CA2336789A1 - Injectable sodium acetylsalicylate composition and method - Google Patents

Abstract

An injectable composition of sodium acetylsalicylate is provided having very high purity and which is extremely stable and having pharmaceutical acceptability for use as a subdermal injectable medication. The anhydrous sodium acetylsalicylate is formed by removing water vapor from the dihydrate form of the sodium acetylsalicylate at the same rate as that of its formation.
The shelf life of such sodium acetylsalicylate is at least about 2-3 years, exhibiting between 97 % and 100 % purity after three years at room temperature. A method of making the anhydrous sodium acetylsalicylate injectable aspirin composition is also provided.

Description

INJECTABLE SODIUM ACETYLSALICYLATE
COMPOSITION AND METHOD
s BACKGROUND OF THE INVENTION
The present invention relates in general to aspirin compositions, and in particular to a new and useful aspirin composition which may be administered to mammals by subdermal injection, a method of making the composition, and a method of using such composition.
Aspirin is the most widely used drug in the world. It has a number of important uses in medicine: It is a valuable analgesic. antipyretic, and heart-attack and stroke-preventive.
Aspirin is also one of the most potent anti--inflammatory agents, and is the drug of choice and mainstay of arthritis therapy. It stimulates the immune system, reduces opportunistic infections and is potentially useful as an adjunct in treating cancer, AIDS, and other immune disorders. It shows promise in treatment of Alzheimer's Disease; it is used in rheumatic fever, gout and cataracts; it provides pain relief from tendinitis, headaches, backaches, muscle strains, and other injuries. It has a specific analgesic effect in migraine headaches, a condition in which acetaminophen and ibuprofen show no activity. No other drug in the history of medicine has exhibited such an array of multifaceted therapeutic properties.
Due to the large doses required when taken orally, aspirin is not 2s widely used as an anti-inflammatory agent, even though it is actually the mainstay and-drug of choice in arthritis -- a disease directly caused by SUBSTITUTE SHEET (RULE 2B)

2 inflammation. Instead, its use in arthritis is limited mostly to alleviating pain, for which low 325-500 mg dosages suffice. To be an effective anti-inflammatory agent, daily aspirin dosages of 5,000+ mg are required. Taken orally, at such levels, large amounts of undissolved aspirin particles adhere to S the gastrointestinal mucosa, greatly aggravating topical irritation and side effects.
Narcotics are often used to control arthritis pain. However, narcotics are addictive, depress respiration and can produce other serious adverse reactions as osteoporosis peptic ulcers, convulsions, hypertension and allergic reactions. Steroids are often used to treat arthritis and control pain.
However, either or both of these may produce such adverse reactions as narcotic addiction, osteoporosis, peptic ulcers. respiration depression, convulsions, hypertension and allergic reactions.
Clearly, the potential advantages of aspirin in an injectable form have great pharmacological potential. For instance, the potency of aspirin injected directly into the spinal column of patients has shown to be 100 to times greater than orally administered aspirin. An injectable form of aspirin thus provides a non-addictive and safe alternative to steroids and narcotics now commonly used to treat arthritic and injured joints. Injectable aspirin could also be used for post-surgery treatments to control pain, fever and inflammation. Aspirin also shows promise in cancer treatments for treating pain, as it has effects comparable to morphine, without the narcotic side effects. Injectable aspirin could also be used in sports medicine and in veterinary applications, i.e. the invention is broadly applicable to mammals.
The FDA imposes stringent requirements of fundamental pharmaceutical purity for compositions acceptable for use in administration by subdermai injection. Accordingly, approval for such an acceptable subdermal injectable aspirin composition can be expected to require a high level of purity and significant stability at ambient temperatures for an extended shelf life.
SUBSTITUTE SHEET (RULE 2R)

3 Among the potentially available soluble compounds of aspirin (i.e., sodium and potassium), the most promising as an injectable salt is sodium acetylsalicylate. It is non-toxic, essentially neutral, readily soluble in water, and is compatible with blood serum.
However, preparation of sodium acetylsalicylate of a purity and stability suitable for injection has not been possible in the past. Sodium acetylsalicylate prepared according to prior art is unstable and deteriorates on storage.
For example, the sodium acetylsalicylate composition described in U.S. Patent 3,985,792, the disclosure of which in its entirety is incorporated herein by reference, is made by reacting aspirin with sodium bicarbonate in water, cooling the mixture, isolating the crystalline dihydrate from the aqueous solution by addition of an organic polar solvent (generally a lower alkanol), followed by filtering of the crystals, washing with cold isopropanol and subsequently with benzene, drying the crystals and then removing the water of hydration from the di-hydrate to produce anhydrous sodium acetylsalicylate.
Thus, the old art ignored the criticality of the rate of water removal, with no special precaution being instructed or taken in this regard during the dehydration step. The compositions as thus produced, while then thought to have good stability, actually tended to deteriorate via decomposition at a compound rate of about 3.5% per year, or at least about 5-7-% within two years of storage.
Such a decomposition of sodium acetylsalicylate resulrJ in formation of byproducts such as salicylic acid, sodium salicylate, acetic acid and others. While a limited amount of deterioration may be considered acceptable for orally administered drugs, it is unacceptable for injectable drugs. Injectable drugs are held to higher standards of purity and stability since the danger of toxic reactions and allergic reactions is much greater.
SUBSTITUTE SHEET (RULE 26)

