the subject invention will now be described in greater detail for preferred embodiments of the invention , it being understood that these embodiments are intended only as illustrative examples and the invention is not to be limited thereto . as will be illustrated through exemplary embodiments 1 - 16 , gabapentin may be prepared from the hydrochloride salt of gabapentin ( gabapentin hydrochloride ) and that in purified form gabapentin may have a ph in the range of 6 . 8 - 7 . 3 , and preferably in the range of 7 . 0 - 7 . 2 . the gabapentin formulation may also contain more than 20 ppm of chloride ion in the composition as measured by the amount of chloride ion in the composition . exemplary embodiments 17 - 19 illustrate formulations of gabapentin containing varying amounts of chloride ion , some of which are greater than 20 ppm and some less , and all of which initially contain less than 0 . 5 % of lactam and after one year of storage at 25 ° c . and 60 % humidity , the conversion of gabapentin to its corresponding lactam is measured not to exceed 0 . 2 % by weight of gabapentin . commonly known adjuvants ( also referred to as excipients ) which can be utilized in a gabapentin formulation of the present invention may include for example , modified maize starch , sodium croscarmelose , titanium dioxide , and ( silica gel such as aerosil 200 . hydroxypropylmethylcellulose , polyvinylpyrrolidone , crospovidon , poloxamer 407 , poloxamer 188 , sodium starch glycolate , copolyvidone , maize starch , cyclodexterin , lactose , talc , co - polymers of dimethylamino - methacrylic acid and neutral methacrylic acid ester may also be used . the list of adjuvants is not an exhaustive list and it would be within the scope of the claimed invention to use any known adjuvant that would behave similar to those enumerated herein . certain specific representative embodiments of the invention are described in detail below , the materials , apparatus and process steps being understood as examples that are intended for illustrative purposes only . consequently , it will be noted that the invention is not intended to be limited to the methods , materials , conditions , precess parameters , apparatus and the like specifically recited herein . in the examples below chloride ion concentration is measured by any commonly known method , such as for example , by titration with agno 3 , ph electrode or chromatography . gabapentin hydrochloride 18 . 2 g isopropanol for dissolution 160 ml active carbon sx1 1 . 1 g ethylacetate 268 ml tributylamine 19 . 5 g methanol for washing 23 ml gabapentin hydrochloride was dissolved in 130 ml of dry isopropanol at 25 ° c . by mixing . next , 1 . 1 grams of active carbon was added and the suspension was heated to 40 ° c . and maintained at this temperature for 2 hours . the suspension was then filtered at 40 ° c . and the filter cake was washed twice with additional 15 ml of isopropanol each time . the washings were added to the already separated solution of gabapentin hydrochloride in isopropanol . the solution was concentrated to dryness in vacuum ( approximately 10 mm hg ) to a constant weight . the temperature of the heating bath was maintained ( maximally ) at 35 ° c . during this operation . thereafter , 245 ml of ethylacetate was added to the dry residue of gabapentin hydrochloride and the solution was mixed . after half an hour of mixing at 25 ° c ., an amount of 19 . 5 grams of tributylamine was added during the subsequent 30 minutes . the mixing continued for an additional two hours at the same temperature . the gabapentin base which was formed during this operation was separated from the suspension through filtration . the filter cake was washed with 23 ml of ethylacetate and 23 ml of methanol to give crude gabapentin . wet crude gabapentin prepared according to step a was suspended in 52 . 5 ml of methanol for 14 hours at approximately 25 ° c . and stirred . thereafter , the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 15 ml of methanol and than dried under vacuum giving pure gabapentin . the yield was 72 %. the following data regarding the chlorine anion content of the above - prepared gabapentin were obtained : gabapentin purified according to these procedures contains less than 0 . 5 % lactam as measured by hplc vs . standard . after a year of storage at 25 ° c . and 60 % relative humidity , the conversion of gabapentin to its corresponding lactam is measured not to exceed 0 . 2 % by weight of gabapentin . for a better control of the ph of pure gabapentin several basic agents were added . some examples of added basic agents are given in the following examples . methanol for suspending 52 . 5 ml methanol for washings 2 × 15 ml tributylamine ˜ 0 . 3 equivalents the wet crude gabapentin ( as in step 1a ) was suspended in 52 . 