent with a local site of
action of the compound.
Opioid receptors are expressed throughout the GI tract [115]
and the three major subtypes (d, k and µ) implicated in GI
transit, secretory activity and visceral hyperalgesia. Lopera-
mide is a gut-restricted selective µOR agonist [116] which is
often used to reduce diarrhea and urgency of IBS-D [117] but
may be associated with constipation [118]. Eluxadoline was
the molecule that resulted from efforts to ‘dial in’ d OR mod-
ulation as well as potent µ OR agonism as an option that
reduces the likelihood of constipation for IBS patients.
The preclinical and clinical experience confirm that µOR
agonists contribute to constipation [119] by reducing secretion
and motility but a role of the dOR in propulsive motility and
secretion is confounded by species and GI regional differen-
ces. The dOR is expressed in enterocytes and enteric neurons
in mice and human intestinal tissue and, when mice were
mildly stressed for 90 min prior to being killed, there was
an increase in dOR immunohistochemical staining in entero-
cytes of the distal colon [105]. Functionally, dOR agonism
decreased colonic propulsion in guinea pig colon and a
dOR antagonist (naltrindole) was synergistic with 5-HT4 ago-
nists to increase propulsion [93]. Some other reports show little
effect of dOR activation [120,121]. That dOR antagonism could
oppose the inhibition exerted by µOR agonism was confirmed
by the addition of a dOR antagonist that reduced the inhibi-
tory contractility µOR effect in electrical field stimulation
guinea pig isolated ileum and in mice GI transit in vivo [105].
Eluxadoline, which combined these attributes in one mole-
cule, was assessed in two models of increased GI transit (novel
environment stress and post-inflammatory altered GI
function 3--4 weeks after intracolonic OM) where it normal-
ized GI transit and fecal output over a wide dose-range com-
pared to potent inhibition by loperamide in these models [105]
and in castor oil-induced diarrhea in mice [98]. In a subsequent
study using castor oil-induced diarrhea, the lower doses of
eluxadoline did slightly reduce fecal output [99] perhaps
because acute administration of this irritant is a different trig-
ger than stress, which, as noted above, increased mucosal dOR
expression [105,122,123].
Delta OR antagonists enhance morphine-induced analge-
sia [124] and a molecular mechanism that could explain the dif-
ferent effects of loperamide and eluxadoline may be through
engagement of µOR and dOR heterodimers. Activation of
µOR alone on heterodimers activates b-arrestin 2 signaling
but a combination of µOR and dOR agonists/antagonists
drives signaling through G protein signaling [125]. Therefore,
eluxadoline differentiation from loperamide may be due to
µOR/dOR signaling through G protein reducing the likeli-
hood of constipation associated with b-arrestin 2 signaling.
Recently, the ability of eluxadoline and loperamide to activate
G-protein- and b-arrestin-mediated signaling was investigated
in vitro in heterologous cells. Eluxadoline was more potent
than loperamide in eliciting G-protein activity and b-arrestin
recruitment in µOR expressing cells. However, in cells
expressing µOR/dOR heterodimers, the potency of eluxado-
line was higher, and signaling was reduced by antibodies
that immunoneutralized µOR/dOR heterodimers selectively.
This led to the conclusion that in the presence of dORs, elux-
adoline appeared to act through µ/d heteromer [99].
In a Phase II clinical trial, the percentage of IBS-D patients
treated with eluxadoline who met the primary end point of
clinical response at Week 4 was greater than those treated
with placebo. The clinical response definition was a compos-
ite of pain and stool consistency and response rates were mod-
est based on the requirement that a patient met the pre-
specified improvements in both worst abdominal pain and
stool consistency in the same week. After 4 weeks, the
response rates for eluxadoline of abdominal pain was approx-
imately 40% across groups and not significantly different
from placebo [126]. Two Phase III trials with a total of
2428 IBS-D patients (66% females; mean age of 45 years)
and two doses of eluxadoline met the primary endpoints
(ClinicalTrials.gov Identifiers NCT01553591 and
NCT01553747). The 12-week efficacy was an improvement
over placebo in the composite endpoint of the simultaneous
improvement in both pain and diarrhea at 75- and 100-mg
doses, and more patients receiving 100 mg eluxadoline were
FDA and EMA composite responders than control-treated
patients, as reported in abstract form [127]. In the higher
dose (100-mg b.i.d.) group over the period of 1 -- 12 weeks,
42% of patients reported being urgency free in over half the
days, compared with 21% of controls. In the same group,
bowel movement frequency improved from 4.9 to 2.9/day
(week 26) compared to 3.3/day in controls and abdominal
pain score improved by at least 40% at Week 12 in 43.2%
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of patients versus 35.8% of controls [127]. The most common
adverse effects were constipation and nausea. Hepatobiliary
sphincter of Oddi spasm was noted in patients with prior cho-
lecystectomy, where seven of the eight affected patients were
in the high dose arm. All eight cases were rapidly reversed
upon prompt drug discontinuation. Another eluxadoline-
associated adverse event was five confirmed cases of pancreati-
tis, which were mild, and all in patients with pancreatitis risk
factors [127].
