Development of drugs for celiac disease: review of endpoints for Phase 2 and 3 trials

Introduction

Research into therapy for celiac disease is currently at a particularly interesting point. There are now several different drugs in development but no agreement exists on the best endpoints for registration trials. We will review drugs in the ‘pipeline’, examine the experience to date, and discuss possible regulatory endpoints in detail.

Pathology and incidence of celiac disease

Celiac disease is an immune-mediated small intestinal enteropathy, triggered by the ingestion of gluten in the genetically susceptible, which results in villous atrophy. The presenting symptoms may range from diarrhea, constipation, vomiting, malnutrition, or failure to thrive, to chronic fatigue, joint pain, anemia, osteoporosis, or migraines [1]. The prevalence of celiac disease has increased over the past 50 years and the rate of diagnosis has risen over the past two decades [2].

Celiac disease affects some two million Americans, of whom around 83% are not diagnosed [3, 4], and 3.5 million Europeans [5]. This makes celiac disease one of the most common food-related, lifelong disorders worldwide [6]. Although many individuals with this disease remain undiagnosed, others who have been given the diagnosis do not actually have the disease; additionally, there is an increasing number of people who have no diagnosis of celiac disease but who nevertheless adhere to a gluten-free diet (GFD). Rubio-Tapia et al. reported that, among the general population of the USA that has not been diagnosed with celiac disease, the prevalence of gluten exclusion was similar to that of actual celiac disease [7]. The environmental trigger (gluten derived from wheat, rye and barley), the genetic predisposition conferred by the human leukocyte antigen (HLA)-DQ2 and HLA-DQ8 haplotypes, and many steps in the disease pathogenesis are known. Novel alternative treatments or adjunctive therapies to a gluten-free diet—which is currently the only available and effective treatment for the condition—are increasingly being suggested [8].

This paper covers celiac disease per se, which is the best-characterized of the spectrum of gluten-related disorders [9]. A gluten-free diet is central to the management of celiac disease and has historically been the only treatment, although several potential therapies are now under development. There is also a widespread public interest in gluten-free foods reflected, for example, in recent U.S. Food and Drug Administration (the FDA) food labeling regulations [10]. Market research by the National Purchase Diary Group (NPD) shows a gradual but steady increase in the percentage of adults who say they are cutting down on gluten or avoiding it completely—currently more than one in every four adults [11].

Celiac disease is typically detected by serological testing of celiac-specific antibodies, and the diagnosis is usually confirmed by duodenal mucosal biopsies [12]. Both serology and biopsy should be performed while the individual is following a gluten-containing diet [12]; however, potential celiac disease sufferers who have started themselves on a celiac free-diet before a work-up was completed, may be harder to diagnose. This is especially true if the patient carries the HLA-DQ2 and HLA-DQ8 haplotypes, as do 30–50% of the unaffected population [13]. In these circumstances, a new diagnostic test—currently still under development and based on cytokine release assay in response to a gluten challenge—could be helpful [14]. Although duodenal biopsies are still considered essential in adults, an alternative diagnostic strategy has been suggested, which avoids biopsies in children who have tissue transglutaminase levels higher than 10 times the upper limit of normal [15].

It is generally believed that the IgA anti-tissue transglutaminase (tTG) is the single best serological test to use for the detection of celiac disease. While celiac disease can be recognized endoscopically by visual inspection, especially if water immersion is used to enhance the detection of villi, a normal endoscopic appearance does not preclude the diagnosis [16].

At present, treatment primarily involves a GFD, an approach that demands significant patient education, motivation, and follow-up. The GFD also imposes a heavy treatment burden on patients, given the requirement for constant vigilance for gluten contamination. Not surprisingly, a large proportion of patients report inadvertent or deliberate exposure to gluten [17]. Non-responsive celiac disease is frequently reported, especially among individuals diagnosed in adulthood, and may be in part related to trace gluten contamination [18]. In many adult patients—perhaps the majority—the intestine fails to heal even after two years of the GFD [19].

Research and development ‘pipeline’ for celiac disease drugs & continuing clinical trials

Clinical trials on drugs to treat celiac disease are at an early stage of evolution, with no products having reached Phase 3 to date (Figure 1). Products in development include:

• ALV003, Alvine Pharmaceuticals’ lead clinical candidate for celiac disease, which is currently being studied in Phase 2b [20]. ALV003 is an orally administered mixture of two recombinant gluten-specific proteases—a cysteine protease (EP-B2) and a prolyl endopeptidase (PEP)—which have been shown in vitro to degrade gluten.