5 ml of methanol for 14 hours and at 25 ° c . and stirred . tributylamine was added to the suspension . after 14 hours of stirring the solid gabapentin was separated from the suspension by filtration . the filter cake was then washed twice , each time with 15 ml of methanol and than dried under vacuum resulting in pure gabapentin with a yield of 87 %, ph of 7 . 15 and chlorine anion content of 50 ppm . gabapentin so prepared initially contained less than 0 . 5 % by weight of lactam , and , after a year of storage at 25 ° c . and 60 % relative humidity , the conversion of gabapentin to its corresponding lactam is measured not to exceed 0 . 2 % by weight of gabapentin . methanol for suspending 52 . 5 ml methanol for washing 2 × 15 ml sodium methoxide ˜ 0 . 001 equivalents the wet crude gabapentin ( as in example 1 , step a ) was suspended in 52 . 5 ml of methanol for 14 hours and kept at 25 ° c . sodium methoxide was added to the suspension . after 14 hours of stirring , the solid gabapentin was separated from the suspension by filtration . the filter cake was then washed twice with 15 ml of methanol , then dried under vacuum , resulting in pure gabapentin having a yield of 85 %, ph of 6 . 8 , and chlorine anion content of 50 ppm . gabapentin so prepared contained less than 0 . 5 % by weight of lactam , and , after a year of storage at 25 ° c . and 60 % relative humidity , the conversion of gabapentin to its corresponding lactam is measured not to exceed 0 . 2 % by weight of gabapentin . it should be noted that the solvents and the base used in example 1a were not unique . in addition , it should be noted that in examples 4 - 9 gabapentin pure was always prepared as in example 1b and the results ( cl − content and yield ) refer to gabapentin pure . gabapentin hydrochloride ( 100 %) 18 . 2 g isopropanol for dissolution 160 ml active carbon sx1 1 . 1 g tributylamine 19 . 5 g methanol for washing 23 ml in this example , gabapentin hydrochloride was dissolved in 130 ml of dry isopropanol at 25 ° c . then 1 . 1 grams of active carbon was added and the suspension was heated to 40 ° c . and maintained at this temperature for 2 hours . the suspension was filtered at 40 ° c . and the filter cake was then washed twice , each time with an additional 15 ml of isopropanol . the washings were added to the already separated solution of gabapentin hydrochloride in isopropanol . after half an hour of mixing at 25 ° c ., 19 . 5 grams of tributylamine was added during half an hour and the mixing was continued for two hours at the same temperature . the formed gabapentin base was separated from the suspension by filtration and washed with 23 ml of methanol to give gabapentin crude . after reslurry as in example 1b gabapentin pure was obtained at a yield of 58 . 8 % and chloride anion content of 7 ppm cl − . gabapentin so prepared contained less than 0 . 5 % by weight of lactam , and , after a year of storage at 25 ° c . and 60 % relative humidity , the conversion of gabapentin to its corresponding lactam is found not to exceed 0 . 2 % by weight of gabapentin . gabapentin hydrochloride ( 100 %) 18 . 2 g isopropanol for dissolution 160 ml active carbon sx1 1 . 1 g ethylacetate 268 ml trihexylamine 28 . 3 g methanol for washing 23 ml gabapentin hydrochloride was dissolved in 130 ml dry isopropanol at 25 ° c . by mixing , then 1 . 1 g of active carbon was added and the suspension was heated to 40 ° c . and maintained for two hours at 40 ° c . the suspension was filtered at 40 ° c . and the filter cake was washed twice with additional 15 ml of isopropanol each time . the washings were added to the already separated solution of gabapentin hydrochloride in isopropanol . the solution was concentrated to dryness in vacuum ( approximately 10 mm hg ) to constant weight . the temperature of the heating bath was maintained at maximum 35 ° c . during this operation . next , 245 ml of ethylacetate was added to the dry residue of gabapentin hydrochloride and the mixing was started . after half an hour of mixing at 25 ° c ., an amount of 28 . 3 grams of trihexylamine was added during half an hour and the mixing was continued for an additional two hours at the same temperature . the formed gabapentin base was separated from the suspension by filtration . the filter cake was washed with 23 ml of ethylacetate and 23 ml of methanol to give gabapentin crude . after reslurry as in example 1b , gabapentin pure was obtained having a yield of 75 . 0 % and chloride anion content of 213 ppm . gabapentin hydrochloride ( 100 %) 18 . 2 g isopropanol for dissolution 160 ml active carbon sx1 1 . 1 g ethylacetate 268 ml tripropylamine 15 g methanol for washing 23 ml gabapentin hydrochloride was dissolved in 130 ml dry isopropanol at 25 ° c . by mixing , then 1 . 