Field of the Invention
The present invention relates to a crystalline polymorphic form of 6-fluoro-1-methyl-7-{4-[(5-methyl-2-oxo-1,3-dioxol-4-yl)methyl]piperazin-1-yl}-4-oxo-4H-[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylic acid (prulifloxacin). More specifically, the invention relates to a crystalline form of prulifloxacin (herein referred to as Form A), and a method for preparing the crystalline Form A. The present invention further provides a pharmaceutical formulation comprising the novel form of prulifloxacin.
Background of the Invention
Prulifloxacin is chemically known as 6-fluoro-1-methy!-7-{4-[(5-methyl-2-oxo-1,3-dioxol-4-yl)methyl]piperazin-1-yl}-4-oxo-4H-[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylic acid, and it has the structure as shown below as formula I:

Prulifloxacin has significant antibacterial activity and has been marketed as a synthetic antibacterial agent.
Prulifloxacin was first disclosed in US 5,086,049. The patent discloses a process for the preparation of prulifloxacin by the condensation of ulifloxacin with a 4-halomethyl-5-methyl-1,3-dioxolen-2-one of formula III

wherein X is halo selected form chloro, bromo or iodo, in the presence or absence
of an aprotic solvent and a base to obtain pruiifloxacin free base which is
recrytallised with chloroform-methanol. In an exemplified process, ethyl 6,7-
difluoro-1-methyl-4-oxo-4H-(1,3)-thiazeto-(3,2-a)-quinoline-3-carboxylate is
condensed with piperazine in the presence of dimethyl formamide and purified by column chromatography followed by basic hydrolysis to give ulifloxacin, which is then converted to pruiifloxacin.
The above process involves column chromatography. Pruiifloxacin prepared by this method has a purity of 60-65% containing impurities in unacceptable levels. Removal of these impurities by usual purification procedures, such as recrystallisation, distillation and washing, is difficult and requires extensive and expensive multiple purification processes. This further decreases the overall yield. A method involving column chromatographic purifications and multiple purifications cannot be used for large-scale operations, thereby making the process commercially non-viable.
A need still remains for an improved and commercially-viable process for preparing pure pruiifloxacin that will solve the aforesaid problems associated with the process described in the prior art and that will be suitable for large-scale preparation, in terms of simplicity, purity and yield of the product.
EP1626051 A1 mentions that Type I, Type II and Type III crystals of pruiifloxacin are obtained by crystallization from acetonitrile as reported in lyakuhin Kenkyu, Vol. 28 (1), (1997), 1-11. However, the conditions of crystallization from acetonitrile for preparing Type I, Type II and Type III crystals are not disclosed in lyakuhin Kenkyu, Vol. 28 (1), (1997), 1-11. EP1626051A1 further mentions that

Type III crystals have been marketed by considering the solubility, absorbability, therapeutic effect and the like of the respective crystal forms.
US 2007/0149540 discloses a crystal of prulifloxacin acetonitrile solvate (Compound B) which is an intermediate for producing preferentially the type III crystal of prulifloxacin. A crystal of Compound B can be preferentially precipitated by controlling the supersaturation concentration in crystallization using acetonitrile as a solvent, subsequently; the type III crystal of Compound A can be produced by performing desolvation of the crystal.
WO 2008/111018 discloses processes for the preparation of Type I, Type II and Type III crystals of prulifloxacin. There is disclosed a process for preparing Type I crystals by controlled cooling over a period of 7 to 9 hours and prolonged drying over 24 hours. The inventors of the present invention have found that Type I and Type III crystals prepared according to the WO 2008/111018 process are unstable and the process is non-reproducible.
WO 2010/0084508 discloses processes for the preparation of Type I, Type II and Type III crystals of prulifloxacin.
WO 2008/059512 discloses a process for the preparation of prulifloxacin using novel intermediates.
WO 2008/111016 discloses a process for the preparation of prulifloxacin having purity of about 99% or above.
It would be a significant contribution to the art to provide a crystalline form of prulifloxacin, which is consistent and to provide industrially viable methods of preparation, pharmaceutical formulations, and methods of use thereof.
Object of the invention

