SECOND-GENERATION SULFONYLUREAS
The
second-generation sulfonylureas are prescribed more frequently in the USA than
are the first-generation agents because they have fewer adverse effects and
drug interactions. These potent sulfonylu-rea compounds—glyburide, glipizide,
and glimepiride—should be used with caution in patients with cardiovascular
disease or in elderly patients, in whom hypoglycemia would be especially
dangerous.
Glyburide is metabolized in the liver into products with verylow hypoglycemic activity.
The usual starting dosage is 2.5 mg/d or
less, and the average maintenance dosage is 5–10 mg/d given as a single morning
dose; maintenance dosages higher than 20 mg/d are not recommended. A
formulation of “micronized” glyburide (Glynase PresTab) is available in a
variety of tablet sizes. However, there is some question as to its
bioequivalence with nonmicron-ized formulations, and the FDA recommends careful
monitoring to re-titrate dosage when switching from standard glyburide doses or
from other sulfonylurea drugs.
Glyburide
has few adverse effects other than its potential for causing hypoglycemia.
Flushing has rarely been reported after ethanol ingestion, and the compound
slightly enhances free water clearance. Glyburide is contraindicated in the
presence of hepatic impairment and in patients with renal insufficiency.
Glipizide has
the shortest half-life (2–4 hours) of the morepotent agents. For maximum effect
in reducing postprandial hyper-glycemia, this agent should be ingested 30
minutes before breakfast because absorption is delayed when the drug is taken
with food. The recommended starting dosage is 5 mg/d, with up to 15 mg/d given
as a single dose. When higher daily dosages are required, they should be
divided and given before meals. The maximum total daily dosage recommended by
the manufacturer is 40 mg/d, although some studies indicate that the maximum
therapeutic effect is achieved by 15–20 mg of the drug. An extended-release
preparation (Glucotrol XL) provides 24-hour action after a once-daily morning
dose (maximum of 20 mg/d). However, this formulation appears to have sacrificed
its lower propensity for severe hypoglycemia com-pared with longer-acting
glyburide without showing any demon-strable therapeutic advantages over the
latter (which can be obtained as a generic drug).
Because
of its shorter half-life, the regular formulation of glip-izide is much less
likely than glyburide to produce serious hypo-glycemia. At least 90% of
glipizide is metabolized in the liver to inactive products, and 10% is excreted
unchanged in the urine. Glipizide therapy is therefore contraindicated in
patients with significant hepatic or renal impairment, who would be at high
risk for hypoglycemia.
Glimepiride is
approved for once-daily use as monotherapy orin combination with insulin.
Glimepiride achieves blood glucose lowering with the lowest dose of any
sulfonylurea compound. A single daily dose of 1 mg has been shown to be
effective, and the recommended maximal daily dose is 8 mg. Glimepiride has a
long duration of effect with a half-life of 5 hours, allowing once-daily dosing
and thereby improving compliance. It is completely metab-olized by the liver to
metabolites with weak or no activity.
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