The present patent application is an application for patent of addition with respect to the patent application No. 201617041112 dated l-December-2016. Applicants have found that-extended release liquid compositions of metformin in the form of suspensions or reconstituted powder for suspensions comprising cores of metformin coated with a release-controlling agent, wherein the coated cores are dispersed in a suspension base provide the desired uniform extended release profile throughout the shelf-life of the compositions. Furthermore, said extended release liquid compositions are stable and bioequivalent to a reference composition. Patent application No. .201617041112 relates to extended release liquid compositions of metformin. The extended release liquid compositions are in the form of suspensions or reconstituted powder for suspensions. Said extended release liquid compositions comprise cores of metformin coated with a release-controlling polymer, wherein the cores are dispersed in a suspension base. It also relates to processes for the preparation of said extended release liquid compositions. Field of the Invention The present invention relates to stable extended release liquid compositions of metformin. The stable extended release liquid compositions are in the form of suspensions or reconstituted powder for suspensions. Said extended release liquid compositions comprise cores of metformin coated with a release-controlling agent, wherein the coated cores are dispersed in a suspension base. Said extended release liquid compositions provide the desired uniform extended release profile throughout the shelflife of the compositions. Furthermore, said extended release liquid compositions are bioequivalent to a reference composition. Background of the Invention Metformin, an effective anti-diabetic drug known for decades, acts by reducing glucose production by the liver and by decreasing intestinal absorption of glucose. Metformin improves glucose tolerance in patients with Type II diabetes and lowers both basal and post-prandial plasma glucose. U.S. Patent No. 8,197,850 discloses a medicament comprising microcapsules of a biguanide antihyperglycemic agent which comprises a core containing the biguanide antihyperglycemic agent and a prolonged release coating film which does not have a nitrogenous polymer, wherein the mean fraction by mass of the biguanide in the microcapsules is greater than 50%. U.S. Patent No. 7,214,387 discloses sustained-release pharmaceutical compositions of metformin comprising xanthan gum, locust bean gum, and a diluent, wherein therapeutically beneficial blood levels of metformin are maintained over a period of time from about I hour to about 24 hours. U.S. Patent No. 6,676,966 discloses an extended release formulation of metformin hydrochloride and an encasement coat in the form of one or more layers of pH-sensitive polymeric film, wherein said polymeric film is soluble in a pH of above 5.0. Q D.E.LMX 2 1 - I I - 1 Q I 7 L7 : 2 .a 2 Immediate release tablets of metformin marketed under the trade name of Glucophage® and an immediate release oral solution of metformin marketed under the trade name of Riomet® are administered multiple times a day, leading to a high level of non-compliance. Further, extended release tablets of metformin marketed under the trade name of Glucophage XR®, Fortamet®, and Glumetza® reduces the frequency of administration, but due to the large size of the tablets, the problem of patient compliance still remains. This problem is further aggravated in patients who have difficulty in swallowing, such as pediatric and geriatric patients. Furthermore, as metformin is indicated for chronic administration, such high levels of non-compliance may lead to non-adherence to the prescribed therapy, resulting in serious medical problems. There exists a need in the art for extended release liquid compositions of metformin which provide effective control of blood glucose levels over a prolonged period of time, thereby leading to enhanced patient compliance and ease in administration. In view of this, extended release liquid compositions such as suspensions and reconstituted powder for suspensions provide the best alternative over the available dosage forms. However, it remains a great challenge to formulate extended release liquid composition of metformin. The key hurdle remains to avoid the release of metformin from controlled release units into the suspension base during storage, and to begin release only when the dosage form enters the gastrointestinal tract. Because of its high solubility, metformin tends to leach out from the controlled release units into the suspension base during storage, thus obliterating the whole objective of the extended release. Furthermore, the irregular release may lead to sub-therapeutic or toxic effects leading to serious medical conditions. The present invention addresses this problem by providing a simplified technology in which suspension base is used to prevent leaching of the metformin from the controlled release units into the suspension base. The present invention provides consistent in-vitro extended release of metformin which further ensures steady plasma concentrations throughout the shelf life of the composition. Furthermore, said extended release liquid compositions are bioequivalent to a reference composition. Therefore, the present invention is a significant advance over the available dosage forms of metformin and also fulfills the long felt need to improve patient compliance by providing extended release liquid compositions of metformin. Extended release liquid compositions of metformin of the present invention are simple, and easy to manufacture with functional reproducibility. The extended release liquid compositions are provided with a pleasant mouth feel thereby further aiding to patient compliance and ease of administration. Summary of the Invention The present invention relates to stable extended release liquid compositions of metformin. The s t g b | | ^ ^ ^ form of suspensions or reconstituted powder for 3. suspensions. Said extended release liquid compositions comprise cores of metformin coated with a release-controlling agent, wherein the cores are dispersed in a suspension base. Said extended release liquid, compositions provide the desired uniform extended release profile throughout the shelf-life of the composition. Furthermore, said extended release liquid compositions are bioequivalent to a reference composition. It also relates to processes for the preparation of said extended release liquid compositions. The stable extended release liquid compositions of the present invention allow for dosing flexibility based on age and body weight of the patients. Said extended release liquid compositions provide high levels of patient compliance particularly for pediatric and geriatric group of patients. Also, as hypoglycemic therapy is frequently based on combinations of anti-diabetic drugs, the extended release liquid compositions of the present invention can incorporate any additional drug suitable for combination with metformin. Brief Description of the Drawings Figure 1 shows the in-vitro dissolution release on day 0, day 30, and day 66 of the extended release liquid composition prepared according to Example 1 upon storage at room temperature. The figure also shows the in-vitro dissolution release on day 0, day 36, and day 66 of the extended release liquid composition (at room temperature) formed after reconstituting the powder stored for one month at accelerated conditions. Figure 2 shows the in-vitro dissolution release on day 0 and day 30 of the extended release liquid composition prepared according to Example 2 upon storage at room temperature. The figure also shows the in-vitro dissolution release on day 0 and day 32 of the extended release liquid composition (at room temperature) formed after reconstituting the powder stored for three months and six months at accelerated conditions. Figure 3 shows the in-vitro dissolution release on day 0 and day 30 of the extended release liquid composition prepared according to Example 3 upon storage at room temperature. The figure also shows the in-vitro dissolution release on day 0 and day 30 of the extended release liquid composition (at room temperature) formed after reconstituting the powder stored for one month at accelerated conditions. Detailed Description of the Invention A first aspect of the present invention provides a stable extended release liquid composition of metformin characterized by a Tmax in a range from about 3 hours to about 12 hours after a single dose administration under fed conditions. According to one embodiment of the above aspect, the Tmax ranges from about 4 hours to about 10 hours. In a preferred embodiment of the above aspect, the Tmax ranges from about 5 hours to about 9 hours. 