The present invention relates to a pharmaceutical composition containing therapeutically effective amount of gabapentin and its derivatives in combination with effective carriers. More particularly, the present invention relates to a composition and a process for manufacturing pure and stable gabapentin having greater than 20 ppm of chloride ion.
Gabapentin is 1-(aminomethyl)-1-cyclohexaneacetic acid, having the chemical structure of formula I: 
Gabapentin is used for treating cerebral diseases such as epilepsy, faintness attacks, hypokinesis and cranial traumas. U.S. Pat. No. 4,024,175 to Satzinger et al., incorporated herein by reference, discloses that gabapentin of formula (I) shows hypothermal and, in some cases, narcosis-potentiating or sedating properties as well as protective effect against cardiozole cramp in animals. Finally, gabapentin has been found especially useful in treating geriatric patients. As such, there has been a need for producing pure and stable gabapentin.
U.S. Pat. No. 6,054,482 to Augart et al. discloses that preparation and long-term storage of gabapentin presents several problems since (i) during the preparation the compounds shows considerable variations without apparent reason, and (ii) the long-term storage of even very pure gabapentin showed differing stabilities with progressively long storage times. Augart further discloses that the toxic lactam compound of formula (II) 
forms during the preparation and storage of gabapentin. According to Augart, because the lactam has a higher toxicity than gabapentin, its presence in gabapentin should be limited if not eliminated. To combat lactam formation and provide product stability, Augart stresses the importance of (i) starting with gabapentin raw material that contains 0.5% or less of corresponding lactam, (ii) not allowing the anion of a mineral acid in the composition to exceed 20 ppm, and (iii) using a specifically selected adjuvant that is not adverse to gabapentin stability.
According to Augart, the following adjuvants (or excipients) had no noticeable influence on the stability of gabapentin, and as such, they were taught to be acceptable adjuvants for use with gabapentin: hydroxypropylmethylcellulose, polyvinylpyrrolidone, crospovidon, poloxamer 407, poloxamer 188, sodium starch glycolate, copolyvidone, maize starch, cyclodextrin, lactose, talc, as well as co-polymers of dimethylamino-methacrylic acid and neutral methacrylic acid ester.
Conversely, Augart discloses that the following adjuvants reduce the stability of gabapentin and should be avoided: modified maize starch, sodium croscarmelose, glycerol behenic acid ester, methacrylic acid co-polymers (types A and C), anion exchangers titanium dioxide and silica gels such as Aerosil 200.
The composition and method disclosed in Augart are industrially impractical and technically unnecessary. It has now been found that Augart""s reliance on maintaining the anion of a mineral acid as not exceeding 20 ppm is misplaced. Thus, gabapentin and pharmaceutical formulations of gabapentin can be prepared and stored such that initially they do not contain more than 0.5% of the lactam, and even after one year of storage at 25xc2x0 C. and 60% atmospheric humidity, the conversion of gabapentin to its corresponding lactam does not exceed 0.2% by weight of gabapentin. That is, gabapentin and pharmaceutical formulations of gabapentin have been found to be stable even though such formulations do not meet Augart""s requirements (ii) and (iii).
The specific mineral acid disclosed by Augert is hydrochloric acid (column 3, lines 61-63; column 5, lines 24-29; exampes 1 and 2). The specification states, in particular
The active materials of formula (I) [including gabapentin] must be prepared as highly purified, nonderivatized free amino acids, for example, from the corresponding hydrochloride by ion exchange. The proportion of remaining hydrochloride admixtures should thereby not exceed 20 ppm.
(Column 5, lines 24-29).
20 ppm of gabapentin hydrochloride corresponds to roughly 3 ppm of chloride ion, due to the higher molecular weight of gabapentin.
Augert""s claims require gabapentin with xe2x80x9cless than 20 ppm of the anion of a mineral acidxe2x80x9d, e.g. chloride.
Accordingly, the present invention relates to a pharmaceutical composition containing a pharmaceutically effective amount of gabapentin containing more than 20 ppm of an anion of a mineral acid and which initially contains less than 0.5% of a corresponding lactam and after one year of storage at 25xc2x0 C. and 60% atmospheric humidity the conversion of gabapentin to its corresponding lactam does not exceed 0.2% by weight of gabapentin.
The present invention also relates to a process for preparing a stable pharmaceutical formulation containing gabapentin with more than 20 ppm of the anion of a mineral acid and which initially contains less than 0.5% of a corresponding lactam and after storage for one year at 55xc2x0 C. and 60% atmospheric humidity the conversion of gabapentin to its corresponding lactam does not exceed 0.2% by weight of gabapentin.
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 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 25xc2x0 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 gels such as Aerosil 200. Hydroxypropylmethylcellulose, polyvinylpyrrolidone, crospovidon, poloxamer 407, poloxamer 188, sodium starch glycolate, copolyvidone, maize starch, cyclodextrin, 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 AgNO3, or pH electrode or chromatography.