1 g of active carbon was added and the suspension was heated to 40 ° c . and maintained during two hours at 40 ° c . the suspension was filtered at 40 ° c . and the filter cake was washed twice with additional 15 ml of isopropanol each time . the washings were added to the already separated solution of gabapentin hydrochloride in isopropanol . the solution was concentrated to dryness in vacuum (˜ 10 mm hg ) to constant weight . the temperature of the heating bath was maintained at maximum 35 ° c . during this operation . next , 245 ml of ethylacetate was added to the dry residue of gabapentin hydrochloride and mixing commenced . after half an hour of mixing at 25 ° c ., fifteen grams of tripropylamine was added during half an hour and the mixing was continued for two hours at the same temperature . the formed gabapentin base was separated from the suspension by filtration . the filter cake was washed with 23 ml of ethylacetate and 23 ml of methanol to give gabapentin crude . after a reslurry process , as in example 1b , gabapentin pure was obtained having a yield of 68 . 0 % and chloride anion content of 142 ppm . gabapentin hydrochloride ( 100 %) 18 . 2 g isopropanol for dissolution 160 ml active carbon sx1 1 . 1 g acetonitrile 268 ml tributylamine 19 . 5 g methanol for washing 23 ml gabapentin hydrochloride was dissolved in 130 ml of dry isopropanol at 25 ° c . by mixing , then 1 . 1 g of active carbon was added and the suspension was heated to 40 ° c . and maintained for two hours at 40 ° c . the suspension was filtered at 40 ° c . and the filter cake was washed twice with additional 15 ml of isopropanol . the washings were added to the already separated solution of gabapentin hydrochloride in isopropanol . the solution was concentrated to dryness in vacuum (− 10 mm hg ) to constant weight . the temperature of the heating bath was maintained at a maximum temperature of 35 ° c . during this operation . next , 245 ml of acetonitrile was added to the dry residue of gabapentin hydrochloride and mixing commenced . after half an hour of mixing at 25 ° c ., an amount of 19 . 5 g of tributylamine was added during 30 minutes and the mixing was continued for two hours at the same temperature . the formed gabapentin base was separated from the suspension by filtration . the filter cake was washed with 23 ml of acetonitrile and 23 ml of methanol to give gabapentin crude . after reslurry as in example 1b , gabapentin pure was obtained having a yield of 67 . 8 %, and anion content of 142 ppm . gabapentin hydrochloride ( 100 %) 18 . 2 g isopropanol for dissolution 160 ml active carbon sx1 1 . 1 g dimethylcarbonate 268 ml tributylamine 19 . 5 g methanol for washing 23 ml gabapentin hydrochloride was dissolved in 130 ml of dry isopropanol at 25 ° c . by mixing , then 1 . 1 g of active carbon was added and the suspension was heated to 40 ° c . and maintained at 40 ° c . for two hours . the suspension was filtered at 40 ° c . and the filter cake was washed twice with additional 15 ml of isopropanol . the washings were added to the already separated solution of gabapentin hydrochloride in isopropanol . the solution was concentrated to dryness in vacuum (− 10 mm hg ) to constant weight . the temperature of the heating bath was maintained at maximum of 35 ° c . during this operation . next , 245 ml of dimethylcarbonate was added to the dry residue of gabapentin hydrochloride and the mixing was started . after half an hour of mixing at 25 ° c ., an amount of 19 . 5 g of tributylamine was added during half an hour and the mixing was continued for two hours at the same temperature . the formed gabapentin base was separated from the suspension by filtration . the filter cake was washed with 23 ml of dimethylcarbonate and 23 ml of methanol to give gabapentin crude . after reslurry as in example 1b , gabapentin pure was obtained , having a yield of 57 . 9 %, and anion content of 142 ppm . gabapentin hydrochloride ( 100 %) 18 . 2 g isopropanol for dissolution 160 ml active carbon sx1 1 . 1 g isopropylacetate 268 ml tributylamine 19 . 5 g methanol for washing 23 ml gabapentin hydrochloride is dissolved in 130 ml of dry isopropanol at 25 ° c . by mixing , then 1 . 1 g of active carbon was added and the suspension was heated to 40 ° c . and maintained for two hours at 40 ° c . the suspension was filtered at 40 ° c . and the filter cake was washed twice with additional 15 ml of isopropanol each time . the washings were added to the already separated solution of gabapentin hydrochloride in isopropanol . the solution was concentrated to dryness in vacuum (− 10 mm hg ) to constant weight . the temperature of the heating bath was maintained at maximum 35 ° c . during this operation . next , 245 ml of isopropylacetate was added to the dry residue of gabapentin hydrochloride and mixing commenced . after half an hour of mixing at 25 ° c ., an amount of 19 . 