In a Phase 2 study with ALV003, adults with biopsy-proven celiac disease were randomly assigned to groups receiving ALV003 (n = 20) or placebo (n = 21), together with a daily 2 g gluten challenge. Duodenal biopsies were collected at baseline and after the gluten challenge. The ratio of villus-height-to-crypt-depth and densities of intra-epithelial lymphocytes were the primary endpoints. Biopsies from subjects in the placebo group showed evidence of mucosal injury after gluten challenge (mean villus height-to-crypt depth ratio changed from 2.8 before challenge to 2.0 afterward; P = 0.0007; density of CD3+ intraepithelial lymphocytes changed from 61 to 91 cells/mm after challenge; P = 0.0003). In contrast, no significant mucosal deterioration was observed in biopsies from the ALV003 group. Between groups, morphologic changes and CD3+ intraepithelial lymphocyte counts differed significantly from baseline to Week 6 (P = 0.0133 and P = 0.0123, respectively). Interestingly, there were no statistically significant differences in symptoms between groups [21].

Based on the promising Phase 2a results, a Phase 2b study is now being conducted. The study is evaluating the safety and efficacy of ALV003 at different dose levels administered over a twelve-week period in 500 celiac disease patients in the USA, Canada and Europe, who are symptomatic despite attempting to follow a GFD. The primary efficacy endpoint for the study is the change in small intestinal mucosal morphology, as measured by the change in villus-height-to-crypt-depth ratio (Vh:Cd) from baseline and week 12 assessments. Secondary endpoints are the changes in density in intestinal intraepithelial lymphocytes and celiac disease-specific symptoms during the study. Other outcomes to be evaluated include changes in celiac disease serologies and quality-of-life measures [20].

Aspergillus niger prolyl endoprotease (AN-PEP) is an endopeptidase, like the PEP component of ALV003, which can break down gluten. The enzyme is active between pH 2 and pH 8, with an optimum activity at pH 4–5, and is therefore effective at the pH levels present in the stomach and small intestine [22]. AN-PEP was evaluated in a recent small, double-blind, placebo controlled, randomized trial on 16 patients with a diagnosis of celiac disease—as confirmed by positive serology—with sub-total or total villous atrophy on duodenal biopsies, who adhered to a strict GFD, resulting in normalized antibodies and mucosal healing classified as Marsh 0 or I [22]. In a randomized, double-blind, placebo-controlled pilot study, patients consumed toast (approximately 7 g/day gluten) with AN-PEP for 2 weeks (safety phase). After a 2-week washout period with adherence to the usual GFD, 14 patients were randomized to gluten intake with either AN-PEP or placebo for 2 weeks (efficacy phase). No serious adverse events occurred and no patients withdrew during the trial. The mean score for the gastrointestinal subcategory of the celiac disease quality (CDQ) was relatively high throughout the study, indicating that AN-PEP was well tolerated. In the efficacy phase, the CDQ scores of patients consuming gluten with placebo or gluten with AN-PEP did not significantly deteriorate and, moreover, no differences between the groups were observed. The authors conclude in their discussion that “with hindsight, the study should possibly have been designed for a much longer period of time with many more patients” [22].

• Larazotide acetate (AT-1001) is Alba Therapeutics Corporation’s investigational product, a first-in-class tight junction regulator, intended for the treatment of patients with celiac disease. It has been hypothesized that celiac disease is accompanied by raised paracellular permeability, accompanied by an inflammatory cascade within the bowel, which is controlled by tight junctions. Alba has reported positive results from a double-blind, placebo-controlled, Phase 2b trial in February 2014 [23]. This evaluated the efficacy and safety of larazotide acetate in 342 celiac disease patients who had symptoms despite being on a GFD. The trial consisted of a four-week placebo run-in, 12 weeks of randomized therapy, and four weeks of post-treatment follow-up. Patients were randomized to four groups: a placebo group or larazotide 0.5, 1.0, or 2.0 mg, three times per day. Treatment with the lowest of three doses of larazotide was associated with significant improvement in the primary outcome, i.e. the average on-treatment score in the Celiac Disease Gastrointestinal Symptom Rating Scale (CeD GSRS) domains of Diarrhea, Indigestion, and Abdominal pain. Analysis of individual components of the rating scale, proprietary to Alba Therapeutics [24], showed consistent improvement with larazotide for each parameter [25].

