The invention relates to a process for the synthesis of carglumic acid by addition reaction of sodium cyanate to L-glutamic acid in an aqueous medium in the presence of a compound containing lithium, and acidification.

Background to the invention

Carglumic acid (N-carbamoyl-L-glutamic acid, CGA) is an analogue of N-acetylglutamate, a natural activator of carbamoyl phosphate synthetase, the enzyme involved in the first reaction of the urea cycle. Although carbamoyl phosphate synthetase has a lower affinity for carglumic acid than N-acetylglutamate, carglumic acid seems to stimulate the enzyme effectively, because it crosses the mitochondrial membrane of the hepatocyte (wherein this stage of the urea cycle takes place) more easily than N-acetylglutamate.

Urea cycle disorders are rare diseases which may occur at birth (total deficiency) or later (partial deficiency), and are due to a deficiency of one or more of the enzymes necessary for urea synthesis. Non-transformation of ammonium to urea is followed by an accumulation of ammonia in the plasma, and consequently in the tissues. Hyperammonaemia gives rise to encephalopathy, leading to neurological consequences and psychomotor retardation, or even the death of the infant; at later stages of life it can cause lethargy, behavioural disorders and, less frequently, encephalopathy and death. N-acetylglutamate synthetase deficiency is the rarest hereditary urea cycle abnormality. Patients suffering from said disorder are generally treated with sodium phenylbutyrate, which promotes the elimination of nitrogenous metabolites, bypassing the urea cycle. However, phenylbutyrate is only partly effective, and must be supported by strict dietary protein restrictions. Conversely, in the majority of patients treated with carglumic acid before the appearance of permanent brain lesions, growth and psychomotor development are normal. On an indirect comparison, carglumic acid proves more effective than sodium phenylbutyrate and does not require concomitant protein restrictions. Moreover, it has no serious adverse effects.

Carglumic acid can be synthesised from L-glutamic acid or the sodium salt thereof, both of which are commercially available at a low cost. The other reactive species is usually a cyanate, which is preferable to urea due to its higher reactivity, as a result of which it does not require drastic reaction conditions.

L-glutamic acid, the starting cyanate and carglumic acid are reactive species in a basic medium, usually giving rise to the formation of considerable by-products due to the orthogonal reactivity of the reagents or to the breakdown of the product, and significantly reduce both the yield and the quality of the product obtained.

In fact, none of the procedures described in the literature reports that carglumic acid was obtained with a purity level satisfactory for pharmaceutical use.

CN101168518 describes a process wherein glutamic acid, potassium cyanate and potassium hydroxide are reacted before acidifying with hydrochloric acid and hydantoic acid. Said process requires an additional crystallisation. Moreover, the use of hydantoic acid is not straightforward from the industrial standpoint, and traces thereof, or of its by-products, can easily be found in the end product.

CN107033035 describes a process that uses sodium glutamate and sodium cyanate, acidifying with sulfuric acid. The purities reported are unsatisfactory, and additional purifications are also required.

CN105601542 describes an improvement in the quality of the product compared with CN101168518 due to the use of a mixture of inorganic acids instead of hydrochloric acid alone. However, the purities obtained are only just over 90%.

CN101440042 describes the reaction between glutamic acid, ammonium formate and sodium hydroxide at l00°C. Said procedure is hazardous, because ammonium formate is a source of gaseous hydrogen; the drastic reaction conditions contribute to increasing the risks of the process. Moreover, the yields reported are modest, and there are no purity data.

CN101481336 describes the reaction between sodium glutamate and urea in the presence of microwaves, and is therefore only applicable on a laboratory scale for exploratory or analysis purposes.

In view of the factors described above, the need is still felt for a process able to limit or wholly eliminate the formation of certain impurities, thus enabling carglumic acid to be obtained with a purity sufficient for pharmaceutical use, and at the same time requiring a smaller number of operations than the known processes.

Description of the invention

It has now been found that carglumic acid can be obtained with high yields and high purity by using the particular reactivity of its lithium salt with 1:1 stoichiometry (monolithium carglutamate or lithium monocarglutamate). Said salt is novel, and constitutes a further subject of the invention.

The process according to the invention produces carglumic acid with a purity exceeding 99.8% directly by filtration of the solid from the reaction mixture, with no need for further purifications.

The formation of carglumic acid lithium salt as intermediate drastically reduces or actually eliminates some degradation impurities of the end product which tend to form in the presence of bases containing sodium or potassium, and therefore notoriously make at least two further crystallisations necessary, thus increasing the processing time in industrial plants and leading to a significant increase in the costs of the process and the cost of the product obtained therefrom.

