Detailed Description of the Invention

In the last decade, 2-propylρentanoic acid and its alkali or earth alkali salts (hereinafter referred to as valproic acid and valproates or valproate salts, re- spectively) have been introduced in the arsenal of drugs useful for treating epileptic seizures or convulsions. Most commonly used are valprioc acid itself or its sodium salt. The former is a liquid and as such is less desir¬ able for preparing an oral dosage form while the latter is a solid that has poor stability characteristics partially due to pronounced hygroscopicity.

It has now been found that a highly stable, non- hygroscopic, solid entity can be prepared from valproic acid and its salts, representing a single chemical mole- cule with well-defined physical characteristics, although a definite structure has not been assigned to this entity. The new compound represents a single crystalline entity consisting of one molecule each of valproic acid or diethylacetic acid and a valproate salt, the cation of said salt being sodium or calcium. While it has not been determined for sure whether the new compound represents a solution of the valprioc acid in the valproate salt or a complex between the two compounds, the new material has been tentatively assigned the following structure:

wherein M represents Na or 1/2 Ca and n is 1 or 2.

In the simplest embodiment, the above compound is prepared by dissolving one mole each of [Me(CH ₂ )_] ~ CHCOOH and M-valproate in 1000 ml. of acetone at about 50°C. After cooling the solution to 0°C. or below, the formed new compound is filtered, washed if desired with pre-cooled acetone, and dried under reduced pressure to remove all traces of acetone. Alternately, the new com¬ pound wherein n = 2 can be made in a two-component liquid

medium which includes acetone. In this instance, M- alproa is formed in situ by adding OH at a level ^' of one half of a molecular equivalent of the valproic acid present, preferabl as a solution in an acetone-miscible solvent for said M-OH, e.g., water. Another method consists in simply admixing an aqueous solution of sodium hydroxide of at least 10% concen¬ tration, preferably 35-50%, and valproic acid, said sodium hydroxide being used in an amount corresponding to 48-52% of the stoichi ^' ometric amount of said valproic acid, and removin the water from the reaction mixture by evaporation. A fur¬ ther modification comprises the replacement of the above ace tone by sufficient acεtonitrile to form a clear solution at 50°C or above. The new dimer can be recovered from the liqu phase by evaporating the solvent(s) and, if desired, the new dimer can be recrystallized, for instance from acetone/wate from acetonitrile or others, or the material may be spray- dried, lypholized or purified by chromatograph .

The new dimer represents a single chemical molecu as can be determined by microanalysis, n r spectrum, mixed melting point determination, IE. spectrum and/or X-ray diffra tion. The new compound does not have the aforementioned de¬ trimental physical characteristics of either of the two star ing materials; it is a crystalline, stable solid. Surprisin ly, such a useful dimer can be made only from valproic acid and diethylacetic acid on one side of the molecule, ^' with the sodium or calcium salt of valproic acid. When other valproa salts are used, i.e., the potassium, ammonium or magnesium salts, the resulting dimer, either does not crystallize, doe not form or is highly unstable in the presence of any atmos- pheric moisture. Conversely, when other fatty acids are use to replace ICH3 (CH ₂ ) _n J 2 ^HC00H -* including other branched fatt acids, similarly deficient products or product mixtures are obtained.

The process for making the compounds of this in- vention are best illustrated by reference to the following examples which, however, are not intended to limit the in¬ vention in any respect. _> „-

OMP

/., IPO

Example 1 In 1000 ml. of acetone at about 50°C. is dissolved 166 g. of sodium valproate and 144 g. of valproic acid. The solution is cooled to about 0°C. , filtered and the crystalline precipitate is washed with pre-cooled acetone at about OOC. The new compound is obtained in a yield of 90% of theory. Additional material can be obtained by using the acetone fil¬ trate in a subsequent batch. ^'

The new material is a .stable,white, crystalline powder which melts at 98-lOOOC Its moisture stability is established by placing samples of the material for 45 minutes in a controlle environment at room temperature and 80% relative humidity. No weight gain is observed, while under the same condition, the simple sodium salt of valproic acid gains between 17 and 24% in weight.

The infrared spectrum is consistent with proposed structure I and has the following characterizing absorption bands: strong bands at 2957, 2872, 2932, 1685, 1555 and 1370 cm""**-. The first two of these indicate the various methyl groups, the last two are due respectively to the anti¬ symmetric and symmetric 0-C-O-stretching vibrations of the carboxyl salt. The remaining strong bands indicate the stretching vibrations of the various methylene groups and the C=0 in the carboxylic acid group, while the weak, broad bands at 2450 and 1900 cm" ¹ are due to intramolecularly bounded OH groups of the carboxylic acid.

