Is an enzyme supplement for celiac disease finally on the cards?

There is an increasing number of dietary enzyme supplements on the market that claim to effectively degrade gluten, and in this way support gluten digestion in individuals suffering from gluten-related disorders. The mode of action of these enzymes is based on studies that have shown that the immunogenicity of gluten can be reduced by breaking down the gluten protein into smaller peptides [1]. Gluten contains high amounts of the amino acid proline. Human gastrointestinal proteases do not accept proline at their cleavage sites, which is why gluten proteins remain largely undigested. The immunogenicity of gluten in wheat can mostly be attributed to epitopes in α-, γ-, and ω-gliadin fragments. In barley and rye, similar epitopes can be found as part of the gluten-like peptides hordein and secalin [2]. The commercially available enzymatic products contain different types of bacteriaor fungi-derived proline-specific proteases, and it is important to consider the scientific evidence regarding their efficacy before use. Often, their glutendegrading capacity has not been tested, or alternatively showed to be rather low. For example, the commonly used fungal enzyme DPPIV, a dipeptidyl peptidase, is not active at a low pH and, hence, not very effective in an acidic gastric environment [3,4]. Furthermore, DPPIV can only act on the N-terminus of proteins and has no endoprotease activity, and thus leaves the large immunogenic gluten fragments mostly intact [3,5]. Kuma030 is an enzyme specifically designed by computational enzyme design to achieve 99% reduction of the immunogenic gliadin fractions even under gastric conditions, which was successfully shown in a laboratory-simulated gastric digestion model [6]. However, this enzyme has not been tested in vivo so far and is not commercially available yet. A proline protease that has been well investigated and is commercially available in the USA, Canada and Europe is the Aspergillus niger-derived prolyl endoprotease AN-PEP. It is active at a low pH (pH 2-8, optimal activity pH 4-5), and is resistant to degradation by pepsin [3]. Its capacity to degrade gluten into non-immunogenic peptides has been shown in a number of in vitro and in vivo studies [7–11]. Salden et al. showed that AN-PEP efficiently degraded gluten (4 g) in the stomach of healthy subjects, when added to an intragastrically infused meal [10]. It was recently shown that this was also the case when AN-PEP was administered in tablet form together with a composite meal in self-reported non-celiac glutensensitive subjects [11]. This study investigated the efficacy of AN-PEP to degrade a small amount of gluten (circa 0.5 g), which was intended to represent unintentional intake of gluten. Even if the enzyme has not specifically been tested in celiac disease patients for this purpose, it can be assumed that the physiology behind the gluten degradation by such an enzyme in the stomach does not differ between patients with celiac disease, non-celiac gluten sensitivity, or healthy subjects. Investigating the effect of gluten-degrading enzymes on symptoms and biomarkers in celiac disease patients in randomized, placebo-controlled studies is unfortunately not as straightforward as it might seem. In a pilot study, Tack et al. randomized 14 celiac disease patients to a daily intake of 7 g of gluten intake together with either AN-PEP or placebo over 2 weeks [12]. However, this relatively high amount of gluten did not result in clinical deterioration even in the placebo group, so that an effect of the enzyme on clinical markers and symptoms could not be shown. Increasing the amount of gluten or prolonging the exposure to gluten can raise ethical issues. It is rather common that celiac disease patients do not show symptoms when consuming gluten, especially after a longer gluten-free period, even if immunologic responses and small bowel tissue damage are triggered [13]. An additional, well-investigated gluten-degrading enzyme is latiglutenase (formerly known as ALV003). It is a mixture of two recombinant gluten-targeting proteases that has been studied in Phase 1 and 2 clinical trials. When administered together with 2 g of gluten over a period of 6 weeks in celiac disease patients, this enzyme significantly attenuated mucosal damages found in the placebo group (villus height to crypt depth ratio, amount of intraepithelial lymphocytes), while no effects were reported on symptoms [14]. However, when tested in a large dose-ranging study with almost 500 celiac disease patients, no difference in histologic or symptom scores were found between latiglutenase and placebo [15]. Both the treatment and the placebo groups showed significant improvements in those scores, and the authors of this study suggest that this could be due to a large trial effect. Amongst other factors affecting