4 More recently, other pharmaceutical agents have been touted as providing a substitute for the effects of aspirin. However, these newer medications have yet to withstand the test of prolonged treatments, and may not be as safe or effective as now viewed.
It-is thus clear that there is a present need for an injectable form of aspirin, which is of sufficient stability to have a suitable shelf life of at least about two years under normal conditions, while retaining a high level of purity, and remaining substantially free of toxic compounds which cause side-effects.
1 o SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an anhydrous sodium acetylsalicylate composition suitable for subcutaneous injection, having a very high purity and a shelf life of 2-3 years with little or no deterioration.
15 It is a further object of the invention to provide a method for making the aforesaid anhydrous sodium acetylsalicylate composition.
Accordingly, the invention provides a composition of sodium acetylsalicylate having very high purity and substantial shelf life. It has been discovered that anhydrous sodium acetylsalicylate of such high purity and 20 stability suitable for injection can be formed by a dehydration procedure involving removing water of hydration frorr~ the dihydrate form of the sodium acetylsalicylate at a rate which is no less than the rate at which the free water vapor is formed. The free water vapor rate of formation may be monitored by various known means, for instance by using a calibrated gas-flow meter. On a 25 small scale this may be accomplished by weighing a flask containing the sodium acetylsalicylate dihydrate, and then balancing the weight-loss therefrom with the weight gain of a (tared) connected vessel containing a SUBSTITUTE SNEET (RULE 26)

5 PCTNS99/15434 water-absorbing agent such as calcium chloride or sulfuric acid. Conditions for the dehydration process, such as vacuum pump capacity and applied temperature, should then be adjusted so that the rate of removal of water from the dehydrate is at the rate of formation of the water vapor, and of its removal from the atmosphere above the dehydrate salt itself.
The shelf life of such anhydrous sodium acetylsalicylate as thus prepared according to this invention is about 2-3 years, and will continue to exhibit between about 97% and 100% purity after three years storage at room temperature, provided that it is stored under a substantially anhydrous atmosphere. Sodium acetyl salicylate has therapeutic value substantially equivalent to aspirin itself. .
One method for preparing anhydrous sodium acetylsalicylate is by using a vacuum dehydration technique. Sodium acetylsalicylate dehydrate in particulate form is placed in a suitable container or vessel connected to a vacuum pump. A strong vacuum pump with a large capacity is then used to evacuate and remove the water of hydration, as water vapor, from the dehydrate form of the sodium acetylsalicylate at no less than the same rate as the rate of formation of the water vapor. A sufficiently large-diameter tube or other conduit is provided between the vessel and the pump to allow a free and unimpeded flow of water vapor to be removed.
The various features of novelty that characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying descriptive matter in which preferred embodiments of the invention are illustrated.
sussTrruTE sHF~r RULE 26~

6 BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of a device used in certain of the experiments disclosed herein;
FIGURE 2 is a kinetic plot for drying the dehydrate at 22° C.;
FIGURE 3 is a kinetic plot for drying the dehydrate at 40° C.;
FIGURE 4 is a kinetic plot for drying the dehydrate at 56.5° C.;
and FIGURE 5 is a comparative kinetic plot for drying the dehydrate at the above three indicated temperatures.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The injectable aspirin composition of the invention is produced from sodium acetylsalicylate dehydrate by removing the water formed during dehydration thereof at the same rate at which it is formed. The sodium acetylsalicylate dehydrate may be prepared by first reacting aspirin with sodium bicarbonate in water, and then crystallizing the sodium acetylsalicylate in the form of the dehydrate using, for instance, a suitable solvent/non-solvent medium. An improved modification, described hereinbelow, of the procedure described in U.S. Patent 3,985,792 may be employed for preparation of the dehydrate as the starting material for the present invention.
This step is then followed by dehydration under carefully controlled conditions.
The anhydrous sodium acetylsalicylate obtained by the process of this invention is extremely stable. After being stored three years at room temperature, the injectable composition exhibits virtually no decomposition.
The composition retains a purity level of between about 97% and 100%.
SUBSTITUTE SHEET' (RULE 26) WO OOlOZ565 PCT/US99/15434

7 The following examples are provided to illustrate the invention.
Example I
One hundred grams (IOOg.) of sodium acetylsalicylate dehydrate, prepared as described herein, and in fine particulate form were placed in a layer of between 3 and 4 cm deep in a shallow dish inside a vacuum desiccator. A suitable dehydrating agent may be employed in a separate or connected vessel downstream of the desiccator. The desiccator is in tum connected to a vacuum pump. The water-collecting dehydrating agent may be either calcium chloride or sulfuric acid, or one of the phosphorus oxides, for instance. A vacuum of 3-15 mm Hg was imposed in the desiccator for about one hour. At this point, the desiccator was opened and the particulate partially dehydrated sodium acetylsalicylate dehydrate platelets contents of the dish were briefly stirred. The vacuum was then reapplied for about another 4-5 hours.
A stable and acceptably substantially pure anhydrous sodium acetylsalicylate, pharmaceutically suitable for injection into a mammal, was obtained.
SUBSTITUTE SHEET (RULE 26)

