1. Introduction

Diabetes mellitus is characterized by chronically elevated serum glucose levels resulting in damage of several tissues (e. g. retina, kidney, nerves) due to higher protein glycation, retardation of wound healing, impaired insulin secretion, enhanced insulin resistance, cell apoptosis, and increased oxidative stress. Type 2 diabetes (T2DM), representing 90-95 % of all diabetic cases, is a multifactorial disease where impaired insulin secretion and the development of insulin resistance ultimately leads to hyperglycemia (Hengesh, 1995). The end of the 20th century has witnessed a dramatic increase in the number of patients diagnosed with diabetes worldwide. The predicted number for the year 2025 is well over 300 million representing a 4-5 % yearly increase of the population above 20 years of age (Treadway et al., 2001). This striking prevalence can even be an underestimate due to methodological uncertainties as well as undiagnosed cases (Green et al., 2003). The highest increases are expected in the developing countries of Africa, Asia, and South America, while European populations seem to be less affected (Diamond, 2003). T2DM has been considered as the adult- or late-onset variant, however, the recent decade has seen the appearance and spreading of the disease among young people including children: this forecasts severe economic and health service burdens in the near future (Alberti et al., 2004; Ehtisham & Barrett, 2004).

The epidemic of T2DM is in conjunction with genetic susceptibility: evidence for a genetic component to the disease are accumulating, and the potential of these factors in the treatment and prevention of diabetes has been reviewed (Barroso, 2005; Bonnefond et al., 2010; Sladek et al., 2007; Toye & Gauguier, 2003). A similarly high contribution to this epidemic may originate from behavioral factors such as sedentary lifestyle, overly rich nutrition, and obesity (Bloomgarden, 2004).

Especially due to its long term complications (Brownlee, 2001) like retinopathy, neuropathy, nephropathy, and in particular cardiovascular diseases, as well as significantly higher risk of myocardial infarction, stroke, gangrene, and limb amputation diabetes has become one of the largest contributors to disability and mortality. Although several pathomechanisms (Lowell & Shulman, 2005;Panunti et al., 2004; Stumvoll et al., 2005) are under investigation, no firm understanding of the molecular origins (Ross et al., 2004) of the disease exists. Thereby, all available and investigational treatments are symptomatic. As the complications can first of all be attributed to the high blood glucose levels, current antidiabetic therapies (Table 1) aim at reaching normoglycemia. However, most of the applied oral hypoglycemic agents (Cheng & Fantus, 2005; Krentz & Bailey, 2005; Mizuno et al., 2008; Padwal et al., 2005; Rendell, 2004; Uwaifo & Ratner, 2005) have several side effects and are inadequate for 30-40 % of the patients (Wagman & Nuss, 2001). On the other hand, their efficacy is lost over the time, and several concerns exist regarding their safety (Israili, 2011).

The complexity of T2DM offers many potential points of intervention for pharmacotherapy for which the main molecular targets and strategies such as insulin secretagogues, insulin sensitizers, hormones, inhibitors of PTP-1B, GSK3, and hepatic glucose production, methods for altering lipid metabolism, combination therapies, etc. have been reviewed in details (Israili, 2011; Morral, 2003; Nourparvar et al., 2004; Wagman et al., 2004).

Among the numerous methods used to treat type 2 diabetes and investigated to find new therapeutic possibilities there are several approaches which apply carbohydrate (especially glucose) derivatives as well as compounds mimicking the properties of sugars. Based on our experience in the chemistry of carbohydrates and glycomimetics, in this survey we summarize the roles of such compounds in combatting type 2 diabetes relying on the review literature and very recent primary scientific papers.

2. Inhibitors of α-glucosidase enzymes

Starch and sucrose are the most important dietary carbohydrates but they are not directly available for the cells. They are digested in the gastrointestinal tract to monosaccharides which can be absorbed to the circulation to raise the serum concentration (Hanhineva et al., 2010). The normal blood glucose level (3.6–5.8 mM) fluctuates throughout the day, is usually lowest in the morning, before the first meal of the day, and rises after meals for an hour or two.

A medically applied treatment of diabetes is to retard the absorption of glucose by inhibition of the carbohydrate hydrolyzing enzymes α-amylase and α-glucosidase in the digestive tract. In humans the digestion of starch, maltodextrins, and maltooligosaccharides includes several stages: degradation of the polymeric substrates results in shorter oligomers which are than cleaved by α-amylase into smaller oligosaccharides. This mixture is broken down to monosaccharides by α-glucosidase from the non-reducing end of the oligosaccharides. By inhibition of these enzymes the rate of glucose production can be reduced that contributes to diminishing the blood glucose levels, too (Tundis et al., 2010). Such inhibitors decrease postprandial hyperglycaemia and hyperinsulinaemia, thereby may improve sensitivity to insulin and release the stress on β-cells (Scheen, 2003).

Glycosidases are a long known and studied class of glycoenzymes for which an enormous number of compounds have been tested as inhibitors (El Ashry et al., 2000a; El Ashry et al., 2000b; El Ashry et al., 2000c; Lillelund et al., 2002). Analogues of monosaccharides in which the ring oxygen is replaced by a nitrogen atom are known as iminosugars (or less properly azasugars) comprising both natural and synthetic molecules (Table 2) which, as the most potent inhibitors of glycosidases, have high pharmacological potential not only in the context of T2DM (Asano, 2009; Compain & Martin, 2007).

The naturally occurring salacinol and analogous sugar mimics with a 4-thiofuranoid type ring (Table 2) belong to a growing class of zwitterionic glycosidase inhibitors, which attract great interest both as synthetic targets and applications for α-glucosidase inhibition (Praly & Vidal, 2010).

The positive charge on the sulfur atom in the thiosugar derivatives and in the iminosugar-based glycosidase inhibitors at physiological pH is facilitating the binding in the active sites of glycosidase enzymes as a mimicry of the charge of the oxocarbeniumion-like transition state formed during hydrolysis of the natural enzyme substrate (Zechel & Withers, 2000). The stabilizing electrostatic interactions between the ammonium (protonated nitrogen) or sulfonium (positively charged sulfur) moieties and an active-site carboxylate residue are considered to be a possible mechanism of action of these inhibitors (Mohan & Pinto, 2007).

Three competitive inhibitors of α-glucosidases: acarbose, miglitol, and voglibose (de Melo et al., 2006) (Table 3) are used as drugs in the treatment of T2DM under various brand names. These compounds are known to inhibit a wide range of glycosidases. In the absence of specificity and because of the known serious side effects, the applications of these first generation iminosugar drugs are limited. Current investigations aim at discovering safer, more specific, and effective iminosugar based derivatives not only as hypoglycemic agents but for several other purposes among others in oncology, as antivirals, and against cystic fibrosis as reviewed in (Home et al., 2011).

TABLE 2.

Select iminosugar and thiosugar type inhibitors and their effect againstα-glucosidases originating from mammalian gastrointestinal tract

Name	Structure	Side-effect
Acarbose Approved in 1995		Flatulence (78% of the patients) Diarrhea (14% of the patients)
Miglitol Approved in 1996		Diarrhea, gas, soft stools, stomach pain
Voglibose Approved in 1997		Diarrhea, stool loss, meteorism, upset stomach

TABLE 3.

α-Glucosidase inhibitors in the clinical practice against T2DM

 

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