The Phase 2b study discussed above was preceded by a separate, dose-ranging, placebo-controlled study of 86 patients with celiac disease controlled through diet [26]. The aim of this study was to evaluate the efficacy and tolerability of larazotide in protecting against gluten-induced intestinal permeability and worsening gastrointestinal symptoms. Study participants were randomly assigned to larazotide acetate (0.25, 1, 4, or 8 mg) or placebo three times a day, with or without gluten challenge (2.4 g/day) for 14 days. The primary efficacy outcome, an improvement in the lactulose/mannitol (LAMA) fractional excretion ratio (an experimental biomarker for intestinal permeability) was not met; however, the 0.25 and 4.0 mg doses of larazotide acetate showed statistically significant prevention of severe worsening of gastrointestinal symptoms.

An exploratory study published in 2013 [27] examined the effect of larazotide acetate on intestinal permeability, development of antibodies to tTG and celiac disease symptoms during a gluten challenge that exceeded the likely level of accidental gluten ingestion in individuals whose disease was well controlled by a GFD. In the larazotide acetate 1 mg group, a reduction in the expected increase was seen in the urinary LAMA ratio but the difference was not statistically significant as compared with placebo. Changes in pre-specified secondary endpoints suggest that larazotide acetate reduced antigen exposure, as shown by lowered production of anti-tTG antibodies. Larazotide acetate also reduced gastrointestinal symptoms upon gluten challenge.

• Nexvax2^®: ImmusanT’s peptide-based therapeutic celiac disease vaccine.

According to press releases from ImmusanT [28], the therapeutic vaccine Nexvax2 combines three proprietary peptides that elicit an immune response in celiac disease patients who carry the immune recognition gene HLA-DQ2. Similarly to treatments for allergies, the vaccine is designed to reprogram gluten-specific T cells triggered by the patient’s immune response to the protein. According to ImmusanT, the objective is for Nexvax2 to restore celiac patients’ immune tolerance to gluten, reduce inflammation in the nutrient-absorbing villi that line the small intestine, return the intestine to a healthy state, and allow patients to eat a normal diet [28].

The company says that early clinical trials have so far proven promising, with Phase 1b trial results demonstrating clear proof of mechanism and Phase 2 trials expected to begin in 2015—but details are not known at the time of this writing (December 2014) [29].

• BL-7010: BiolineRx’s non-absorbable, high molecular weight polymer with a high affinity for gliadins, the immunogenic peptides present in gluten that cause celiac disease. The product acts by sequestering gliadins. Experiments in vivo in a murine model of celiac disease have shown that BL-7010 prevents pathological damage to the small intestine, helps to preserve the integrity of the intestinal mucosa and reduces inflammation [30]. Although the company website lists BL-7010 as being in pre-clinical development, clinicaltrials.gov shows an active recruiting Phase 1 safety study [31].

• AVX176, from Avaxia Biologics, is an investigational oral antibody drug that is the subject of U.S. composition of matter patent 8,071,101, “Antibody Therapy for Treatment of Diseases Associated with Gluten Intolerance.” The patent, which expires on May 27 2029, provides broad coverage for treating celiac disease using orally administered antibodies produced by Avaxia’s proprietary platform technology [32].

• ActoGenX is carrying out discovery research in celiac disease with its range of ActoBiotics^™, which use Lactococcus lactis as an expression system to locally secrete bio-therapeutics such as cytokines, antibodies, hormones, etc. [33]. Early pre-clinical work with a genetically altered L. lactis secreting a peptide derived from gliadin demonstrated an in vivo suppression of gluten sensitization. Specifically, Huigbregtse et al. engineered L. lactis to secrete a deamidated DQ8 gliadin epitope (LL-eDQ8d) and studied the induction of Ag-specific tolerance in NOD ABo DQ8 transgenic mice [34]. Although apparently not part of the ActoGenX development program, recent work by Galipeau et al. also deserves mention in this context. The group treated gluten-sensitive mice with elafin, a serine protease inhibitor, delivered by the L. lactis vector, and found normalization of inflammation, improved permeability, and maintained ZO-1 expression. There is speculation that this is due to reduced deamidation of gliadin peptide [31].

• Chemocentryx’s CCR9 (vercirnon, which is also known as Traficet-EN, or CCX282B)—originally intended for patients with moderate-to-severe Crohn’s disease—has completed one Phase 2 trial in 67 patients with celiac disease [35]; however, despite the completion of the trial several years ago, no results relating to celiac disease have been made public or published.

A patent search (see Appendix Table A1) revealed at that at least a dozen patents (including U.S. and European patents) were granted for potential celiac disease-related therapies and diagnostics in 2013 and 2014. These were assigned to organizations including Alvine Pharmaceuticals Inc., Curemark LLC, Medarex Inc., Aesku Diagnostics GmbH & Co. KG, Immco Diagnostics Inc., Alba Therapeutics Corp., Nestec SA, BTG International Ltd., ImmusanT Inc., DSM IP Assets BV, Sigma Tau Ind Farmaceuti, and various institutions, groups and individuals.

