METFORMIN HYDROCHLORIDE (metformin hydrochloride 500 mg) Dailymed

BOXED WARNING SECTION

Lactic Acidosis:

Lactic acidosis is a rare, but serious, metabolic complication that can occur due to metformin accumulation during treatment with metformin; when it occurs, it is fatal in approximately 50% of cases. Lactic acidosis may also occur in association with a number of pathophysiologic conditions, including diabetes mellitus, and whenever there is significant tissue hypoperfusion and hypoxemia. Lactic acidosis is characterized by elevated blood lactate levels (>5 mmol/L), decreased blood pH, electrolyte disturbances with an increased anion gap, and an increased lactate/pyruvate ratio. When metformin is implicated as the cause of lactic acidosis, metformin plasma levels >5 µg/mL are generally found.

The reported incidence of lactic acidosis in patients receiving metformin hydrochloride is very low (approximately 0.03 cases/1000 patient-years, with approximately 0.015 fatal cases/ 1000 patient-years). In more than 20,000 patient-years exposure to metformin in clinical trials, there were no reports of lactic acidosis. Reported cases have occurred primarily in diabetic patients with significant renal insufficiency, including both intrinsic renal disease and renal hypoperfusion, often in the setting of multiple concomitant medical/surgical problems and multiple concomitant medications. Patients with congestive heart failure requiring pharmacologic management, in particular those with unstable or acute congestive heart failure who are at risk of hypoperfusion and hypoxemia, are at increased risk of lactic acidosis. The risk of lactic acidosis increases with the degree of renal dysfunction and the patient’s age. The risk of lactic acidosis may, therefore, be significantly decreased by regular monitoring of renal function in patients taking metformin and by use of the minimum effective dose of metformin. In particular, treatment of the elderly should be accompanied by careful monitoring of renal function. Metformin treatment should not be initiated in patients ≥80 years of age unless measurement of creatinine clearance demonstrates that renal function is not reduced, as these patients are more susceptible to developing lactic acidosis. In addition, metformin should be promptly withheld in the presence of any condition associated with hypoxemia, dehydration, or sepsis. Because impaired hepatic function may significantly limit the ability to clear lactate, metformin should generally be avoided in patients with clinical or laboratory evidence of hepatic disease. Patients should be cautioned against excessive alcohol intake, either acute or chronic, when taking metformin, since alcohol potentiates the effects of metformin hydrochloride on lactate metabolism. In addition, metformin should be temporarily discontinued prior to any intravascular radiocontrast study and for any surgical procedure (see also PRECAUTIONS).

The onset of lactic acidosis often is subtle, and accompanied only by nonspecific symptoms such as malaise, myalgias, respiratory distress, increasing somnolence, and nonspecific abdominal distress. There may be associated hypothermia, hypotension, and resistant bradyarrhythmias with more marked acidosis. The patient and the patient’s physician must be aware of the possible importance of such symptoms and the patient should be instructed to notify the physician immediately if they occur (see also PRECAUTIONS). Metformin should be withdrawn until the situation is clarified. Serum electrolytes, ketones, blood glucose, and if indicated, blood pH, lactate levels, and even blood metformin levels may be useful. Once a patient is stabilized on any dose level of metformin, gastrointestinal symptoms, which are common during initiation of therapy, are unlikely to be drug related. Later occurrence of gastrointestinal symptoms could be due to lactic acidosis or other serious disease.

Levels of fasting venous plasma lactate above the upper limit of normal but less than 5 mmol/L in patients taking metformin do not necessarily indicate impending lactic acidosis and may be explainable by other mechanisms, such as poorly controlled diabetes or obesity, vigorous physical activity, or technical problems in sample handling (see also PRECAUTIONS).

Lactic acidosis should be suspected in any diabetic patient with metabolic acidosis lacking evidence of ketoacidosis (ketonuria and ketonemia).

Lactic acidosis is a medical emergency that must be treated in a hospital setting. In a patient with lactic acidosis who is taking metformin, the drug should be discontinued immediately and general supportive measures promptly instituted. Because metformin hydrochloride is dialyzable (with a clearance of up to 170 mL/min under good hemodynamic conditions), prompt hemodialysis is recommended to correct the acidosis and remove the accumulated metformin. Such management often results in prompt reversal of symptoms and recovery (see also CONTRAINDICATIONSand PRECAUTIONS).

Description

Metformin hydrochloride tablets and metformin hydrochloride extended-release tablets are oral antihyperglycemic drugs used in the management of type 2 diabetes. Metformin hydrochloride (N,N-dimethylimidodicarbonimidic diamide hydrochloride) is not chemically or pharmacologically related to any other classes of oral antihyperglycemic agents. The structural formula is as shown:

Metformin hydrochloride, USP is a white crystalline compound with a molecular formula of C₄H₁₁N₅ • HCl and a molecular weight of 165.63. Metformin hydrochloride is freely soluble in water, slightly soluble in alcohol and is practically insoluble in acetone and methylene chloride. The pK_a of metformin is 12.4. The pH of a 1% aqueous solution of metformin hydrochloride is 6.68.

Metformin hydrochloride tablets contain 500 mg or 850 mg or 1000 mg of metformin hydrochloride. Each tablet contains the inactive ingredients colloidal silicon dioxide, hypromellose, magnesium stearate, microcrystalline cellulose, polyethylene glycol, povidone and sodium starch glycolate.

Metformin hydrochloride extended-release tablets contain 500 mg or 750 mg of metformin hydrochloride as the active ingredient.

Metformin hydrochloride extended-release tablets 500 mg and 750 mg contain the inactive ingredients glyceryl behenate, hypromellose, microcrystalline cellulose and povidone.

System Components and Performance:

Metformin hydrochloride extended-release tablets comprise a monolithic hydrophilic polymer matrix system. Metformin hydrochloride is combined with a drug release-controlling polymer to form the core of the matrix. After administration fluid from the Gastro Intestinal tract (GI tract) enters the tablet causing the polymers to hydrate and swell. Due to hydration and swelling of the polymer, the size of the tablet in the stomach increases. As a result, the dosage may remain in the gastrointestinal tract for a prolonged period with slow release of the drug. Drug is released from the dosage form by a process of diffusion or erosion through the gel matrix that is essentially independent of pH. Hydrated polymer can be expected to be broken up by normal peristalsis in the GI tract. The biologically inert components of the tablet may occasionally remain intact during GI transit and will be eliminated in the feces as a soft, hydrated mass.

The methodology used for dissolution testing is USP Test 1 for metformin hydrochloride tablets, USP.

The methodology used for dissolution testing is USP Test 8 for metformin hydrochloride extended-release tablets, USP.