8 Example 2 Four hundred grams (400g) of particulate sodium acetylsalicylate dehydrate, prepared as described above and in the form of large crystalline plates was placed on a stainless steel drying pan and dried overnight in a Stokes Vacuum Oven in air at room temperature without heat under a 1.5 mm Hg vacuum. The product was then removed from the oven, stirred and broken apart. It was observed that at this stage there was some caking of the particulate material. Thereafter, the vacuum was reapplied for 3 more hours.
Again, a highly pure particulate dehydrated sodium acetylsalicylate suitable for injection was obtained.
Example 3 Particulate sodium acetylsalicylate dehydrate was prepared as described above and suspended in current of dry air at about 20°C. The air temperature was gradually raised to 45°C over one hour, and then raised again to 65°C during one additional hour. Anhydrous sodium acetylsalicylate having a purity and stability suitable for injection was obtained.
Examples 1-3 above illustrate different methods for obtaining suitably pure and stable anhydrous sodium acetylsalicylate. In each case, the technique employed is to remove the water vapor expelled from the dehydrate at a rate such as to be no less than the rate of its formation from the water of hydration. A further important factor in dehydrating the sodium acetylsalicylate dehydrate is ensuring that the particulated surface of the sodium acetylsalicylate dehydrate is constantly exposed to a (relatively) dry atmosphere wherefrom the water vapor is evacuated. One way in which this is accomplished is to use thin layers of the particulate sodium acetylsalicylate dehydrate. Alternatively, the product may be stirred, either intermittently or continuously by any suitable mechanical device.
SUBSTITUTE SHEET (RULE 26)

9 If the dehydration is run under vacuum, the diameter of the tubing leading from the drying vessel to the vacuum source can be critical, and should be of sufficient dimension so as to permit unimpeded rapid removal of the water vapor containing atmosphere above the particulate material. For S instance, if either the tubing by which the water vapor is removed from the system is of insufficient diameter, or if the vacuum-pump used for the removal does not have sufficient capacity or is inefficient, the resulting anhydrous sodium acetylsalicylate may in fact be unstable and may decompose too rapidly to be useful. On the other hand, if the vacuum is sufficiently high and the pump capacity is sufficiently large, but a tubing of, for example, only 3-mm in diameter is used, the product may be unstable because the removal/flow of water vapor is impeded. However, in such instance, use of a tubing, or in general an evacuation conduit, having a diameter of at least 10 mm or greater produces a stable product suitable for injection, due to the unimpeded removal or flow of the water vapor.
When the condition is met that the water of hydration is removed at the same rate as the rate of water vapor formation, the resulting product is stable, and pure anhydrous sodium acetylsalicylate suitable for use as injectable aspirin is obtained.
~,0 Stated another way, the technique of this invention is believed to achieve the dehydration under conditions such that the partial water vapor pressure in the atmosphere above the sodium acetylsalicylate dihydrate is maintained at minimum levels throughout the process.
While I do not wish to be bound by any specific theory, I
believe that this procedure substantially prevents re-hydration of the anhydrous sodium acetylsalicylate crystals and that the latter then form a stable anhydrous crystal lattice structure.
It is particularly valuable in the process of this invention that the crystal lattice structure for both the sodium acetylsalicylate dihydrate and SUBSTITUTE SHEET (RULE 26) for the anhydrous sodium acetylsalicylate should be in the form of platelets rather than in a needle form. On occasion in the process of forming the sodium acetylsalicylate dihydrate, it may tend to have a portion, or even all, of its crystal structure in the form of needles. Much preferred is the practice of 5 the process whereby the crystal structure is substantially entirely composed of crystals in a platelet form.
Other techniques for the preparation of the anhydrous sodium acetylsalicylate may also employed. For instance, various inert, that is inactive, gases may be employed such as nitrogen or argon, in place of air. A batch or

10 continuously operated fluidized bed technique may be employed wherein the bed is formed from the particulate sodium acetylsalicylate dihydrate itself.
In such a case, the apparatus desirably includes equipment to permit collection of fines developed during the fluidization and for the removal and recovery of the anhydrous sodium acetylsalicylate. Alternatively, a constant stirring may be employed using a micro-perforated elastomeric support for the particulate aspirin for passage therethrough of air, or any inert dry gas, optionally also utilizing a continuous belt system. Such various forms of apparatus may of course also be used in sequential combination, in any desired order.
Additionally, an azeotropic distillation technique may be employed for the removal of the water of hydration in a suitable, preferably non-polar, azeotrope-forming organic liquid. The azeotropic boiling point may be lowered somewhat by use of a somewhat reduced pressure, desirably with reflux to accelerate the process while maintaining a lower temperature.
This can be useful where azeotrope-forming non-polar or weakly polar solvents are employed to avoid decomposition of the starting material while also achieving removal of the water of hydration.
The product obtained is thereafter stored in moisture proof air tight containers under an atmosphere of less than 50% relative humidity. The product is not hygroscopic under these conditions. The table following is SUBSTITUTE SKEET (RULE 26)

11 illustrative of the stability of the material obtained by the various techniques as described above.
Example 4 SODIUM ASPIRIN PREPARATION
The following procedure was used to produce 0.55-mole laboratory batches of sodium acetylsalicylate:
Reagents: Aspirin 100 gm Sodium bicarbonate 50 gm Toluene 8 ml Water 45 ml Isopropanol 1 liter 1. Aspirin, sodium bicarbonate, toluene, and water are initially mixed in a 400 ml beaker.
2. The beaker was set in a water bath at 40°-45° C. and the mixture was stirred until the effervescence ceased (approximately 60 minutes). The foam was controlled by adding small portions (in addition to the 8 ml) of toluene as needed.
3. Isopropanol (300 ml) was added, the mixture stirred and filtered from unreacted sodium bicarbonate, (the filter was not washed).
The solution was then cooled to 5-6°C and seeded with platelet crystals of sodium acetylsalicylate dihydrate (for seed preparation, see below). Isopropanol (525 ml, T=5-6°C) was added. The mixture was then stirred in a water-bath (ice+water) for 2 hours during which crystallization of sodium SUBSTITUTE SHEET (RULE 2G)