Clinical trial endpoints

An endpoint is quite simply a measure believed to quantify the potential effect of the treatment or intervention under study. Effect is, of course, a term that can be broadly interpreted. The effect is articulated in the claims made on the official drug label and such claims need to be supported by the study results, which are quantified by the endpoints. The process of thinking in terms of a drug’s mechanism of action (what it could do in theory), what it does in practice (clinical effect and efficacy), how this is demonstrated (endpoints) and how this relates into drug labeling claims (what is on the label) is not linear; in fact, regulatory consultants often advise that the thinking should begin around appropriate endpoints, with the end (labeling claims) in mind. In fact, this approach has found its canonization in what is known as the target product profile (TPP). Although not much used in a formal way, it nevertheless reflects how regulators think: the TPP is organized according to the key sections in the drug label and links drug development activities to specific concepts intended for inclusion in the drug labeling [36].

Clinical trial endpoints are well established in many major disease areas where a fair number of registration trials have already been conducted and drugs approved for marketing. Emerging fields, such as celiac disease—where there is little experience and no approved products—often lack agreed endpoints. Consequently, both sponsors and regulators will need to come to a new agreement for each development program. Many small biotechnology companies lack in-house regulatory experience and much frustration could be avoided by understanding, at an early stage, some general principles concerning primary endpoints for registration trials. In the following, we will review important concepts.

For registration trials, the FDA requires ‘clinically meaningful endpoints’ defined as endpoints that are direct measures of how patients feel, function, and survive [37] unless a validated surrogate biomarker acceptable to the FDA is available, which is rare. In disease in which there is a large subjective component, co-primary endpoints are increasingly employed: typically, a patient-reported outcome (PRO) is combined with an instrument that reflects disease activity more directly—for example, a biomarker. This approach is currently being implemented for Crohn’s disease and ulcerative colitis. Although the inflammatory bowel diseases have little in common with celiac disease, they are still similar with regard to ‘treatment success’ which would be incompletely captured if one were to focus on a single endpoint; for example histology or how the patient feels, but not both.

In celiac disease, conceptually, one type of treatment could control symptoms and prevent worsening of damage while another is, at least initially, focused primarily on healing and maintenance of healing, with little effect on symptoms. Obviously, in both cases, different endpoints or endpoint instruments are needed.

Several other food- and allergy-related disorders share a lack of well-defined clinical endpoints. An example is eosinophilic esophagitis, which occurs in response to an as-yet unknown allergen in the diet, has histological manifestations, has histology that does not correlate well with symptoms, and in which there is a requirement for an endpoint instrument that combines a PRO with objective response criteria [38].

Co-primary endpoints require that, to be an overall responder, patients meet the responder definitions for each of the individual endpoints that comprise the co-primary endpoint (see Glossary). Composite endpoints, in contrast, are endpoints that are composed of different measurements (e.g. several different scales, instruments or components) that are aggregated to an overall endpoint (see Glossary).

Recommendations for endpoints in clinical trials of drugs for celiac disease

Phase 2 POC trials offer more flexibility on outcome measures, needing only to provide stakeholders with evidence to make a go/no-go decision. Ideally, this outcome instrument should also inform the development of potential endpoints in Phase 3 trials. To be truly useful, even in the POC stage, POC endpoints need to be able to reasonably predict ‘clinical benefit’, that is, in practical terms, correlate with outcome instruments acceptable for future registration trials. POC endpoints also need to be statistically efficient, i.e. take advantage of as much information as the data contain and be suitable for the most powerful hypothesis tests. Typically, this would favor continuous data over ordinal data or scores.

Conclusion

Research into celiac disease has entered an exciting phase: for the first time there may be alternatives to the gluten-free diet. As drugs move out of the proof-of-concept stage to Phase 3 confirmatory trials, they need to be evaluated by endpoints that are tailored to the drug, disease, and target patient population. Perhaps most importantly, these endpoints need to be acceptable to regulatory agencies. We have attempted to open a discussion of the issues that we find are relevant and important, which we hope academic researchers, industry and regulators will continue.

Conflict of interest statement: Drs. Gottlieb, Dawson and Hussain work for Quintiles, a company that provides bio-pharmaceutical development services and consulting. They have no other relevant disclosures.

Joseph A. Murray has consulting arrangements with AMAG Pharmaceuticals, Entera Health, Inc, Sonomaceuticals, LLC, BioLineRx and is on the Alvine Pharmaceuticals, Inc. advisory board. Dr. Murray has received grants or research support from Alvine Pharmaceuticals, Inc., and Alba Therapeutics.