Clinical Pharmacology

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 fasting insulin levels and day-long plasma insulin response may actually decrease.

Pharmacokinetics

Absorption and Bioavailability

The absolute bioavailability of a metformin hydrochloride tablet 500 mg given under fasting conditions is approximately 50-60%. Studies using single oral doses of metformin hydrochloride tablets 500 mg to 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. Food decreases the extent of and slightly delays 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.

Following a single oral dose of metformin hydrochloride extended-release tablets, C_max is achieved with a median value of 7 hours and a range of 4 hours to 8 hours. Peak plasma levels are approximately 20% lower compared to the same dose of metformin hydrochloride tablets, however, the extent of absorption (as measured by AUC) is similar to metformin hydrochloride tablets.

At steady state, the AUC and C_max are less than dose proportional for metformin hydrochloride extended-release tablets within the range of 500 mg to 2000 mg administered once daily. Peak plasma levels are approximately 0.6, 1.1, 1.4, and 1.8 (g/mL for 500, 1000, 1500, and 2000 mg once-daily doses, respectively. The extent of metformin absorption (as measured by AUC) from metformin hydrochloride extended-release tablets at a 2000 mg once-daily dose is similar to the same total daily dose administered as metformin hydrochloride tablets 1000 mg twice daily. After repeated administration of metformin hydrochloride extended-release tablets, metformin did not accumulate in plasma.

Within-subject variability in C_max and AUC of metformin from metformin hydrochloride extended-release tablet is comparable to that with metformin hydrochloride tablet.

Although the extent of metformin absorption (as measured by AUC) from the metformin hydrochloride extended-release tablets increased by approximately 50% when given with food, there was no effect of food on C_max and T_max of metformin. Both high and low fat meals had the same effect on the pharmacokinetics of metformin hydrochloride extended-release tablets.

Distribution

The apparent volume of distribution (V/F) of metformin following single oral doses of metformin hydrochloride tablets 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 metformin hydrochloride tablets, steady state plasma concentrations of metformin are reached within 24-48 hours and are generally <1 µg/mL. During controlled clinical trials of metformin hydrochloride tablets, maximum metformin plasma levels did not exceed 5 µg/mL, even at maximum doses.

Metabolism and Elimination

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 1) 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.

Special Populations

Patients with Type 2 Diabetes

In the presence of normal renal function, there are no differences between single- or multiple-dose pharmacokinetics of metformin between patients with type 2 diabetes and normal subjects (see Table 1), nor is there any accumulation of metformin in either group at usual clinical doses.

The pharmacokinetics of metformin hydrochloride extended-release tablets in patients with type 2 diabetes are comparable to those in healthy normal adults.

Renal Insufficiency

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 (see Table 1, also see WARNINGS)

Hepatic Insufficiency

No pharmacokinetic studies of metformin have been conducted in patients with hepatic insufficiency.

Geriatrics

Limited data from controlled pharmacokinetic studies of metformin hydrochloride tablets in healthy elderly subjects suggest that total plasma clearance of metformin is decreased, the half-life is prolonged, and C_max 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 (see Table 1). Metformin 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 and DOSAGE AND ADMINISTRATION).

Table 1:Select Mean (±S.D.) Metformin Pharmacokinetic Parameters Following Single or Multiple Oral Doses of Metformin Hydrochloride Tablets

Subject  Groups :  Metformin  Hydrochloride  Tablets  dosea  ( number  of  subjects )	Cm a x b ( µg / mL )	Tm a x c ( hrs )	Renal  Clearance  ( mL / min )
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 Combined results (average means) of five studies: mean age 32 years (range 23-59 years)
e Kinetic study done following dose 19, given fasting
f Elderly subjects, mean age 71 years (range 65-81 years)
g CLc r = creatinine clearance normalized to body surface area of 1.73 m2
Healthy ,  nondiabetic  adults : 500 mg single dose (24)850 mg single dose (74) d 850 mg three times daily for 19 dosese  (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 single dose (23)850 mg three times daily for 19 dosese  (9)	1.48 (±0.5)1.90 (±0.62)	3.32 (±1.08) 2.01 (±1.22)	491 (±138)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 (CLc r g 61-90 mL/min) (5) Moderate (CLc r 31-60 mL/min) (4) Severe (CLc r 10-30 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)

Pediatrics

After administration of a single oral metformin hydrochloride tablets, 500 mg with food, geometric mean metformin C_max and AUC differed less than 5% between pediatric type 2 diabetic patients (12 to 16 years of age) and gender- and weight-matched healthy adults (20 to 45 years of age), all with normal renal function.

Gender

Metformin pharmacokinetic parameters did not differ significantly between normal subjects and patients with type 2 diabetes when analyzed according to gender (males = 19, females= 16). Similarly, in controlled clinical studies in patients with type 2 diabetes, the antihyperglycemic effect of metformin hydrochloride tablet was comparable in males and females.

Race

No studies of metformin pharmacokinetic parameters according to race have been performed. In controlled clinical studies of metformin hydrochloride tablets in patients with type 2 diabetes, the antihyperglycemic effect was comparable in whites (n=249), blacks (n=51), and Hispanics (n=24).

Clinical Studies

Metformin hydrochloride tablets

In a double-blind, placebo-controlled, multicenter U.S. clinical trial involving obese patients with type 2 diabetes whose hyperglycemia was not adequately controlled with dietary management alone (baseline fasting plasma glucose [FPG] of approximately 240 mg/dL), treatment with metformin hydrochloride tablets (up to 2550 mg/day) for 29 weeks resulted in significant mean net reductions in fasting and postprandial plasma glucose (PPG) and hemoglobin A_1c (HbA_1c) of 59 mg/dL, 83 mg/ dL, and 1.8%, respectively, compared to the placebo group (see Table 2).