5 g of tributylamine was added during half an hour and the mixing was continued for two hours at the same temperature . the formed gabapentin base was separated from the suspension by filtration . the filter cake was washed with 23 ml of isopropylacetate and 23 ml of methanol to give gabapentin crude . after reslurry as in example 1b , gabapentin pure was obtained having a yield of 57 . 9 % and an anion content of 142 ppm . ( the neutralization reaction as in example 1 , however , the reslurry in methanol is replaced by a crystallization in methanol .) the gabapentin crude ( step 1a ) was suspended in 180 ml of methanol at 25 ° c . the suspension was heated while mixing to 55 ° c . when gabapentin was dissolved . the solution was then cooled slowly for an hour to 25 ° c . at 25 ° c . the solution was concentrated to a volume of 50 ml . the suspension was stirred for twelve hours at 25 ° c . after 12 hours , the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 12 ml of methanol then dried under vacuum to give gabapentin pure ( yield : 72 %). following cl − contents of gabapentin and ph values were obtained and tabulated in table 2 as follows : for a better control of the ph of gabapentin pure several basic agents were added . some examples of added basic agents are given in the following examples methanol for suspending 52 . 5 ml methanol for washings 2 × 15 ml tributylamine ˜ 0 . 34 equivalents the gabapentin crude was suspended in 180 ml of methanol at 25 ° c . the suspension was then heated , while mixing , to 55 ° c . when gabapentin was dissolved . tributylamine was added to the solution and the solution was cooled slowly during an hour to a temperature of 25 ° c . at 25 ° c . the solution was concentrated to a volume of 50 ml . the suspension was stirred for twelve hours at 25 ° c . after 12 hours the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 12 ml of methanol and then dried under vacuum to give gabapentin pure having a yield of 81 . 4 %, ph of 7 . 25 and chlorine anion content of 35 ppm . gabapentin so prepared initially contained less than 0 . 5 % by weight of lactam , and , after a year of storage at 25 ° c . and 60 % relative humidity , the conversion of gabapentin to its corresponding lactam is measured not to exceed 0 . 2 % by weight of gabapentin . methanol for suspending 52 . 5 ml methanol for washings 2 × 15 ml sodium methoxide ˜ 0 . 001 equivalents crude gabapentin was suspended in 180 ml of methanol at 25 ° c . the suspension was heated under mixing to 55 ° c . when gabapentin was dissolved . sodium methoxide was added to the solution and the solution was cooled slowly during one hour to 25 ° c . at 25 ° c . the solution was concentrated to a volume of 50 ml . the suspension was stirred for twelve hours at 25 ° c . after 12 hours the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 12 ml of methanol , then dried under vacuum to give gabapentin pure at a yield of 81 . 4 %, ph of 7 . 08 and anion content of cl − 20 ppm . gabapentin so prepared contained less than 0 . 5 % by weight of lactam , and , after a year of storage at 55 ° c . and 50 % relative humidity , the amount of lactam remained less than 0 . 5 % by weight . after a year of storage at 25 ° c . and 60 % relative humidity , the conversion of gabapentin to its corresponding lactam is found not to exceed 0 . 2 % by weight of gabapentin . methanol for suspending 52 . 5 ml methanol for washings 2 × 15 ml sodium bicarbonate ˜ 0 . 05 equivalents crude gabapentin was suspended in 180 ml of methanol at 25 ° c . the suspension was heated under mixing to 55 ° c . when gabapentin was dissolved . sodium bicarbonate was added to the solution and the solution was cooled slowly for one hour to 25 ° c . at 25 ° c . the solution was concentrated to a volume of 50 ml . the suspension was stirred for twelve hours at 25 ° c . after 12 hours the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 12 ml of methanol , then dried under vacuum to give gabapentin pure having a yield of 72 . 4 %, ph of 7 . 28 and anion ( cl − ) content of 20 ppm . gabapentin so prepared contained less than 0 . 5 % by weight of lactam , and , after a year of storage at 25 ° c . and 50 % relative humidity , the amount of lactam remained less than 0 . 5 % by weight . after a year of storage at 25 ° c . and 60 % relative humidity , the conversion of gabapentin to its corresponding lactam is found not to exceed 0 . 2 % by weight of gabapentin . methanol for suspending 52 . 5 ml methanol for washing 2 × 15 ml tetramethylammoniumhydroxide ˜ 0 . 002 equivalents crude gabapentin was suspended in 180 ml of methanol at 25 ° c . the suspension was heated under mixing to 55 ° c . when gabapentin was dissolved . tetramethylammoniumhydroxide was added to the solution and the solution was cooled slowly for one hour to 25 ° c . at 25 ° c . the solution was concentrated to a volume of 50 ml . the suspension was stirred for 12 hours at 25 ° c . after 12 hours the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 12 ml of methanol than dried under vacuum to give gabapentin pure having a yield of 75 . 