Example 2 In a 500 ml. flask containing 7.2 g. of valproic acid is added a slurry of 0.926 g. of fresh calcium hydroxide in 100 ml. of water. After increasing the volume by the addition of water to 250 ml. and warming the mixture on a steam bath to about 50oc, some undissolved residue remains which, however, dissolves upon cooling. The water is then evaporated under vacuum and the residue is dissolved in 200 ml. of hot acetone. The solution is filtered, concentrated to about 100 ml. and cooled in an ice bath to yield 5.4 g. of fine, white, ^' crystalline calcium dihydrogen valproate melt¬ ing at 175°C. The n r- and IR-spectra and microanalγsis^s-Mξ- _j ^

firm the named single-molecule chemical identification and the material is stable to normal storage conditions.

Example 3 In the fashion of Example 1 but using sodium val- proate with the molar equivalents of dibutylaσetic acid or diethylacetic acid, respectivel ,the corresponding hydrogen sodium, mixed salts of the assumed Structure I with n=3 o-r 1, respectively,are obtained. In the instance of dibutylacetic acid, .a very hygroscopic product is obtained which is very difficult to handle and therefore unsuitable for pharmaceu¬ tical dosage forms. The mixed salt obtained with diethyl¬ acetic acid is a white crystalline powder which is stable to ordinary storage conditions and essentially nonhygroscop

Example 4 In a comparison of anticonvuls nt activities of

A valproic acid (stable, liquid) B sodium valproate (hygroscopic solid) C mixed dimer (stable solid) of Example 1 the oral ED50 based on equimolar valproic acid equivalents are established by standard procedures. The results are as follows: A B C

Audiogenic seizures (mice) 154 141 81 mg/kg Metrazol seizures (mice) <800 282 178 mg/kg Metrazol seizures (rats) 355 415 362 mg/kg In a bioavailability study carried out with (A) and (C) above in various animal species, the peak blood plasma levels of oral, equimolar doses are determined accor ing to standard procedures, 30 minutes after drug ad¬ ministration. A C Mouse (200 mg/kg) 133.7 207.4 mg/kg Rat (200 mg/kg) 84.1 63.0 mg/kg Dog ( 25 mg/kg) 65.2 73.6 mg/kg Dog ( 25 mg/kg) AUC* 82.3 95.0 hr.mcg/ml *Area under the curve value for ^• 0 - 7 hours.

O PI

From the above examples it will be seen that the new material has equal or better physiological pro¬ perties than either valproic acid or sodium valproate. Since the new dimer salt has far superior physical characteristics than either "monomer" from which it is made, it greatly facilitates the preparation of solid pharmaceutical dosage forms, and specific amounts can be weighed out and blended with starch and/or other binders to form a flowable powder which can be forward- ed to standard tableting machines after granulation. Neither the hygroscopic sodium salt of valproic acid nor the liquid valproic acid itself can be processed in this fashion without special precautions or ab¬ sorbents. The new compounds can be tableted in accord¬ ance with Example XIII of ϋ. S. 3,325,361 and analogous methods. In these procedures, one or more diluents and/or excipients are used, e.g., starch, talcum powder, lubri¬ cants, disintegrators, flavoring agents, coloring agents and the like. These additives, of course, are the usual pharmaceutically acceptable carriers or diluents employed in routine fashion by tablet for ulators.

While the above structure I is the most likely true two-dimensional view of the sodium/hydrogen divalpro- ate or valproate-diethylacetate and seems to be confirmed by IR and nmr spectra, by molecular weight and microanaly- tic values, it is possible that the two molecules bind to one another in some other fashion. Thus, the new material should be characterized not by depicting a structural formula but by reference to a single compound of formula

(C ₃ H ₇ ) ₂ CHC0 ₂ Na/R ₂ CHC0 ₂ H or [ (Pr ₂ CHC0 ₂ ) (R ₂ CHC0 ₂ )]Na,H where¬ in each R is ethyl or propyl , or by reference to sodium/hy¬ drogen divalproate or sodium/hydrogen dipropylacetate di- ethylacetate, or the corresponding dihydrogen/calcium te- travalproate or -bis(diethylacetate)divalproate.

OMPI _/ ., IPO . )