Table 2:Metformin Hydrochloride Tablets vs Placebo Summary of Mean Changes from BaselineAll patients on diet therapy at Baseline in Fasting Plasma Glucose, HbA_{l c}, and Body Weight, at Final Visit(29-week study)

	Metformin  Hydrochloride  Tablets ( n  =  141 )	Placebo ( n  =  145 )	p - ValueÂ
FPG  ( mg / dL )
BaselineÂ	241.5Â	237.7Â	NSNot statistically significant
Change at FINAL VISIT	-53.0Â	6.3Â	0.001Â
Hemoglobin  Al c  (%)
BaselineÂ	8.4Â	8.2Â	NS
Change at FINAL VISIT	-1.4Â	0.4Â	0.001Â
Body  Weight  ( lbs )
BaselineÂ	201.0Â	206.0Â	NS
Change at FINAL VISITÂ	-1.4Â	-2.4Â	NS

A 29-week, double-blind, placebo-controlled study of metformin hydrochloride tablets and glyburide, alone and in combination, was conducted in obese patients with type 2 diabetes who had failed to achieve adequate glycemic control while on maximum doses of glyburide (baseline FPG of approximately 250 mg/dL) (see Table 3). Patients randomized to the combination arm started therapy with metformin hydrochloride tablets 500 mg and glyburide 20 mg. At the end of each week of the first four weeks of the trial, these patients had their dosages of metformin hydrochloride tablets increased by 500 mg if they had failed to reach target fasting plasma glucose. After week four, such dosage adjustments were made monthly, although no patient was allowed to exceed metformin hydrochloride tablets 2500 mg. Patients in the metformin hydrochloride tablets only arm (metformin plus placebo) followed the same titration schedule. At the end of the trial, approximately 70% of the patients in the combination group were taking metformin hydrochloride tablets 2000 mg/glyburide 20 mg or metformin hydrochloride tablets 2500 mg/glyburide 20 mg. Patients randomized to continue on glyburide experienced worsening of glycemic control, with mean increases in FPG, PPG, and HbA_1c of 14 mg/dL, 3 mg/dL, and 0.2%, respectively. In contrast, those randomized to metformin hydrochloride tablets (up to 2500 mg/day) experienced a slight improvement, with mean reductions in FPG, PPG, and HbA_1c of 1 mg/dL, 6 mg/dL, and 0.4%, respectively. The combination of metformin hydrochloride tablets and glyburide was effective in reducing FPG, PPG, and HbA_1c levels by 63 mg/dL, 65 mg/dL, and 1.7%, respectively. Compared to results of glyburide treatment alone, the net differences with combination treatment were –77 mg/dL, –68 mg/dL, and –1.9%, respectively (see Table 3).

Table 3:Combined Metformin Hydrochloride Tablets/Glyburide (Comb) vs Glyburide (Glyb) or Metformin Hydrochloride Tablets (Met) monotherapy: Summary of Mean Changes from BaselineAll patients on glyburide, 20 mg/day, at Baseline in Fasting Plasma Glucose, HbA_{1 c}, and Body Weight, at Final Visit (29-week study)

					p - valuesÂ
	Comb ( n  =  213 )Â	Glyb ( n  =  209 )Â	Met ( n  =  210 )Â	Glyb  vs CombÂ	Met  vs CombÂ	Met  vs GlybÂ
Fasting  Plasma  Glucose  ( mg / dL )
BaselineÂ	250.5Â	247.5Â	253.9Â	NSNot statistically significant	NS	NS
Change at FINAL VISIT	-63.5Â	13.7Â	-0.9Â	0.001Â	0.001Â	0.025Â
Hemoglobin  Al c  (%)
Baseline	8.8Â	8.5Â	8.9Â	NS	NS	0.007Â
Change at FINAL VISIT	-1.7Â	0.2Â	-0.4Â	0.001Â	0.001Â	0.001Â
Body  Weight  ( lbs )
BaselineÂ	202.2Â	203.0Â	204.0Â	NS	NS	NS
Change at FINAL VISIT	0.9Â	-0.7Â	-8.4Â	0.011Â	0.001Â	0.001Â

The magnitude of the decline in fasting blood glucose concentration following the institution of metformin hydrochloride tablets therapy was proportional to the level of fasting hyperglycemia. Patients with type 2 diabetes with higher fasting glucose concentrations experienced greater declines in plasma glucose and glycosylated hemoglobin.

In clinical studies, metformin hydrochloride tablets, alone or in combination with a sulfonylurea, lowered mean fasting serum triglycerides, total cholesterol, and LDL cholesterol levels and had no adverse effects on other lipid levels (see Table 4).

Table 4: Summary of Mean Percent Change From Baseline of Major Serum Lipid Variables at Final Visit (29-week studies)

	Metformin  Hydrochloride  Tablets  vs  PlaceboÂ		Combined  Metformin  Hydrochloride  Tablets / Glyburide  vs  MonotherapyÂ
	Metformin  Hydrochloride  Tablets ( n  =  141 )Â	Placebo  ( n  =  145 )Â	Metformin  Hydrochloride  Tablets  ( n  =  210 )Â	Metformin  Hydrochloride  Tablets /  Glyburide ( n  =  213 )Â	Glyburide ( n  =  209 )Â
Total  Cholesterol  ( mg / dL )Â
BaselineÂ	211.0Â	212.3Â	213.1Â	215.6Â	219.6Â
Mean % Change at FINAL VISITÂ	-5%Â	1%Â	-2%Â	-4%Â	1%Â
Total  Triglycerides  ( mg / dL )Â
BaselineÂ	236.1Â	203.5Â	242.5Â	215.0Â	266.1Â
Mean % Change at FINAL VISITÂ	-16%Â	1%Â	-3%Â	-8%Â	4%Â
LDL - Cholesterol  ( mg / dL )Â
BaselineÂ	135.4Â	138.5Â	134.3Â	136.0Â	137.5Â
Mean % Change at FINAL VISITÂ	-8%Â	1%Â	- 4%Â	-6%Â	3%Â
HDL - Cholesterol  ( mg / dL )Â
BaselineÂ	39.0Â	40.5Â	37.2Â	39.0Â	37.0Â
Mean % Change at FINAL VISITÂ	2%Â	-1%Â	5%Â	3%Â	1%Â

In contrast to sulfonylureas, body weight of individuals on metformin hydrochloride tablets tended to remain stable or even decrease somewhat (see Tables 2 and 3).

A 24-week, double-blind, placebo-controlled study of metformin hydrochloride tablets plus insulin versus insulin plus placebo was conducted in patients with type 2 diabetes who failed to achieve adequate glycemic control on insulin alone (see Table 5). Patients randomized to receive metformin hydrochloride tablets plus insulin achieved a reduction in HbA_1c 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 vs 110.6 U/day, metformin hydrochloride tablets plus insulin versus insulin plus placebo, respectively, p=0.04.

