BACKGROUND OF THE INVENTION Diabetes mellitus is a metabolic disorder of multiple aetiology characterized by chronic hyperglycemia resulting from defects in insulin secretion, insulin action, or both. Clinically, diabetes mellitus manifesting itself in pathologically elevated plasma glucose levels and can be assessed by measurement of glucose concentration in blood.

Metformin has long been known as an antidiabetic compound, which is useful in lowering the sugar content in blood and treating insulin resistance, especially under type 2 diabetes, obesity and aging. However, there is a particular disadvantage in its use, i. e., metformin has a relatively short duration in respect of its activity and therefore it is required to administer metformin to the patient frequently. Hence it has become desirable to provide a hypoglycemic which is characterized by an improved duration of activity.

Metfbrmin is used as such in the form of salts such as the hydrochloride, the sulfate, the phosphate, the adamantanoate, the dichloroacetate, or the citrate. Until the invention described herein, there was no report of use of metfbrmin in the form of salts such as the succinate.

Succinic acid or salts thereof were described in JP patent 61171417 as antidiabetics useful for stimulating insulin secretion. Contrary to JP 61171417, MacDonald et al. found that succinic acid and salts thereof did not stimulate insulin secretion from pancreas, and only esters of succinic acid are potent insulin

secretagogues. MacDonald, M. J. and Fahien, L. A. Diabetes 37 (7): 997-999 (1988). Also, succinic acid was described in JP patent 6062798 as a component of food for person with poor glucose-tolerance in combination with acetic, lactic, and gluconic acid. Disodium succinate was described as a component of the composition for decreasing blood glucose in rabbits with alloxan diabetes in combination with citric and acetic acid. Dzvonlcevich, N. D. et al., Ukr. Biokhim. Zh., 46 (5): 547-552 (1974).

Until the invention described herein, there was no report of use of metformin together with succinic acid or a pharmaceutically acceptable salt thereof.

It has now been discovered that combination therapy with metformin and succinic acid or a pharmaceutically salts thereof results in synergistic improvement in glycemic control. Accordingly, such combinations are especially useful in treating diabetes.

It is an object of the present invention to provide the pharmaceutical composition comprising an effective amounts of metformin and succinic acid or a pharmaceutically acceptable salt thereof, preferably for synergistic treatment of diabetes.

It is an object of the present invention to provide a method of treating diabetes in a mammal, comprising administering to the mammal in need thereof an effective amount of metformin stepwise or in combination with an effective amount of succinic acid or a pharmaceutically acceptable salt thereof.

SUMMARY OF THE INVENTION This invention relates to compositions comprising metformin and succinic acid or a pharmaceutically acceptable salt thereof. More specifically, this invention relates to compositions comprising metfbrmin and succinic acid or a pharmaceutically acceptable salt thereof, which are presented in amounts sufficient to cause synergistic effects for the treatment of diabetes, particularly for synergistic improvement in glycemic control. Further, the invention relates to new salts of metformin, more specifically to metformi succinates and process for producing the same. Further, this invention relates to methods of treating diabetes in mammal in need thereof, which methods comprise administering to said mammal, either stepwise or in combination, synergistically effective amounts of metfbrmin and succinic acid or a pharmaceutically acceptable salt thereof.

DETAILED DESCRIPTION OF THE INVENTION The term"synergistic", as used herein, means that the effect achieved with the methods and compositions of this invention is greater than the sum of the effects that result from methods and compositions comprising metfbrmin and succinic acid or a pharmaceutically acceptable salt thereof separately and in the amounts employed in the methods and compositions hereof.

Accordingly to this invention, it is now possible to achieve a synergistic effect in treating diabetes in a mammal with amounts of metformin and succinic acid or a pharmaceutically acceptable salt thereof which, if administered in said amounts singly, are not capable of achieving said effect and which effect is greater than the sum of the effects achieved for metformin and succinic acid or a pharmaceutically acceptable salt thereof separately.

The present invention provides a pharmaceutical composition, which comprises from about 0.05 to 2000 mg of metformin and from about 0.05 to 2000 mg of succinic acid or a pharmaceutically acceptable salt thereof. Preferably, said amounts of metformin and succinic acid or a pharmaceutically acceptable salt thereof are synergistic for the treatment of diabetes mellitus in a mammal in need thereof.

Metformin is widely employed to treat diabetes, for example as described in USP Dispensing Information. It typically is used clinically as a pharmaceutically acceptable salt, preferably the hydrochloride salt. A commercial form of metformin hydrochloride is available, and its chemical name is N, N-dimethylimidodicarbonimidic diamide hydrochloride. Metfbrmin hydrochloride has the chemical formula As used herein,"metformin"means the base compound as well as its pharmaceutically acceptable salts.

