The present invention relates to a novel formate salt of gemifloxacin, which is chemically 7-[(4Z)-3-(aminomethyl)-4-(methoxyimino)-l-pyrrolidinyl]-l-cyclopropyl-6-fluoro-l, 4-dihydro-4-oxo-l, 8-naphthyridine-3-carboxylic acid or its hydrate. Further process for its preparation, pharmaceutical compositions comprising it and method of treatment comprising administration of formate salt of gemifloxacin or its hydrate are also provided.
Gemifloxacin, represented by Formula I
(Formula Removed)
is a naphthyridine carboxylic acid derivative. It has potent antibacterial activity and is known from US patent no. US 5633262.
Several processes for the preparation of gemifloxacin have been described in the literature such as in US 5633262, WO 01/18002, US 6307059, Journal of Medicinal Chemistry (1997), 40 (22), 3584, which are herein incorporated by reference. While the US patent no. US 5633262 mentions that the naphthyridine carboxylic acid derivatives including gemifloxacin can form salts with inorganic acid such as hydrochloric acid, hydrobromic acid, phosphoric acid, sulphuric acid, etc as well as organic acid such as acetic acid, succinic acid, benzoic acid, sulphonic acids, etc, no such salt has been prepared.
US 6723734 discloses methane sulphonic acid salt of gemifloxacin alongwith its process of preparation. WO 00/17199 and WO 01/18002 disclose improved processes for the preparation of methane sulphonate salt of gemifloxacin and its hydrates.
WO 03/087100 discloses a process for the preparation of acid salts of gemifloxacin, which does not include the formate salt. Although, the application mentions in the specification that the process disclosed is applicable for different acid salts of gemifloxacin such as salts with hydrochloric acid, methane sulphonic acid, sulphuric acid, acetic acid, only methane sulphonic acid salt is prepared specifically.
In the light of the above prior art, no salt other than methane sulphonic acid has been prepared or characterized earlier and formate salt of gemifloxacin is not known in the literature. The salts known in the prior art have a low solubility in water and thus the in-vivo antibacterial activity is not so high as the in-vitro activity. Accordingly, numerous studies have been continuously conducted to improve the solubility in water and to improve the pharmacokinetic property. Therefore, it is an object of the present invention to provide a novel formate salt of gemifloxacin of Formula II
(Formula Removed)
or its hydrate which shows a potent antibacterial activity against broad pathogenic strains including both gram-positive and gram-negative strains with comparable solubility in water so as to exhibit a good antibacterial activity in the biological systems. The inventors of the present application have prepared a novel formate salt of gemifloxacin or its hydrate. It has been observed that the formate salt of gemifloxacin of the present invention has comparable solubility and stability with methane sulphonic acid salt of gemifloxacin reported earlier.
There is also provided a process for preparing formate salt of gemifloxacin or its hydrate, which comprises contacting gemifloxacin with formic acid in a suitable solvent (s) and isolating the formate salt of gemifloxacin or its hydrate.
Further aspects include a method of treating bacterial infections comprising administering formate salt of gemifloxacin or its hydrate, and a pharmaceutical composition that comprises formate salt of gemifloxacin or its hydrate along with one or more pharmaceutically acceptable excipients.
Gemifloxacin formate or its hydrate is a crystalline solid. Crystalline gemifloxacin formate hydrate may be characterized by strong X-ray peaks at about 26.96, 24.52, 23.10, 9.80, 7.62 ± 0.2 degrees two-theta and weak peaks at 25.30, 25.18, 23.72, 21.36, 14.80, 8.04, 4.88 ± 0.2 degrees two-theta. Crystalline gemifloxacin formate may also be characterized by its DSC and TGA graph.
Gemifloxacin formate obtained by the present invention has an endotherm at about 160°C in the DSC thermogram and has solubility in water at room temperature of at least Igm in 15 ml water.
FIG. I depicts a powder X-ray powder diffraction pattern of gemifloxacin formate hydrate. FIG. II depicts a DSC graph of gemifloxacin formate hydrate. FIG. Ill depicts a TGA graph of gemifloxacin formate hydrate.
The gemifloxacin free base to be used for the preparation of formate salt can be obtained by any of the methods known in the art including those described in US 5633262, WO 01/18002, US 6307059, Journal of Medicinal Chemistry (1997), 40 (22), 3584 which are herein incorporated by reference. The starting gemifloxacin free base may be obtained as a solution directly from a reaction in which gemifloxacin is formed and used as such without isolation.