12 aspirin dihydrate took place. It was then kept in refrigerator overnight. The product obtained was in the form of platelet crystals.
4. The mixture was stirred, filtered and washed with isopropanol ( 150 ml, T=5-6°C) and benzene ( 100 ml) and dried at room temperature or under a lamp in a large shallow dish. During drying, the mixture was frequently stirred and lumps were broken up with a spatula (such lumps may become very hard unless continuously broken).
Weight = about 55 gm, nearly 50 % of theory.
5. The dried product was placed in vacuum desiccator under vacuum using calcium chloride as desiccant in a separate chamber from the sodium acetylsalicylate dihydrate product.
The following day, the product was stirred and placed under vacuum again for another 24 hours. These operations were ail conducted in an air atmosphere of at most about 40% or less relative humidity. The resulting anhydrous product in the form of platelet crystals was then placed in airtight bottles.
PREPARATION OF SEED CRYSTALS
6. Sodium acetylsalicylate (5 gm) from an earlier run, and 1.5 ml of water were stirred with a glass rod in a wide test-tube for several minutes, forming a fluid mixture. Isopropanol (5 ml) was added and stirred in a warm water-bath until almost all dissolved. It was then cooled in ice-water, stirred and rubbed until crystallization was induced. The crystallized material was kept in an ice-bath with occasional stirring until used above (#3). The dihydrate platelet seed cr<~stals should be freshly SUBSTITUTE SHEET (RULE 26)

13 made and kept in a refrigerator or freezer until used. On long standing, however, they will hydrolyze.
For the initial runs, when seed crystals are not available, they are prepared by placing a small portion of the solution (#3, above) in a test tube, and scratching the walls with a glass rod in an ice-water bath to induce initial crystallization, followed by allowing the mixture to further crystallize over a 2-3 hour period with occasional stirring.
Example 5.
The following procedure was used to produce 8.9 mole laboratory batches:
Reagents:
Aspirin 1,600 gm Sodium bicarbonate 800 gm Toluene 120 ml Water 720 ml Isopropanol 16 liters Benzene 300 ml 1. The aspirin and sodium bicarbonate were mixed in a 5-liter beaker, and 120 ml of toluene was added with stirring, followed by 720 ml of water.
2. The beaker was set in a water-bath at 40°C. t2°C. and mechanical stirring was begun and continued until effervescence ceased (approximately 75minutes).
SUBSTITUTE SHEET (~;JLE 26)

14 Note: During the reaction, the mixture became a syrupy fluid with a slight pink hue. Some sodium bicarbonate remained unreacted.
3. Isopropanol (4,800 ml) was added to the beaker and stirred.
S The suspension was then filtered through a Watman #2 paper using suction to remove the unreacted sodium bicarbonate, and the filtrate divided into two equal portions.
4. Each portion was placed in a 12-liter round-bottom flask placed in an ice-salt bath and equipped with a paddle-stirrer, addition funnel, and thermometer.
5. The solutions were stirred for a half hour or until the temperature reached 2-5° C. Crystallization was induced by seeding with sodium acetylsalicylate dihydrate platelets.
Note: Immediate crystallization does not usually occur, and the solution should be stirred until crystals form. The preparation of sodium acetylsalicylate dihydrate platelet crystal seeds is described elsewhere herein.
6. Approximately 15 minutes after crystallization had developed, 5-6 liters of isopropanol were added dropwise to each flask over a period of 3 hours with constant stirring.
7. The crystal content of both flasks was suction-filtered each through the same Buchner funnel using Watman #2 filter paper, then washed with approximately 300 ml of benzene.
8. The two lots of crystals were combined, then subdivided into 4 stainless-steel drying pans, and were dried overnight in a Stokes vacuum-oven without heat at 1.5 mm vacuum.
SUBSTITUTE SHEET (RULE 26) IS
9. The product was removed from the oven, stirred, any caking was broken apart, and then replaced under vacuum for an additional 3 hours.
10. The now dry dehydrated platelet crystals were removed from the oven and passed through a #20 screen, placed in bottles containing silica-gel drying bags, and sealed with plastic tape.
Wide-mouth amber bottles complete with plastic-coated cardboard liners were used for packaging the anhydrous sodium acetylsalicylate. The bottles were flushed with dry air and allowed to equilibrate overnight in the low-humidity room.
Steps #9 and #10 were conducted in a low-humidity environment with relative humidity at 15%.
Results:
Theory 1,782 grams Actual Yield 1,344 grams NOTES:
a. The method of preparation described herein is based along the general lines of US Patent No. 3,985,782. However, this method has been further improved to make the product more suitable for the present injection-grade preparation.
b. For successful preparation, it is important to avoid formation of the needle-like crystal form of sodium acetylsalicylate. When a concentrated aqueous solution of sodium acetylsalicylate is treated with such solvents as isopropanol, while taking no special precautions, the tendency is to get the needle-like form ereferentially. This form is inferior in many respects to the SUBSTITUTE SHEET (RULE 26~