The object of the present invention is to provide a novel crystalline polymorphic form of prulifloxacin.
Another object of the present invention is to provide an improved process for the synthesis of a novel crystalline polymorphic form of prulifloxacin.
Yet another object of the present invention is to provide an improved process for preparing prulifloxacin.
Yet another object of the invention is to provide an industrially-advantageous, cost-effective and environmentally-friendly process for preparing highly-pure prulifloxacin in high yields.
Summary of the Invention
The present invention provides novel crystalline polymorphic Form A of prulifloxacin, a method of preparing the novel form of prulifloxacin and an improved process for producing prulifloxacin in high purity and high yield. The present invention also provides pharmaceutical compositions comprising the novel form and the known forms of prulifloxacin prepared by the novel processes. The advantages of the process include simplicity, eco-friendliness and suitability for commercial use. Further, the crystalline forms are produced without adding seed crystals.
The invention relates to a crystalline prulifloxacin Form A. Thus, according to a first aspect of the present invention, there is provided crystalline Form A of prulifloxacin.

In an embodiment, the crystalline Form A of prulifloxacin is characterized by having an XRD pattern comprising characteristic peaks at 6.1, 13.7, 16.5,19.8 and 26.8 °26± 0.2 °20.
In another embodiment, crystalline Form A of prulifloxacin is characterized by having an XRD pattern as shown in Figure 1.
Crystalline Form A of prulifloxacin may also be characterized by having an IR spectrum comprising characteristic IR spectra peaks at about 2940 cm"1, 2821 cm" \ 1812 cm1, 1739 cm"1, 1706 cm"1, 1631 cm"1, 1598 cm"1, 1497 cm"1, 1450 cm"1, 1380 cm"1, 1325 cm"1, 1308 cm"1, 1288 cm"1, 1206 cm"1, 1125 cm"1, 1102 cm"1,
1049 cm'1,1002 cm"1, 937 cm"1, 892 cm"1, 862 cm"1, 822 cm"1 and 804 cm"1 ± 2cm"
1
In an embodiment, crystalline Form A of prulifloxacin is characterized by having an IR spectrum as shown in Figure 2.
According to another aspect of the present invention, there is provided a process for preparing crystalline Form A of prulifloxacin, the process comprising dissolving prulifloxacin in a solvent of DMF or DMSO at a temperature ranging from room temperature (about 20°C to about 25 °C) to the boiling point of the solvent, quenching the solution in water, filtering the solids, suspending the solids in an alcohol or water or a mixture thereof for a period sufficient to crystallize Form A of prulifloxacin.
In an embodiment, the solvent is DMSO.
The alcoholic solvent may be selected from methanol, ethanol, isopropanol, t-butanol and isobutanol. In a preferred embodiment the solvent is methanol.

After quenching, the reaction mass may be stirred at 25-30°C, for a period of time ranging from 30 minutes to about 3 days. The period of time may range from about 30 minutes to about 5 hours, preferably from about 30 minutes to about 3 hours, more preferably from about 1 hour to about 3 hours, most preferably from about 1 hour to about 2 hours.
The suspension may be stirred at a temperature ranging from about 25° to about 30°C, for a period of time ranging from 30 minutes to 2 days. The period of time may range from about 30 minutes to about 5 hours, preferably from about 30 minutes to about 3 hours, more preferably from about 1 hour to about 3 hours, most preferably from about 1 hour to about 2 hours.
The precipitated Form A may be isolated by filtration, for example by gravity or suction. The precipitate may be dried at room temperature and/or in vacuum to obtain Form A.
In an embodiment, the polar solvent is DMSO, the prulifloxacin is dissolved in the DMSO at a temperature ranging from about 70°C to about 100°C, preferably from about 80°C to about 90°C, quenching in water, filtering and the filtered solids are suspended in water or an aqueous solution of methanol for a period of time ranging from about 30 minutes to about 3 hours, preferably from about 1 hour to about 2 hours, to crystallize Form A of prulifloxacin.
The prulifloxacin starting material may be in any polymorphic form or in a mixture of any polymorphic forms. In an embodiment, the prulifloxacin is in Type I crystal form. The Type I crystals may be prepared by dissolving/suspending prulifloxacin in a polar aprotic solvent at a temperature ranging from room temperature to boiling point of the solvent, cooling the solution/suspension under stirring for a period sufficient to crystallize Type I crystals of prulifloxacin.
Preferably, the temperature of the solution/suspension is reduced to 0°C.