4 A second aspect of the present invention provides a stable extended release liquid composition of metformin providing therapeutic effectiveness for at least about 12 hours after a single dose administration under fed conditions. More particularly, the composition provides therapeutic effectiveness for at least about 24 hours after a single dose administration under fed conditions. A third aspect of the present invention provides a stable extended release liquid composition of metformin characterized by a pharmacokinetic profile having one or more features of: a Cmax from about 0.4 ng/mL/mg to about 2.5 ng/mL/mg; an area under the curve (AUC) o—oo from about 2.0 ng.h/mL/mg to about 22.0 ng.h/mL/mg; and an area under the curve (AUC) o—t from about 2.5 ng.h/mL/mg to about 22.0 ng.li/mL/mg after a single dose administration under fed conditions. A fourth aspect of the present invention provides a stable extended release liquid composition of metformin characterized in that the liquid composition is bioequivalent to a marketed extended release tablet of metformin. According to one embodiment of the above aspect, the marketed extended release tablet of metformin is Glucophage XR® tablet, Fortamet® tablet, or Glumetza® tablet. Preferably, the marketed extended release tablet of metformin is Glucophage XR® tablet. A fifth aspect of the present invention provides a stable extended release liquid composition of metformin comprising metformin hydrochloride in a concentration from about 25 mg/mL to about 500 mg/mL of the composition. According to an embodiment of the above aspect, the composition comprises metformin hydrochloride in a concentration from about 50 mg/mL to about 200 mg/mL of the composition. A sixth aspect of the present invention provides a method of lowering blood glucose levels in non-insulin dependent diabetes mellitus patients by administering a stable extended release liquid composition of metformin once daily or twice daily. A seventh aspect of the present invention provides a stable extended release liquid composition of metformin characterized by having a substantially similar in-vitro dissolution release profile when tested in a dissolution media with a pH ranging from about 1 to about 7. According to one embodiment of the above aspects, the composition comprises: (i) cores of metformin coated with a release-controlling agent; and (ii) a suspension base. According to another embodiment of the above aspects, the composition is characterized by having an in-vitro dissolution release profile as determined by USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with a pH 6.8 at 37°C as follows: - less than 30% of metformin released after 0.5 hour; - less than 70% of metformin released after 2 hours; and - more than 85% of metformin released after 12 hours. 5 According to another embodiment of the above aspects, the in-vitro dissolution release profile of the composition remains substantially similar to the initial in-vitro dissolution release profile upon storage for at least seven days. According to another embodiment of the above aspects, the composition is characterized by having an osmolality ratio of at least about 1. According to another embodiment of the above aspects, the suspension base has an osmolality of about 1 osmol/kg or more than about 1 osmol/kg of the suspension base. In a particular embodiment, the suspension base has an osmolality of more than about 2 osmol/kg of the suspension base. In a more particular embodiment, the suspension base has an osmolality of more than about 3 osmol/kg of the suspension base. According to another embodiment of above aspects, the suspension base is responsible for creating a hypertonic environment. According to another embodiment of the above aspects, the suspension base comprises an osmogent. According to another embodiment of the above aspects, the composition is a suspension or a reconstituted powder for suspension. According to another embodiment of the above aspects, the release-controlling agent is selected from the group comprising a pH-dependent release-controlling agent, a pH-independent releasecontrolling agent, or mixtures thereof. According to another embodiment of the above aspects, the core is in the form of a bead, a pellet, a granule, a spheroid, or the like. According to another embodiment of the above aspects, metformin is layered onto an inert particle to form the core. According to another embodiment of the above aspects, the composition has a pH ranging from about 4 to about 10. According to another embodiment of the above aspects, the suspension base has a viscosity ranging from about 500 cps to about 15,000 cps. Preferably, the viscosity of the suspension base ranges from about 1,000 cps to about 10,000 cps. More preferably, the viscosity of the suspension base ranges from about 2,000 cps to about 7,000 cps. The viscosity of the suspension base of the present invention is measured by using a Brookfield Viscometer having a # 2 spindle rotating at 5 rpm at 25°C. According to another embodiment of the above aspects, the average diameter of the cores of metformin coated with a release-controlling agent ranges from about 10 \xm to about 2000 |am, preferably from about 100 \xm to about 1000 |am, and more preferably from about 150 (am to about 500 ^im. According to another embodiment of the above aspects, the suspension base does not comprise asa^r^ted^lu^o§of^i|tfoiTmgi.i r IT l 2J3 6 According to another embodiment of the above aspects, the composition is a taste masked composition. i An eight aspect of the present invention provides a process for the preparation of a stable extended release liquid composition of metformin, wherein the process comprises the steps of: (i) preparing cores comprising metformin and one or more pharmaceutically acceptable excipients; (ii) dissolving/dispersing a release-controlling agent and one or more pharmaceutically acceptable coating additives in a suitable solvent; (iii) applying the coating composition of step (ii) over the cores of step (i); (iv) dissolving/dispersing one or more osmogents and pharmaceutically acceptable excipients in a pharmaceutically acceptable vehicle to form a suspension base; and (v) dispersing the coated cores of step (iii) in the suspension base of step (iv) to obtain the extended release liquid composition. A ninth aspect of the present invention provides a process for the preparation of a stable extended release liquid composition of metformin, wherein the process comprises the steps of: (A) preparing a powder for suspension comprising the steps of: (i) preparing cores comprising metformin and one or more pharmaceutically acceptable excipients; (ii) dissolving/dispersing a release-controlling agent and one or more pharmaceutically acceptable coating additives in a suitable solvent; (iii) applying the coating composition of step (ii) over the cores of step (i); (iv) blending the coated cores of step (iii) with pharmaceutically acceptable excipients to form the powder for suspension; (B) dissolving/dispersing one or more osmogents and pharmaceutically acceptable excipients in a pharmaceutically acceptable vehicle to form a suspension base; and (C) reconstituting the powder for suspension of step (A) with the suspension base of step (B) to obtain the extended release liquid composition. A tenth aspect of the present invention provides a process for the preparation a stable extended release liquid composition of metformin, wherein