5.2 Guanylate cyclase C receptor agonists
Linaclotide was launched in the US in 2012 and in the EU in
2013 after it was approved by FDA and EMA, respectively,
for the treatment of IBS-C in adults. This was largely based
on the positive endpoints in two Phase III trials where patients
with IBS-C were administered linaclotide or placebo daily for
12 weeks [128] and 26 weeks [129]. The approach to its discov-
ery exploited the mechanisms of enterotoxigenic Escherichia
coli to produce traveler’s diarrhea and the discovery of the
endogenous hormones guanylin and uroguanylin regulating
intestinal fluid homeostasis.
Heat stable enterotoxin precursors are proteolytically
cleaved intracellularly to the active peptides which, similar
to the endogenous host guanylin and uroguanylin, activate
the guanylyl cyclase C (GC-C) receptor located on the lumi-
nal surface in the intestine that results in the conversion of
GTP to cGMP [130]. Linaclotide is a 14-amino-acid synthetic
peptide agonist of GC-C that increases intracellular cGMP,
which binds and activates the cGMP-dependent protein
kinase II highly expressed in intestinal epithelium [131] similar
to guanylin and uroguanylin. Protein kinase II causes cystic
fibrosis transmembrane conductance regulator phosphoryla-
tion and results in increased secretion of bicarbonate and
chloride ions into the intestinal lumen. These unique proper-
ties of the peptides activating GC-C locally in the intestine for
the treatment of IBS-C and chronic constipation have been
reviewed [132].
It is not surprising based on its mechanism that linaclotide
increases intestinal fluid secretion and accelerates GI transit
but this GC-C/cGMP pathway has also been explored for
reducing visceral hyperalgesia. In rat models of visceral hyper-
sensitivity after intracolonic trinitrobenzene sulfonic acid
inflammation and stress, oral administration of cGMP and
uroguanylin decreased the affective response to colorectal dis-
tension [133]. Linaclotide activates GC-C expressed on muco-
sal epithelial cells that results in increased extracellular
concentrations of cGMP [128,129,134]. Linaclotide treatment
improved abdominal pain scores in IBS-C [135] and in a post
hoc analysis of data from Phase III, double-blind study of
IBS-C patients, quantification of abdominal pain revealed
that 70% patients had at least a 30% reduction in abdominal
pain compared with 50% patients given placebo [134]. In the
other Phase III study [128], the 12-week RCT had a 4-week
randomized withdrawal period during which patients who
had been receiving placebo were assigned to receive linaclotide
and patients who had been receiving linaclotide were random-
ized to either placebo or continued receiving linaclotide.
When patients were switched to placebo, the abdominal
pain relief obtained during treatment was lost [128]. The mech-
anism for this benefit is not clear. In mouse, both linaclotide
and cGMP inhibited colonic serosal afferents in vitro and
intracolonic linaclotide reduced nociceptive colonic mechano-
sensory input to the spinal cord in vivo [134]. Thus, mechanis-
tically, linaclotide-activated cGMP release from epithelial cells
could reduce nociceptive signaling to the CNS [83]. But
whether determination of this mechanism, a priori, in rodents
accounts for the clinical response is unclear. Overall, the
development path for linaclotide lends support to targeting
an anti-constipatory mechanism in order to get it into clinical
testing, and subsequent research demonstrates other beneficial
effects in IBS.
5.3 Cystic fibrosis conductance regulator/calcium-
activated chloride channel
Crofelemer was extracted from the stem bark latex of the
Croton lechleri tree from South America where local indige-
nous populations used the bark in a number of herbal reme-
dies, including diarrhea. It inhibits the epithelial cystic
fibrosis conductance regulator and calcium-activated chloride
channel with no effect on the activity of epithelial sodium or
potassium channels or on cAMP and calcium signaling [136].
Originally identified as SP-303, it was efficacious against
in vivo cholera toxin-induced fluid secretion [137] due to the
dual chloride channel effect that modulates fluid secretion.
Crofelemer has very low bioavailability when given orally
and has minimal toxicity. In diarrhea associated with a secre-
tory component, such as cholera, travelers’ diarrhea and acute
infectious diarrhea, crofelemer improves stool consistency and
duration of symptoms. Less clear are the effects in diarrhea in
HIV-associated diarrhea and IBS-D patients [138].
In IBS-D clinical trial, the primary efficacy measure was
improvement in stool consistency (primary endpoint), with
urgency and pain scores also evaluated. Crofelemer did not
improve in stool frequency, but did increase in the number
of pain- and discomfort-free days [139]. Whether this identifies
an unexpected anti-hyperalgesic effect, similar to the case for
linaclotide, remains to be explored.
