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Mechanism of Action
Metformin is an antihyperglycemic agent which improves glucose tolerance in patients with type 2 diabetes, lowering both basal and postprandial plasma glucose. Its pharmacologic mechanisms of action are different from other classes of oral antihyperglycemic agents. Metformin decreases hepatic glucose production, decreases intestinal absorption of glucose, and improves insulin sensitivity by increasing peripheral glucose uptake and utilization. Unlike sulfonylureas, metformin does not produce hypoglycemia in either patients with type 2 diabetes or normal subjects (except in special circumstances, see PRECAUTIONS) and does not cause hyperinsulinemia. With metformin therapy, insulin secretion remains unchanged while fastingplasma insulin levels and day-long plasma insulin response may actually decrease.
Pharmacokinetics And Drug Metabolism
Absorption and Bioavailability
The appearance of metformin in plasma from a FORTAMET® (metformin hcl) Extended-Release Tablet is slower and more prolonged compared to immediate-release metformin. In a multiple-dose crossover study, 23 patients with type 2 diabetes mellitus were administered either 2000 mg FORTAMET® (metformin hcl) once a day (after dinner) or immediate-release (IR) metformin hydrochloride 1000 mg twice a day (after breakfast and after dinner). After 4 weeks of treatment, steady-state pharmacokinetic parameters, area under the concentration-time curve (AUC), time to peak plasma concentration (Tmax), and maximum concentration (Cmax) were evaluated. Results are presented in Table 1.
Table 1: FORTAMET® (metformin hcl) vs. Immediate-Release Metformin Steady-State Pharmacokinetic Parameters at 4 Weeks
| FORTAMET® (metformin hcl)
| Immediate-Release Metformin
(1000 mg b.i.d.)
| AUC 0-24 hr
|26,811 ± 7055||27,371 ± 5,781|
|Tmax (hr)||6 (3-10)||3 (1-8)|
|Cmax (ng/mL)||2849 ± 797||1820 ± 370|
In four single-dose studies and one multiple-dose study, the bioavailability of FORTAMET® (metformin hcl) 2000 mg given once daily, in the evening, under fed conditions [as measured by the area under the plasma concentration versus time curve (AUC)] was similar to the same total daily dose administered as immediate release metformin 1000 mg given twice daily. The geometric mean ratios (FORTAMET® (metformin hcl) / immediate-release metformin) of AUC0-24hr, AUC0-72hr, and AUC0-inf. for these five studies ranged from 0.96 to 1.08.
In a single-dose, four-period replicate crossover design study, comparing two 500 mg FORTAMET® (metformin hcl) tablets to one 1000 mg FORTAMET® (metformin hcl) tablet administered in the evening with food to 29 healthy male subjects, two 500 mg FORTAMET® (metformin hcl) tablets were found to be equivalent to one 1000 mg FORTAMET® (metformin hcl) tablet.
In a study carried out with FORTAMET® (metformin hcl) , there was a dose associated increase in metformin exposure over 24 hours following oral administration of 1000, 1500, 2000, and 2500 mg.
In three studies with FORTAMET® (metformin hcl) using different treatment regimens (2000 mg after dinner; 1000 mg after breakfast and after dinner; and 2500 mg after dinner), the pharmacokinetics of metformin as measured by AUC appeared linear following multiple-dose administration.
The extent of metformin absorption (as measured by AUC) from FORTAMET® (metformin hcl) increased by approximately 60% when given with food. When FORTAMET® (metformin hcl) was administered with food, Cmax was increased by approximately 30% and Tmax was more prolonged compared with the fasting state (6.1 versus 4.0 hours).
Distribution studies with FORTAMET® (metformin hcl) have not been conducted. However, the apparent volume of distribution (V/F) of metformin following single oral doses of immediate-release metformin 850 mg averaged 654 ± 358 L. Metformin is negligibly bound to plasma proteins, in contrast to sulfonylureas, which are more than 90% protein bound. Metformin partitions into erythrocytes, most likely as a function of time. At usual clinical doses and dosing schedules of immediate-release metformin, steady state plasma concentrations of metformin are reached within 24-48 hours and are generally < 1 µg/mL. During controlled clinical trials of immediate-release metformin, maximum metformin plasma levels did not exceed 5 µg/mL, even at maximum doses.
Metabolism and Excretion
Metabolism studies with FORTAMET® (metformin hcl) have not been conducted. 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.
In healthy nondiabetic adults (N=18) receiving 2500 mg q.d. FORTAMET® (metformin hcl) , the percent of the metformin dose excreted in urine over 24 hours was 40.9% and the renal clearance was 542 ± 310 mL/min. After repeated administration of FORTAMET® (metformin hcl) , there is little or no accumulation of metformin in plasma, with most of the drug being eliminated via renal excretion over a 24-hour dosing interval. The t1/2 was 5.4 hours for FORTAMET® (metformin hcl) .
Renal clearance of metformin (Table 2) 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.