It has also surprisingly been found that the formation of the intermediate lithium salt drastically reduces the known tendency of L-glutamic acid to cyclise intramolecularly in basic aqueous solution to give L-pyro glutamic acid (scheme 1), one of the impurities often present in the final carglumic acid. As reported in the experimental examples, when the same reaction is conducted with lithium hydroxide, sodium hydroxide and potassium hydroxide, a tendency to inhibit said cyclisation is observed, which reduces the percentage of L-pyroglutamic acid about 10-fold, with a significant impact on both the yield and the quality of the solid carglumic acid, which is filtered directly from the reaction mixture; in fact, unlike the solid obtained when a lithium base is used, the solids obtained in the presence of sodium or potassium bases not only present considerably lower isolated yields, but also require further purification stages.

The process according to the invention comprises the addition reaction of sodium cyanate to L-glutamic acid in an aqueous medium in the presence of a compound containing lithium, optional concentration of the aqueous phase until precipitation of carglumic acid lithium salt with 1 : 1 stoichiometry, and acidification to obtain carglumic acid having an HPLC purity > 99.8%, with no need for further purifications.

Scheme 2 illustrates the two embodiments of the process according to the invention (with or without isolation of the lithium salt).

Detailed description of the invention

The order of addition of the solvents, raw materials, acids or bases may differ from that reported below.

In one embodiment of the invention, the process is conducted as follows:

1 mole of L-glutamic acid is reacted with 0.8-2 moles of sodium cyanate, preferably 0.9-1.2 moles, in the presence of 0.6-1.5 moles, preferably 0.8-1.2 moles, of a compound containing lithium selected from lithium hydroxide monohydrate, anhydrous lithium hydroxide, metallic lithium, lithium carbonate, lithium hexamethyldisilazide, tribasic lithium phosphate, dibasic lithium phosphate, monobasic lithium phosphate, lithium bicarbonate, lithium sulfate, lithium hydrogen sulfate, lithium chloride, lithium acetate, lithium formate, lithium trifluoroacetate, lithium fluoride, lithium bromide, lithium iodide, lithium oxide, lithium tert-butoxide, straight or branched C1-C6 alkyllithium, lithium diisopropylamide (LDA) and lithium diethylamide, in 1-50 volumes of water, preferably 3-20 volumes, at a temperature ranging between 0 and 90°C, preferably between 20 and 60°C. When the disappearance of the L-glutamic acid has been monitored, an amount of an acid selected from aqueous HC1, nitric acid, concentrated sulfuric acid and aqueous sulfuric acid sufficient to reach a pH value between 0 and 3 is added. The carglumic acid thus filtered presents an HPLC purity > 99.8%, and therefore does not need any further purifications.

Alternatively, when the disappearance of the L-glutamic acid has been monitored, the aqueous phase is concentrated until the carglumic acid lithium salt precipitates with 1 : 1 stoichiometry, and the solid is filtered. An amount of an acid selected from aqueous HC1, concentrated sulfuric acid and aqueous sulfuric acid is added to the salt obtained, dissolved in 1-30 volumes of water, preferably 3-10 volumes, until a pH of between 0 and 3 is reached. The carglumic acid is filtered, and presents an HPLC purity > 99.8%, and therefore does not need any further purifications.

The invention is illustrated in detail in the examples below. Example 1 (Scheme 2 step a)

57.1 g (1.36 moles) of lithium hydroxide monohydrate is added to a reactor containing 1.2 L of water. After complete dissolution, 88.4 g (1.36 moles) of sodium cyanate is added, followed by 200 g (1.36 moles) of L-glutamic acid, and the mixture is stirred at 30°C for 24 h. 30% by weight of aqueous HC1 is then added until a pH value below 3 is reached, at which point precipitation of the product is observed. The filtered solid is washed with water (3 x 30 mL) and dried under vacuum at 50°C for 24 h. 237 g of carglumic acid is obtained (yield 91.7%), with an HPLC purity of 99.86%.

The percentage of L-pyroglutamic acid in the reaction mixture after 24 h is 1.06%.

Example 2 (Scheme 2 step b)

47.6 g (1.13 moles) of lithium hydroxide monohydrate is added to a reactor containing 1.0 L of water. After complete dissolution, 73.7 g (1.13 moles) of sodium cyanate is added, followed by 167 g (1.13 moles) of L-glutamic acid, and the mixture is stirred at 30°C for 24 h. The solution is then concentrated at low pressure until the precipitation of carglumic acid lithium salt with 1 : 1 stoichiometry, which is filtered and dried under vacuum at 50°C for 24 h. 200 g of intermediate is isolated (yield 90.4%).