the process comprises the steps of: (A) preparing a powder for suspension comprising the steps of: (i) preparing cores comprising metformin and one or more pharmaceutically acceptable excipients; (ii) dissolving/dispersing a release-controlling agent and one or more pharmaceutically acceptable coating additives in a suitable solvent; (iii) applying the coating composition of step (ii) over the cores of step (i); (iv) mixing one or more osmogents and one or more pharmaceutical^ acceptable excipients with the coated cores of step (iii) to obtain the powder for suspension; and (B) reconstituting the powder for suspension of step (A) with a pharmaceutical^ acceptable vehicle to obtain the extended release liquid composition. According to another embodiment of the above aspects, the suspension base comprises an immediate release metformin component. According to another embodiment of the above aspect, the extended release liquid composition further comprises one or more anti-diabetic drugs selected from the group comprising acarbose, miglitol, voglibose, repaglinide, nateglinide, glibenclamide, glimepride, glipizide, gliclazide, chloropropamide, tolbutamide, phenformin, aloglitin, sitagliptin, linagliptin, saxagliptin, rosiglitazone, pioglitazone, troglitazone, faraglitazar, englitazone, darglitazone, isaglitazone, zorglitazone, liraglutide, muraglitazar, peliglitazar, tesaglitazar, canagliflozin, dapagliflozin, remogliflozin, sergliflozin, and combinations thereof. The term "extended release," as used herein, refers to a release profile of metformin over an extended period of time, e.g., over a period of 4, 6, 8, 12, 24 hours, or more. The in-vivo release profile can be assessed using pharmacokinetic parameters of (Cmax), (Tmax) and area under the curve (AUC). The term "Cmax," as used herein, refers to the maximum plasma concentration of metformin hydrochloride, calculated as the geometric mean for individuals participating in the bioequivalence study. The term "Tmax," as used herein, refers to the time to the maximum observed plasma metformin hydrochloride concentration, represented as a median for individuals participating in the bioequivalence study. The term "AUCo—oo," as used herein, refers to the area under the plasma concentration-time curve extrapolated to infinity, calculated as the geometric mean for individuals participating in the bioequivalence study. The term "AUCo—t," as used herein, refers to the area under the plasma concentration-time curve till time t, calculated as the geometric mean for individuals participating in the bioavailability study. The term "bioequivalent," as used herein, is a term of art and is defined to mean the term used by the drug approval agencies, such as the US Food and Drug Administration: "the absence of a significant difference in the rate and extent to which the active ingredient or active moiety in pharmaceutical equivalents or pharmaceutical alternatives becomes available at the site of drug action when administered at the same molar dose under similar conditions in an appropriately designed stBSJg.JL ffte raficS of2h^lo£®rfefprmad7me^©alues for Cmaxand AUC for the test and reference 8 product (T/R ratio) is a measure of the bioequivalence between the test and reference product. Values between 80 and 125% for these intervals indicate bioequivalence as recommended by the US FDA. The term "single dose," as used herein means the dose of metformin hydrochloride ranging from about 500 mg to about 2500 mg. In particular, the dose of metformin hydrochloride is 750 mg. The stable extended release liquid composition of metformin provides a pharmacokinetic profile characterized by one or more features of: a Cmax from about 0.4 ng/mL/mg to about 2.5 ng/mL/mg, more particularly a Cmax from about 0.5 ng/mL/mg to about 1.5 ng/mL/mg; an area under the curve (AUC) o—«> from about 2.0 ng.h/mL/mg to about 22.0 ng.h/mL/mg, more particularly an area under the curve (AUC)o—a> from about 5.0 ng.h/mL/mg to about 15.0 ng.h/mL/mg; an area under the curve (AUC) o->t from about 2.5 ng.h/mL/mg to about 22.0 ng.h/mL/mg, more particularly an area under the curve (AUC)o—t from about 5.0 ng.h/mL/mg to about 15.0 ng.h/mL/mg after a single dose administration under fed conditions. Preferably, the stable extended release liquid composition of metformin comprising of the present invention provides a pharmacokinetic profile characterized by one or more features of: a Cmax from about 600 ng/mL to about 800 ng/mL, an area under the curve (AUC) o—«> from about 7500 ng.h/mL to about 9500 ng.h/mL, and an area under the curve (AUC)o->t from about 7000 ng.h/mL to about 9000 ng.h/mL upon administration of a single dose equivalent to 750 mg of metformin hydrochloride under fed conditions. The term "hypertonic environment,"" as used herein, means the suspension base has a higher solute concentration which helps to generate high osmotic pressure such that there is no leaching of metformin from the extended release coated cores into the suspension base. In the present invention, the solutes are osmogents, i.e., pharmaceutical^ acceptable inert water-soluble compounds that contribute towards generating hypertonic environment in the suspension base. The term "osmolality ratio," as used herein, means the ratio of osmolality of the external phase to the osmolality of the internal phase. The external phase herein means the suspension base without multiple coated cores of metformin. The internal phase herein means the coated cores of metformin. The osmolality of the internal phase is represented as the osmolality of the solution which prevents significant leaching of metformin from the coated cores into the solution. The leaching of metformin from the coated cores is determined by the difference in the osmolalities across the coating layer and the absence of any significant leaching from the coated cores directs that the osmolality of the solution has become equal to the osmolality of the coated cores. The osmolality ratio of the extended release liquid compositions of present invention is at least about 1. The term "osmolality," as used herein, means the concentration of an osmogent, i.e., any pharmaceutical^ acceptable inert water-soluble compound present in the suspension base. In the present invention, the osmolality is expressed as number of moles of any water-soluble compound per k©84ji!c|uS' P%^"T|eliqu^l5:l&^ cafLBS a s&i^ension base or a solution. In the present invention, 9 the osmolality may be measured according to known methods, such as using a Vapor pressure Osmometer, a Colloid Osmometer, or a Freezing Point Depression Osmometer such as Osmomat® 030-D or Osmomat® 3000, in particular by a Freezing Point Depression Osmometer. The osmolality of suspension base of the extended release liquid compositions of the present invention remains equivalent upon storage for at least seven days. Particularly, the osmolality of the suspension base measured after one month remains equivalent to the osmolality of the suspension base measured as soon as practicable after preparation of the extended release liquid compositions. More particularly, the osmolality of the suspension base measured after three months remains equivalent to the osmolality of the suspension base measured as soon as practicable after preparation of the extended release liquid compositions. More particularly, the osmolality of the suspension base measured after six months remains substantially similar to the osmolality of the suspension base measured as soon as practicable after preparation of the extended release liquid compositions. The equivalent osmolality of the suspension base ensures that there is no leaching of the metformin from the coated cores into the suspension base. The in-vitro dissolution release profile of the extended release liquid compositions of the present invention upon storage for at least seven days remains substantially similar to the initial invitro dissolution release profile obtained as soon as practicable after preparation of the extended release liquid compositions. Particularly, the in-vitro dissolution release profile of the extended release liquid compositions