Suitable solvents for preparing formate salt of gemifloxacin or its hydrate are the customary inert solvents that do not change under the reaction conditions. Examples of suitable solvent(s) for carrying out the process include water, water-miscible solvents and mixtures thereof. The term
"water-miscible" solvents, as used herein, is meant to include solvents which do not form a two-phase system with water under the given reaction conditions. Examples of water-miscible solvents include alcohols such as straight and branched-chain lower alcohols such as methanol, ethanol, isopropanol; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone; nitriles such as acetonitrile; ethers such as tetrahydofuran; dipolar aprotic solvents such as dimethylformamide, dimethyl sulphoxide and mixtures thereof.
In some particular embodiments, mixture of water and alcohol may be used. Preferably, water and isopropanol may be used in some particular embodiments. The ratio of water and isopropanol may be from 1:1 to 1:5, preferably 1:2.
Formic acid used for the preparation of gemifloxacin formate may be in the range of about 0.8 to about 2.5 equivalents.
The term "contacting" includes dissolving, slurrying, stirring or a combination thereof.
The mixture of gemifloxacin free base, formic acid and solvent (s) may be heated to room temperature to 60°C to obtain a clear solution for a time period sufficient to complete the reaction, preferably for about 5 to 15 minutes.
The solution of gemifloxacin free base, formic acid and solvent(s) may be treated with a decolorizing agent such as activated charcoal before precipitation.
Generally formate salt of gemifloxacin or its hydrate may be precipitated out of the solution or the reaction mixture. The precipitation may be spontaneous depending upon the solvent used and reaction conditions. The precipitation may also be facilitated by adding seeds of formate salt of gemifloxacin or its hydrate. The seeds of gemifloxacin formate may be added at a temperature range of about 25°C to 35°C. The precipitation may also be induced by reducing the temperature.
Isolation of the formate salt of gemifloxacin may be accomplished by concentration, precipitation, cooling, filtration or centrifugation or a combination thereof followed by drying.
The precipitated formate salt of gemifloxacin or its hydrate may be isolated in a solid state by conventional methods such as filtration or centrifugation, optionally followed by washing and/or drying and may be purified by crystallization.
Solvates and isomers of formate salt of gemifloxacin are also included within the scope of this invention.
The formate salt of gemifloxacin or its hydrate has a broad antibacterial activity in comparison with the early stage antibacterial compounds, and therefore, has been widely and practically used for treatment of diseases in clinical field such as acute bacterial exacerbation of chronic bronchitis and community acquired pneumonia. The salt is usually administered as part of a pharmaceutical composition. Accordingly, in a further aspect, there is provided a pharmaceutical composition that comprises formate salt of gemifloxacin or its hydrate, and one or more pharmaceutically acceptable carriers, diluents or excipients and optionally other therapeutic ingredients. The salt may be conventionally formulated into tablets, capsules, suspensions, dispersions, injectables and other pharmaceutical forms. Any suitable route of administration may be employed for example peroral or parental.
In the foregoing section embodiments are described by way of examples to illustrate the process of invention. However, these are not intended in any way to limit the scope of the present invention. Several variants of these examples would be evident to persons ordinarily skilled in the art.
Methods
Powder XRD
X-Ray Difractometer, Rigaku Coorperation, RU-H3R
Goniometer CN2155A3
X-Ray tube with Cu target anode
Divergence slits 1 0, Receiving slit 0.15mm, Scatter slit 1 0
Power: 40 KV, 100mA
Scanning speed: 2 deg/min step: 0.02 deg
Wavelength: 1.5406 A
TGA
Perkin Elmer T4A-7 instrument
DSC
Mettler Toledo instrument
Example 1
To a suspension of (R, S)-7-(3-aminomethyl-4-synmethoxyiminopyrrolidin-l-yl)-l-cyclopropyl-
6-fluoro-4-oxo-l, 4-dihydro-l, 8-naphthyridine-3-carboxylic acid (100 g) in a mixture of
isopropanol (800 ml) and water (400 ml) was added formic acid (11.8 g) at 45-55°C and stirred
for 10 minutes. To the resulting solution was added activated charcoal (20 g) and the solution
was again stirred for 30 minutes at 45-55°C and then filtered. The filtrate was allowed to cool to
35°C and seed crystal of formate salt (0.5 g) was added. The suspension was cooled to 25°C and
stirred at 20-25°C for 6-8 hours. It was further cooled to 5°C for 1 hour and then filtered. The
precipitate was washed with isopropanol: water (2:1) mixture (200 ml) and then with isopropanol
(200 ml). The wet material was dried at 50-55°C to give the formate salt of gemifloxacin.
