Acarbose: A Preliminary Review of its Pharmacodynamic and Pharmacokinetic Properties, and Therapeutic Potential

Abstract

Synopsis Acarbose delays the production of monosaccharides (notably glucose) by inhibiting the α-glucosidases associated with the brush-border membrane of the small intestine which are responsible for the digestion of complex polysaccharides and sucrose. In healthy subjects acarbose 100 to 200mg significantly inhibits postprandial glucose, insulin and triglyceride responses, with some evidence of carbohydrate malabsorption with the higher dose. Clinical trials in patients with non-insulin-dependent diabetes mellitus showed that acarbose improved diabetic control, especially postprandial blood glucose levels, independent of whether the patients were receiving concomitant oral antidiabetic drugs in addition to dietary management. In comparative studies acarbose was significantly superior to placebo, and comparable to biguanides, when used alone or as an adjuvant to sulphonylurea therapy. Trials in patients requiring insulin to control their diabetes demonstrated that acarbose significantly reduced postprandial blood glucose concentrations, resulting in a smoother diurnal blood glucose-time curve and improved symptoms associated with nocturnal hypoglycaemia. Daily insulin requirements were sometimes reduced. In large multicentre trials acarbose up to 600 mg/day for 3 to 12 months improved glycaemic control in approximately 55% of patients with non-insulin-dependent or insulin-dependent diabetes mellitus. Apart from its use in diabetes, encouraging preliminary results have been obtained with acarbose in other therapeutic areas such as dumping syndrome, reactive hypoglycaemia, and types IIb and IV hyperlipoproteinaemias — however, further clinical experience is needed in these settings before clear conclusions can be drawn. No serious side effects have been reported during treatment with acarbose, although it is associated with a high incidence of troublesome gastrointestinal symptoms such as flatulence, abdominal distension, borborygmus and diarrhoea. The incidence of these reactions usually decreases with time. Thus, acarbose represents the first of a new class of oral antidiabetic drugs — the α-glucosidase inhibitors. It has proven useful for improving glycaemic control when used as an adjunct to standard therapy involving dietary restriction, oral antidiabetic drugs and/or subcutaneous insulin. That being the case, acarbose should provide the clinician with an interesting treatment option which can be used in a broad range of patients with diabetes mellitus in whom ‘traditional’ management approaches produce suboptimal glycaemic control. Pharmacodynamic Studies Acarbose is a complex oligosaccharide which reversibly inhibits α-glucosidases present in the brush-border of the small intestinal mucosa. It has a rank order of inhibitory potency of glucoamylase > sucrase > maltase > isomaltase. The net effect of acarbose-induced inhibition of α-glucosidases is a slowing down in the rate at which complex polysaccharides and sucrose are digested and a subsequent delay in the absorption of glucose. In one study involving 12 healthy volunteers the extent of sucrose absorption was 90% within 200 minutes under control conditions and 60% within 400 minutes following administration of acarbose 100mg. Studies in rodents and in healthy volunteers have clearly shown that acarbose reduces postprandial hyperglycaemia and plasma insulin concentrations. In healthy subjects the reductions in plasma insulin concentrations were dose dependent, while strict dose-dependency was not always observed for decreases in blood glucose levels. Acarbose 100 to 200mg with each meal appeared to provide optimal inhibition of postprandial increases in glucose, insulin and triglycerides although there is evidence that the higher dosage caused some malabsorption of carbohydrate. Results from studies in patients with diabetes mellitus have generally confirmed the effects of acarbose on postprandial blood glucose concentrations whereas changes in plasma insulin and triglyceride concentrations have been equivocal. In studies evaluating hormonal responses to a test meal in healthy volunteers, acarbose 200mg 3 times daily for 7 days significantly reduced insulin (37%), C-peptide (35%), gastrin (71%) and pancreozymin (38%) plasma concentrations, and significantly increased somatostatin levels. Furthermore, single-dose and short term studies have reported that the response of gastric inhibitory polypeptide to a test meal decreased in parallel with that of insulin in healthy subjects receiving acarbose. Acarbose produced a dose-dependent reduction of serum triglyceride, cholesterol and free fatty acid concentrations in rodents. In man the major change in lipid metabolism during acarbose administration is a reduction in serum triglyceride concentrations which seems to be mediated by suppressing the biosynthesis of very low density lipoproteins. A large number of animal studies have generally shown a consistent bodyweight-lowering action for acarbose (even when energy consumption was maintained or, sometimes, increased). However, studies in obese patients have been disappointing — although there is preliminary evidence that acarbose may help prevent relapse weight gain in obese