Glyburide and Metformin Hydrochloride
In bioavailability studies of glyburide and metformin hydrochloride 2.5 mg/500 mg and 5 mg/500 mg, the mean area under the plasma concentration versus time curve (AUC) for the glyburide component was 18% and 7%, respectively, greater than that of standard particle-size glyburide coadministered with metformin. The pharmacokinetics of metformin HCl component of glyburide and metformin hydrochloride was consistent with that of metformin HCl coadministered with glyburide.
Effect of food: Following administration of a single glyburide and metformin hydrochloride 5 mg/500 mg tablet with either a 20% glucose solution or a 20% glucose solution with food, there was no effect of food on the C max and a relatively small effect of food on the AUC of the glyburide component. The T max for the glyburide component was shortened from 7.5 hours to 2.75 hours with food compared to the same tablet strength administered fasting with a 20% glucose solution. The effect of food on the pharmacokinetics of the metformin component of glyburide and metformin hydrochloride was indeterminate. However, food is known to decrease the extent of and slightly delay the absorption of metformin, as shown by approximately a 40% lower mean peak plasma concentration (C max ), a 25% lower area under the plasma concentration versus time curve (AUC), and a 35-minute prolongation of time to peak plasma concentration (T max ) following administration of a single 850 mg tablet of metformin with food, compared to the same tablet strength administered fasting. The clinical relevance of these decreases is unknown.
Single-dose studies with standard particle-size glyburide tablets in normal subjects demonstrate significant absorption of glyburide within 1 hour, peak drug levels at about 4 hours, and low but detectable levels at 24 hours. Mean serum levels of glyburide, as reflected by areas under the serum concentration-time curve, increase in proportion to corresponding increases in dose. Bioequivalence has not been established between glyburide and metformin hydrochloride and single-ingredient standard particle-size glyburide products.
The absolute bioavailability of a 500 mg metformin tablet given under fasting conditions is approximately 50% to 60%. Studies using single oral doses of metformin tablets of 500 mg and 1500 mg, and 850 mg to 2550 mg, indicate that there is a lack of dose proportionality with increasing doses, which is due to decreased absorption rather than an alteration in elimination. At usual clinical doses and dosing schedules of metformin, steady-state plasma concentrations of metformin are reached within 24 to 48 hours and are generally <1 mcg/mL.
Sulfonylurea drugs are extensively bound to serum proteins. Displacement from protein binding sites by other drugs may lead to enhanced hypoglycemic action. In vitro , the protein binding exhibited by glyburide is predominantly non-ionic, whereas that of other sulfonylureas (chlorpropamide, tolbutamide, tolazamide) is predominantly ionic. Acidic drugs, such as phenylbutazone, warfarin, and salicylates, displace the ionic-binding sulfonylureas from serum proteins to a far greater extent than the non-ionic binding glyburide. It has not been shown that this difference in protein binding results in fewer drug-drug interactions with glyburide tablets in clinical use.
The apparent volume of distribution (V/F) of metformin following single oral doses of 850 mg averaged 654±358 L. Metformin is negligibly bound to plasma proteins. Metformin partitions into erythrocytes, most likely as a function of time.
Metabolism and Elimination
The decrease of glyburide in the serum of normal healthy individuals is biphasic; the terminal half-life is about 10 hours. The major metabolite of glyburide is the 4-trans-hydroxy derivative.
A second metabolite, the 3-cis-hydroxy derivative, also occurs. These metabolites probably contribute no significant hypoglycemic action in humans since they are only weakly active (1/400 and 1/40 as active, respectively, as glyburide) in rabbits. Glyburide is excreted as metabolites in the bile and urine, approximately 50% by each route. This dual excretory pathway is qualitatively different from that of other sulfonylureas, which are excreted primarily in the urine.
Intravenous single-dose studies in normal subjects demonstrate that metformin is excreted unchanged in the urine and does not undergo hepatic metabolism (no metabolites have been identified in humans) nor biliary excretion. Renal clearance (see Table 4) is approximately 3.5 times greater than creatinine clearance, which indicates that tubular secretion is the major route of metformin elimination. Following oral administration, approximately 90% of the absorbed drug is eliminated via the renal route within the first 24 hours, with a plasma elimination half-life of approximately 6.2 hours. In blood, the elimination half-life is approximately 17.6 hours, suggesting that the erythrocyte mass may be a compartment of distribution.
No pharmacokinetic studies have been conducted in patients with hepatic insufficiency for either glyburide or metformin [see Warnings and Precautions (8.7)].
No information is available on the pharmacokinetics of glyburide in patients with renal insufficiency.
In patients with decreased renal function the plasma and blood half-life of metformin is prolonged and the renal clearance is decreased (Table 4); [see Dosage and Administration (2), Contraindications (4), and Warnings and Precautions (5.1)].
There is no information on the pharmacokinetics of glyburide in elderly patients.
Limited data from controlled pharmacokinetic studies of metformin in healthy elderly subjects suggest that total plasma clearance is decreased, the half-life is prolonged, and C max is increased, when compared to healthy young subjects. From these data, it appears that the change in metformin pharmacokinetics with aging is primarily accounted for by a change in renal function (Table4); [see Dosage and Administration (2.3) and Warnings and Precautions (5.1)].
Table 4: Select Mean (±SD) Metformin Pharmacokinetic Parameters Following Single or Multiple Oral Doses of Metformin HCl
|a All doses given fasting except the first 18 doses of the multiple-dose studies b Peak plasma concentration c Time to peak plasma concentration d SD=single dose e Combined results (average means) of 5 studies: mean age 32 years (range 23 to 59 years) f Kinetic study done following dose 19, given fasting g Elderly subjects, mean age 71 years (range 65 to 81 years) h CL cr =creatinine clearance normalized to body surface area of 1.73 m 2|
|Subject Groups: Metformin HCl Dose a (number of subjects)||C max b (mcg/mL)||T max c (hrs)||Renal Clearance (mL/min)|
|Healthy, nondiabetic adults: 500 mg SD d (24) 850 mg SD (74) e 850 mg t.i.d. for 19 doses f (9)||1.03 (±0.33) 1.60 (±0.38) 2.01 (±0.42)||2.75 (±0.81) 2.64 (±0.82) 1.79 (±0.94)||600 (±132) 552 (±139) 642 (±173)|
|Adults with type 2 diabetes: 850 mg SD (23) 850 mg t.i.d. for 19 doses f (9)||1.48 (±0.5) 1.90 (±0.62)||3.32 (±1.08) 2.01 (±1.22)||491 (±138) 550 (±160)|
|Elderly g , healthy nondiabetic adults: 850 mg SD (12)||2.45 (±0.70)||2.71 (±1.05)||412 (±98)|
|Renal-impaired adults: 850 mg SD Mild (CL cr h 61 to 90 mL/min) (5) Moderate (CL cr 31 to 60 mL/min) (4) Severe (CL cr 10 to 30 mL/min) (6)||1.86 (±0.52) 4.12 (±1.83) 3.93 (±0.92)||3.20 (±0.45) 3.75 (±0.50) 4.01 (±1.10)||384 (±122) 108 (±57) 130 (±90)|
There is no information on the effect of gender on the pharmacokinetics of glyburide.
Metformin pharmacokinetic parameters did not differ significantly in subjects with or without type 2 diabetes when analyzed according to gender (males=19, females=16).
No information is available on race differences in the pharmacokinetics of glyburide.
No studies of metformin pharmacokinetic parameters according to race have been performed.
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