Table 5: Combined Metformin Hydrochloride Tablets/Insulin vs Placebo/Insulin Summary of Mean Changes from Baseline in HbA_{1 c} and Daily Insulin Dose

	Metformin  Hydrochloride  Tablets  /  Insulin n  = 26Â	Placebo /  Insulin n  = 28Â	Treatment  Difference  Mean  ±  SEÂ
Hemoglobin  Al c  (%)Â
BaselineÂ	8.95Â	9.32Â	-Â
Change at FINAL VISITÂ	-2.10Â	-1.56Â	- 0.54 ± 0.43Statistically significant using analysis of covariance with baseline as covariate (p=0.04) Not significant using analysis of variance (values shown in table)
Insulin  Dose  ( U / day )Â
BaselineÂ	93.12Â	94.64Â	-Â
Change at FINAL VISITÂ	-0.15Â	15.93Â	- 16.08 ± 7.77Statistically 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 HbA_1c of 7.46 ± 0.97%, the addition of metformin hydrochloride tablets maintained similar glycemic control (HbA_1c 7.15 ± 0.61 versus 6.97 ± 0.62 for metformin hydrochloride tablets 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 metformin hydrochloride tablets plus insulin and placebo plus insulin, p<0.01). In addition, this study demonstrated that the combination of metformin hydrochloride tablets 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.

Metformin hydrochloride extended-release tablets

A 24-week, double-blind, placebo-controlled study of metformin hydrochloride extended-release tablets, taken once daily with the evening meal, was conducted in patients with type 2 diabetes who had failed to achieve glycemic control with diet and exercise (HbA_lc 7.0-10.0%, FPG 126-270 mg/dL). Patients entering the study had a mean baseline HbA_lc of 8.0% and a mean baseline FPG of 176 mg/dL. After 12 weeks treatment, mean HbA_1c had increased from baseline by 0.1% and mean FPG decreased from baseline by 2 mg/dL in the placebo group, compared with a decrease in mean HbA_1c of 0.6% and a decrease in mean FPG of 23 mg/dL in patients treated with metformin hydrochloride extended-release tablets 1000 mg once daily. Subsequently, the treatment dose was increased to 1500 mg once daily if HbA_1c was ≥7.0% but <8.0% (patients with HbA_1c ≥8.0% were discontinued from the study). At the final visit (24-week), mean HbA_1c had increased 0.2% from baseline in placebo patients and decreased 0.6% with metformin hydrochloride extended-release tablets.

A 16-week, double-blind, placebo-controlled, dose-response study of metformin hydrochloride extended-release tablets, taken once daily with the evening meal or twice daily with meals, was conducted in patients with type 2 diabetes who had failed to achieve glycemic control with diet and exercise (HbA_1c 7.0-11.0%, FPG 126-280 mg/dL). Changes in glycemic control and body weight are shown in Table 6.

Table 6: Summary of Mean Changes from BaselineAll patients on diet therapy at Baseline in HbA_{l c} , Fasting Plasma Glucose, and Body Weight at Final Visit (16-week study)

	Metformin  Hydrochloride  Extended - release  Tablets
	500  mg Once  Daily	1000  mg Once  Daily	1500  mg Once  Daily	2000  mg Once  Daily	1000  mg Twice  Daily	Placebo
Hemoglobin  Al c  (%)	( n = 115 )	( n = 115 )	( n = 111 )	( n = 125 )	( n = 112 )	( n  = 111 )
Baseline	8.2	8.4	8.3	8.4	8.4	8.4
Change at FINAL VISIT	-0.4	-0.6	-0.9	-0.8	-1.1	0.1
p – valueAll comparisons versus Placebo	<0.001	<0.001	<0.001	<0.001	<0.001	-
FPG  ( mg / dL )	( n = 126 )	( n = 118 )	( n = 120 )	( n = 132 )	( n = 122 )	( n = 113 )
Baseline	182.7	183.7	178.9	181.0	181.6	179.6
Change at FINAL VISIT	-15.2	-19.3	-28.5	-29.9	-33.6	7.6
p – value	<0.001	<0.001	<0.001	<0.001	<0.001	-
Body  Weight  ( lbs )	( n = 125 )	( n = 119 )	( n = 117 )	( n = 131 )	( n = 119	( n = 113 )
Baseline	192.9	191.8	188.3	195.4	192.5	194.3
Change at FINAL VISIT	-1.3	-1.3	-0.7	-1.5	-2.2	-1.8
p – value	NSNot statistically significant	NS	NS	NS	NS	-

Compared with placebo, improvement in glycemic control was seen at all dose levels of metformin hydrochloride extended-release tablets and treatment was not associated with any significant change in weight (see DOSAGE AND ADMINISTRATION for dosing recommendations for metformin hydrochloride tablets and metformin hydrochloride extended-release tablets).

A 24-week, double-blind, randomized study of metformin hydrochloride extended-release tablets, taken once daily with the evening meal, and metformin hydrochloride tablets, taken twice daily (with breakfast and evening meal), was conducted in patients with type 2 diabetes who had been treated with metformin hydrochloride tablets 500 mg twice daily for at least 8 weeks prior to study entry. The metformin hydrochloride tablets dose had not necessarily been titrated to achieve a specific level of glycemic control prior to study entry. Patients qualified for the study if HbA_lc was (8.5% and FPG was (200 mg/dL. Changes in glycemic control and body weight are shown in Table 7.

Table 7: Summary of Mean Changes from BaselineAll patients on metformin hydrochloride tablets 500 mg twice daily at Baseline in HbA_{1 c}, Fasting Plasma Glucose, and Body Weight at Week 12 and at Final visit (24 week study)

	Metformin  Hydrochloride  Tablets 500  mg  Twice  DailyÂ	Metformin  Hydrochloride  Extended - release  TabletsÂ
		1000  mg  Once  DailyÂ	1500  mg Once  DailyÂ
Hemoglobin  Al c  (%)Â	( n =  67 )Â	( n = 72 )Â	( n = 66 )Â
BaselineÂ	7. 06Â	6.99Â	7.02Â
Change at 12 WeeksÂ	0.14Â	0.23Â	0.04Â
(95% CI)Â	(-0.03, 0.31)Â	(0.10, 0.36)Â	(-0.08, 0.15)Â
Change at FINAL VISITÂ	0.14n = 68	0.27Â	0.13Â
(95% CI)Â	(-0.04, 0.31)Â	(0.11, 0.43)Â	(-0.02, 0.28)Â
FPG  ( mg / dL )Â	( n = 69 )Â	( n = 72 )Â	( n = 70 )Â
BaselineÂ	127.2Â	131.0Â	131.4Â
Change at 12 WeeksÂ	12.9Â	9.5Â	3.7Â
(95% CI)Â	(6.5, 19.4)Â	(4.4, 14.6)Â	(-0.4, 7.8)Â
Change at FINAL VISITÂ	14.0Â	11.5Â	7.6Â
(95% CI)Â	(7.0, 21.0)Â	(4.4, 18.6)Â	(1.0, 14.2)Â
Body  Weight  ( lbs )Â	( n = 71 )Â	( n = 74 )Â	( n = 71 )Â
BaselineÂ	210.3Â	202.8Â	192.7Â
Change at 12 WeeksÂ	0.4Â	0.9Â	0.7Â
(95% CI)Â	(-0.4, 1.5)Â	(0.0, 2.0)Â	(-0.4, 1.8)Â
Change at FINAL VISITÂ	0.9Â	1.1Â	0.9Â
(95% CI)Â	(-0.4, 2.2)Â	(-0.2, 2.4)Â	(-0.4, 2.0)Â

After 12 weeks of treatment, there was an increase in mean HbA_1c in all groups; in the metformin hydrochloride extended-release tablets 1000 mg group, the increase from baseline of 0.23% was statistically significant (see DOSAGE AND ADMINISTRATION).

