alpha-Glucosidase inhibitors in diabetes: lessons from animal studies

Peripheral Neuropathy Phenotyping in Rat Models of Type 2 Diabetes Mellitus: Evaluating Uptake of the Neurodiab Guidelines and Identifying Future Directions

Diabetic peripheral neuropathy (DPN) affects over half of type 2 diabetes mellitus (T2DM) patients, with an urgent need for effective pharmacotherapies. While many rat and mouse models of T2DM exist, the phenotyping of DPN has been challenging with inconsistencies across laboratories. To better characterize DPN in rodents, a consensus guideline was published in 2014 to accelerate the translation of preclinical findings. Here we review DPN phenotyping in rat models of T2DM against the 'Neurodiab' criteria to identify uptake of the guidelines and discuss how DPN phenotypes differ between models and according to diabetes duration and sex. A search of PubMed, Scopus and Web of Science databases identified 125 studies, categorised as either diet and/or chemically induced models or transgenic/spontaneous models of T2DM. The use of diet and chemically induced T2DM models has exceeded that of transgenic models in recent years, and the introduction of the Neurodiab guidelines has not appreciably increased the number of studies assessing all key DPN endpoints. Combined high-fat diet and low dose streptozotocin rat models are the most frequently used and well characterised. Overall, we recommend adherence to Neurodiab guidelines for creating better animal models of DPN to accelerate translation and drug development.

Comparison of drug efficacy in two animal models of type 2 diabetes: A systematic review and meta-analysis

Previous qualitative research has suggested there are only minor differences between the db/db mouse and the Zucker Diabetic Fatty (ZDF) rat, both animal models of type 2 diabetes. However, it is not known whether these models are also comparable regarding drug response in quantitative terms (effect size). To investigate the extent of these differences, we conducted a systematic review and meta-analysis of approved drugs in these models. We searched on PubMed and Embase on July 3, 2019 for studies including either model, a monotherapy arm with an EMA/FDA approved drug for the treatment of type 2 diabetes, HbA1c assessment and a control group. Studies aimed at diabetes prevention or with surgical interventions were excluded. We calculated the Standardised Mean Difference (SMD) to compare effect sizes (HbA1c reduction) per drug and drug class across models. We included a risk of bias assessment for all included publications. A total of 121 publications met our inclusion criteria. For drugs with more than two comparisons, both models predicted the direction of the effect regarding HbA1c levels. There were no differences between the db/db mouse and ZDF rat, except for exenatide (P = 0.02) and GLP-1 agonists (P = 0.03) in which a larger effect size was calculated in the ZDF rat. Our results indicate the differences between the db/db mouse and ZDF rat are not relevant for preliminary efficacy testing. This methodology can be used to further differentiate between animal models used for the same indication, facilitating the selection of models more likely to predict human response.

Effects of methimazole on the onset of type 2 diabetes in leptin receptor-deficient rats

We investigated the effects of methimazole, an anti-thyroid drug, on the onset of type 2 diabetes in Zucker diabetic fatty (ZDF) rats. For this, 0.03% methimazole was administered to 7-week-old, pre-diabetic ZDF rats in drinking water for 5 weeks and the animals were sacrificed at 12 weeks of age. Methimazole treatment to ZDF rats significantly reduced blood glucose levels, food intake, body weight, and serum T3 levels. Hepatocytes in ZDF-methi rats were more densely stained with eosin than those in ZDF rats because of low fat accumulation in ZDF-methi hepatocytes. The pancreatic islet in ZDF-methi rats was normal compared to that in ZDF rats. Glucagon, not insulin, immunoreactivity in ZDF-methi rats was significantly higher than that in ZDF-methi rats. These suggest that methimazole treatment may delay the onset of type 2 diabetes in leptin receptor-deficient rats and also suggests that thyroid hormones may be necessary for the onset of diabetes.

Adenoviral vector-mediated glucagon-like peptide 1 gene therapy improves glucose homeostasis in Zucker diabetic fatty rats

BACKGROUND

Glucagon-like peptide-1 (GLP-1) is a gut-derived incretin hormone that plays an important role in glucose homeostasis. Its functions include glucose-stimulated insulin secretion, suppression of glucagon secretion, deceleration of gastric emptying, and reduction in appetite and food intake. Despite the numerous antidiabetic properties of GLP-1, its therapeutic potential is limited by its short biological half-life due to rapid enzymatic degradation by dipeptidyl peptidase IV. The present study aimed to demonstrate the therapeutic effects of constitutively expressed GLP-1 in an overt type 2 diabetic animal model using an adenoviral vector system.