8 %, ph of 7 . 03 and anion content of ( cl − ) 20 ppm . gabapentin so prepared initially contained less than 0 . 5 % by weight of lactam . methanol for suspending 52 . 5 ml methanol for washing 2 × 15 ml tetrabutylammoniumhydroxide ˜ 0 . 002 equivalents crude gabapentin was suspended in 180 ml of methanol at 25 ° c . the suspension was heated under mixing to 55 ° c . when gabapentin was dissolved . tetrabutylammoniumhydroxide was added to the solution and the solution was cooled slowly during one hour to 25 ° c . at 25 ° c . the solution was concentrated to a volume of 50 ml . the suspension was stirred for 12 hours at 25 ° c . after 12 hours the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 12 ml of methanol , then dried under vacuum to give gabapentin pure having a yield of 77 . 6 %, ph of 7 . 22 and anion ( cl − ) content of 20 ppm . methanol for suspending 52 . 5 ml methanol for washing 2 × 15 ml sodiumtetraborate ˜ 0 . 05 equivalents crude gabapentin was suspended in 180 ml of methanol at 25 ° c . the suspension was heated under mixing to 55 ° c . when gabapentin was dissolved . sodiumtetraborate was added to the solution and the solution was cooled slowly for one hour to 25 ° c . at 25 ° c . the solution was concentrated to a volume of 50 ml . the suspension was stirred for 12 hours at 25 ° c . after 12 hours the solid gabapentin was separated from the suspension by filtration . the filter cake was washed twice with 12 ml of methanol and then dried under vacuum to give gabapentin pure having a yield of 75 %, ph of 7 . 17 and anion content ( cl − ) of 10 ppm . the following gabapentin tablet formulation is prepared using gabapentin containing chloride ion ranging from 5 to 40 ppm and ph in the range of 6 . 84 - 7 . 04 according to example 1 . the following material is used : ingredients amounts gabapentin 125 g corn starch nf 200 g cellulose , microcrystalline 46 g sterotex powder hm 4 g purified water q . s . or 300 ml combine corn starch , cellulose , and gabapentin together in a mixer and mix for 2 - 4 minutes . add water to this combination and mix for an addition 1 - 3 minutes . the resulting mix is spread on trays and dried in convection oven at 45 - 55 ° c . until a moisture level of 1 to 2 % is obtained . the dried mix is then milled and added back to the mill mixture and the total is blended for additional 4 - 5 minutes . compressed tables of 150 mg , 375 mg and 750 mg are formed using appropriate punches from the total mix . the formulation is measured to contain less than 0 . 5 % lactam and after one year of storage at 25 ° c . and 60 % atmospheric humidity , the conversion of gabapentin to its corresponding lactam is measured not to exceed 0 . 2 % by weight of gabapentin . gabapentin of example 2 ( having chloride ion content of 50 ppm and ph of 7 . 15 ) is used to formulate tablets as in example 17 , except that corn starch is replaced in each sample by one of the following adjuvants : pregelatinized starch , croscarmelose sodium , silica gel , titanium dioxide , talc , modified maize starch and maize starch . the resulting gabapentin tablet of each sample is initially measured to have 0 . 5 % by weight of a corresponding lactam , more than 50 ppm of chloride anion , and ph exceeding 6 . 8 . the tablet is stored for one year at 25 ° c . and 60 % atmospheric humidity and the conversion of gabapentin to its corresponding lactam is found not to exceed 0 . 2 % by weight of gabapentin . example 18 is repeated except that gabapentin of example 4 , having chloride ion of 7 ppm is used for formulating tablets . the resulting gabapentin tablet of each sample is initially measured to have 0 . 5 % by weight of lactam and approximately 7 ppm of chloride anion . the tablet is stored for one year at 25 ° c . and 60 % atmospheric humidity and the increase in the lactam concentration is found not to exceed 0 . 2 % by weight . examples 17 - 19 show that , contrary to augart &# 39 ; s disclosure , the presence of anion of a mineral acid in an amount greater than 20 ppm does not adversely affect the stability of gabapentin when stored for one year at 25 ° c . and 60 % humidity ( or higher ). in addition , the examples also show that gabapentin having ph in the range of 6 . 8 to 7 . 3 , and preferably in the range of 7 . 0 - 7 . 2 is stable when stored for one year at 25 ° c . and 60 % humidity . further , the examples show that the gabapentin formulations prepared in accordance with the invention showed equally stable result regardless of the type of adjuvant that were used .
a pharmaceutical composition containing substantially pure and stable gabapentin are disclosed wherein gabapentin has a ph of between 6 . 8 to 7 . 3 .