Field of the Invention

The present invention relates to a new process for the preparation of organic salts of N,N' diacetylcystine [DiNAC].

Background of the Invention

N-acetyl-L-cysteine is a well-known compound which is used as a therapeutic agent against chronic obstructive pulmonary diseases and chronic bronchitis. Although the first patent was filed in 1964 (GB 954268), the mechanism of action of the compound has not been established. It is also known that the corresponding disulphide of N-acetyl-L-cysteine, i.e. L-DiNAC, acts as a potent immunostimulator (SE patent appliprotonated form No. 9002067-8), showing an activity comparable to contemporary immunostimulants such as sodium diethyl dithiocarbamate or 2,2'-dithiobisethanol.

It has recently been found that certain salts of DiNAC with organic bases exhibit a favourable combination of non-hygroscopicity and crystallinity which permits the isolation and formulation of these salts in solid form. They have the advantages of ease of crystallisation, non-hygroscopicity and chemical stability, as well as the immunomodulating activity of DiNAC, and are thus medically useful. These salts are described in WO 93/11104.

The organic salts described in WO 93/11104 are generally prepared by mixing DiNAC and an organic amine, each dissolved or dispersed in a solvent or solvent mixture. Solvents such as water, alcohols, glycols, ketones, amides, sulphoxides or other polar solvents or solvent mixtures may be used. The salt either precipitates directly from the reaction mixture, or is obtained by the addition of a less polar solvent or by evaporation or lyophilisation. Alternatively, the salt can be prepared by oxidation of the appropriate N-

acetylcysteine salt in an aqueous or alcoholic solution, followed by precipitation as above. The oxidation may be effected chemically, using, e.g., hydrogen peroxide or a halogen; alternatively, the oxidation may be effected electrochemically.

Example 13 in WO 93/11104 describes oxidation of N-acetyl-L-cysteine with hydrogen peroxide in the presence of 1 mole equivalent of sodium hydroxide at a temperature below 10°C, followed by protonation of the product formed with the aid of an activated protonated form exchanger. The filtrate is collected, L-lysine is added and di-L-lysinium- N,N'-diacetyl-L-cystinate is crystallised from refluxing ethanol. The method involves many steps and is therefore time-consuming. It is important that the temperature during the exothermic oxidation is kept below 25°C, which requires intensive cooling. Otherwise too much decomposition will occur, resulting in a low yield of an impure product.

Example 19 in WO 93/11104 describes a simpler process for the preparation of di-L- lysinium-N,N'-diacetyl-L-cystinate by dissolving N-acetyl-L-cysteine and L-lysine in deionised water, adding hydrogen peroxide dropwise while stirring and keeping the temperature below 25°C, and stirring the reaction mixture for an additional 4 hours before the product, di-L-lysinium-N.N'-diacetyl-L-cystinate, can be precipitated. This procedure is also time consuming.

Disclosure of the Invention

It has now been found that drawbacks of the previous methods for the preparation of organic salts if DiNAC can be eliminated by the process according to the invention.

The present invention provides a new process for the preparation of organic salts of DiNAC, the process being faster than the previous methods, generally giving rise to low decomposition and thus purer products. Furthermore, the new process makes it possible to work at relatively high temperatures, thus requiring less cooling during the oxidation reaction.

The invention provides a process for the preparation of organic salts of N,N' diacetylcystine, having the formulae

or

wherein the N,N'-diacetylcystine is the D-, L- or meso form, or a mixture thereof, and each of R ⁺ and R ²⁺ is the protonated form of an organic base, preferably selected from lysine, ethylenediamine, N.N'-dibenzylethylene-diamine, adamantanamine, N-benzyl-2- phenylethylamine, piperazine or ammonia. The process includes providing a solutioa comprising N-acetyl-L-cysteine, an organic base, a catalytic amount of an alkali metal hydroxide, and a solvent, where the solvent is water or alcohol, and applying an oxidant to the solution.

An example of a catalytic amount is 0.001 mole equivalents. The temperature may be controlled during the oxidation; for example, the temperature may be kept below about 45°C. The product may then be precipitated from an alcohol or aqueous alcohol, and isolated.

A preferred product prepared by the process of the invention is di-L-lysinium-N,N'- diacetyl-L-cystinate.

By this process, the oxidation is immediate and the need for prolonged stirring of the reaction solution is eliminated, thus minimising the decomposition of the product formed.

The organic salts prepared according to the invention include hydrated and solvated salts, e.g., solvated with lower alkanols.

By "catalytic amount" is meant a trace amount of alkali metal hydroxide sufficient to catalyse the described reaction; preferably at least 0.001 mole equivalent compared to the N-acetyl-L-cysteine is used, or up to 0.01 or 0.1 mole equivalent. Preferably no more than 0.5 mole equivalent is used. In one embodiment of the invention, about 0.1 mole equivalent of the alkali metal hydroxide is used.

As mentioned above, the temperature during oxidation can be up to the reflux temperature of the chosen solvent, but is preferably up to about 45°C, above which decomposition starts to be noticeable. Below about 5°C, the reaction mixture becomes difficult to stir, and the lower limit for the temperature during oxidation will depend on the stirring equipment. With appropriate stirring equipment, temperatures of below 0°C may be possible, although temperatures of at least 0°C are preferred, more preferably temperatures of 5°C or above.

Suitable alkali metal hydroxides are, e.g., sodium, potassium or lithium hydroxide.

The oxidation may be effected either chemically, using, e.g., hydrogen peroxide or halogen as oxidising agent, or electrochemically.

The salt precipitates directly from the solution or it may be obtained, for example, by the addition of or to a less polar solvent or by evaporation or lyophilisation.

The process may be aided using crystals of Di-L-lysinium-N,N'-diacetyl-L-cystinate to promote crystallisation. The product may be recrystallised for higher purity.

The invention is illustrated by the following non-limiting examples.

Example I

Dι-L-lysimum-N,N'-diacetyl-L-cystinate

L-lysine monohydrate (200 kg, 1.0 eq.), N-acetyl-L-cysteine (200 kg, 1.0 eq.) and potassium hydroxide (6.8 kg, 0.08 eq.) were dissolved in water (300 L) at approx. 25°C. To the solution was added hydrogen peroxide (60 kg, 35% eq.) with cooling to maintain the reaction temperature at 25-30°C. The solution was then cooled to approx. 10°C.

The reaction solution was then added to a refluxing slurry of di-L-lysiniurn-N,N'-diacetyl- L-cystinate (16 kg) in ethanol (2600 L).

After complete addition (approx. 3 h) the resulting slurry was cooled to approx. 25°C. Crude di-L-lysinium-N,N'-diacetyl-L-cystinate was isolated by centrifugation and washed with ethanol (a total of 800 L).

The crude di-L-lysinium-N,N'-diacetyl-L-cystinate (680 kg, approx. 320 kg calculated as dry weight (81%) was dissolved in water (300 L) at approx. 25°C.

The aqueous solution of crude di-L-lysinium-N,N'-diacetyl-L-cystinate was then added to a refluxing slurry of di-L-lysinium-N,N'-diacetyl-L-cystinate (16 kg) in ethanol (2300 L\

After complete addition (approx. 3 h), the resulting slurry was cooled to approx. 25°C. The product was isolated by centrifugation and washed with ethanol (a total of 800 L), which yielded 540 kg. The product was then dried in vacuo at approx. 45°C for 60 h. The yield of the product was approx. 270 kg, 66%, calculated on the starting N-acetyl-L-cysteine.