of the present invention upon storage at room temperature for at least one month remains substantially similar to the initial in-vitro dissolution release profile obtained as soon as practicable after preparation of the extended release liquid compositions. More particularly, the in-vitro dissolution release profile of the extended release liquid compositions of the present invention upon storage at room temperature for at least three months remains substantially similar to the initial in-vitro dissolution release profile obtained as soon as practicable after preparation of the extended release liquid compositions. More particularly, the in-vitro dissolution release profile of the extended release liquid compositions of the present invention upon storage for at least six months remains substantially similar to initial in-vitro dissolution release profile obtained as soon as practicable after preparation of the extended release liquid compositions. The composition of the present invention shows an in-vitro dissolution release profile as determined by USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with a pH 6.8 at 37°C as follows: - less than 30% of metformin released after 0.5 hour; - less than 70% of metformin released after 2 hours; and - more than 85% of metformin released after 12 hours. The above in-vitro dissolution release profile is determined by using a composition comprising metformin hydrochloride in a dose range from about 500 mg to about 2500 mg. In particular, the cO®fesM®. c(^frise|^ftetf@T®i^ydr<^51ol,i^|a a dose of about 750 mg. .*•• 10 The extended release liquid composition of the present invention also provides the consistent in-vivo release which ensures steady and predictable metformin release with minimal inter and intra subject variation throughout the shelf life of the composition. The term "substantial," as used herein refers to any value which lies within the range as defined by a variation of up to ±15 from the average value. The term "stable," as used herein, refers to chemical stability, wherein not more than 5% w/w of total related substances are formed on storage at 40°C and 75% relative humidity (R.H.) or at 25°C and 60% R.H. for a period of at least three months to the extent necessary for the sale and use of the composition. The extended release liquid composition of the present invention may be in the form of a suspension or a reconstituted powder for suspension. The term "suspension base," as used herein, refers to a medium which is used to suspend the coated cores of the metformin. The suspension base comprises a pharmaceutically acceptable vehicle, one or more osmogents, and pharmaceutically acceptable excipients. The powder for suspension having coated cores of metformin of the present invention may be reconstituted with the suspension base having osmogents, pharmaceutically acceptable excipients, and a pharmaceutically acceptable vehicle. Alternatively, osmogents and pharmaceutically acceptable excipients may be mixed with the coated cores of metformin which may then be reconstituted with a pharmaceutically acceptable vehicle. In case of powder for suspension, the suspension base may be pre-formed or formed at the time of reconstitution. The suspension base of the present invention does not include a saturated solution of metformin. The suspension base has an osmolality of about 1 osmol/kg or more than about 1 osmol/kg of the suspension base. The suspension base of the present invention has an osmolality ranging from about 1 osmol/kg to about 20 osmol/kg of the suspension base. The pharmaceutically acceptable vehicle as used herein means an aqueous vehicle. The term "inert particle," as used herein, refers to a particle made from a sugar sphere also known as a non-pareil seed, a microcrystalline cellulose sphere, a dibasic calcium phosphate bead, a mannitol bead, a silica bead, a tartaric acid pellet, a wax based pellet, and the like. The term "about," as used herein, refers to any value which lies within the range defined by a variation of up to ±10% of the value. The term "equivalent" as used herein, refers to any value which lies within the range defined by a variation of up to ±30% of the value. The term "significant leaching," as used herein means more than 20% of the metformin is leached out from the coated cores into the solution, The term "metformin," as used herein, refers to metformin as well as its pharmaceutically | a®<||1||^ chelates, and complexes. The preferred 11 salt of metformin is metformin hydrochloride. The extended release liquid compositions of the present invention comprise metformin in a range of from 1% to about 30% w/w based on total weight of the composition. In particular, the extended release liquid compositions of the present invention comprise metformin in a range of from 5% to about 20% w/w based on total weight of the composition. The suspension base may additionally include an immediate release component of the metformin. However, the suspension base of the present invention does not include any saturated solution of the metformin. The suspension base may include an immediate release component of the metformin, wherein the metformin is present in an amount that does not exceed the amount required to form the saturated solution either initially or during storage. The metformin may be present in the form of a powder, a pellet, a bead, a spheroid, or a granule, or in the form of immediate release coating over the extended release coated cores. Alternatively, the amount of metformin may exceed the amount required to form the saturated solution. However, the saturated solution of metformin is not formed, as the release of metformin into the suspension base is prevented during storage. This is achieved by using a coating layer over cores of metformin, wherein the coating layer comprises a polymer that remain insoluble in the suspension base during storage and which releases the metformin "in an immediate release form once ingested. Alternatively, this can also be done by using an ion-exchange resin complex approach, wherein the complex prevent any release of the metformin into the suspension base during storage, and release the metformin only when exposed to the physiological conditions upon ingestion. The polymer can be a water-soluble polymer in which the release of metformin is prevented by using a high molar concentration of the solutes in the suspension base, wherein the solutes have a higher affinity towards water in comparison to the polymer. Further, the polymer can be having a pH dependent solubility in which the release of metformin is prevented by using a pre-adjusted pH of the suspension base such that the polymer does not get dissolved in the suspension base but get dissolved when exposed to the physiological conditions. Metformin, being a cationic drug can be complexed with any of the known cation exchange resin like indion 244 and indion 264. The immediate release component may help in providing an immediate therapeutic effect which could be subsequently followed by an extended therapeutic effect over a longer duration of time. In the present invention, the metformin may be present in the immediate release form in an amount of the about 0.5% to about 10 % w/w based on total weight of the composition, particularly in an amount of about 1% to about 5% w/w based on total weight of the composition. The extended release liquid compositions of the present invention may further include one or more anti-diabetic drugs such as acarbose, miglitol, voglibose, repaglinide, nateglinide, glibenclamide, glimepride, glipizide, gliclazide, chlorpropamide, tolbutamide, phenformin, aloglitin, sitagliptin, linagliptin, saxagliptin, rosiglitazone, pioglitazone, troglitazone, faraglitazar, englitazone, darglitazone, isaglitazone, zorglitazone, liraglutide, muraglitazar, peliglitazar, tesaglitazar, cffi^Efyzfii, <&ff|glilLo&in, Z^idgMflo^j^ setgKIlozin, or any other known anti-diabetic drug The 12 extended release liquid composition of the present invention are particularly suitable for anti-diabetic drugs which are incompatible with metformin. Additionally, extended release