1H-NMR (D ₂ 0, 400 MHz):

6 4.00 - 3.92 (m, 1H), 2.30 - 2.18 (m, 2H), 2.09 - 1.97 (m, 1H), 1.91 - 1.78 (m, 1H).

¹³ C-NMR (D2O, 100 MHz):

d 182.5, 180.1, 160.9, 55.6, 34.1, 29.1.

Figure 1 shows the X-ray powder difffactogram, having characteristic peaks at the diffraction angles 9.8°, 14.5°, 15.1°, 16.0°, 19.8°, 21.2°, 27.' 7°, 28.8°, 30.1°, 30.4°, 32.3°, 35.7°, 38.7° and 40.0° (2Q ± 0.1°, CuK).

Example 3 (Scheme 2, step c)

200 g of carglumic acid lithium salt with 1:1 stoichiometry (1.02 moles) is dissolved in 1 L of water, and 30% by weight of aqueous HC1 is added to the solution until a pH value below 3 is reached, at which point precipitation of the product is observed. The filtered solid is washed with water (3 x 20 mL) and dried under vacuum at 50°C for 24 h. 192 g of carglumic acid is obtained (yield 99.0%), with an HPLC purity of 99.88%.

Example 4 (Scheme 2, step a)

50.3 g (0.68 moles) of lithium carbonate is added to a reactor containing 0.6 L of water. After complete dissolution, 44.2 g (0.68 moles) of sodium cyanate is added, followed by 100 g (0.68 moles) of L-glutamic acid, and the mixture is stirred at 30°C for 24 h. 30% by weight of aqueous HC1 is then added until a pH value below 3 is reached, at which point precipitation of the product is observed. The filtered solid is washed with water (3 x 10 mL) and dried under vacuum at 50°C for 24 h. 113.6 g of carglumic acid is obtained (yield 88.0%), with an HPLC purity of 99.82%.

Example 5 (Scheme 2, step a)

6.6 g (0.95 moles) of metallic lithium is added to a reactor containing 0.84 L of water. After complete dissolution, 61.9 g (0.95 moles) of sodium cyanate is added, followed by 140 g (0.95 moles) of L-glutamic acid, and the mixture is stirred at 30°C for 24 h. 30% by weight of aqueous HC1 is then added until a pH value below 3 is reached, at which point precipitation of the product is observed. The filtered solid is washed with water (3 x 30 mL) and dried under vacuum at 50°C for 24 h. 162.6 g of carglumic acid is obtained (yield 90.1%), with an HPLC purity of 99.89%.

Example 6 (Scheme 2, step a)

89.8 g (1.36 moles) of lithium acetate is added to a reactor containing 1.2 L of water. After complete dissolution, 88.4 g (1.36 moles) of sodium cyanate is added, followed by 200 g (1.36 moles) of L-glutamic acid, and the mixture is stirred at 30°C for 24 h. 30% by weight of aqueous HC1 is then added until a pH value below 3 is reached, at which point precipitation of the product is observed. The filtered solid is washed with water (3 x 30 mL) and dried under vacuum at 50°C for 24 h. 226 g of carglumic acid is obtained (yield 87.5%), with an HPLC purity of 99.84%. Comparative example 7

54.4 g (1.36 moles) of sodium hydroxide is added to a reactor containing 1.2 L of water. After complete dissolution, 88.4 g (1.36 moles) of sodium cyanate is added, followed by 200 g (1.36 moles) of L- glutamic acid, and the mixture is stirred at 30°C for 24 h. 30% by weight of aqueous HC1 is then added until a pH value below 3 is reached, at which point precipitation of the product is observed. The filtered solid is washed with water (3 x 30 mL) and dried under vacuum at 50°C for 24 h. 214.5 g of carglumic acid is obtained (yield 83.0%), with an HPLC purity of 97.3%. The percentage weight of the L-pyroglutamic acid in the reaction mixture after 24 h amounts to 5.06%.

Comparative example 8

76.3 g (1.36 moles) of potassium hydroxide is added to a reactor containing 1.2 L of water. After complete dissolution, 88.4 g (1.36 moles) of sodium cyanate is added, followed by 200 g (1.36 moles) of L-glutamic acid. The mixture is stirred at 30°C for 24 h. 30% by weight of aqueous HC1 is then added until a pH value below 3 is reached, at which point precipitation of the product is observed. The filtered solid is washed with water (3 x 30 mL) and dried under vacuum at 50°C for 24 h. 189.1 g of carglumic acid is obtained (yield 73.2%), with an HPLC purity of 96.7%. The percentage weight of the L-pyroglutamic acid in the reaction mixture after 24 h amounts to 10.40%.