plate form: The needle form is difficult to manage, stir, filter, wash and dry. Also, the yield is considerably lower than with plates. Needle form crystals may also present difficulties in filling vials for injection.
Formation of the needle-form is prevented by a strict control of conditions: proportion of reactants, amounts of water and isopropanol, temperature, speed of stirring, and employment of sufficient amounts of carefully prepared crystalline seeds of the platelet form.
c. Conditions for preparing a stable product were discovered on observing that controlling the rate of removal of water of hydration from sodium acetylsalicylate di-hydrate is critical; the water of hydration must be removed from the system at no less than the same rate as it is formed. This is achieved by use of good vacuum and use of large-diameter tubing or conduits. The dehydration may also be accomplished very efficiently by use of a fluidized-bed.
d. As to storage of anhydrous sodium acetylsalicyate. the product was kept in moisture-proof airtight glass containers. This product is not hygroscopic at RH<50% and was therefore packed at this condition.
e. Regarding stability and decomposition, the following table indicates stability of the product prepared under diverse conditions: The column % Free Salicylate shows the quantity of non-aspirin salicylate content in various prepared lots. Thus, after a year's storage at room temperature stability was, on the average, very high with purity of about 999. Accelerated aging tests (30 days at 50° C.) showed similar stability (see following table, Lots A, B, C, D).
SUBSTITUTE SHEET (RULE 26~

f. As to dehydration, the water-content may be determined by Karl Fisher or IR-assay, and was found to be about 0.1 % (see following table.) g. Ambient temperatures were used in Examples unless otherwise indicated.
h. As to drying agents, CaS04, P205, MgS04, and molecular sieves may all be suitably employed for removing water of hydration inasmuch as the dihydrate starting material quite easily loses its water of hydration. Even so, the most effective and industrially practical method may be the fluidized bed, or other suitable, at least semi-continuous, method.
i. Regarding Example 2, as the product is dehydrated, it tends to cake. Here again, the use of a fluidized-bed technique can prevent this. With other methods, caking is prevented during dehydration by various stirring methods. For instance, one method described is the use of a rotating flask under vacuum.
Owing'to the constant movement of particles, caking can be avoided.
ANALYTICAL PROCEDURES FOR
ANHYDROUS SODIUM ASPIRIN
The procedure described here is manual. Most stability assays can be practiced with an automated form of this procedure. All solutions are kept ice-cold.
1. REAGENTS
(a). Buffer - mix equal volumes of 2-propanol and a pH 2.2 HCl/KCl solution- The latter is prepared by dissolving 3.72 gm SUBSTITUTE SHEET (RULE 26) KC1 in distilled water, and adding 39 ml or 0.2 molar HCI; the mixture is diluted to 1 liter.
(b). Ferric nitrate - five grams of ferric nitrate and 2.5 ml of concentrated nitric acid are dissolved in water and diluted to 500 ml.
2. PREPARATION OF STANDARD
Approximately 40 mg of accurately weighed sodium salicylate is dissolved in the buffer solution and diluted to 25.0 ml with the buffer.
3. ASSAY
Described here for initial assays. Stability samples may require different dilutions.
Accurately weigh approximately 60 mg of sample into a 10 ml volumetric flask. Dissolve and dilute the sample to mark with cold buffer solution. Keep the solution in ice. In a spectrophotometer curette, mix 4.0 ml of the sample solution and 2.0 ml of the cold ferric nitrate solution. Read the absorbences of these solutions at 520 mp.
using a mixture of 4.0 ml of buffer and 2.0 ml of ferric nitrate solution as a blank.
A~~k x Conc. 5.e. * x 100 = % salicylate as sodium salicylate Astd Wt. unk *Concentration of standard in 10 mi of final dilution.
SUBSTITUTE SHEET (RULE 26) WO 00/02565 PC'f/US99/15434 ANALYSIS FOR SALICYLATE CONTENT
1. REAGENTS
a) Ferric Solution: Fe Alum (1 g) in 200 ml water containing 1 ml of conc. HCl b? Standard Salicylic Solution: 1 gm in 1 1. water 0.( 1 wt.
2. COLOR COMPARISON TEST-TUBES (Nessler) length: 154 mm; >D; 19 MM; od: 22 mm 3. PROCEDURE:
(a). dissolve 50 mg of compound in 20 ml of water, place into first Nessler tube; add 4 drops of glacial acetic acid, followed by 8 drops of ferric solution (b). place 20 ml of water in the second Nessler tube, add 4 drops of glacial acetic acid, add 8 drops of ferric solution; then, with shaking, add dropwise the 0.190 salicylic solution until the color matches that of the solutions in the first Nessler tube (viewing through the width, not the depth, of the Nessler, and against a white background.) Each drop of 0.1 % salicylic solution corresponds to 0.05 mg of salicylic acid.
The concentration or the solution being tested should be adjusted so that not more than b-7 drops of salicylic solution is required; otherwise the color is too deep for comparison.
SUBSTITUTE SHEET (RULE 26) 4. NOTE:
This test requires less than 5 minutes to perform. Since aspirin salts hydrolyze in water, the test should be completed as fast as possible.

(Physical Properties) a) Appearance: Free-flowing white granular powder;
monoclinic plates.

b) Identity: Melting/Decomposition point: 210-213 C.

10 (literature: 217.5 C.) c) Purity (as measured): 97-103% (by aspirin content) d) Water Content: Maximum: 0.1% as determined by Karl Fisher or IR assay.

e) Solubility: Extremely soluble in water, and as follows,

15 Methanol: 3.50% w/v Ethanol: 0.30% w/v Isopropanol: 0.05% w/v f) Free Salicylate: Less than 1 % salicylate.

g) Mol. Wt. = 202 Sodium = 12.8 wt. %

20 h) Hygroscopic at RH>50%

Stability data are indicated in the following table.
SUBSTITUTE SHEET (RULE 26) Table I
STABILITY OF ANHYDROUS SODIUM ACETYLSALICYLATE
Lot No. Time Wt. % Free- Dehydration Conditions Salicylate A 30 days @50 0.7 RV: 3 hrs, vac desiccator 14 months @ 0.8 (4 days, sulfuric RT acid) B 30 days @ 50 1.0 RV: 3 hrs.