liquid composition of the present invention can incorporate anti-diabetic drugs having a low dose e.g., glibenclamide without altering the homogeneity of the composition. These anti-diabetic drugs may be present in the form of a powder, a pellet, a bead, a spheroid, or a granule providing immediate release or in the form controlled release coated cores providing the extended release. The extended release liquid compositions of the present invention are homogeneous which means the compositions provide the content uniformity and deliver the desired dose of the metformin in every use without any risk of overdosing or underdosing. The average diameter of the cores of metformin coated with a release-controlling agent ranges from about 10 jam to about 2000 |im, particularly from about 100 ^im to about 1000 |im, and more particularly from about 150 [im to about 500 \im. Finer sizes of the coated cores help in avoiding grittiness in the mouth and thereby are easy and are more acceptable. The cores of metformin coated with a release-controlling agent comprise metformin in an amount of about 10% to about 70% w/w based on the total weight of the coated cores, particularly from about 30% to about 50% w/w based on the total weight of the coated cores. The cores may comprise one or more pharmaceutically acceptable excipients such as binders. The release-controlling agents used to form the extended release coating are selected from a group comprising a pH-dependent release-controlling agent, a pH-independent release-controlling agent, or mixtures thereof. For an extended release coating comprising a pH-dependent releasecontrolling agent, the pH of the suspension base is pre-adjusted such that the coating remains insoluble in the suspension base during the storage. The extended release coating comprising a pH-dependent release-controlling agent may alternatively be coated with a coating layer comprising a polymer such that said coating layer remain insoluble in the suspension base during storage. The core may comprise release-controlling agents in the form of a matrix with metformin, which can be coated with a coating layer that remain insoluble in the suspension base during storage. Suitable examples of pH-dependent release-controlling agents are selected from the group comprising acrylic copolymers such as methacrylic acid and methyl methacrylate copolymers, e.g., Eudragit® L 100 and Eudragit® S 100, methacrylic acid and ethyl acrylate copolymers, e.g., Eudragit® L 100-55 and Eudragit® L 30 D-55, dimethylaminoethyl methacrylate and butyl methacrylate and methyl methacrylate copolymers e.g., Eudragit® E 100, Eudragit® E PO, methyl acrylate and methacrylic acid and octyl acrylate copolymers, styrene and acrylic acid copolymers, butyl acrylate and styrene and acrylic acid copolymers, and ethylacrylate-methacrylic acid copolymer; cellulose acetate phthalate; cellulose acetate succinates; hydroxyalkyl cellulose phthalates such as hydroxypropylmethyl cellulose phthalate; hydroxyalkyl cellulose acetate succinates such as hsd@^gf^ylffl€#iyl Se|lulcg©aS^te SiScinSt© vinyl acetate phthalates; vinyl acetate succinate; 13 CD Q. CN E o CN o CO cellulose acetate trimelliate; polyvinyl derivatives such as polyvinyl acetate phthalate, polyvinyl alcohol phthalate, polyvinyl butylate phthalate, and polyvinyl acetoacetal phthalate; zein; shellac; and mixtures thereof. Suitable examples of pH-independent release-controlling agents are selected from the group comprising cellulosic polymers such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethylmethyl cellulose, hydroxypropylmethyl cellulose, and carboxy methylcellulose; acrylic copolymers such as methacrylic acid copolymers, e.g., Eudragit® RS, Eudragit® RL, Eudragit® NE 30 D; cellulose acetate; polyethylene derivatives e.g., polyethylene glycol and polyethylene oxide; polyvinyl alcohol; polyvinyl acetate; gums e.g., guar gum, locust bean gum, tragacanth, carrageenan, alginic acid, gum acacia, gum arabic, gellan gum, and xanthan gum; triglycerides; waxes, e.g., Compritol®, Lubritab®, and Gelucires®; lipids; fatty acids or their salts/derivatives; a mixture of polyvinyl acetate and polyvinyl pyrrolidone, e.g., Kollidon® SR; and mixtures thereof. In particular, the pH-independent release-controlling agent used in the present invention is ethyl cellulose. The term "osmogent," as used herein, refers to all pharmaceutically acceptable inert watersoluble compounds that can imbibe or dissolve in water and/or aqueous biological fluids. Suitable examples of osmogents or pharmaceutically acceptable inert water-soluble compounds are selected from the group comprising carbohydrates such as xylitol, mannitol, sorbitol, arabinose, ribose, xylose, glucose, fructose, mannose, galactose, sucrose, maltose, lactose, dextrose and raffinose; water-soluble salts of inorganic acids such as magnesium chloride, magnesium sulfate, potassium sulfate, lithium chloride, sodium chloride, potassium chloride, lithium hydrogen phosphate, sodium hydrogen phosphate, potassium hydrogen phosphate, lithium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, and sodium phosphate tribasic; water-soluble salts of organic acids such as sodium acetate, potassium acetate, magnesium succinate, sodium benzoate, sodium citrate, and sodium ascorbate; water-soluble amino acids such as glycine, leucine, alanine, o) methionine; urea or its derivatives; propylene glycol; glycerin; polyethylene oxide, xanthan gum, 2 hydroxypropylmethyl cellulose; and mixtures thereof. Particularly, the osmogents used in the present invention are xylitol, mannitol, glucose, lactose, sucrose, and sodium chloride. The term "pharmaceutically acceptable excipients," as used herein, refers to excipients that are routinely used in pharmaceutical compositions. The pharmaceutically acceptable excipients may comprise glidants, sweeteners, suspending agents, anti-caking agents, wetting agents, preservatives, buffering agents, flavoring agents, anti-oxidants, chelating agents, and combinations thereof. jN Suitable glidants are selected from the group comprising silica, calcium silicate, magnesium CO jo silicate, colloidal silicon dioxide, cornstarch, talc, stearic acid, magnesium stearate, calcium stearate, o sodium stearyl fumarate, hydrogenated vegetable, and mixtures thereof. ? 1PQ. Q.ELKI 2 3 - I I - 2.Q17 1? : 2 S o z 14 CN Suitable sweeteners are selected from the group comprising saccharine or its salts such as sodium, potassium, or calcium; cyclamate or its salt; aspartame; alitame; acesulfame or its salt; stevioside; glycyrrhizin or its derivatives; sucralose; and mixtures thereof Suitable suspending agents are selected from the group comprising cellulose derivatives such as co-processed spray dried forms of microcrystalline cellulose and carboxymethyl cellulose sodium, hydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropylmethyl cellulose, methylcellulose, carboxymethyl cellulose and its salts/derivatives, and microcrystalline cellulose; carbomers; gums such as locust bean gum, xanthan gum, tragacanth gum, arabinogalactan gum, agar gum, gellan gum, guar gum, apricot gum, karaya gum, sterculia gum, acacia gum, gum arabic, and carrageenan; pectin; dextran; gelatin; polyethylene glycols; polyvinyl compounds such as polyvinyl acetate, polyvinyl alcohol, and polyvinyl pyrrolidone; sugar alcohols such as xylitol and mannitol; colloidal silica; and mixtures, thereof. The co-processed spray dried forms of microcrystalline cellulose and carboxymethyl cellulose sodium have been marketed under the trade names Avicel® RC-501, Avicel® RC-581, Avicel® RC-591, and Avicel®CL-611. The suspending agent is present in an amount of not more than about 20% w/w, based on the total weight of the suspension base. Suitable anti-caking agents are selected from the group comprising colloidal silicon dioxide, tribasic calcium phosphate, powdered cellulose, magnesium trisilicate, starch, and mixtures thereof. Suitable wetting agents are selected from the group comprising anionic, cationic, nonionic, or zwitterionic surfactants, or combinations thereof Suitable examples of wetting agents are sodium lauryl