patients who have lost weight, it would not appear to be of significant value in subjects who need to shed weight. Pharmacokinetic Studies In healthy volunteers, acarbose is minimally absorbed in unchanged form and has extremely limited systemic availability, 0.5 to 1.7%. Following intravenous administration, steady-state volume of distribution was 0.32 L/kg. Approximately 35% of a 14C-labelled dose of acarbose 200mg is recovered in the urine (only 1.7% as unchanged acarbose) and about 50% in faeces. Acarbose is extensively metabolised by gastrointestinal amylases, which probably accounts for the very high value for total body clearance of 600 L/h. Using the equation for a 2-compartment model the elimination half-life of acarbose was about 2.8 hours, although there is some evidence that a more slowly declining terminal phase exists with a half-life of about 9 hours. Autoradiographic studies in animals have confirmed the relatively low level of absorption and distribution of acarbose. Therapeutic Trials Non-comparative multicentre clinical trials involving almost 1000 diabetic patients (both insulin- and non-insulin-dependent) demonstrated that acarbose up to 600 mg/day for 3 to 12 months improved glycaemic control (as estimated by fasting and postprandial glucose concentrations, glycosuria and serum triglyceride concentrations) in about 55% of those treated, with the most promising results tending to occur in those patients treated by dietary restriction alone at baseline. In smaller clinical trials involving patients with non-insulin-dependent diabetes mellitus, acarbose (usually 300 to 600 mg/day) for periods of up to 6 months usually improved diabetic control independent of whether the patients were receiving concomitant oral antidiabetic drugs in addition to dietary management. The most clinically significant finding was a reduction of postprandial blood glucose concentrations, although some studies did also report reductions in other parameters of metabolic regulation such as glycosylated haemoglobin (HbA1) concentrations and occasionally fasting blood glucose and serum triglyceride concentrations. Plasma insulin levels were not generally affected by acarbose. In comparative studies involving non-insulin-dependent diabetic patients, acarbose 300 to 600 mg/day significantly improved one or more indices of carbohydrate metabolism and/or the degree of diabetic control compared with placebo, and was found to be of comparable efficacy to biguanide drugs (buformin, metformin and phenformin) and guar gum when used alone or as adjuvants to sulphonylurea therapy. Preliminary studies involving small numbers of non-insulin-dependent diabetics demonstrated that acarbose up to 600 mg/day was as effective as glisoxepide 8 mg/day and glibenclamide 7 mg/day during periods of short term (8 to 12 weeks) therapy. Studies in patients with insulin-dependent diabetes mellitus have shown that acarbose up to 600 mg/day for 6 months significantly reduced postprandial blood glucose concentrations, produced a much smoother diurnal blood glucose-time curve, and in some cases reduction in daily insulin requirements. These effects were generally confirmed in placebo-controlled short to medium term (up to 12 weeks) clinical trials. In one study involving 14 insulin-dependent patients, acarbose 100mg at night was found to provide significant improvement in symptoms associated with nocturnal hypoglycaemia. Acarbose has been evaluated in a few clinical trials in small numbers of patients with dumping syndrome, reactive hypoglycaemia, and types IIb and IV hyperlipoproteinaemias. Although some encouraging preliminary results have been reported, further clinical experience is needed in these therapeutic areas to help define the role it may play in their long term management. Side Effects Gastrointestinal symptoms such as flatulence, abdominal distension, borborygmus and diarrhoea are the most frequently reported adverse effects (usually in about 60% of patients), and they are symptomatic of fermentation of unabsorbed carbohydrate in the bowel with a resultant increase in intestinal gas. Such symptoms have been shown to decrease with time and may be alleviated by a reduction in acarbose dosage. Results to date indicate that about 11% of patients will discontinue treatment with acarbose but in only 3 to 5% of cases is this due to gastrointestinal complaints. Approximately 2% of patients treated with acarbose (alone or in combination with other antidiabetic drugs) develop hypoglycaemia while a further 3% exhibit symptoms suggestive of hypoglycaemia (sleepiness, weakness, dizziness, headache, vertigo, etc.). These effects are probably related to poor metabolic control rather than being specific adverse reactions, and may be reduced by modifying the dosage of insulin or oral antidiabetic drugs. Dosage and Administration Acarbose dosage should be tailored to meet the needs of the individual. Experience from clinical trials indicates that postprandial blood glucose concentrations are decreased by acarbose 50 to 200mg 3 times daily taken at the beginning of meals. To reduce the level of gastrointestinal adverse symptoms it would seem prudent to initiate treatment with lower doses, with gradual increases until an adequate response is achieved. © 1988, ADIS Press Limited. All rights reserved.