C 30 days @ 50 1.2 Vacuum desiccator 14 months @ 0.6 RT

(5 days, CaCl2) D 30 days @ 50 0.8 Vacuum desiccator 14 months @ 0.9 ( 12 days,, CaCl2) RT

E 13 months @ 1.2 Vacuum desiccator RT

( 17 hrs. CaCl2) then RV

F 13 months @ 0.8 Vacuum desiccator RT

(21 days, CaCl2) G 13 months @ 1.2 FB
RT

SUBSTITUTE SHEET (RULE 2S) WO 00/02565 PCTNS99/i5434 Lot No. Time Wt. % Free- Dehydration Conditions Salicylate X45' @ 55;

35' @ 65) H 12 months 0.8 FB
@ RT

(I hr @ 45;

2 hrs @ 85) I 12 months I .5 FB
@ RT

(5 hrs @ 55) 9 months @ 1.0 FB
RT

J

( I hr @ 20 C.

I hr @ 45 C.

1 hr @ 65 C.) NOTES: "RT" = Room Temperature "RV" = Rotating flask, vacuum, water-bath 20°- 50°-85°
"FB" = Fluid-bed The initial free-salicylate in all runs is <0.05%.
Further Examples of improved preparations are described hereinafter.
S1JBSTfTUTE S~i~~' (RULE 26).

Example 6 A mixture of aspirin (25.0 g, 0.139 mol) and sodium bicarbonate ( 12.5 g, 0.147 mol) and toluene (2.0 ml) was placed in a 250 ml ehrlenmeyer flask and water ( 12.2 ml) was added. The reaction mixture was stirred magnetically in a water bath at 40-45°C. Effervescence and frothing were observed. Additional portions of toluene (3 ml) were added to reduce the frothing. After 45 minutes the effervescence and frothing ceased, and isopropanol (75 ml) was added portionwise. The reaction mixture was filtered, and the clear filtrate was cooled in an ice bath and treated with an additional portion of isopropanol (130 ml). The clear solution was scratched with a stirring rod to induce. crystallization and placed in the refrigerator overnight. The resultant crystalline product was filtered and washed with isopropanol (37 ml) and dried at room temperature. A total of 8.73 g (26.4%
yield) of dihydrate product was obtained; mp: softens at 143°C., resolidified at 165°C. and melts with decomposition at 238°C.
A portion of this product (5.50 g) was recrystallized by dissolving in water (2.0 ml) and warming in a water bath at 45°C to give a clear solution. Isopropanol (7 ml) was added and the clear solution was cooled in an ice bath for 30 minutes. During this time there was a crystallization of dihydrate in the form of beautiful white platelets which were filtered and dried at room temperature. A total of 2.54 g of product was obtained; mp: begins to soften at 74°C, completely softens at 90°C, and melts with decomposition at 237°C. Anal: H20 content (14.51%) [Karl Fischer AnalysisJ.
The dihydrate sodium acetylsalicylate product from this preparation was subsequently subjected to kinetic studies using various drying conditions to form the desired anhydrous sodium acetyl salicylate, as described hereinafter.
SUBSTITUTE SHEET (RUt F ~,~

Example 7 Sodium acetylsalicylate dehydrate was placed in a tared vial and introduced into an abderholden drying chamber which was charged with dry calcium chloride. This device is illustrated in FIGURE 1. The abderholden was connected to a vacuum pump at a reduced pressure of 45 mm. At various times the vial containing the sodium acetylsalicylate dehydrate was weighed and the loss of weight (equivalents of water) was plotted versus time (minutes). For sodium acetylsalicylate dehydrate the theoretical number of equivalents of water is 2.00. The following tables summarize the drying curves for each temperature studied.
As generally shown in Figure l, the abderholden device 10 has a flask 12 for a distillable fluid in communication with an outer chamber 14 through the connecting tube 16. Outer chamber 14 has a generally concentrically mounted inner chamber 18 for containing the sample 20 to be treated and isolated from any communication with outer chamber 14. Outer chamber 14 is in turn fitted with a reflux condensor 22 for return of the condensed refluxing liquid in flask 12. The inner chamber 18 is fitted with a second flask 24 for containing, in this instance, a drying agent. Flask 24 is further provided with means permitting a connection to a vacuum pump (not shown) via line 26 fitted with a stopcock 28 or other valve device.
Connections between flask 12, outer chamber 14, condensing column 22. and inner chamber 18 with flask 24 are conveniently provided with respectively ground glass joints 30. By placing a liquid of suitable boiling point in flask the sample (here, sodium acetyl salicylate dehydrate) placed in inner chamber 18, which is isolated from outer chamber 14 (as by a ring seal at 32), can be maintained at a suitable predetermined temperature via the function of the condensor 22. The drying agent in flask 24 is selected so as to effectively absorb the water of hydration as it is removed from the initial dehydrate starting material by the applied heat at the temperature of the refluxing medium and under the imposed vacuum.
SUBSTITUTE SHEET (RULE 2~) Table II
Drying at 22.0°C
Sodium acetylsalicylate dehydrate (485.3 mg, 2.037 mmol) Amount of HBO removed from dihvdrate Time (min)Sample wt ~mg) Mmols Equivalents (m~) 0 485.3 0.0 0.000 0.000 20 478.8 6.5 0.361 0.177 40 477.0 8.3 0.461 0.226 60 475.6 9.7 0.539 0.265 80 474.5 10.8 0.600 0.295 145 471.2 14.1 0.783 0.384 1300 419.4 65.9 3.660 1.800 4140 410.3 75.0 4.167 2.045 5 FIGURE 2 is a plot of the resulting drying process, illustrating the asymptotic curve as the procedure forms the anhydrous sodium acetyl salicylate of this invention.
SUBSTITUTE SHEET (RULE 2&) Table III
Drying at 40.0°C
Sodium acetylsalicylate dihydrate (477.5 mg, 2.003 mmol) Amount of HBO removed from dihvdrate Time (min)Sample wt ~mg~ rnmols Equivalents (m~