Changes in lipid parameters in the previously described placebo-controlled dose-response study of metformin hydrochloride extended-release tablets are shown in Table 8.

Table 8: Summary of Mean Percent Changes from BaselineAll patients on diet therapy at Baseline in Major Lipid Variables at Final Visit (16-week study)

	Metformin  Hydrochloride  Extended - release  TabletsÂ
	500  mg Once  DailyÂ	1000  mg Once  DailyÂ	1500  mg Once  DailyÂ	2000  mg Once  DailyÂ	1000  mg Twice  DailyÂ	PlaceboÂ
Total  Cholesterol  ( mg / dL )	( n = 120 )Â	( n = 113 )Â	( n = 110 )Â	( n = 126 )Â	( n = 117 )Â	( n = 110 )Â
Baseline Â	210.3Â	218.1Â	214.6Â	204.4Â	208.2Â	208.6Â
Mean % Change at FINAL VISITÂ	1.0%Â	1.7%Â	0.7%Â	- 1.6%Â	- 2.6%Â	2.6%Â
Total  Triglycerides  ( mg / dL )	( n = 120 )Â	( n = 113 )Â	( n = 110 )Â	( n = 126 )Â	( n = 117 )Â	( n = 110 )Â
Baseline Â	220.2Â	211.9Â	198.0Â	194.2Â	179.0Â	211.7Â
Mean % Change at FINAL VISIT Â	14.5%Â	9.4%Â	15.1%Â	14.9%Â	9.4%Â	10.9%Â
LDL -  Cholesterol  ( mg / dL ) Â	( n = 119 )Â	( n = 113 )Â	( n = 109 )Â	( n = 126 )Â	( n = 117 )Â	( n = 107 )Â
Baseline Â	131.0Â	134.9Â	135.8Â	125.8Â	131.4Â	131.9Â
Mean % Change at FINAL VISITÂ	-1.4%Â	-1.6%Â	-3.5%Â	-3.3%Â	-5.5%Â	3.2%Â
HDL  –  Cholesterol  ( mg / dL )	( n = 120 )Â	( n = 108 )Â	( n = 108 )Â	( n = 125 )Â	( n = 117 )Â	( n = 108 )Â
Baseline Â	40.8Â	41.6Â	40.6Â	40.2Â	42.4Â	39.4Â
Mean % Change at FINAL VISITÂ	6.2%Â	8.6%Â	5.5%Â	6.1%Â	7.1%Â	5.8%Â

Changes in lipid parameters in the previously described study of metformin are shown in Table 9.

Table 9: Summary of Mean Percent Changes from BaselineAll patients on metformin hydrochloride tablets 500 mg twice daily at Baseline in Major Lipid Variables at Final Visit (24-week study)

	Metformin  Hydrochloride  Tablets 500  mg  Twice  DailyÂ	Metformin  Hydrochloride  Extended - release  TabletsÂ
		1000  mg  Once  DailyÂ	1500  mg  Once  DailyÂ
Total  Cholesterol  ( mg / dL )Â	( n = 68 )Â	( n = 70 )Â	( n = 66 )Â
BaselineÂ	199.0Â	201.9Â	201.6Â
Mean % Change at FINAL VISITÂ	0.1%Â	1.3%Â	0.1%Â
Total  Triglycerides  ( mg / dL )Â	( n = 68 )Â	( n = 70 )Â	( n = 66 )Â
BaselineÂ	178.0Â	169.2Â	206.8Â
Mean % Change at FINAL VISITÂ	6.3%Â	25.3%Â	33.4%Â
LDL -  Cholesterol  ( mg / dL )Â	( n = 68 )Â	( n = 70 )Â	( n = 66 )Â
BaselineÂ	122.1Â	126.2Â	115.7Â
Mean % Change at FINAL VISITÂ	-1.3%Â	-3.3%Â	-3.7%Â
HDL  - Cholesterol  ( mg / dL )Â	( n = 68 )Â	( n = 70 )Â	( n = 65 )Â
BaselineÂ	41.9Â	41.7Â	44.6Â
Mean % Change at FINAL VISITÂ	4.8%Â	1.0%Â	-2.1%Â

Pediatric Clinical Studies

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 metformin hydrochloride tablets (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 (see Table 10).

Table 10: Metformin Hydrochloride Tablets Vs Placebo (PediatricsPediatric patients mean age 13.8 years (range 10-16 years)) Summary of Mean Changes from BaselineAll patients on diet therapy at Baseline in Plasma Glucose and Body Weight at Final Visit

	Metformin  Hydrochloride  TabletsÂ	PlaceboÂ	p -  ValueÂ
FPG  ( mg / dL )Â	( n = 37 )Â	( n = 36 )Â
BaselineÂ	162.4Â	192.3Â	-Â
Change at FINAL VISITÂ	-42.9Â	21.4Â	< 0.001Â
Body  Weight  ( lbs )Â	( n = 39 )Â	( n = 38 )Â
BaselineÂ	205.3Â	189.0Â	-Â
Change at FINAL VISITÂ	-3.3Â	-2.0Â	NSNot statistically significant

Indications And Usage

Metformin hydrochloride tablet is indicated as an adjunct to diet and exercise to improve glycemic control in adults and children with type 2 diabetes mellitus.

Metformin hydrochloride extended-release tablet is indicated as an adjunct to diet and exercise to improve glycemic control in adults with type 2 diabetes mellitus.

Contraindications

Metformin hydrochloride tablets and metformin hydrochloride extended-release tablets are contraindicated in patients with:

• Renal disease or renal dysfunction (e.g., as suggested by serum creatinine levels ≥1.5 mg/dL [males], ≥1.4 mg/dL [females] or abnormal creatinine clearance) which may also result from conditions such as cardiovascular collapse (shock), acute myocardial infarction, and septicemia (see WARNINGS and PRECAUTIONS).
• Known hypersensitivity to metformin hydrochloride.
• Acute or chronic metabolic acidosis, including diabetic ketoacidosis, with or without coma. Diabetic ketoacidosis should be treated with insulin.