In an embodiment the polar aprotic solvent is selected from tetrahydrofuran, acetone, 1,4-dioxane, dimethylformamide (DMF), dimethylsulfoxide and N-methyl-2-pyrolidone. In a preferred embodiment the solvent is DMF.
According to another aspect of the present invention, there is provided a process for preparing Type I crystals of prulifloxacin, the process comprising:
(a) dissolving or suspending prulifloxacin in DMF at a temperature ranging from about 70°C to about 90°C,
(b) cooling the solution/suspension to a temperature ranging from about 20°C to about 30°C, and
(c) further cooling the solution/suspension to a temperature ranging from about 0°C to about 10°C.
In an embodiment, the process comprises:
(a) dissolving or suspending prulifloxacin in DMF at a temperature ranging from about 70°C to about 90°C for a period of time ranging from about 30 minutes to about 2 hours,
(b) cooling the solution/suspension to a temperature ranging from about 20°C to about 30aC for a period ranging from about 1 hour to about 3 hours,
(c) further cooling the solution/suspension to a temperature ranging from about 0°C to about 10°C for a period of time ranging from about 1 hour to about 3 hours, and crystallizing the Type I crystals of prulifloxacin.
In an embodiment, the process comprises:
(a) dissolving or suspending prulifloxacin in DMF at a temperature ranging from about 80°C to about 85°C for a period of time ranging from about 30 minutes to about 2 hours,

(b) cooling the solution/suspension to a temperature ranging from about 25°C to about 30°C for a period ranging from about 1 hour to about 3 hours,
(c) further cooling the solution/suspension to a temperature ranging from about 0°C to about 5°C for a period of time ranging from about 1 hour to about 3 hours, and crystallizing the Type I crystals of prulifloxacin.
in an embodiment, the process comprises:
(a) dissolving or suspending prulifloxacin in DMF at a temperature ranging from about 80°C to about 85°C for about 1 hour,
(b) cooling the solution/suspension to a temperature ranging from about 25°C to about 30°C for about 2 hours,
(c) further cooling the solution/suspension to a temperature ranging from about 0°C to about 5°C for about 2 hours, and crystallizing the Type I crystals of prulifloxacin.
The prulifloxacin starting material may be in any polymorphic form (for example Type II or Type III crystals) or in a mixture of any polymorphic forms. The inventors have surprisingly and advantageously found that the process described above is consistent in preparing Type I crystals of prulifloxacin that are stable. In the prior art crystallization involving acetonitrile, any of the Type I, II or III crystals may be formed, and the resulting polymorphs are not stable. However, the Type I crystals prepared by the process described above are stable.
According to another aspect of the present invention, there is provided Type I crystals of prulifloxacin prepared by the process described above.
In an embodiment, Type I crystals of prulifloxacin are characterized by having an XRD pattern comprising peaks at 7.13, 12.3, 12.8, 14.3, 16.28, 17.8, 21.9, 23.1, 23.7 and 25.1°29 ± 0.2 °29. The XRD pattern may comprise further peaks at

11.81, 18.8, 20.3, 21.0, 24.6, and 25.8°29 ± 0.2 °29. The XRD pattern comprise still further peaks at 15.48, 27.0, 27.7, 28.5, 28.8,30.7,32.7 and 32.9 °29 ± 0.2 °29.
In another embodiment, Type I crystals of prulifloxacin are characterized by having an XRD pattern as shown in Figure 3.
Type I crystals of prulifloxacin may also be characterized by having an IR spectrum comprising characteristic IR spectra peaks at about 2922 cm"1, 2854 cm" \ 1814 cm"1, 1709 cm"1, 1628 cm"1,1601 cm"1, 1496 cm1, 1467 cm"1, 1371 cm"1, 1349 cm"1, 1332 cm1, 1304 cm"1, 1283 cm"1, 1250 cm"1, 1232 cm"1, 1205 cm" \1133 cm"1, 1097 cm"1, 1046 cm"1, 1007 cm"1, 930 cm"1, 894 cm"1, 862 cm"1, 847 cm"1 and 806cm"1 ±2.
In an embodiment, Type I crystals of prulifloxacin is characterized by having an IR spectrum as shown in Figure 4.
According to another aspect of the present invention, there is provided the use of Type I crystals of prulifloxacin to prepare Form A of prulifloxacin.
According to yet another aspect of the present invention, there is provided a process for preparing prulifloxacin, the process comprising:
a) condensation of 5,6-difluoro-1-methyl-4-oxo-4H-[1,3]thiazeto-[3,2-a]-quinoline-3-carboxylic acid ethyl ester (formula II)
o
FYVYC00C2HS
CH3 Formula II

