Eugen J. Verspohl

Novel therapeutics for type 2 diabetes: incretin hormone mimetics (glucagon-like peptide-1 receptor agonists) and dipeptidyl peptidase-4 inhibitors.

Known treatments of type 2 diabetes mellitus have limitations such as weight gain, and hypoglycaemias. A new perspective is the use of incretin hormones and incretin enhancers. Incretins are defined as being responsible for the higher insulin release after an oral glucose load compared to an intravenous glucose load. The delicate balance of glucose homeostasis, in which incretin hormones are involved, is disturbed in type 2 diabetes mellitus. The incretin GLP-1 helps to maintain glucose homeostasis through stimulation of insulin secretion and inhibition of glucagon release in a glucose-dependent manner. This is associated with reductions in body weight, and no risk of hypoglycaemias. When classical oral agents have failed to maintain adequate glycaemic control, incretin mimetics may be of particular value for obese patients and those who have little control over meal sizes. Exenatide was marketed as a GLP-1 analogue and longer acting incretin mimetics such as liraglutide, albiglutide and others have the same pharmacological profile. In addition to incretin mimetics incretin enhancers which inhibit/delay degradation of incretins were developed: so-called DPP-4 inhibitors such as sitagliptin and vildagliptin are approved in Europe. Their differences from incretin mimetics include: oral bioavailability, less side effects with overdose, no direct CNS effects (nausea and vomiting) and no effect on weight. In rodent models of diabetes, but not yet in humans, GLP-1 receptor agonists and DPP-4 inhibitors increase islet mass and preserve beta-cell function. Incretin mimetics and enhancers expand type 2 diabetes treatment, are still not first line therapy and it is discussed if they are to be prophylactically used.

Novel Pharmacological Approaches to the Treatment of Type 2 Diabetes

The huge increase in type 2 diabetes is a burden worldwide. Many marketed compounds do not address relevant aspects of the disease; they may already compensate for defects in insulin secretion and insulin action, but loss of secreting cells (β-cell destruction), hyperglucagonemia, gastric emptying, enzyme activation/inhibition in insulin-sensitive cells, substitution or antagonizing of physiological hormones and pathways, finally leading to secondary complications of diabetes, are not sufficiently addressed. In addition, side effects for established therapies such as hypoglycemias and weight gain have to be diminished. At present, nearly 1000 compounds have been described, and approximately 180 of these are going to be developed (already in clinical studies), some of them directly influencing enzyme activity, influencing pathophysiological pathways, and some using G-protein-coupled receptors. In addition, immunological approaches and antisense strategies are going to be developed. Many compounds are derived from physiological compounds (hormones) aiming at improving their kinetics and selectivity, and others are chemical compounds that were obtained by screening for a newly identified target in the physiological or pathophysiological machinery. In some areas, great progress is observed (e.g., incretin area); in others, no great progress is obvious (e.g., glucokinase activators), and other areas are not recommended for further research. For all scientific areas, conclusions with respect to their impact on diabetes are given. Potential targets for which no chemical compound has yet been identified as a ligand (agonist or antagonist) are also described.

Large molecules as anti-adhesive compounds against pathogens.

Anti‐adhesive compounds are potential prophylactic tools in alternative treatment regimes against bacterial infection, as bacterial adhesion is commonly mediated by carbohydrate‐protein interactions between surface adhesions of microorganisms and the host cell. The use of exogenous polyvalent, high‐molecular carbohydrates and tannin‐like plant‐derived compounds should antagonize the adhesive interaction. A range of carbohydrates and carbohydrate‐ and proanthocyanidin‐enriched plant extracts were screened for potential anti‐adhesive effects against Helicobacter pylori, Campylobacter jejuni, Porphyromonas gingivalis and Candida albicans in different in‐situ assays on primary tissue. The adhesion of H. pylori on human stomach tissue was effectively blocked by glucuronic acid‐enriched polysaccharides from immature okra fruits (Abelmoschus esculentus). These compounds also had strong in‐vitro effects against C. jejuni (inhibition up to 80%), but were ineffective in an in‐vivo study in infected chicken broilers due to metabolism in the gastrointestinal system. Polysaccharides from Glycyrrhizia glabra, also enriched with glucuronic acid, showed strong anti‐adhesive properties against H. pylori and P. gingivalis (inhibition 60–70%). Pelargonium sidoides extract, containing mainly polymeric proanthocyanidins, was effective against H. pylori in a dose‐dependent manner. Due to the multifunctional adhesive strategy of C. albicans, no effective compounds were detected against this yeast. Structure‐activity relationships are presented and the potential in‐vivo use of carbohydrate‐based anti‐adhesives is discussed.

Cinchonain Ib isolated from Eriobotrya japonica induces insulin secretion in vitro and in vivo.