Metformin hydrochloride tablets and metformin hydrochloride extended-release tablets should be temporarily discontinued in patients undergoing radiologic studies involving intravascular administration of iodinated contrast materials, because use of such products may result in acute alteration of renal function (see also PRECAUTIONS).

Warnings

BOXED WARNING SECTION

Lactic Acidosis:

Lactic acidosis is a rare, but serious, metabolic complication that can occur due to metformin accumulation during treatment with metformin; when it occurs, it is fatal in approximately 50% of cases. Lactic acidosis may also occur in association with a number of pathophysiologic conditions, including diabetes mellitus, and whenever there is significant tissue hypoperfusion and hypoxemia. Lactic acidosis is characterized by elevated blood lactate levels (>5 mmol/L), decreased blood pH, electrolyte disturbances with an increased anion gap, and an increased lactate/pyruvate ratio. When metformin is implicated as the cause of lactic acidosis, metformin plasma levels >5 µg/mL are generally found.

The reported incidence of lactic acidosis in patients receiving metformin hydrochloride is very low (approximately 0.03 cases/1000 patient-years, with approximately 0.015 fatal cases/ 1000 patient-years). In more than 20,000 patient-years exposure to metformin in clinical trials, there were no reports of lactic acidosis. Reported cases have occurred primarily in diabetic patients with significant renal insufficiency, including both intrinsic renal disease and renal hypoperfusion, often in the setting of multiple concomitant medical/surgical problems and multiple concomitant medications. Patients with congestive heart failure requiring pharmacologic management, in particular those with unstable or acute congestive heart failure who are at risk of hypoperfusion and hypoxemia, are at increased risk of lactic acidosis. The risk of lactic acidosis increases with the degree of renal dysfunction and the patient’s age. The risk of lactic acidosis may, therefore, be significantly decreased by regular monitoring of renal function in patients taking metformin and by use of the minimum effective dose of metformin. In particular, treatment of the elderly should be accompanied by careful monitoring of renal function. Metformin treatment should not be initiated in patients ≥80 years of age unless measurement of creatinine clearance demonstrates that renal function is not reduced, as these patients are more susceptible to developing lactic acidosis. In addition, metformin should be promptly withheld in the presence of any condition associated with hypoxemia, dehydration, or sepsis. Because impaired hepatic function may significantly limit the ability to clear lactate, metformin should generally be avoided in patients with clinical or laboratory evidence of hepatic disease. Patients should be cautioned against excessive alcohol intake, either acute or chronic, when taking metformin, since alcohol potentiates the effects of metformin hydrochloride on lactate metabolism. In addition, metformin should be temporarily discontinued prior to any intravascular radiocontrast study and for any surgical procedure (see also PRECAUTIONS).

The onset of lactic acidosis often is subtle, and accompanied only by nonspecific symptoms such as malaise, myalgias, respiratory distress, increasing somnolence, and nonspecific abdominal distress. There may be associated hypothermia, hypotension, and resistant bradyarrhythmias with more marked acidosis. The patient and the patient’s physician must be aware of the possible importance of such symptoms and the patient should be instructed to notify the physician immediately if they occur (see also PRECAUTIONS). Metformin should be withdrawn until the situation is clarified. Serum electrolytes, ketones, blood glucose, and if indicated, blood pH, lactate levels, and even blood metformin levels may be useful. Once a patient is stabilized on any dose level of metformin, gastrointestinal symptoms, which are common during initiation of therapy, are unlikely to be drug related. Later occurrence of gastrointestinal symptoms could be due to lactic acidosis or other serious disease.

Levels of fasting venous plasma lactate above the upper limit of normal but less than 5 mmol/L in patients taking metformin do not necessarily indicate impending lactic acidosis and may be explainable by other mechanisms, such as poorly controlled diabetes or obesity, vigorous physical activity, or technical problems in sample handling (see also PRECAUTIONS).

Lactic acidosis should be suspected in any diabetic patient with metabolic acidosis lacking evidence of ketoacidosis (ketonuria and ketonemia).

Lactic acidosis is a medical emergency that must be treated in a hospital setting. In a patient with lactic acidosis who is taking metformin, the drug should be discontinued immediately and general supportive measures promptly instituted. Because metformin hydrochloride is dialyzable (with a clearance of up to 170 mL/min under good hemodynamic conditions), prompt hemodialysis is recommended to correct the acidosis and remove the accumulated metformin. Such management often results in prompt reversal of symptoms and recovery (see also CONTRAINDICATIONSand PRECAUTIONS).

Precautions

General

Macrovascular Outcomes

There have been no clinical studies establishing conclusive evidence of macrovascular risk reduction with metfomin hydrochloride tablets or metformin hydrochloride extended-release tablets or any other anti-diabetic drug.

Monitoring of renal function

Metformin is known to be substantially excreted by the kidney, and the risk of metformin accumulation and lactic acidosis increases with the degree of impairment of renal function. Thus, patients with serum creatinine levels above the upper limit of normal for their age should not receive metformin. In patients with advanced age, metformin should be carefully titrated to establish the minimum dose for adequate glycemic effect, because aging is associated with reduced renal function. In elderly patients, particularly those ≥80 years of age, renal function should be monitored regularly and, generally, metformin should not be titrated to the maximum dose (see WARNINGS and DOSAGE AND ADMINISTRATION).

Before initiation of metformin therapy and at least annually thereafter, renal function should be assessed and verified as normal. In patients in whom development of renal dysfunction is anticipated, renal function should be assessed more frequently and metformin discontinued if evidence of renal impairment is present.

Use of concomitant medications that may affect renal function or metformin disposition

Concomitant medication(s) that may affect renal function or result in significant hemodynamic change or may interfere with the disposition of metformin, such as cationic drugs that are eliminated by renal tubular secretion (see PRECAUTIONS: Drug Interactions), should be used with caution.

Radiologic studies involving the use of intravascular iodinated contrast materials (for example, intravenous urogram, intravenous cholangiography, angiography, and computed tomography (CT) scans with intravascular contrast materials)— Intravascular contrast studies with iodinated materials can lead to acute alteration of renal function and have been associated with lactic acidosis in patients receiving metformin (see CONTRAINDICATIONS). Therefore, in patients in whom any such study is planned, metformin should be temporarily discontinued at the time of or prior to the procedure, and withheld for 48 hours subsequent to the procedure and reinstituted only after renal function has been re-evaluated and found to be normal.