with piperazine in an aprotic solvent to yield ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-thiazeto-[3,2-a]-quino!ine-3-carboxylate (formula III);

Formula
b) hydrolysis of ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-thiazeto-[3,2-a]-quinoline-3-carboxylate (formula III) in the presence of a base and a solvent to yield 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylic acid (formula IV);

Formula IV c) condensation of 6-Fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylic acid (formula IV) with 4-halomethyl-5-methyl-1,3-dioxol-2-one [DMDO-Hal] (formula V - 4-bromo depicted below), wherein Hal is a halo leaving group



in the presence of a base and a solvent to yield prulifloxacin (formula I);
u Formula I
[PRULIFLOXACIN]
and optionally d) crystallization of prulifloxacin from a solvent or mixture thereof.
In step a), the ethyl ester (II) may be in the form of a different alkyl ester, for example a CrC6, preferably C1-C4 straight- or branched-chain alkyl ester. Suitably, the alkyl ester is a methyl, ethyl, n-propyl or i-propyl ester. Preferably, the ester is the ethyl ester depicted above.
In step b, the Hal group is preferably chloro or bromo, more preferably bromo.
In an embodiment, the crystallization step d) is carried out, and the step d) comprises the process for preparing crystalline Form A of prulifloxacin as described above.
According to another aspect of the present invention, there is provided crystalline prulifloxacin Form A prepared by a process according to the process described above.

According to another aspect of the present invention, there is provided a pharmaceutical composition comprising crystalline prulifloxacin Form A as described above, together with one or more pharmaceutical^ acceptable excipients.
Suitably, the pharmaceutical composition is in the form of a tablet suitable for oral delivery.
According to another aspect of the present invention, there is provided crystalline prulifloxacin Form A as described above for use in medicine.
According to another aspect of the present invention, there is provided crystalline prulifloxacin Form A as described above for use in treating infectious diarrhea, including travelers' diarrhea, which can be caused by a broad range of bacteria.
According to another aspect of the present invention, there is provided the use of crystalline prulifloxacin Form A as described above for use in the manufacture of a medicament for treating infectious diarrhea, including travelers' diarrhea.
According to another aspect of the present invention, there is provided a method of treating infectious diarrhea, including travelers' diarrhea comprising administering to a patient in need thereof a therapeutically effective amount of crystalline prulifloxacin Form A as described above.
Brief Description of the Drawings
Figure 1: Powder X-ray diffractogram (XRD) of crystalline Form A of prulifloxacin of the present invention.
Figure 2: Infra-Red (IR) absorption spectra of crystalline Form A of prulifloxacin of the present invention.

Figure 3; Powder X-ray diffractogram (XRD) of Type I crystals of prulifloxacin prepared by the process of the present invention.
Figure 4: Infra-Red (IR) absorption spectra of Type I crystals of prulifloxacin prepared by the process of the present invention.
Detailed Description of the Invention
In one aspect, the present invention relates to a novel polymorphic Form A of prulifloxacin which is substantially non-hygroscopic and has good flow characteristics.
In another aspect, the present invention provides a process for preparing the novel Form A of prulifloxacin in high yield and purity. The present invention also provides pharmaceutical compositions comprising Form A of prulifloxacin.
In an embodiment, the process involves isolation of the Form A of pruliffoxacin in a solvent-free media, thus producing prulifloxacin free of solvents or having a negligible solvent content.
The crystalline nature of polymorphic Form A of prulifloxacin is analysed, characterized and differentiated by X-ray diffractogram, Infrared spectrum and Differential scanning calorimetry thermogram techniques, known per se.
The X-ray powder diffraction pattern of crystalline polymorph Form A of prulifloxacin was measured on a Rigaku Dmax 2200 advanced X-ray powder diffractometer using a copper-K-a radiation source.
In an embodiment, the crystalline polymorph Form A of prulifloxacin has an XRD pattern with characteristic peaks at 6.2, 13.7, 16.5, 19.8 and, 26.8 °2G (± 0.2°2e).