AIMS OF THE STUDY Eriobotrya japonica leaves had been used traditionally for the treatment of diabetes mellitus by immersing the dried leaves in a hot water drink. Few studies have shown the hypoglycemic effect of Eriobotrya japonica using crude alcoholic extract and isolated methanolic compounds. These studies proposed that the mechanism of action could be by stimulating the beta-islets of Langerhans to secrete insulin, however with no scientific evidence. METHODS Eriobotrya japonica water extract (EJWE) and the compounds derived from it: cinchonain Ib, procyanidin B-2, chlorogenic acid and epicatechin, were tested for their effects on insulin secretion from INS-1 cells and following oral administration in rats. RESULTS The present study showed that EJWE increased significantly (p<0.05) insulin secretion from INS-1 cells in dose-dependent manner. Oral administration of EJWE at 230 mg/kg to rats, however, decreased plasma insulin level for as long as 240 min post-administration and caused a transient drop of blood glucose at 15 and 30 min post-administration. On the other hand, cinchonain Ib enhanced significantly (p<0.05) insulin secretion from INS-1 cells, whereas epicatechin inhibited significantly (p<0.05) insulin secretion from INS-1 cells. In addition, cinchonain Ib enhanced significantly (150%: p<0.05) plasma insulin level in rats for as long as 240 min after 108 mg/kg oral administration but did not induce any change in blood glucose level. CONCLUSION These data indicate that cinchonain Ib has an insulinotropic effect and suggest the possible use of cinchonain Ib for managing type 2 diabetes.

Impact of the 5-HT3 receptor channel system for insulin secretion and interaction of ginger extracts.

The relevance of serotonin and in particular that of 5-HT(3) receptors is unequivocal with respect to emetic/antiemetic effects, but it is controversial with respect to antidiabetic effects. The effects of tropisetron (5-HT(3) receptor antagonist) and various ginger (Zingiber officinale) extracts (known to interact with the 5-HT(3) receptor channel system) were investigated. Serotonin (32 to 500 microM) inhibits insulin release (RIA) from INS-1 cells which is reversed by tropisetron (10 to 100 microM) and two different ginger extracts (spissum and an oily extract). Their effects are obvious even in the absence of serotonin but are more pronounced in its presence (doubled to tripled). Specific 5-HT(3) binding sites are present in INS-1 cells using 0.4 nM [3H] GR65630 in displacement experiments. The in vitro data with respect to ginger are corroborated by in vivo data on glucose-loaded rats showing that blood glucose (Glucoquant) is decreased by approximately 35% and plasma insulin (RIA) is increased by approximately 10%. Both the spissum extract and the oily ginger extract are effective in two other models: they inhibit [(14)C] guanidinium uptake into N1E-115 cells (model of 5-HT(3) effects) and relax rat ileum both directly and as a serotonin antagonistic effect. Other receptors addressed by ginger are 5-HT(2) receptors as demonstrated by using methysergide and ketanserin. They weakly antagonize the serotonin effect as well. It may be concluded that serotonin and in particular the 5-HT(3) receptor channel system are involved in modulating insulin release and that tropisetron and various ginger extracts can be used to improve a diabetic situation.

The impact of adenosine and A2B receptors on glucose homoeostasis

Adenosine and adenosine receptor antagonists are involved in glucose homoeostasis. The participating receptors are not known, mainly due to a lack of specific agonists and antagonists, but are reasonable targets for anti‐diabetic therapy. The stable, albeit nonselective, adenosine analogue NECA (5′‐N‐ethylcarboxamidoadenosine) (10 μM) reduced glucose‐stimulated insulin release from INS‐1 cells. This was mimicked by A1‐(CHA), A2A‐(CGS‐21680) and A3‐receptor agonists (Cl‐IB‐MECA). Two newly synthesized A2B‐receptor antagonists, PSB‐53 and PSB‐1115, counteracted the inhibitory effect of NECA. These in‐vitro effects were mirrored by in‐vivo data with respect to CHA, CGS and Cl‐IB‐MECA. Distinct concentrations of either PSB‐53 or PSB‐1115 reversed the decrease in plasma insulin induced by NECA. This was not mimicked by a corresponding change in blood glucose. The effect of PSB‐1115 was also obvious in diabetic GotoKakizaki rats: plasma insulin was increased whereas blood glucose was unchanged. During most experiments the effects on blood glucose were not impressive probably because of the physiologically necessary homoeostasis. The adenosine levels were not different in normal Wistar rats and in diabetic GotoKakzaki rats. Altogether the A2B‐receptor antagonists showed an anti‐diabetic potential mainly by increasing plasma insulin levels under conditions when the adenosine tonus was elevated in‐vivo and increased insulin release in‐vitro.