sulphate; cetrimide; polyethylene glycols; polyoxyethylene-polyoxypropylene block copolymers such as poloxamers; polyglycerin fatty acid esters such as decagiyceryl monolaurate and decagiyceryl monomyristate; sorbitan fatty acid esters such as sorbitan monostearate; polyoxyethylene sorbitan fatty acid ester such as polyoxyethylene sorbitan monooleate; polyethylene glycol fatty acid ester such as polyoxyethylene monostearate; polyoxyethylene alkyl ether such as polyoxyethylene lauryl ether; polyoxyethylene castor oil; and mixtures thereof. CD o> Suitable preservatives are selected from the group comprising parabens such as methyl paraben i. "fl H -fW ?iTi 18 The powder for suspension prepared as per Example 1 (till step 6) was kept for one month at accelerated conditions i.e., 40°C/75% R.H. After one month, the powder for suspension was reconstituted with required amount of purified water and this extended release suspensions composition was kept for 66 days at room temperature. The in-vitro dissolution was determined at 0, 30, and 66 days using USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 2. Table 2: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer, pH 6.8,1000 mL, and 100 Number of Days After Reconstitution Time (hours) 0.5 1 2 3 4 5 6 8 10 12 rpm) 0 36 66 Percentage of Metformin Release 28.8 32.4 57.6 74.8 83.1 89.2 91.3 95.2 96.6 98.6 26.2 33.0 50.5 70.3 80.7 85.9 91.2 - - 101.3 27.0 32.0 53.0 67.0 83.0 87.0 92.0 95.0 97.0 100.0 From the above data, it is clear that the extended release powder prepared according to Example 1 stored at accelerated conditions for one month, upon reconstitution and storage for 66 days at room temperature provides substantially similar in-vitro metformin release for 66 days. The results are shown in Figure 1. Example 2 ff"n. «••• LA • ££ 1 B « L_ ft Ingredients Quantity (mg/mL) Core Metformin hydrochloride Macrocrystalline cellulose spheres Hydroxypropylmethyl cellulose Purified water 80.00 56.00 4.00 q.s. Extended Release Coating Ethyl cellulose Dibutyl sebacate Acetone Purified water Total Weight of Extended Release Beads Metformin hydrochloride Xylitol Microcrystalline cellulose - sodium carboxymethyl cellulose (Avicel® CL-611) Xanthan gum Strawberry flavor Sucralose 50.40 5.60 q.s. q.s. 196.00 rag 20.00 450.00 20.00 1.50 2.00 0.50 Vehicle ^ Purified water ^ ~ si ,if—a*—= 3 a *s j - i . f.—s * s—^ _— ." at a. 1 q.s. to 1 mL 19 Procedure: 1. Metformin hydrochloride and hydroxypropylmethyl cellulose were dissolved in purified water. 2. Microcrystalline cellulose spheres were coated with the solution of step 1. 3. Ethyl cellulose and dibutyl sebacate were dispersed in a mixture of acetone and purified water. 4. The beads of step 2 were coated with the coating dispersion of step 3. 5. Metformin hydrochloride, xylitol, microcrystalline cellulose - sodium carboxymethyl cellulose, xanthan gum, strawberry flavor, and sucralose were mixed. 6. The coated beads of step 4 were mixed with the mixture of step 5 to form a powder for suspension. 7. The powder for suspension of step 6 is reconstituted with purified water when required to form the extended release liquid composition. In-Vitro Studies The extended release liquid composition prepared as per Example 2 was stored at room temperature for 30 days. The in-vitro dissolution was determined at 0 and 30 days using a USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 3. Table 3: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: / Phosphate Buffer, pH 6.8,1000 mL, and 100 mm) Number of Days Time (hours) 0.5 1 2 4 5 6 8 10 12 0 30 Percentage of Metformin Release 22 31 58 83 86 91 95 97 99 24 34 61 89 93 96 101 102 103 From the above data, it is clear that the extended release liquid composition prepared according to Example 2 provides a substantially similar in-vitro metformin release profile for 30 days. The powder for suspension prepared as per Example 2 (until step 6) was kept for three months at accelerated conditions 40°C/75% R.H. After three months, the powder for suspension was reconstituted with required amount of purified water and this extended release suspensions composition was kept for 30 days at room temperature. The in-vitro dissolution was determined at 0 and 32 days using USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 4. •TT-- PQ « e S JL_ Is? JL *** 1 1 JL »* 20 Table 4: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer, DH 6.8,1000 mL, and 100 Number of Days After Reconstitution Time (hours) 0.5 1 2 4 5 6 8 10 12 rpm) 0 32 Percentage of Metformin Release 22 33 60 85 89 92 96 98 101 26 . 37 66 90 94 97 101 103 103 The powder for suspension prepared as per Example 2 (till step 6) was kept for six months at accelerated conditions i.e., 40°C/75% R.H. After six months, the powder for suspension was reconstituted with required amount of purified water and this extended release suspensions composition was kept for 32 days at room temperature. The in-vitro dissolution was determined at 0 and 32 days using USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 5. Table 5: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer, pH 6.8, 1000 mL, and 100 rpm) Number of Days After Reconstitution Time (hours) 0.5 1 2 4 5 6 8 10 12 0 32 Percentage of Metformin Release 24 35 • 63 87 91 94 97 99 99 25 34 60 86 91 94 98 101 101 From the above data, it is clear that the extended release powder prepared according to Example 2 and stored at accelerated conditions for three or six months, upon reconstitution and storage for 30 days at room temperature, provides a substantially similar in-vitro metformin release for 30 days. The results are presented in Figure 2. Example 3 .a™ a; A- a -ar Ingredients Core Metformin hydrochloride Macrocrystalline cellulose spheres Hydroxypropylmethyl cellulose Purified water Extended Release Coating E#iy4 celkitese^-^ -*=- * — ** ~ ^— Quantity (mg/mL) 80.00 56.00 4.00 q.s. 61.48 21 Dibutyl sebacate Acetone Purified water Total Weight of Extended Release Beads Metformin hydrochloride Xylitol Microcrystalline cellulose - sodium carboxymethyl cellulose (Avicel® CL-611) Xanthan gum Strawberry flavor Sucralose Colloidal silicon dioxide Vehicle Purified water 1.52 q.s. q.s. 203.00 mg 20.00 450.00 20.00 1.50 2.00 0.50 3.50 q.s. to 1 mL Procedure: 1. Metformin hydrochloride and hydroxypropylmethyl cellulose were dissolved in purified water! 2. Microcrystalline cellulose spheres were coated with the solution of step 1. 3. Ethyl cellulose and dibutyl sebacate were dispersed in a mixture of acetone and purified water. 4. The beads of step 2 were coated with the coating dispersion of step 3. 5. Metformin hydrochloride, xylitol, microcrystalline cellulose - sodium carboxymethyl cellulose, xanthan gum, strawberry flavor, sucralose, and colloidal silicon dioxide were mixed. 6. The coated beads of step 4 were mixed with the mixture of step 5 to form a powder for suspension. 7. The powder for suspension of step 6 is reconstituted with required amount of purified water when required to form the extended release liquid composition. In-Vitro Studies The extended release liquid composition prepared as per Example 3 was stored at room temperature for 30 days. The in-yitro dissolution was determined at 0 and 30 days using a USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 6. Table 6: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer, pH 6.8,1000 mL, and 100 rpm) Number of Days Time (hours) 0.5 1 2 3 4 5 6 8 10 12 0 30 Percentage of Metformin Release 20 27 59 77 84 88 92 95 97 98 22 28 64 80 89 93 95 99 101 103 u e «_ r% x.. 4., & •B Iff '7 A. 22 From the above in-vitro release data, it is evident that the extended release liquid composition prepared according to Example 3 provides the substantially similar in-vitro release for 30 days. The powder for suspension prepared as per Example 3 (till step 6) was kept for one month at accelerated conditions i.e., 40°C/75% R.H. After one month, the powder for suspension was reconstituted with purified water and kept for 30 days at room temperature. The in-vitro dissolution profile was determined at 0 and 30 days using a USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 7. Table 