0 477.5 0.0 0.000 0.000 30 472.1 5.4 0.300 0.150 135 454.0 23.5 1.306 0.652 275 443.2 34.3 1.906 0.952 1445 412.0 65.5 3.638 1.816 1535 412.1 65.4 3.633 1.814 FIGURE 3 is a plot similar to Figure 2 of the resulting drying curve, again showing the asymptotic shape.
SUBSTITUTE SHEET (RULE 26) WO OOlOZ565 PCT/US99/15434 Table N
Drying at 56.5°C
Sodium acetylsalicylate dehydrate (527.8.mg, 2.215 mmol) Amount of Hz0 removed from dihvdrate Time (min)Sample wt ,(mg,~ mmols Equivalents (mgl 0 527.8 0.0 0.000 0.000 40 471.8 56.0 3.111 1.404 65 466.6 61.2 3.400 1.534 90 463.1 64.7 3.594 1.622 115 461.8 66.0 3.667 1.655 175 458.6 69.2 3.844 1.736 235 458.6 69.2 3.844 1.736 295 458.6 69.2 3.844 1.736 FIGURE 4 is a plot, similar to that of Figures 2 and 3, again showing the asymptotic shape of the drying curve.
FIGURE 5 is a comparative plot of the curves of Figures 2, 3 and 4 and illustrates that the process of this invention and the rate of conversion of the sodium acetylsalicylate dehydrate to the anhydrous form is dependent on temperature (as noted above, for this series of plots, the same t 0 reduced pressure of 45 mm Hg was employed). At 22° C., conversion to the anhydrous form proceeded to 50% completion in about 10 hours. At 40°
C., conversion to the anhydrous form proceeded to 50% completion in about 4 SUBSTITUTE SHEET (RULE 2fi) WO OO/OZSbS PCTNS99/15434 hours. At 56.5° C., the highest temperature used in this study, 50%
conversion of the dehydrate to the anhydrous form proceeded in less than 40 minutes.
The dehydrate samples used in the drying experiments at 40°
and 56.5° C. (tables III and IV) show a less than theoretical ultimate removal of water of hydration probably due to the fact that prior to use in the above experiments they were stored overnight in a desiccator in a refrigerator over calcium chloride and had therefore undergone some initial loss of water of hydration. The validity of the experiment is unaffected by this circumstance.
There was no indication of any decomposition of sodium acetylsalicylate dehydrate at the temperatures studied over the course of these kinetic studies. However, iE is generally advised that drying should not be carried out at temperatures that are much greater than about 60°C since sodium acetylsalicylate dehydrate begins to soften and may decompose at higher temperatures (see, mp information below).
As obtained by the above procedure, after drying, the anhydrous sodium acetylsalicylate was recovered as white platelets; mp: begins to soften at 107°C, completely softens at 121°C, and begins to decompose at 174°C and completely decomposes at 205°C. Anal: Hz0 content (0.85%) [Karl Fischer Analysis].
The Karl Fischer analysis is very useful for measuring the course of the conversion of the dehydrate to the anhydrous form of sodium acetylsalicylate. For example, a sample of partially dehydrated sodium acetylsalicylate dehydrate (taken at the 1300 minute point during the course of drying a sample at 22.0°C (see, Table In, as expected, had a water content of 6.11 % by the Karl Fisher analysis).
It can also be concluded that azeotropic removal of the water from sodium acetylsalicylate dehydrate is possible. However, in practice, especially on large scale, the boiling point of such azeotropic mixtures could SUBSTfTUTE SHEET (RULE 26) lead to decomposition of the product unless carried out under a reduced pressure. Drying at no more than about 60°C. (i.e. at about 56°
C. which is the by of refluxing acetone as used in the abderholden device referred to above) is the preferred method for conversion to the anhydrous form since the process proceeds rapidly (complete within 2 hours). Of course, larger scale drying processes may require periodic mixing or rotation of sample and may require longer times and/or lower temperatures.
The foregoing Examples demonstrate that drying to a constant weight is a sufficient measurement of the complete conversion to anhydrous sodium acetylsalicylate.
Thus, this invention provides a novel advantageous process for the dehydration of sodium acetylsalicylate dihydrate to produce a novel anhydrous sodium acetylsalicylate composition which is particularly distinguished by the dominant presence of a platelet crystal morphology and by a high purity which is retained with an extended shelf life of at least 2-3 years. This product is of sufficient purity and stability to permit its use for subdermal injection into mammals for therapeutic purposes. The dehydration process is especially characterized by the rapid removal of the water vapor from the system as it is formed so as to prevent re-hydration of the sodium acetylsalicylate and the ultimate deterioration of its desired characteristics.
Such process is to be conducted at temperatures insufficiently high to induce decomposition of the fundamental sodium acetylsalicylate molecular structure so that high purity is achieved. Various specific techniques to accomplish this goal are described in the foregoing description which are in accord with the application of the principles of the invention. For instance, in place of isopropanol there may be used other C3 to Ca alcohols in the precipitation and crystallization of the dihydrate, as is also indicated in the process of the above-mentioned U.S. 3,985,792. It will therefore be understood that the invention may be embodied otherwise without departing from such principles, and is therefore limited only by the spirit and scope of the following claims.
SUBSTITUTE SHEET (RULE ~6)

