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Diabetes Drugs: Alpha-Glucosidase Inhibitors
September 4, 2009
Editor’s Note: This is the fifth post in our miniseries about diabetes drugs. Tune in on September 11 for the next installment.
Alpha-glucosidase inhibitors, a class of drugs also known as “starch blockers,” function by slowing the absorption of certain carbohydrates in the gastrointestinal tract. Two drugs in this class — acarbose (brand name Precose) and miglitol (Glyset) — are approved in the United States, while two others — voglibose (Volix and others) and emiglitate — are available only outside of the United States.
The use of alpha-glucosidase inhibitors results in a lower, delayed rise in blood glucose after a meal. Since these drugs lower blood glucose by preventing the immediate breakdown of carbohydrate from a meal, they are only effective when taken with the first bite of a meal. Alpha-glucosidase inhibitors are particularly useful for managing mild hyperglycemia (high blood glucose) in newly diagnosed people with Type 2 diabetes, as well as for managing after-meal hyperglycemia. The joint guidelines issued by the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) list these drugs under the category of “other” therapy. This means that they are not included in the first two tiers of recommendations, having been found less effective than metformin (Glucophage and others) or sulfonylureas (Diabinese, DiaBeta, Glynase, Micronase, Glucotrol, Glucotrol XL, Amaryl) at managing blood glucose.
Carbohydrate comes in many forms, but all types of carbohydrate are essentially chains of very simple sugar molecules. Since the gastrointestinal tract absorbs simple sugar molecules, but not complex or long-chain carbohydrates, very efficiently, there are enzymes in the intestines that serve to break the long chains into shorter chains. These enzymes can be very specific with regard to the type of long-chain carbohydrate they work on. Alpha-glucosidase is an intestinal enzyme that breaks down a variety of carbohydrates, but two of the more important ones are sucrose (table sugar), which is broken down into glucose and fructose, and lactose (milk sugar), which is broken down into glucose and galactose. None of the alpha-glucosidase inhibitors have any direct effect on the absorption of glucose. Rather, the drugs in this class delay and reduce the amount of glucose that is ready for absorption by interfering with the breakdown of the long-chain carbohydrates mentioned above, allowing the pancreas more time to secrete insulin to cover the meal.
Almost all of the data for these drugs is from people with Type 2 diabetes, and the majority of that information deals with acarbose. Accordingly, most of the following numbers are from studies done with acarbose. The guidelines issued by the ADA and EASD suggest that the expected decrease in HbA1c (an indicator of blood glucose control over the previous 2–3 months) would be between 0.5% and 0.8% when this drug is used alone. This is not as large a reduction as one would expect when metformin or sulfonylureas are used alone. When acarbose is used along with insulin, metformin, or sulfonylureas, an average additional HbA1c lowering of 0.5% is found. The largest study that included acarbose was the United Kingdom Prospective Diabetes Study, which included almost 1,000 people using this medicine. This study demonstrated a 0.5% reduction in HbA1c over a 2-year period with use of this medicine.
Acarbose does not cause weight gain, and when given by itself, it has a very low risk of hypoglycemia (low blood glucose). The most common side effects of this drug class are gastrointestinal. By blocking the absorption of carbohydrate into the bloodstream, alpha-glucosidase inhibitors leave more carbohydrate available to the gastrointestinal flora (bacteria that live in the intestines). As a result, these bacteria increase in number. This increase, along with the delay the drugs cause in the breakdown of carbohydrate, can result in bloating, diarrhea, excess gas, nausea, and abdominal pain. In studies, these effects occurred in approximately one-third of people taking acarbose, with higher doses of the medicine causing more side effects. The severity of these effects tends to decrease over time with use of the drug.
In approximately 3% of people, the drug is associated with elevation in liver enzymes, which may indicate potential liver damage. In almost all of these cases, the enzyme levels return to normal once the drug is discontinued.
Acarbose is not absorbed into the bloodstream, but instead remains in the intestinal tract. It is partially broken down by intestinal bacteria and absorbed and eliminated by the kidneys. Miglitol is almost completely absorbed and eliminated by the kidneys. (This drug is not recommended for people with severe kidney dysfunction.)
The recommended starting dose for acarbose is 25 milligrams taken three times a day with the first bite of each meal. This can be gradually increased to the greatest tolerated dose, but should not exceed 100 milligrams taken three times a day for people greater or equal to 132 pounds and 50 milligrams taken three times a day for people less than 132 pounds. The recommended starting dose for miglitol is 25 milligrams taken three times a day with the first bite of each meal, which can be increased to 100 milligrams taken three times a day. To reduce the chances of gastrointestinal side effects, this drug can also be started at a dose of 25 milligrams taken once a day and then gradually increased.
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