7: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer, PH 6.8,1000 mL, and 100 rpm) Number of Days After Reconstitution Time (hours) 0.5 1 2 3 4 5 6 8 10 12 0 30 Percentage of Metformin Release 20 26 57 74 82 86 90 92 94 96 19 26 57 74 80 85 88 91 93 94 From the above data, it is clear that the extended release powder prepared according to Example 3 stored at accelerated condition for one month, upon reconstitution and storage for 30 days at room temperature provides substantially similar in-vitro metformin release for 30 days. The results are presented in Figure 3. Osmolality Measurement of the Extended Release Suspension The metformin extended release powder prepared according to the Example 3 (till step 6) was reconstituted with purified water. This suspension was shaken manually for at least 20 minutes. This suspension was then filtered and diluted with purified water and the osmolality was measured using Osmomat® 030-D. The osmolality of the suspension base was found to be 4.112 osmol/kg of the suspension base on day 0. The osmolality of the suspension base was found to.be 4.328 osmol/kg of the suspension base on day 7. It is evident from the above data that the osmolality of the suspension base of the extended release liquid composition as per Example 3 remains equivalent for seven days. Osmolality Measurement of the External Phase The metformin hydrochloride, xylitol, microcrystalline cellulose - sodium carboxymethyl cellulose, xanthan gum, strawberry flavor, sucralose, and colloidal silicon dioxide were mixed as per I P O DELHI' 2 S - I I - 2 Q 1 7 17 : 2Q23 step 6 of Example 3. This powder was reconstituted with purified water. This suspension was then filtered and diluted with purified water and the osmolality was measured using Osmomat® 030-D. The osmolality of the suspension base i.e., external phase was found to be 4.204 osmol/kg of the suspension base. Osmolality Measurement of the Internal Phase Various solutions having various concentrations of osmogent (sodium chloride) were prepared as per Examples 3A-3F. The osmolalities of these solutions were measured using Osmomat® 030-D. Ingredient Sodium Chloride (mg) Purified water Osmolality (osmol/kg) Example 3A 30.00 q.s. to 1 mL 0.910 Example 3B 60.00 q.s. to 1 mL 1.787 Example 3C 120.00 q.s. to 1 mL 3.574* Example 3D 180.00 q.s. to 7.5 mL 5.361* Example 3E 240.00 q.s. to 1 mL 7.148* Example 3F 300.00 q.s. to 1 mL 8.935* * Extrapolated using values of dilute solutions The coated beads of step 4 of Example 3 were dispersed in different solutions as per Examples 3 A-3F. These solutions were kept for seven days at room temperature. After seven days, each solution was analyzed by HPLC for metformin content. The results are represented in Table 8. Table 8: Effect of Osmolality on Metformin Leaching Example 3A 3B 3C 3D 3E 3F Osmolality (osmol/kg) of the solution 0.910 1.787 3.574* 5.361* 7.148* 8.935* Metformin Content (%) 67.3 30.3 2.9 1.8 1.7 1.0 * Extrapolated using values of dilute solutions From the above data, it is evident that the leaching of metformin from the coated beads into the solution was decreasing as the osmolality of the solution was increasing from Examples 3A-3F. The leaching is found to be significantly reduced from Example 3C onwards. The osmolality of the formulation prepared according to Example 3C is considered to be the osmolality of the internal phase. Osmolality Ratio 1.176 Dose Uniformity Data The extended release suspension equivalent to 100 mL was prepared according to formula given in Example 3. This suspension was shaken manually for at least 20 minutes and then ten 7.5 mL samples were taken with a graduated syringe. The metformin content of each sample is determined by HPLC method [Inertsil ODS column (250 x 4.6 mm, 5 |am); mobile phase-buffer (pH 3.5):acetonitrile (95:5 v/v); flow rate of 1.5 mL/min; UV detection at 233 nm] The results are shown in Table 9. Table 9: Metformin Content (%) For Each 7.5 mL of Suspension Sample Number 1 2 3 4 5 6 7 8 9 10 Mean value Metformin content for 7.5 mL of suspension (%) 98.6 97.9 96.6 97.2 99.7 96.4 95.9 97.3 98.8 96.9 97.5 Form the above data, it is evident that the extended release liquid composition prepared according to Example 3 is homogeneous. Assay Data The assay for the extended release liquid composition prepared as per Example 3 was determined at 0 day and after storage at room temperature for 30 days. The powder for suspension prepared as per Example 3 (till step 7) was kept for one month at accelerated conditions i.e., 40°C/75% R.H. After one month, the powder for suspension was reconstituted with purified water and then the assay was determined at day 0 and after storage at room temperature for 30 days. The assay of metformin was determined by HPLC [Inertsil ODS column (250 x 4.6 mm, 5 (im); mobile phasebuffer (pH 3.5):acetonitrile (95:5 v/v); flow rate of 1.5 mL/min; UV detection at 233 nm]. The results are shown in Table 10. Table 10: Assay for Metformin Condition Initial 1 month (40°C/75% R.H.) % Assay (After reconstitution) Oday 97.0 97.4 30 days 99.5 98.9 It is evident from the above data that the extended release liquid composition prepared as per Example 3 is stable. The in-vitro dissolution of the formulation prepared according to Example 3 (7.5 mL of extended release suspension equivalent to 750 mg of metformin hydrochloride) was compared with commercially available Glucophage® XR Tablets 750 mg. The in-vitro dissolution was determined by using a USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 11. D E L H I 2H- \Ar 2.Q17 1 7 : 2Q 25 Table 11: Comparative Dissolution Release Data of Example 3 and Glucophage® XR Tablets Tiirie (hours) 0.5 1 2 3 4 5 6 8 10 12 Percentage of Metformin Release from Example 3 18 22 45 66 76 83 86 90 92 94 Percentage of Metformin Release from Glucophage® XR Tablets 21 33 49 61 70 77 83 90 94 96 Example 4 Ingredients Quantity (mg/mL) Core Metformin hydrochloride Macrocrystalline cellulose spheres Hydroxypropyimethyl cellulose Purified water 80.00 56.00 4.00 q.s. Extended Release Coating Ethyl cellulose Dibutyl sebacate Acetone Purified water Total Weight of Extended Release Beads 68.31 1.69 q.s. q.s. 210.00 mg Suspension Base Metformin hydrochloride Xylitol Microcrystalline cellulose - sodium carboxymethyl cellulose (Avicel® CL-611) Xanthan gum Methyl paraben Propyl paraben Strawberry flavor Sucralose Colloidal silicon dioxide Purified water 20.00 450.00 20.00 1.50 1.80 0.20 2.00 0.50 3.50 472.00 mg Procedure: 1. Metformin hydrochloride and hydroxypropyimethyl cellulose were dissolved in purified water. 2. Microcrystalline cellulose.spheres were coated with the solution of step 1. 3. Ethyl cellulose and dibutyl sebacate were dispersed in a mixture of acetone and purified water. 4. The beads of step 2 were coated with the coating dispersion of step 3 and dried to form a powder for suspension. 5. Purified water was heated to dissolve methyl paraben and propyl paraben. D E L H I . 2 9 - i r - 2 0 1 7 17 I-2Q . 26 6. Metformin hydrochloride, xylitol, microcrystalline cellulose - sodium carboxymethyl cellulose, xanthan gum, strawberry flavor, sucralose, and colloidal silicon dioxide were mixed in the solution of step 5 to form a suspension base. 7. The powder for suspension of step 4 was prefilled in the second chamber of a dual-chamber pack. 8. The suspension base of step 7 was prefilled in a container of a first chamber of a dual-chamber pack. 9. The two chambers were assembled and the pack was activated to form the extended release liquid composition when required. In-Vitro Studies The extended release liquid composition prepared as per Example 4 (for a dose equivalent to 750 mg of metformin hydrochloride) was stored at room temperature for 120 days. The in-vitro dissolution was determined at 0, 45, 90, and 120 days using USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 12. Table 12: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer Number of Days Time (hours) 0.5 1 2 3 4 5 6 8 10 12 , D H 6.8,1000 m L , and 100 rpm) 0 45 90 Percentage of Metformin Release 20 27 55 74 83 85 87 91 93 94 21 25 52 72. 