Claims (17)

WHAT IS CLAIMED IS:

1. A therapeutic sodium acetylsalicylate composition having a long shelf life and pharmaceutical-acceptability suitable for mammalian subcutaneous injection, comprising the anhydrous form of sodium acetylsalicylate, formed by removing the water of hydration from the sodium acetylsalicylate dehydrate, at a rate which corresponds to no less than the rate of formation of water vapor from the water of hydration, and obtained in the form of monoclinic platelets.

2. The composition of claim 1 wherein the product is composed of anhydrous sodium acetylsalicylate having a making point of about 210° -213° C.

3. A composition according to claim 2, wherein the anhydrous sodium acetylsalicylate is stable with a purity of at least about 97% to 100%
after storage at room- temperature under an air atmosphere of no more than 50%
relative humidity for at least three years.

4. A therapeutic acetylsalicylate composition composed of anhydrous sodium acetyl salicyiate in the form of white monoclinic platelets and having a making point of 210° - 213° C., and an H2O content of less than 1 wt. %, and a free salicylate content of less than 1 wt. %.

5. A composition according to claim 4, wherein the anhydrous sodium acetylsalicylate is stable with a purity of at least about 97% to 100%
after storage at room- temperature under an air atmosphere of no more than 50%
relative humidity for at least three years

6. A method of preparing a therapeutic composition of acetyl salicylate having a long shelf life and pharmaceutical-acceptability for mammalian subcutaneous injection, the method comprising:
providing an initial amount of solid particulate sodium acetylsalicylate dehydrate composed in large part of platelet crystals; and removing the water of hydration from the said acetylsalicylate dehydrate at a rate that is not less than the rate of water vapor formation from the dehydrate to a level of a residual amount of water of hydration of less than about 2 wt. %, to produce thereby anhydrous sodium acetylsalicylats in the form of white monoclinic platelet crystals and having a melting temperature of about 2 10° - 213°
C.

7. A method according to claim 6, wherein removing the water of hydration further comprises placing the said acetylsalicylate dehydrate under a vacuum.

8. A method according to claim 6, wherein the vacuum is between about 1 mm Hg, and 50 mm Hg.

9. A method according to claim 6, wherein the water of hydration is removed using a vacuum oven.

10. A method according to claim 6, wherein removing the water of hydration further comprises exposing the solid sodium acetylsalicylate dehydrate crystals to a current of dry inactive gas for a required period of time sufficient to effect removal of the water of hydration of the dehydrate.

11. A method according to claim 6, wherein removing the water of hydration further comprises placing the solid sodium acetylsalicylate dehydrate crystals in a current of a substantially dry inactive gas at a temperature of from 50°C., to a temperature of 60°C.

12. A method according to claim 6, wherein removing the water of hydration further comprises agitating or stirring the solid sodium acetylsalicylate dehydrate crystals to prevent caking thereof

13. A method for the production of sodium acetylsalicylate dehydrate crystals which comprises first forming an aqueous suspension of acetylsalicylic acid and treating the same with sodium bicarbonate, thereafter inducing precipitation of sodium acetylsalicylate dehydrate by the addition of a C3 to C4 alcohol to the resulting aqueous solution with cooling the same, inducing the formation of crystals by seeding with pre-formed monoclinic platelet crystals of sodium acetylsalicylate dehydrate, and allowing further crystallization of sodium acetylsalicylate dehydrate to take place whereby a product is formed composed of sodium acetylsalicylate dehydrate in the form of monoclinic platelet crystals.

14. The method of claim 13 wherein said alcohol is isopropanol.

15. A therapeutic acetylsalicylate composition having a long shelf life and pharmaceutically acceptably suitable for mammalian subcutaneous injection, comprising the anhydrous form of sodium acetylsalicylate formed by removing the water of hydration from the sodium acetylsalicylate dehydrate at a rate which corresponds to no less than the rate of formation of water vapor from the water of hydration and obtained in the form of monoclinic platelets having a melting/decomposition temperature of 210° -213° C.

16. A therapeutic acetylsalicylate composition having a prolonged shelf life exhibiting high stability and pharmaceutically acceptably suitable for mammalian subcutaneous injection, comprising the anhydrous form of sodium acetylsalicylate characterized in its preparation by removing the water vapor formed from the water of hydration at a rate which is at least equal to the rate at which said water vapor is formed in the dehydration of said dihydrate.

17. A process for producing an anhydrous sodium acetylsalicylate pharmaceutably suitable for mammalian subcutaneous injection, characterized by first forming an isolated crystalline dihydrate of sodium acetylsalicylate in the form of platelets and characterized by thereafter dehydrating said sodium acetylsalicylate by removing the water of hydration in the form of generated water vapor, with said water vapor being removed from the sodium acetylsalicylate crystals at a rate which is at least as fast as the rate of dissociation of water from said dihydrate.