Limited data from controlled pharmacokinetic studies of immediate-release metformin in healthy elderly subjects suggest that total plasma clearance of metformin is decreased, the half-life is prolonged, and Cmax is increased, 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 (Table 2). FORTAMET® (metformin hcl) treatment should not be initiated in patients ≥ 80 years of age unless measurement of creatinine clearance demonstrates that renal function is not reduced (see WARNINGS, PRECAUTIONS and DOSAGE AND ADMINISTRATION).
No pharmacokinetic data from studies of pediatric patients are currently available (see PRECAUTIONS).
Five studies indicated that with FORTAMET® (metformin hcl) treatment, the pharmacokinetic results for males and females were comparable.
Table 2: Select Mean (±SD) Metformin Pharmacokinetic Parameters Following Single or Multiple Oral Doses of Immediate-Release Metformin
| Subject Groups:
(number of subjects)
|Healthy, nondiabetic adults|
|500 mg single dose (24)||1.03 (±0.33)|
|2.75 (±0.81)||600 (±132)|
|850 mg single dose (74)d||1.60 (±0.38)|
|2.64 (±0.82)||552 (±139)|
|850 mg three times daily for 19 dosese||(9)||2.01 (±0.42)|
|1.79 (±0.94)||642 (±173)|
|Adults with type 2 diabetes:|
|850 mg single dose (23)||1.48 (±0.5)|
|3.32 (±1.08)||491 (±138)|
|850 mg three times daily for 19 dosese||(9)||1.90 (±0.62)|
|2.01 (±1.22)||550 (±160)|
|Elderlyf, healthy nondiabetic adults:|
|850 mg single dose (12)||2.45 (±0.70)|
|2.71 (±1.05)||412 (±98)|
|Renal-impaired adults: 850 mg single dose|
|Mild (CLcrg 61-90 mL/min) (5)||1.86 (±0.52)|
|3.20 (±0.45)||384 (±122)|
|Moderate (CLcr 31-60 mL/min)||(4)||4.12 (±1.83)|
|3.75 (±0.50)||108 (±57)|
|Severe (CLcr 10-30 mL/min)||(6)||3.93 (±0.92)|
|4.01 (±1.10)||130 (±90)|
| a All doses given fasting except
the first 18 doses of the multiple dosestudies
b Peak plasma concentration
c Time to peak plasma concentration
d Combined results (average means) of five studies: mean age 32 years(range 23-59 years)
eKinetic study done following dose 19, given fasting
f Elderly subjects, mean age 71 years (range 65-81 years)
g CLcr = creatinine clearance normalized to body surface area of 1.73 m2
In patients with decreased renal function (based on measured creatinine clearance), the plasma and blood half-life of metformin is prolonged and the renal clearance is decreased in proportion to the decrease in creatinine clearance (Table 2; also see WARNINGS).
No pharmacokinetic studies of metformin have been conducted in patients with hepatic insufficiency.
No studies of metformin pharmacokinetic parameters according to race have been performed. In controlled clinical studies of immediate-release metformin in patients with type 2 diabetes, the antihyperglycemic effect was comparable in whites (n=249), blacks (n=51), and Hispanics (n=24).
In a double-blind, randomized, active-controlled, multicenter U.S. clinical study, which compared FORTAMET® (metformin hcl) q.d. to immediate-release metformin b.i.d., 680 patients with type 2 diabetes who had been taking metformin-containing medication at study entry were randomly assigned in equal numbers to double-blind treatment with either FORTAMET® (metformin hcl) or immediate-release metformin. Doses were adjusted during the first six weeks of treatment with study medication based on patients' FPG levels and were then held constant over a period of 20 weeks. The primary efficacy endpoint was the change in HbA1c from baseline to endpoint. The primary objective was to demonstrate the clinical non- inferiority of FORTAMET® (metformin hcl) compared to immediate-release metformin on the primary endpoint. FORTAMET® (metformin hcl) and metformin patients had mean HbA1c changes from baseline to endpoint equal to +0.40 and +0.14, respectively (Table 3).The least-square (LS) mean treatment difference was 0.25 (95% CI = 0.14, 0.37) demonstrating that FORTAMET® (metformin hcl) was clinically similar to metformin according to the pre-defined criterion to establish efficacy.