81 86 90 94 96 97 20 27 55 74 83 85 87 91 93 94 120 21 25 52 72 81 86 90 94 96 97 From the above in-vitro release data, it is evident that the extended release liquid composition prepared according to Example 4 provides the substantially similar in-vitro metformin release for 120 days. The dual-chamber pack was kept for 1 month at accelerated conditions Le., 40°C/75% R.H. After 1 month, the pack was activated to form an extended release liquid composition which was kept for 120 days at room temperature. The in-vitro dissolution was determined at 0, 45, 90, and 120 days using USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 13. U JL.JL -:? r* T "W *> f27 Table 13: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer, DH 6.8,1000 mL, and 100 rpm) Number of Days Time (hours) 0.5 1 2 3 4 10 0 45 90 120 Percentage of Metformin Release 21 27 56 74 83 96 21 25 55 74 81 96 21 26 52 76 82 97 20 26 54 72 ' 81 94 The dual-chamber pack was kept for 3 months at accelerated conditions i.e., 40°C/75% R.H. After 3 months, the pack was activated to form an extended release liquid composition which was kept for 45 days at room temperature. The in-vitro dissolution was determined at 0 and 45 days using USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with pH 6.8 at 37°C. The results of the release studies are represented in Table 14. Table 14: Percentage (%) of the In-Vitro Metformin Release in USP Type II Apparatus (Media: Phosphate Buffer, PH 6.8, 1000 mL, and 100 rpm) Number of Days Time (hours) 0.5 1 2 3 4 10 0 45 Percentage of Metformin Release 21 26 55 75 80 95 21 25 53 72 80 92 From the above data, it is clear that the powder for suspension and suspension base stored in the dual-chamber pack of the instant invention at accelerated conditions for 1 month and 3 months, upon activation of the pack forms extended release liquid compositions which when stored for 120 days and 45 days respectively at room temperature provides substantially similar in-vitro metformin release. Stability Data The related substances for the extended release liquid composition prepared as per Example 4 were determined at 0 day and after storage at room temperature for 45 and 120 days. The powder for suspension and suspension base was stored in the dual-chamber pack for one month and for three months at 40°C/75% R.H. After one month or three months, the pack was activated to form an extended release liquid composition and then related substances were determined at 0 day and after storage at room temperature for 45 days and 120 days. The assay of metformin was determined by HPLC method. The results are shown in Table 15. > •m -ST > fTt 28 Table 15: Stability Data for Metformin Related Substances (%w/w) Cyanoguainidine Highest unknown impurity Total impurities Initial Oday BLQ 0.05 0.05 45 days 0.001 0.05 0.05 120 days 0.00072 0.04 0.04 1 month (40°C/75% R.H) Oday 0.001 0.05 0.05 45 days 0.001 0.04 0.04 120 days 0.001 0.04 0.04 3 month (40°C/75% R.H) Oday 0.001 0.05 0.09 45 days 0.001 0.04 0.04 *BLQ: Below limit of Quantification It is evident from the above data that the extended release liquid composition prepared as per Example 4 remains stable even after storing at accelerated conditions for 3 months. Osmolality of the suspension base: 3.960 osmol/kg of the suspension base as measured by using Osmomat® 030-D. Viscosity of the suspension base: 2880 cps as measured by using Brookfield Viscometer using a # 2 spindle rotating at 5 rpm at 25°C. Example 5: Pharmacokinetic Study of Metformin Hydrochloride Extended Release Powder for Oral Suspension The extended release liquid composition of Example 3 having a concentration of about 100 mg/ mL was dosed at an amount equivalent to 750 mg metformin hydrochloride and the single-dose pharmacokinetics was determined. This suspension was compared with a commercially available extended release composition of metformin hydrochloride (Glucophage® XR Tablet, 750 mg of Bristol Myers Squibb Company) in healthy adult human subjects under fed conditions. Fourteen healthy adult subjects were enrolled in this open-label, two-treatment, two-period, two-sequence, single dose, crossover bioavailability study and randomly assigned to receive the 750 mg metformin hydrochloride extended release suspension of the present invention (Example 3) or the reference Glucophage® XR Tablet. Blood samples were collected at appropriate time intervals over a period of 48 hours and plasma concentrations of metformin were determined and pharmacokinetic parameters were calculated. Out of fourteen, thirteen subjects completed the study. Pharmacokinetic Analysis Values for metformin pharmacokinetic parameters, including observed Cmax, Tmax, AUCo^t, and AUCo—oo, were calculated using standard noncompartmental method. The pharmacokinetic results obtained after single-dosing in the above study are summarized in Table 16. D E L H I 2 S - 1 1 - 2 . Q 1 7 I 7 : - 2Q 29 Table 16: Comparative Pharmacokinetic Data Pharmacokinetic Parameters AUCo-t(ng.h/mL) AUC0-oo(ng.h/mL) Cmax (ng/mL) Tmax* (h) Geometric mean of Test (T) Metformin Hydrochloride for Extended Release Liquid Composition 100 mg/mL (750 mg) (n=13) 7786.18 8051.95 752.83 6.5 Geometric mean of Reference (R) Glucophage® XR Metformin Hydrochloride Extended- Release Tablets 750 mg (n=13) 8840.66 9125.13 752.97 7.5 Log Transformed T/R ratio of Least Square Means (%) 87.53 87.69 99.99 - 90% Confidence Interval 80.74-94.89 80.72-95.26 92.07-108:59 - Intrasubject CV% 11.47 11.76 11.72 - Cmax: Maximum observed plasma drug concentration AUCo—t: Area under concentration-time curve from zero to time t AUCo—oo: Area under concentration-time curve from zero to infinity Tmax: Time to maximum observed plasma drug concentration, represented as a median From the above table, it is clear that the 90% confidence intervals for the ratios of the log transformed mean values Cmax and AUCo—«> for the test and reference product (T/R ratio) were within the bioequivalence acceptance criteria. Also, it is clear from the above data, the composition of the present invention shows a Cmax of 1.00 hg/mL/mg, AUCo—tof 10.38 ng.h/mL/mg, and AUCo—«>of 10.74 ng.h/mL/mg after a single dose administration under fed conditions. WE CLAIM: 1. A stable extended release liquid composition of metformin characterized by a Tmax in a range from about 3 hours to about 12 hours after a single dose administration under fed conditions. 2. A stable extended release liquid composition of metformin providing therapeutic effectiveness for at least about 12 hours after a single dose administration under fed conditions. 3. A stable extended release liquid composition of metformin characterized by a pharmacokinetic profile having one or more features of: a Cmax from about 0.4 ng/mL/mg to about 2.5 ng/mL/mg; an area under the curve (AUC) o—«> from about 2.0 ng.h/mL/mg to about 22.0 ng.h/mL/mg; and an area under the curve (AUC)o—t from about 2.5 ng.h/mL/mg to about 22.0 ng.h/mL/mg after a single dose administration under fed conditions. 4. A stable extended release liquid composition of metformin comprising metformin hydrochloride in a concentration from about 25 mg/mL to about 500 mg/mL of the composition characterized in that the liquid composition is bioequivalent to a marketed extended release tablet of metformin. 5. A stable extended release liquid composition of metformin comprising cores of metformin coated with a release-controlling agent, wherein the composition is characterized by having an in-vitro dissolution release profile as determined by USP type II apparatus at 100 rpm, in 1000 mL of phosphate buffer with a pH 6.8 at 37°C as follows: - less than 30% of metformin released after 0.5 hour; - less than 70% of metformin released after 2 hours; and - more than 85% of metformin released after 12 hours. 6. The stable extended release liquid composition according to claim 5, wherein the in-vitro dissolution release profile of the composition remains substantially similar to the initial in-vitro dissolution release profile upon storage for at least seven days. 7. The stable extended release liquid composition according to any of the preceding claims, wherein the composition is characterized by having an osmolality ratio of at least about 1.