Succinic acid has the chemical formula HOOCCHiCH2COOH

The pharmaceutically acceptable salt of the succinic acid is prepared by known methods from organic and inorganic bases. Such bases include, but are not limited to, nontoxic alkali metal and alkaline earth bases, for example, calcium, lithium, sodium, and potassium hydroxide ; ammonium hydroxide and nontoxic organic bases, such as triethylamine, butylamine, diethanolamine, and triethanolamine.

Compositions of the invention can be administered in a wide variety of different dosage forms, i. e., they may be formulated with various pharmaceutically acceptable inert carriers in the form of tablets, capsules, lozenges, troches, hard candies, powders, sprays, aqueous solutions, elixirs, syrups and the like.

The particular embodiment of the composition of the invention is metformin succinates, a salts of metformin base with succinic acid.

The present invention provides N, N-dimethylimidodicarbonimidic diamide succinate, a metformin succinate salt (1: 1), which has a chemical formula The present invention provides bis (N, N-dimethylimidodicarbonimidic diamide) succinate, a metformin succinate salt (2: 1), which has a chemical formula Further, the present invention provides a process for producing N, N- dimethylimidodicarbonimidic diamide succinate, which comprises reacting N, N- dimethylimidodicarbonimidic diamide with succinic acid, wherein the mole ratio of N, N-dimethylimidodicarbonimidic diamide to succinic acid entered in reaction is about 1: 1.

Further, the present invention provides a process for producing bis (N, N- dimethylimidodicarbonimidic diamide) succinate, which comprises reacting N, N- dimethylimidodicarbonimidic diamide with succinic acid, wherein the mole ratio of

N, N-dimethylimidodicarbonimidic diamide to succinic acid entered in reaction is about 2: 1.

The reactions of the invention take place at ambient temperatures in a solvent such as water, alcohol, or mixture thereof and the desired product is obtained as a white crystalline powder with reproducible properties.

Further, the present invention provides a method of treating diabetes mellitus in a mammal in need thereof, which comprises administering to said mammal from about 0.05 to 2000 mg of metformin stepwise or in combination with from about 0.05 to 2000 mg of succinic acid or a pharmaceutically acceptable salt thereof, wherein said amounts are synergistic for the treatment of diabetes.

Metfonnin and succinic acid or a pharmaceutically acceptable salt thereof can be administered enterally or parenterally in the methods of the invention.

The dosage of each agent will vary depending upon the severity of the disease, the frequency of administration, the particular agents and combinations utilized, and other factors routinely considered by an attending medical practitioner.

The term"treating"as used herein means the management and care of a mammal for the purpose of combating the disease, condition, or disorder and includes the administration of metformin and succinic acid or a pharmaceutically acceptable salt thereof to prevent the onset of the symptoms or complications, alleviating the symptoms or complications, or eliminating the disease, condition, or disorder. Treating diabetes mellitus includes, but is not limited to, improving glycemic control in mammal in need thereof.

The following examples are presented to demonstrate the invention. The examples are illustrative only and are not intended to limit the scope of the invention in any way.

EXAMPLE 1 Combination of metformin with succinic acid is synergistically effective for treating diabetes mellitus.

Assay. Plasma insulin concentrations were determined using a kit ("Dako", Dutch) with a rat insulin standard (Novo Research Institute, Bagsvard, Denmark). Plasma glucose concentrations were determined by glucose oxidase method using a kit (;'Agat", Russia). Plasma triglycerides and cholesterol concentrations were determined with

reagents FS,"DiaSys", Germany; HDL cholesterol with reagents"Cormay", Poland; and LDL cholesterol with reagents"Boehringer Mannheim", Germany.

Animals. Male Wistar rats 8-10 weeks of age 210-230g of body weight were used. The rats were housed at the temperature of 18 21°C on a 12 hour light-dark cycle. Rats were fed on a stock laboratory diet (59 % carbohydrates; 17 % protein; 3 % fat; 21 % minerals, water, and cellulose) and allowed water ad libitum. Diabetes mellitus was induced in Wistar male rats by twice i. v. injection (tail vein) of alloxan (40 mg/kg body wt) with break of 48 hours. The rats were used in experiments 6 days after the first alloxan injection. Fasting plasma glucose, insulin, total cholesterol (Ch), high-density lipoprotein cholesterol (HDL), low-density lipoprotein cholesterol (LDL), and triglycerides (TG) levels of these animals are presented in Table 1 as Mean+SD in comparison with healthy control. Insulin resistance, hyperinsulinemia, hyperglycemia, hyperlipidemia, and dyslipidemia characterize the rats prepared by this means.