Impact of Thymol in Thyme Extracts on Their Antispasmodic Action and Ciliary Clearance

Thyme is a herb with broncholytic und secretomotoric effects. Its activity on beta (2) receptors as a possible mechanism of action was demonstrated. Major components are thymol and carvacrol which are claimed to be responsible for its effects and, therefore, used for standardization in the German pharmacopoeia (0.03 % phenols calculated as thymol). Our aim was to investigate the impact of thymol by using thyme extracts with either normal or extremely low thymol concentrations (< 0.005 % or > 0.038 %). The antispasmodic effect on smooth muscles of the trachea and the ileum and the effect on ciliary activity (respiratory clearance) were investigated. In addition, pure thymol and carvacrol were investigated separately and in spiking experiments. Thymol and carvacrol had a concentration-dependent antispasmodic effect in the rat trachea either being stimulated by acetylcholine, K (+) or Ba (++). The same result was observed with respect to the increase of mucociliary transport in mice. Extracts with very low thymol contents are effective in all models used except acetylcholine-induced rat ileum contraction. When thyme extracts with normal thymol contents or with very low thymol contents were compared, the extract with normal thymol contents was more effective, both as a relaxant (rat ileum) and as an antispasmodic compound (rat trachea contraction induced by either acetylcholine, Ba (++) or K (+)) and in ciliary transport experiments. Thyme extracts with very low thymol contents (practically free of volatile oil) were equally effective with respect to endothelin effects. When an extract with very low thymol contents is spiked with increasing concentrations of thymol, a concentration-dependent increase concerning the antispasmodic effect (Ba (++)-induced trachea contraction) is observed. In conclusion, the data show that in various models of antispasmodic effect (ileum and trachea) and by measuring ciliary activity, thymol (and carvacrol) is (are) active, although other not identified components of thyme extract appear to be very important as well, since extracts with very low thymol contents are active. On the basis of these results the standardization on thymol alone appears not to be justified.

Synthesis and biological activity of some new flavonyl-2,4-thiazolidinediones

A new series of flavonyl-2,4-thiazolidinediones (Va-c, VIa-c) was prepared by Knoevenagel reaction. The synthesized compounds were tested for their ability to inhibit rat kidney aldose reductase (AR) and for their insulinotropic activities in INS-1 cells. Compound Vb was able to increase insulin release in the presence of 5.6mmol/l glucose. Compounds VIa-c displayed moderate to high AR inhibitory activity levels. Particularly, compound VIa showed the highest AR inhibitory activity (86.57%).

Comparative evaluation of two different Opuntia ficus-indica extracts for blood sugar lowering effects in rats

Stems of Opuntia ficus‐indica (L.) Mill. (OFI) are traditionally used in Mexico to treat diabetes mellitus. Less research data are available for combinations of stem and fruit preparations. The present study was designed to investigate the effects of an aqueous extract prepared from the cladodes and a proprietary stem/fruit skin‐blend (stem/fruit skin ratio 75/25) of OFI on blood glucose and plasma insulin in normal rats. A dose finding study with the traditional cladode OFI extract revealed that maximum effects on blood glucose and insulin were observed after oral administration in a dose range of 6–176 mg/kg. The proprietary OFI blend significantly lowered blood glucose levels in the glucose tolerance test to a similar extent (p < 0.05 vs control) as the traditional aqueous cladode extract when administered in a dose of 6 mg/kg. In contrast to the aqueous extract, the proprietary blend significantly increased basal plasma insulin levels (p < 0.01 vs control) indicating a direct action on pancreatic beta cells. The results suggest that both OFI extracts exert hypoglycemic activities in rats in doses as low as 6 mg/kg but that the effects of the proprietary stem/fruit blend were more pronounced in our model. Copyright

Atrial natriuretic peptide (ANP)-induced inhibition of glucagon secretion: mechanism of action in isolated rat pancreatic islets.

ANP increases insulin levels in vivo. Because in vitro an ANP-induced increase in cGMP levels of islets of Langerhans was observed but no simultaneous increase in insulin release, secreted glucagon may be a candidate for this second messenger affected by ANP. The inhibitory effect of glucose on glucagon secretion was pronounced by 1.0 nM ANP at 3.0 mM glucose as well as at 5.6 and 8.3 mM glucose. Because in other tissues cGMP (the specific second messenger of ANP1 inhibits Ca2+ channels, the uptake of 45Ca2+ was investigated. ANP (1.0 nM) inhibited 45Ca2+ uptake, which was nearly completely abolished by a pertussis toxin (PT) pretreatment. The inhibition of 45Ca2+ uptake fits to inhibitory ANP effects on glucagon secretion but does not fit to insulin secretion. The glucagon secretion coupling cascade affected by ANP probably involves an increase in cGMP because 8-Br-cGMP (a membrane-permeable cGMP analogue) also decreased glucagon secretion. ANP(4-23), a truncated form of ANP, which is selective for the ANP clearance receptor, also inhibited glucagon secretion. HS-42-1, a guanylate cyclase receptor antagonist, tended to reverse the effect of ANP on glucagon release. The data indicate that in the presence of ANP, the in vivo homeostasis of glucose, though plasma insulin levels are increased, is not due to an ANP-mediated increase in glucagon secretion; ANP has a complex inhibitory effect on glucagon release. The data further indicate that the ANP-induced inhibition of glucagon secretion probably involves the cGMP system, an inhibition of Ca2+ uptake and the involvement of PT-sensitive G-proteins. Moreover, an involvement of the clearance receptor seems to be likely. ANP is a valuable tool for investigating glucagon secretion from pancreatic islets because paracrine effects of insulin can be excluded.