Hypoxic states

Cardiovascular collapse (shock) from whatever cause, acute congestive heart failure, acute myocardial infarction and other conditions characterized by hypoxemia have been associated with lactic acidosis and may also cause prerenal azotemia. When such events occur in patients on metformin therapy, the drug should be promptly discontinued.

Surgical procedures

Metformin therapy should be temporarily suspended for any surgical procedure (except minor procedures not associated with restricted intake of food and fluids) and should not be restarted until the patient’s oral intake has resumed and renal function has been evaluated as normal.

Alcohol intake

Alcohol is known to potentiate the effect of metformin on lactate metabolism. Patients, therefore, should be warned against excessive alcohol intake, acute or chronic, while receiving metformin.

Impaired hepatic function

Since impaired hepatic function has been associated with some cases of lactic acidosis, metformin should generally be avoided in patients with clinical or laboratory evidence of hepatic disease.

Vitamin B_l2 levels

In controlled clinical trials of metformin hydrochloride tablets of 29 weeks duration, a decrease to subnormal levels of previously normal serum Vitamin B₁₂ levels, without clinical manifestations, was observed in approximately 7% of patients. Such decrease, possibly due to interference with B₁₂ absorption from the B₁₂-intrinsic factor complex, is, however, very rarely associated with anemia and appears to be rapidly reversible with discontinuation of metformin hydrochloride tablets or Vitamin B₁₂ supplementation. Measurement of hematologic parameters on an annual basis is advised in patients on metformin and any apparent abnormalities should be appropriately investigated and managed (see PRECAUTIONS: Laboratory Tests).

Certain individuals (those with inadequate Vitamin B₁₂ or calcium intake or absorption) appear to be predisposed to developing subnormal Vitamin B₁₂ levels. In these patients, routine serum Vitamin B₁₂ measurements at two- to three-year intervals may be useful.

Change in clinical status of patients with previously controlled type 2 diabetes

A patient with type 2 diabetes previously well controlled on metformin who develops laboratory abnormalities or clinical illness (especially vague and poorly defined illness) should be evaluated promptly for evidence of ketoacidosis or lactic acidosis. Evaluation should include serum electrolytes and ketones, blood glucose and, if indicated, blood pH, lactate, pyruvate, and metformin levels. If acidosis of either form occurs, metformin must be stopped immediately and other appropriate corrective measures initiated (see also WARNINGS).

Hypoglycemia

Hypoglycemia does not occur in patients receiving metformin alone under usual circumstances of use, but could occur when caloric intake is deficient, when strenuous exercise is not compensated by caloric supplementation, or during concomitant use with other glucose-lowering agents (such as sulfonylureas and insulin) or ethanol.

Elderly, debilitated, or malnourished patients, and those with adrenal or pituitary insufficiency or alcohol intoxication are particularly susceptible to hypoglycemic effects. Hypoglycemia may be difficult to recognize in the elderly, and in people who are taking beta-adrenergic blocking drugs.

Loss of control of blood glucose

When a patient stabilized on any diabetic regimen is exposed to stress such as fever, trauma, infection, or surgery, a temporary loss of glycemic control may occur. At such times, it may be necessary to withhold metformin and temporarily administer insulin. Metformin may be reinstituted after the acute episode is resolved.

The effectiveness of oral antidiabetic drugs in lowering blood glucose to a targeted level decreases in many patients over a period of time. This phenomenon, which may be due to progression of the underlying disease or to diminished responsiveness to the drug, is known as secondary failure, to distinguish it from primary failure in which the drug is ineffective during initial therapy. Should secondary failure occur with either metformin or sulfonylurea monotherapy, combined therapy with metformin and sulfonylurea may result in a response. Should secondary failure occur with combined metformin hydrochloride tablets/sulfonylurea therapy or metformin hydrochloride extended-release tablets/sulfonylurea therapy, it may be necessary to consider therapeutic alternatives including initiation of insulin therapy.

Information for Patients

Patients should be informed of the potential risks and benefits of metformin and of alternative modes of therapy. They should also be informed about the importance of adherence to dietary instructions, of a regular exercise program, and of regular testing of blood glucose, glycosylated hemoglobin, renal function, and hematologic parameters.

The risks of lactic acidosis, its symptoms, and conditions that predispose to its development, as noted in the WARNINGS and PRECAUTIONS sections, should be explained to patients. Patients should be advised to discontinue metformin immediately and to promptly notify their health practitioner if unexplained hyperventilation, myalgia, malaise, unusual somnolence, or other nonspecific symptoms occur. Once a patient is stabilized on any dose level of metformin, gastrointestinal symptoms, which are common during initiation of metformin therapy, are unlikely to be drug related. Later occurrence of gastrointestinal symptoms could be due to lactic acidosis or other serious disease.

Patients should be counselled against excessive alcohol intake, either acute or chronic, while receiving metformin.

Metformin alone does not usually cause hypoglycemia, although it may occur when metformin is used in conjunction with oral sulfonylureas and insulin. When initiating combination therapy, the risks of hypoglycemia, its symptoms and treatment, and conditions that predispose to its development should be explained to patients and responsible family members (see Patient Information printed below).

Patients should be informed that metformin hydrochloride extended-release tablets must be swallowed whole and not crushed or chewed, and that the inactive ingredients may occasionally be eliminated in the feces as a soft mass that may resemble the original tablet.

Laboratory Tests

Response to all diabetic therapies should be monitored by periodic measurements of fasting blood glucose and glycosylated hemoglobin levels, with a goal of decreasing these levels toward the normal range. During initial dose titration, fasting glucose can be used to determine the therapeutic response. Thereafter, both glucose and glycosylated hemoglobin should be monitored. Measurements of glycosylated hemoglobin may be especially useful for evaluating long-term control (see also DOSAGE AND ADMINISTRATION).

Initial and periodic monitoring of hematologic parameters (e.g., hemoglobin/ hematocrit and red blood cell indices) and renal function (serum creatinine) should be performed, at least on an annual basis. While megaloblastic anemia has rarely been seen with metformin hydrochloride tablets therapy, if this is suspected, Vitamin B₁₂ deficiency should be excluded.

Drug Interactions

(Clinical Evaluation of Drug Interactions Conducted with Metformin Hydrochloride Tablets)

Glyburide

In a single-dose interaction study in type 2 diabetes patients, co-administration of metformin and glyburide did not result in any changes in either metformin pharmacokinetics or pharmacodynamics. Decreases in glyburide AUC and C_max were observed, but were highly variable. The single-dose nature of this study and the lack of correlation between glyburide blood levels and pharmacodynamic effects, make the clinical significance of this interaction uncertain (see DOSAGE AND ADMINISTRATION: Concomitant Metformin Hydrochloride Tablets or Metformin Hydrochloride Extended-release Tablets and Oral Sulfonylurea Therapy in Adult Patients).