METHODS

A novel plasmid (pAAV-ILGLP-1) and recombinant adenoviral vector (Ad-ILGLP-1) were constructed with the cytomegalovirus promoter and insulin leader sequence followed by GLP-1(7-37) cDNA.

RESULTS

The results of an enzyme-linked immunosorbent assay showed significantly elevated levels of GLP-1(7-37) secreted by human embryonic kidney cells transfected with the construct containing the leader sequence. A single intravenous administration of Ad-ILGLP-1 into 12-week-old Zucker diabetic fatty (ZDF) rats, which have overt type 2 diabetes mellitus (T2DM), achieved near normoglycemia for 3 weeks and improved utilization of blood glucose in glucose tolerance tests. Circulating plasma levels of GLP-1 increased in GLP-1-treated ZDF rats, but diminished 21 days after treatment. When compared with controls, Ad-ILGLP-1-treated ZDF rats had a lower homeostasis model assessment for insulin resistance score indicating amelioration in insulin resistance. Immunohistochemical staining showed that cells expressing GLP-1 were found in the livers of GLP-1-treated ZDF rats.

CONCLUSIONS

These data suggest that GLP-1 gene therapy can improve glucose homeostasis in fully developed diabetic animal models and may be a promising treatment modality for T2DM in humans.

Rosiglitazone prevents the onset of hyperglycaemia and proteinuria in the Zucker diabetic fatty rat

AIM

To investigate the potential of rosiglitazone, a highly potent agonist at the nuclear peroxisome proliferator activated receptor-gamma (PPAR-gamma), to prevent the development of diabetes in the Zucker diabetic fatty (ZDF) rat or to ameliorate the condition at a later stage of the disease.

METHODS

Rosiglitazone (10 micromol/kg body weight daily) was given via the diet to ZDF rats from aged 6 weeks, before the onset of hyperglycaemia (Prevention group), or from aged 21 weeks after hyperglycaemia and proteinuria were established (Intervention group). Untreated ZDF rats and age-matched Zucker lean rats (ZL) served as controls and the experiment was terminated when the animals were aged 28 weeks.

RESULTS

Whilst the combined ZDF control and Intervention groups were already hyperglycaemic (14.6 +/- 1.6 vs. ZL 5.7 +/- 0.1 mmol/l, mean +/- s.e.m.; p < 0.05), glycosuric and polydipsic at aged 11 weeks, and thereafter had a declining plasma insulin concentration, rosiglitazone Prevention treatment maintained normoglycaemia even at aged 27 weeks (3.7 +/- 0.3 mmol/l vs. ZL 3.0 +/- 0.3 mmol/l; NS). Intervention treatment at aged 21 weeks, however, failed to ameliorate the diabetes. These functional data were supported by determinations of pancreatic insulin content (microg/mg tissue as follows: ZL, 43.1 +/- 3.9; ZDF control (28 weeks) + ZDF Intervention control (21 weeks), 6.0 +/- 0.8; Prevention, 63.6 +/- 15.8; Intervention, 6.2 +/- 0.9) and by morphological, immunohistochemical and electron microscopical examination of pancreata at the end of the study. Thus, islets from rosiglitazone Prevention rats were similar to ZL rats, whereas ZDF controls and Intervention rats exhibited islets depleted of insulin, with a disorganized architecture and an ultrastructure indicative of work hypertrophy. ZDF control rats and Intervention rats, though not rosiglitazone Prevention rats, also exhibited marked proteinuria, indicative of renal glomerular damage.

CONCLUSIONS

Our results demonstrate that in ZDF rats, rosiglitazone prevents the progression from insulin resistance to overt diabetes. These data provide a rationale for investigating whether treatment with rosiglitazone of patients with early signs of perturbed glucose metabolism (e.g. impaired fasting glucose (IGT)) may prevent the progression to type 2 diabetes and its associated complications.