Yield: 92 g
HPLC purity: >99%
1HNMR (D2O), ppm: 8.61 (s, 1H); 8.49 (s, 1H); 7.68 (d, 1H); 4.66(brs, 2H), 4.45(m,lH);
4.04(s,3H),3.85(brs,lH);3.71(m,lH);3.60(m,lH);3.45(m,2H); 1.36(m,
2H), 1.10(m,2H) XRD, DSC and TGA graph were similar to those shown in Fig. I, II and III respectively.
Example 2
To a suspension of (R,S)-7-(3-aminomethyl-4-synmethoxyiminopyrrolidin-l-yl)-l-cyclopropyl-6-fluoro-4-oxo-l,4-dihydro-l,8-naphthyridine-3-carboxylic acid (100 g) in a mixture of ethanol (800 ml) and water (400 ml) was added formic acid (11.8 g) at 45-50°C and stirred for 10 minutes. To the resulting solution was added activated charcoal (20 g) and the solution was again stirred for 30 minutes at 45-50°C and then filtered. The filtrate was allowed to cool to 35°C and seed crystal of formate salt (0.5 g) was added. The suspension was further cooled to 25°C and stirred at 20-25°C for about 10 hours. It was further cooled to 5°C and stirred at 0-5°C for Ihour
and then filtered. The precipitate was washed with ethanol: water (2:1) mixture (200 ml) and
then with ethanol (200 ml). The wet material was dried at 50-55°C to give the formate salt of
gemifloxacin.
Yield: 80 g
HPLC purity: >99%
1HNMR (D2O), ppm: 8.61 (s, 1H); 8.49 (s, 1H); 7.68 (d, 1H); 4.66(brs, 2H), 4.45(m, 1H);
4.04(s, 3H), 3.85(brs, 1H); 3.71(m, 1H); 3.60(m, 1H); 3.45(m, 2H);
1.36(m, 2H), 1.10(m, 2H) XRD, DSC and TGA graph were similar to those shown in Fig. I, II and III respectively.

WE CLAIM:
1. A formate salt of gemifloxacin of Formula II,
(Formula Removed)
or its hydrate.
2. The formate salt of gemifloxacin of claim
1 having X-ray powder diffraction pattern essentially as
shown in Figure I. Formula II
3. The salt according to claim 1, which is in a crystalline form and is characterized by a
powder X-ray diffraction pattern with strong peaks at about 26.96, 24.52, 23.10, 9.80,
7.62 ± 0.2 degrees two-theta and weak peaks at 25.3, 25.18, 23.72, 21.36, 14.80, 8.04,
4.88 ± 0.2 degrees two-theta.
4. The formate salt of gemifloxacin of claim 1 having DSC thermogram as shown in Figure
II.
5. The salt according to claim 1, which has an endotherm at about 160°C in its DSC
thermogram.
6. The salt according to claim 1, which has a solubility in water of at least 1 g in 15 ml water
at room temperature.
7. A process for preparing formate salt of gemifloxacin or its hydrate, which comprises
contacting gemifloxacin free base with formic acid in suitable solvent (s) and isolating
formate salt of gemifloxacin or its hydrate.
8. The process according to claim 7, wherein gemifloxacin free base obtained as a solution
directly from a reaction mixture is used.
9. The process according to claim 7, wherein the formic acid is added in the range of about
0.8 to about 2.5 equivalents.
10. The process according to claim 7, wherein the suitable solvent(s) is water, water-miscible
solvents selected from the group consisting of alcohols, ketones, nitriles, ethers, dipolar
aprotic solvents and mixtures thereof.
11. The process according to claim 7, wherein the suitable solvent is a mixture of water and
isopropanol/ethanol in a ratio of 1:2.
12. A method of treating bacterial infections comprising administering a formate salt of
gemifloxacin or its hydrate.
13. An antibacterial composition comprising a formate salt of gemifloxacin or its hydrate and
one or more pharmaceutically acceptable carriers, diluents or excipients.
14. A process for the preparation of formate salt of gemifloxacin of formula II or its hydrate
as herein described and illustrated by the examples herein.