6. Conclusion
The development of the newer therapeutics for IBS highlights
that proximal targets in the intestinal mucosa, or signaling
pathways locally in the gut, are reasonable options until an
understanding of the relationships between the brain,
immune system and intestine can be distilled down to more
distal targets (downstream from the mucosal interface) that
are perturbed in IBS. The unclear relationship within the
brain, gut and immune systems that has led to targeting
motility and secretory mechanisms in the gut mucosa also
has the advantage of restricting drug exposure, with a reduced
P. J. Hornby
Forpersonaluseonly.

likelihood of adverse effects from systemic circulation of the
drug. While more challenging for toxicological evaluation
for safety margins, the requirements for clinical testing can
be met and a chemical strategy for low oral bioavailability
can be realized The examples provided here also illustrate a
bias toward finding new therapies from clinically validated
targets in related (non-IBS) GI disorders (e.g. constipation
or diarrhea). The fact that these drugs provide a measure of
relief for at least a portion of IBS-sufferers lends support to
this approach. Future effort would benefit from a greater
understanding of disease progression biomarkers that may
allow patient segmentation, as well as potential treatments
earlier in disease.
7. Expert opinion
This review reflects a pragmatic viewpoint that, at present,
successful approach is to focus on clinically validated mecha-
nisms in related GI disorders, and optimize interventions
locally (or ‘proximally’) in the gut without systemic exposure,
to safely bring new therapeutics to IBS sufferers. An advantage
of this approach for drug discovery research is that the local
mechanisms in the gut allow for direct assessment using tool
compounds in isolated human tissue, as well as testing in
translatable models of motility and secretion. The compound
can be registered in a related, non-IBS condition, such as
chronic idiopathic constipation to reach patients and optimize
the dosing and development path before testing in IBS.
The challenges with drug discovery to more ‘distal’ targets,
such as in the immune system or CNS, are that we really do
not understand how mechanisms are causal in IBS pathology,
such as visceral hyper-attentiveness, although this can be
quantified in patients. Another challenge currently is the iden-
tification of surrogates for bloating and discomfort since affer-
ent nerve firing or colorectal distention pain does not always
equate to relief from IBS in patients.
What will help in the future for new drug discovery in IBS
is the identification of biomarkers distinctive for patient pop-
ulations of IBS or that associated with disease severity and
progression. The ability to measure these will not only help
clinical trials but also drug discovery if these markers can be
measured in preclinical models.
A strategy is to incorporate known mechanisms of existing
therapeutics, but with a drug designed to target multiple
mechanisms for improved effectiveness in IBS. This was the
case for eluxadoline which builds on a potent µOR agonism
with additional dOR antagonism for improved efficacy and
avoiding constipation in IBS-D. A second strategy can be to
explore mechanisms based on non-pharmacological treat-
ments such as nutritional, natural products and probiotics
for novel host (and bacterial) pathway modulation. Crofe-
lemer and linaclotide are based on natural botanical, bacterial
products and endogenous hormones, respectively. An interest-
ing strategy is to identify novel pathological processes that
may underlie the development of IBS and test the robustness
of the pathophysiology in disease using tool compounds that
are already in clinical testing for other indications. For exam-
ple, if intestinal permeability is causal in IBS, then tight junc-
tion modulators such as larazotide (in clinical trials for celiac
disease) may provide a proof of concept for the development
of more targeted therapeutics. An example of this strategy is
based on the relationship of bile acid pool and IBS pathology
where compounds, such as ileal bile acid transport inhibitors
or sequestrants for metabolic disorders, have shown efficacy
in clinical trials for IBS-C.
In the future, considering interventions at the local luminal
mucosal environments, treatments for IBS will probably
extend beyond pharmaceuticals. Prevention of pathology
and interventions in earlier disease progression will tend to
favor chronic treatment with low safety risk. This would
include traditional pharmaceuticals as well as customized
pro- and pre-biotic mixtures and even non-pharmacology
device interventions locally in the gut.
Addendum to proof: The U.S. Food and Drug Administra-
tion approved Xifaxan (rifaximin) and Viberzi (eluxadoline)
for male and female sufferers of IBS-D on May 27th 2015.
Declaration of interest
P Hornby is an employee of Janssen Pharmamceutical Com-
panies of Johnson & Johnson and was Team Leader for the
discovery and biological characterization of MuDelta (Eluxa-
doline) for diarrhea-predominant Irritable Bowel Syndrome.
She has no other relevant affiliations or financial involvement
with any organization or entity with a financial interest in or
financial conflict with the subject matter or materials dis-
cussed in the manuscript apart from those disclosed.
Forpersonaluseonly.

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Affiliation
Pamela J Hornby
Janssen Research Development, Cardiovascular
and Metabolic Disease, Janssen Pharmaceutical
Companies of Johnson and Johnson,
SH42-2508-A, 1400 McKean Road, Spring
House, PA 19477, USA
Tel: +1 215 628 7187
E-mail: phornby@its.jnj.com
P. J. Hornby
Forpersonaluseonly.

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