Furosemide

A single-dose, metformin-furosemide drug interaction study in healthy subjects demonstrated that pharmacokinetic parameters of both compounds were affected by co-administration. Furosemide increased the metformin plasma and blood C_max by 22% and blood AUC by 15%, without any significant change in metformin renal clearance. When administered with metformin, the C_max and AUC of furosemide were 31% and 12% smaller, respectively, than when administered alone, and the terminal half-life was decreased by 32%, without any significant change in furosemide renal clearance. No information is available about the interaction of metformin and furosemide when co-administered chronically.

Nifedipine

A single-dose, metformin-nifedipine drug interaction study in normal healthy volunteers demonstrated that co-administration of nifedipine increased plasma metformin C_max and AUC by 20% and 9%, respectively, and increased the amount excreted in the urine. T_max and half-life were unaffected. Nifedipine appears to enhance the absorption of metformin. Metformin had minimal effects on nifedipine.

Cationic drugs

Cationic drugs (e.g., amiloride, digoxin, morphine, procainamide, quinidine, quinine, ranitidine, triamterene, trimethoprim, or vancomycin) that are eliminated by renal tubular secretion theoretically have the potential for interaction with metformin by competing for common renal tubular transport systems. Such interaction between metformin and oral cimetidine has been observed in normal healthy volunteers in both single- and multiple-dose, metformin-cimetidine drug interaction studies, with a 60% increase in peak metformin plasma and whole blood concentrations and a 40% increase in plasma and whole blood metformin AUC. There was no change in elimination half-life in the single-dose study. Metformin had no effect on cimetidine pharmacokinetics. Although such interactions remain theoretical (except for cimetidine), careful patient monitoring and dose adjustment of metformin and/or the interfering drug is recommended in patients who are taking cationic medications that are excreted via the proximal renal tubular secretory system.

Other

Certain drugs tend to produce hyperglycemia and may lead to loss of glycemic control. These drugs include the thiazides and other diuretics, corticosteroids, phenothiazines, thyroid products, estrogens, oral contraceptives, phenytoin, nicotinic acid, sympathomimetics, calcium channel blocking drugs, and isoniazid. When such drugs are administered to a patient receiving metformin, the patient should be closely observed for loss of blood glucose control. When such drugs are withdrawn from a patient receiving metformin, the patient should be observed closely for hypoglycemia.

In healthy volunteers, the pharmacokinetics of metformin and propranolol, and metformin and ibuprofen were not affected when co-administered in single-dose interaction studies.

Metformin is negligibly bound to plasma proteins and is, therefore, less likely to interact with highly protein-bound drugs such as salicylates, sulfonamides, chloramphenicol, and probenecid, as compared to the sulfonylureas, which are extensively bound to serum proteins.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Long-term carcinogenicity studies have been performed in rats (dosing duration of 104 weeks) and mice (dosing duration of 91 weeks) at doses up to and including 900 mg/kg/day and 1500 mg/kg/day, respectively. These doses are both approximately four times the maximum recommended human daily dose of 2000 mg based on body surface area comparisons. No evidence of carcinogenicity with metformin was found in either male or female mice. Similarly, there was no tumorigenic potential observed with metformin in male rats. There was, however, an increased incidence of benign stromal uterine polyps in female rats treated with 900 mg/kg/day.

There was no evidence of a mutagenic potential of metformin in the following in vitro tests: Ames test (S. typhimurium), gene mutation test (mouse lymphoma cells), or chromosomal aberrations test (human lymphocytes). Results in the in vivo mouse micronucleus test were also negative.

Fertility of male or female rats was unaffected by metformin when administered at doses as high as 600 mg/kg/day, which is approximately three times the maximum recommended human daily dose based on body surface area comparisons.

Pregnancy

Teratogenic Effects: Pregnancy Category B

Recent information strongly suggests that abnormal blood glucose levels during pregnancy are associated with a higher incidence of congenital abnormalities. Most experts recommend that insulin be used during pregnancy to maintain blood glucose levels as close to normal as possible. Because animal reproduction studies are not always predictive of human response, metformin should not be used during pregnancy unless clearly needed.

There are no adequate and well-controlled studies in pregnant women with metformin. Metformin was not teratogenic in rats and rabbits at doses up to 600 mg/kg/day. This represents an exposure of about two and six times the maximum recommended human daily dose of 2000 mg based on body surface area comparisons for rats and rabbits, respectively. Determination of fetal concentrations demonstrated a partial placental barrier to metformin.

Nursing Mothers

Studies in lactating rats show that metformin is excreted into milk and reaches levels comparable to those in plasma. Similar studies have not been conducted in nursing mothers. Because the potential for hypoglycemia in nursing infants may exist, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother. If metformin is discontinued, and if diet alone is inadequate for controlling blood glucose, insulin therapy should be considered.

Pediatric Use

The safety and effectiveness of metformin hydrochloride tablets for the treatment of type 2 diabetes have been established in pediatric patients ages 10 to 16 years (studies have not been conducted in pediatric patients below the age of 10 years). Use of metformin hydrochloride tablets in this age group is supported by evidence from adequate and well-controlled studies of metformin hydrochloride tablets in adults with additional data from a controlled clinical study in pediatric patients ages 10-16 years with type 2 diabetes, which demonstrated a similar response in glycemic control to that seen in adults (see CLINICAL PHARMACOLOGY: Pediatric Clinical Studies). In this study, adverse effects were similar to those described in adults (see ADVERSE REACTIONS: Pediatric Patients). A maximum daily dose of 2000 mg is recommended (see DOSAGE AND ADMINISTRATION: Recommended Dosing Schedule: Pediatrics).

Safety and effectiveness of metformin hydrochloride extended-release tablets in pediatric patients have not been established.

Geriatric Use

Controlled clinical studies of metformin did not include sufficient numbers of elderly patients to determine whether they respond differently from younger patients, although other reported clinical experience has not identified differences in responses between the elderly and younger patients. Metformin is known to be substantially excreted by the kidney and because the risk of serious adverse reactions to the drug is greater in patients with impaired renal function, metformin should only be used in patients with normal renal function (see CONTRAINDICATIONS, WARNINGS, and CLINICAL PHARMACOLOGY: Pharmacokinetics). Because aging is associated with reduced renal function, metformin should be used with caution as age increases. Care should be taken in dose selection and should be based on careful and regular monitoring of renal function. Generally, elderly patients should not be titrated to the maximum dose of metformin (see also WARNINGS and DOSAGE AND ADMINISTRATION).