In an embodiment, the crystalline polymorph Form A of prulifloxacin has an XRD pattern with peaks at 20-vaiues as shown in Table 1.
Table 1-XRD Peaks

Diffraction angle (°29 ± 0.2 °20) Intensity (%)
6.180 100
9.381 1.8
10.577 1.6
11.900 17.9
12.360 16.4
13.320 5.0
13.681 37.2
14.560 3.7
15.619 14.0
16.141 14.3
16.480 19.6
17.619 9.0
17.819 6.8
18.620 3.4
18.860 7.8
19.760 22.3
20.720 13.5
20.981 11.2
21.240 7.9
21.940 31.5
23.140 39.9
23.600 10.4
24.780 8.5

25.422 3.8
26.419 21.6
26.760 47.5
27.600 7.4
28.480 4.5
29.379 2.1
29.980 4.3
30.379 2.4
In another embodiment, the X-ray powder diffraction spectrum of crystalline polymorphic Form A of prulifloxacin is depicted in Figure 1.
Infrared (FT-IR) spectra were obtained in a KBr disk using a Perkin Elmer FT-IR spectrophotometer, paragon 500 at resolution 4 cm"1. The characteristic absorption bands are expressed in cm"1.
In an embodiment, Form A of prulifloxacin of the present invention is characterized by having characteristic IR spectra peaks at about 2940 cm"1, 2821 cm'1, 1812 cm" \ 1739 cm"1, 1706 cm'1, 1631 cm-1, 1598 cm"1, 1497 cm'1, 1450 cm"1, 1380 cm"1, 1325 cm"1, 1308 cm"1, 1288 cm"1, 1206 cm"1, 1125 cm"1, 1102 cm"1, 1049 cm"1, 1002 cm'1, 937 cm"1, 892 cm'1, 862 cm"1, 822 cm"1 and 804 cm"1 ± 2cm"1 as shown in Figure 2.
In another embodiment, crystalline polymorph Form A of prulifloxacin of the present invention is characterized by having an IR spectrum as shown in Figure 2.
In another aspect the present invention, there is provided a process for preparing crystalline Form A of prulifloxacin. The prulifloxacin used as a starting material can be in any form, e.g. it can be in a reaction solution, suspension, crude or in anhydrous, hydrated or solvated form

Form A prulifloxacin may be prepared by dissolving prulifloxacin in any form or in a mixture of any forms, in a polar aprotic solvent at 80-90°C. The reaction mass may be quenched in water. The solid produced in the reaction solution may be isolated by cooling at 25-30°C and stirring for a prolonged time period. The isolated solid may be further suspended in alcohol or water or mixture thereof for a prolonged time period sufficient to crystallize Form A of prulifloxacin. Examples of prolonged time periods include, but are not limited to, 30 minutes to a few hours such as 2 hours or to a few days such as 2 days.
In an embodiment, the first prolonged period is from about 30 minutes to about 2 days, typically from about 1 hour to about 2 days, preferably from about 1 hour to about 3 hours, more preferably from about 1 hour to about 2 hours, most preferably from about 1 hour to about 2 hours.
In an embodiment, the second prolonged period is from about 30 minutes to 2 days, typically from about 1 hour to about 2 days, preferably from about 1 hour to about 3 hours, more preferably from about 1 hour to about 2 hours, most preferably from about 1 hour to about 2 hours.
The polar aprotic solvent may be selected from dimethylformamide and dimethylsulfoxide. In a preferred embodiment the solvent is dimethylsulfoxide.
The alcoholic solvent may be selected from methanol, ethanol, isopropanol, t-butanol and isobutanol. In a preferred embodiment the solvent is methanol.
The precipitated Form A may be isolated by filtration, for example by gravity or suction. The precipitate may be dried at room temperature and/or in vacuum to obtain Form A.
In an embodiment, the prulifloxacin used to prepare Form A according to the present invention is prepared from Type I crystals of prulifloxacin.