Treatment with acarbose may improve endothelial dysfunction in streptozotocin-induced diabetic rats

We sought to determine whether a single reduction of hyperglycemia and those derivatives from nonenzymatic protein glycosylation may be effective in reducing the development of diabetic endothelial dysfunction. Therefore, we investigated how acarbose, an inhibitor of intestinal alpha-glucosidase that reduce hyperglycemia by lowering glucose absorption, may prevent the impairment of acetylcholine (ACh)-induced endothelium-dependent relaxations observed in isolated vascular segments from untreated streptozotocin-induced diabetic rats. When administered after diabetes induction, 10 mg/kg acarbose decreased modestly the enhancement of blood glucose and glycosylated hemoglobin (HbA1c) levels, but not those of advanced glycosylation end products (AGEs). This effect was linked to a partial improvement of ACh-induced responses both in conductance vessels, such as aortic segments, and resistance vasculature, like mesenteric microvessels. When acarbose was introduced after 6 weeks of untreated diabetes, blood glucose, HbA1c, and AGE levels were not affected and endothelial dysfunction remained unchanged in mesenteric microvessels, whereas a small improvement was observed in aortic segments. The addition of 100 U/ml superoxide dismutase enhanced the impaired relaxations to values similar to vessels from nondiabetic rats, indicating a main role for superoxide anions in diabetes-induced endothelial dysfunction. We conclude that hyperglycemia itself or elevated HbA1c, but not plasma AGEs, are related to enhanced oxidative stress and to the impairment of endothelium function associated to diabetes. This process can be partially prevented by reducing glucose absorption with acarbose.

Effects of long-term treatment with alpha-glucosidase inhibitor on the peripheral nerve function and structure in Goto-Kakizaki rats: a genetic model for type 2 diabetes

BACKGROUND

Continuous hyperglycemia is implicated in the pathogenesis of chronic diabetic complications. It is not well known, however, how and to what extent the development of neuropathy is inhibited by blood glucose control in subjects with Type 2 diabetes. We investigated therefore the effects of an alpha-glucosidase inhibitor (voglibose; Vg) on neuropathic changes in diabetic Goto-Kakizaki (GK) rats, a genetic model for Type 2 diabetes.

METHODS

Twelve week-old male GK rats were given a diet containing Vg (50 ppm) for 24 weeks and monitored for blood glucose, glycated hemoglobin, motor nerve conduction velocity (MNCV). At the end of the administration period (Na(+), K(+))-ATPase activity and the structure of the peripheral nerves were examined. Age- and sex-matched normal Wistar rats were treated similarly and served as controls.

RESULTS

GK rats showed fasting hyperglycemia after 8 weeks of age, and Vg treatment significantly lowered levels of blood glucose and glycated hemoglobin. Slowing of MNCV to 80% of normal control levels was detected in GK rats. Vg treatment inhibited this delay by 24% at 24 weeks and 57% at 36 weeks of age. Nerve (Na(+), K(+))-ATPase activity was reduced to 80% of normal control levels in GK rats and was restored by Vg treatment. Teased fiber studies revealed a higher incidence of fibers with paranodal, segmental demyelination and axonal degeneration in GK rats. Vg treatment significantly inhibited the development of these nerve-fiber abnormalities.

CONCLUSIONS

Lowering of high blood glucose levels achieved by the use of Vg in GK rats improved MNCV and demyelinative nerve changes with restoration of (Na(+), K(+))-ATPase activity.

Effects of the alpha-glucosidase inhibitor acarbose on the development of long-term complications in diabetic animals: pathophysiological and therapeutic implications

Short-term studies with acarbose have demonstrated its efficacy in reducing postprandial blood glucose levels and glycated haemoglobin (HbA(1c)) levels. These effects would be expected to translate into improvements in long-term complications of diabetes, but such data are not yet available due to the long follow-up times required. Animal models of diabetes have, however, demonstrated the efficacy of acarbose in combating the long-term complications of the disease. The 18 animal studies reviewed here showed that acarbose treatment reduced postprandial blood glucose concentrations and decreased protein glycation. Through these actions, acarbose delayed or prevented the onset of renal, retinal, lens and neurological changes and the development of ischaemic myocardial lesions. Acarbose treatment can therefore be expected to benefit patients with Type 2 and, in combination with insulin, Type 1 diabetes. This is being investigated in ongoing clinical studies in patients with Type 2 diabetes and impaired glucose tolerance (IGT).