Table 3: FORTAMET® (metformin hcl) vs. Immediate-Release Metformin Switch Study: Summary of Mean Changes in HbA1c, Fasting Plasma Glucose, Body Weight, Body Mass Index, and Plasma Insulin
(FORTAMET® (metformin hcl)
(2 sided 95% CIa)
| HbA1c (%) N
Baseline (mean± SD)
Change from baseline (mean ± SD)
|3277.04 ± 0.880.40 ±0.75||3327.07 ± 0.760.14± 0.75||0.25 (0.14,0.37)b|
| Fasting Plasma Glucose (mg/dL) N
Baseline (mean± SD)
Change from base line mean ± SD)
|329146.8 ± 32.110.0± 40.8||333145.6 ± 29.54.2 ± 35.9||6.43 (0.57, 12.29)|
| Plasma Insulin (µu/mL) N
Baseline (mean± SD)
Change from baseline base line (mean ± SD)
|304 17.9 ± 15.1 -3.6 ± 13.8||31617.3 ± 10.5-3.2 ± 8.6||0.02 (-1.47, 1.50)|
| Body Weight (kg) N
Baseline (mean± SD)
Change from baseline (mean ± SD)
|31394.1 ± 17.80.3 ± 2.9||320 93.3 ± 17.40.0 ±3.7||0.08 (-0.22, 0.81)|
| a CI= Confidence Interval
b FORTAMET® (metformin hcl) was clinically similar to immediate-release metformin based on the pre-defined criterion to establish efficacy. While demonstrating clinical similarity, the response to FORTAMET® (metformin hcl) compared to immediate-release metformin was also shown to be statistically smaller as seen by the 95% CI for the treatment difference which did not include zero.
Footnote: Patients were taking metformin-containing medications at baseline that were prescribed by their personal physician.
The mean changes for FPG (Table 3) and plasma insulin (Table 3) were small for both FORTAMET® (metformin hcl) and immediate-release metformin, and were not clinically meaningful. Seventy-six (22%) and 49 (14%) of the FORTAMET® (metformin hcl) and immediate-release patients, respectively, discontinued prematurely from the trial. Eighteen (5%) patients on FORTAMET® (metformin hcl) withdrew because of a stated lack of efficacy, as compared with 8 patients (2%) on immediate-release metformin (p=0.047). Results from this study also indicated that neither FORTAMET® (metformin hcl) nor immediate-release metformin were associated with weight gain or increases in body mass index. A 24-week, double blind, placebo-controlled study of immediate-release metformin plus insulin, versus insulin plus placebo, was conducted in patients with type 2 diabetes who failed to achieve adequate glycemic control on insulin alone(Table 4). Patients randomized to receive immediate-release metformin plus insulin achieved a reduction in HbA1c of 2.10%, compared to a 1.56% reduction in HbA1c achieved by insulin plus placebo. The improvement in glycemic control was achieved at the final study visit with 16% less insulin, 93.0 U/day versus 110.6 U/day, immediate-release metformin plus insulin versus insulin plus placebo, respectively, p=0.04.
Table 4: Combined Immediate-Release Metformin /Insulin vs. Placebo/Insulin: Summary of Mean Changes from Baseline in HbA1c and Daily Insulin Dose
(n = 28)
| Treatment difference
Mean ± SE
(n = 26)
| HbA1c (%)
|Change at FINAL VISIT -1.56||-0.54 ± 0.43a||-2.10|
| Insulin Dose (U/day)
|Change at FINAL VISIT 15.93||-16.08 ± 7.77b||-0.15|
| a Statistically significant using analysis of
covariance with baseline as covariate (p=0.04). Not significant using analysisof
variance (values shown in table)
b Statistically significant for insulin (p=0.04)
A second double-blind, placebo-controlled study (n=51), with 16 weeks of randomized treatment, demonstrated that in patients with type 2 diabetes controlled on insulin for 8 weeks with an average HbA1c of 7.46 ± 0.97%, the addition of immediate-release metformin maintained similar glycemic control (HbA1c 7.15 ± 0.61 versus 6.97 ± 0.62 for immediate-release metformin plus insulin and placebo plus insulin, respectively) with 19% less insulin versus baseline (reduction of 23.68 ± 30.22 versus an increase of 0.43 ± 25.20 units for immediate-release metformin plus insulin and placebo plus insulin, p < 0.01). In addition, this study demonstrated that the combination of immediate-release metformin plus insulin resulted in reduction in body weight of 3.11 ± 4.30 lbs, compared to an increase of 1.30 ± 6.08 lbs for placebo plus insulin, p=0.01.
Pediatric Clinical Studies
No pediatric clinical studies have been conducted with FORTAMET® (metformin hcl) . In a double-blind, placebo- controlled study in pediatric patients aged 10 to 16 years with type 2 diabetes (mean FPG 182.2 mg/dL), treatment with immediate-release metformin (up to 2000 mg/day) for up to 16 weeks (mean duration of treatment 11 weeks) resulted in a significant mean net reduction in FPG of 64.3 mg/dL compared with placebo (Table 5).
Table 5: Immediate-Release Metformin vs. Placebo (Pediatricsa): Summary of Mean Changes from Baseline* in Plasma Glucose and Body Weight at Final Visit
|FPG (mg/dL)||(n = 37)||(n = 36)|
|Change at FINAL VISIT||-42.9||21.4||< 0.001|
|Body Weight (lbs)||(n = 39)||(n = 38)|
|Change at FINAL VISIT||-3.3||-2.0||NS**|
| a Pediatric patients mean age 13.8 years (range
* All patients on diet therapy at Baseline
** Not statistically significant
Last reviewed on RxList: 9/25/2008
This monograph has been modified to include the generic and brand name in many instances.
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