Table 1. Fasting plasma parameters in rats at 6th day since the first alloxan injection. Rats Glucose, Insulin, HDL, LDL, TG, Ch, mol/1 µU/l mmol/l mmol/l mmol/l mmol/l Healthy rats (n=10) 4.3 ~ 2.0 ~ 2.0 ~ 0.8 ~ 0.5 ~ 1.7 0.7 0.5 0.3 0.2 0.1 0.3 Diabetic rats 13.9 4.8 1.0 i 1.0 i 1.3 i 2.5 + (n=10) 2. 3* 0.8* 0.3* 0.3 0.3* 0.5* *) Differs significantly from healthy control (P< 0.01) Treatment. The diabetic animals with average plasma glucose level 12.52.4 mmol/1 after overnight fasting were divided into four groups. Rats were treated at 9.00 am by a single i. p. injection of saline (control, n=5), metformin in the form of hydrochloride (0.05 mg, n=5), succinic acid (0.07 mg, n=5), or the combination of metfbrmin (0.05 mg) with succinic acid (0.07 mg, n=6).

Plasma glucose levels were monitored during 10 hours after the injection. Data are presented in Table 2 as Mean~SD.

Table 2. Plasma glucose in diabetic rats treated by metformin in combination with succinic acid. Hours Glucose, mmol/l Metformin Succinic acid Combination Control 0 (just before the 12.5i2. 4 12.5~2.4 12.5~2.4 12.5~2. 4 injection) 2 6.4~1.5 12.4~3.0 5.6~1.9 13.1~3. 0 4 6.2~0.6 8.5~2.3 8.0~1.4 12.9~2. 1 6 4.5~0.4 13.0~1.0 5.9~0.7 13.6~2. 3 '87. 61. 111. 21. 66. 81. 513. 72. 5 10 9. 4~2.3* 13.9~1.6* 5.6~1.1 14.0~2. 4* Differs significantly from the combination (P< 0.01) In diabetic control, the mean of glucose level was found to be 14.0 mmol/1 to 10tl' hour after the injection. Glucose-lowering effect (A glucose), as used herein, is difference between means of plasma glucose in control and treated rats to 10th hour after the injection calculated by equation: A glucose = 14.0-Mean (mmol/1).

Effect of the combination is 8.4 mmol/1 decreasing in glucose level from the control. The sum of effects of metformin and succinic acid administered individually is 4. 7 mmol/1 (4.6 +0.1 =4.7) decreasing in glucose level of control. Since glucose- lowering effect of the combination is greater than the sum of glucose-lowering effects of metformin and succinic acid administered individually, the combination is synergistically effective in lowering pathologically elevated plasma glucose levels.

Moreover, the combination is characterized by an improved duration of activity in comparison with metfonnin alone.

EXAMPLE 2 N, N-dimethylimidodicarbonimidic diamide succinate and the process for producing same.

To a solution of N, N-dimethylimidodicarbonimidic diamide 1.29 g (0.01 mole) in ethanol was added 1.18 g (0.01 mole) of succinic acid. The reaction mixture was stirred at ambient temperature for 1 hour and concentrated under vacuum to dryness at a temperature of 40-50°C. The desired product was obtained at a yield approximating 60 % as a solid white crystalline material: NMR 1H (D20) : 8 2.24 (s, 4H), 2.87 (s, 6H); Anal. Calculated for C8Hl7N5O4 : C, 38.86 ; H, 6.88. Found: C, 38. 79 ; H 6.81.

EXAMPLE 3 Bis (N, N-dimethylimidodicarbonimidic diamide) succinate and the process for producing same.

To a solution of N, N-dimethylimidodicarbonimidic diamide hydrochloride 1.66 g (0.01 mole) and sodium hydroxide 0.4 g (0.01 mole) in water were added 1.18 g (0. 01 mole) of succinic acid. The reaction mixture was stirred at ambient temperature for 1 hour and concentrated under vacuum to dryness at a temperature of 40-50°C. The desired product was obtained at a yield approximating 80 % as a solid white crystalline material after extraction by ethanol and crystallization: NMR lH (D20) : 8 2.20 (s, 4H), 2.85 (s, 12H); Anal. Calculated for Cl2H2sNI004 : C, 38.29 ; H, 7.44. Found: C, 38.17 ; H 7.40.