Positive effects of acarbose in the diabetic rat are not altered by feeding schedule

We previously demonstrated that chronic dietary treatment with acarbose, an alpha-glucosidase inhibitor, improves glucose homeostasis in the streptozotocin (STZ)-induced diabetic rat. In this study we evaluated the effects of 4 weeks of acarbose treatment on glucose homeostasis in STZ-diabetic rats for both meal-fed (three times daily) and ad libitum feeding conditions. Sprague Dawley male rats (n = 58) were started on a daily meal-feeding paradigm consisting of three 2-h feeding periods: 0700 to 0900 hours, 1300 to 1500 hours, and 1900 to 2100 hours. Following 2 weeks of adaptation, half of the animals were switched to ad libitum feeding. The feeding paradigm itself (meal fed versus ad lib.) affected neither body weight nor daily food intake. Twenty animals from each feeding group then received STZ (60 mg/kg i.v.), whereas control animals received vehicle injections only. Two days later, the diet of 10 STZ-treated animals from each paradigm was supplemented with acarbose (40 mg of BAY G 5421/100-g diet), and the groups were treated for 4 weeks. Untreated diabetic rats had lower body weight than vehicle-injected control rats at all time points after STZ treatment. Acarbose treatment delayed this effect on body weight. STZ treatment induced hyperphagia regardless of feeding paradigm, which was significantly attenuated by acarbose only for the first week of treatment. Untreated diabetic rats had fasting blood glucose values 4 times those of vehicle-injected controls in both the meal-fed and ad libitum-fed conditions. Acarbose significantly lowered fasting blood glucose in the treated STZ groups. Blood glucose was also assessed 0, 90, and 180 min following the start of a meal. The postprandial rise in blood glucose was significantly reduced in acarbose-treated meal-fed diabetic rats, to values not significantly different from those of vehicle-injected control rats. During the fourth week of treatment glycated hemoglobin levels were significantly higher in untreated diabetic groups compared to vehicle-injected control groups. Acarbose treatment significantly reduced this rise, regardless of the feeding paradigm. Collectively, the results demonstrate that acarbose reduces diabetes-induced increases of blood glucose and glycated hemoglobin and that the glycemic effects of acarbose are most apparent during the absorptive period. Feeding paradigm (ad lib. versus meal fed) has little or no influence on acarbose's metabolic effects, indicating that large meals are not required to realize the beneficial effects of the drug. The meal-fed STZ-diabetic rat may be a good model with which to test meal-based diabetes treatments.

Pharmacokinetic-pharmacodynamic relationships of Acarbose

Acarbose represents a new pharmacological approach to achieving the metabolic benefits of a slower carbohydrate absorption in diabetes, by acting as a potent, competitive inhibitor of intestinal alpha-glucosidases. Acarbose molecules attach to the carbohydrate binding sites of alpha-glucosidases, with an affinity constant that is much higher than that of the normal substrate. Because of the reversible nature of the inhibitor-enzyme interaction, the conversion of oligosaccharides to monosaccharides is only delayed rather than completely blocked. Acarbose has the structural features of a tetrasaccharide and does not cross the enterocytes after ingestion. Thus, its pharmacokinetic properties are well suited to the pharmacological action directed exclusively towards the intestinal glucosidases. The most important clinical consequence of the delayed carbohydrate digestion caused by acarbose is the attenuation of postprandial increases in blood glucose levels. Other effects have also been described: a decreased beta-pancreatic response to meals, and influences on gut hormone secretion and plasma lipid levels. Gastrointestinal discomfort is frequently reported as an adverse effect of acarbose administration, but incidence usually decreases with time. The suitability of acarbose for improving glucose homeostasis as an adjunct to dietary control or to administration of sulphonylureas or insulin has been extensively studied in patients both with type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes mellitus. Acarbose can be used as first-line therapy in patients with type 2 diabetes which is poorly controlled by diet alone. Moreover, the lack of bodyweight gain or hypoglycaemic effects reported during acarbose treatment may be advantageous for obese or elderly patients. Finally, the reduction in fluctuations of glucose levels throughout the day may help to control type 1 diabetes in patients with 'brittle diabetes'. Long term prospective studies are still needed to confirm these indications and the usefulness of acarbose in conditions other than diabetes, notably reactive hypoglycaemia and dumping syndrome.