Type I crystals of prulifloxacin may be prepared by dissolving/suspending prulifloxacin in a polar aprotic solvent at 85-90°C, cooling the reaction mass to a temperature ranging from about 25-30°C, stirring for period of time ranging from 1-5 hours, preferably 2-3 hours, chilling the reaction mass further to a temperature ranging from about 0°C to 5°C, stirring for period of time ranging from 1-5 hours, preferably 2-3 hours, and isolating the precipitated Type I crystals of prulifloxacin.
The polar aprotic solvent may be selected from tetrahydrofuran, acetone, 1,4-dioxane, dimethylformamide, dimethylsulfoxide and N-methyl-2-pyrolidone. In a preferred embodiment the solvent is DMF.
Type I crystals of prulifloxacin were analyzed, characterized and differentiated by X-ray diffractogram, Infrared spectrum and Differential scanning calorimetry thermogram techniques, known per se.
The X-ray powder diffraction pattern of Type I crystals of prulifloxacin was measured on a Rigaku Dmax 2200 advanced X-ray powder diffractometer using a copper-K-a radiation source.
In an embodiment, Type I crystals of prulifloxacin has and XRD pattern with peaks at 7.13, 11.81, 12.3, 12.8,14.3, 15.5,16.28,17.8, 18.8,20.3,21.0, 21.9, 23.1, 23.7, 24.6, 25.1, 25.8, and 27.0, 27.7,28.5, 28.8,30.7,32.7 and 32.9 ± 0.2°29.
In an embodiment, Type I crystals of prulifloxacin has an XRD pattern with peaks at 2Q-values as shown in Table 2.
Table 2-XRD Peaks

Diffraction angle (°29± O.2°20) Intensity (%)
7.139 46.5

8.899 9.2
10.417 4.3
11.819 33.8
12.321 85.3
12.780 41.5
14.299 63.5
14.861 3.2
15.480 18.6
16.280 89.3
16.979 9.7
17.861 77.6
18.801 18.6
20.379 21.9
20.980 23.4
21.959 55.0
22.515 4.2
23.120 64.6
23.761 100.0
24.680 24.9
25.161 40.3
25.839 31.7
26.521 7.2
27.059 12.4
27.780 18.9
28.580 10.3
28.801 14.0
30.719 12.5
31.280 4.7
32.720 10.1
32.901 16.0

33.520 3.8
33.959 9.1
34.200 6.4
34.620 3.5
35.380 5.8
36.181 5.0
37.999 5.5
38.420 2.2
39.260 3.7
The X-ray powder diffraction spectrum of Type I crystals of prulifloxacin is depicted in Figure 3.
Infrared (FT-IR) spectra were obtained in a KBr disk using a Perkin Elmer FT-IR spectrophotometer, paragon 500 at resolution 4 cm"1. The characteristic absorption bands are expressed in cm"1.
Type i crystals of prulifloxacin prepared as described above may be characterized by having characteristic IR spectra peaks at about 2922 cm"1, 2854 cm'1, 1814 cm" \ 1709 cm"1, 1628 cm"1, 1496 cm*1, 1467 cm'1, 1371 cm'1, 1349 cm"1, 1332 cm"1, 1304 cm'1, 1283 cm"1, 1250 cm"1, 1232 cm"1, 1205 cm"1, 1133 cm'1, 1097 cm'1, 1046 cm"1, 1007 cm"1, 930 cm"1, 894 cm"1, 862 cm"1 and 847 cm"1 ± 2 cm"1 as shown in Figure 4.
According to yet another aspect of the present invention, there is provided a process for preparing prulifloxacin.
The process comprises:
a) condensation of 5,6-difluoro-1-methyl-4-oxo-4H-[1,3]thiazeto-[3,2-a]quinoline-3-carboxylic acid ethyl ester (formula II) with piperazine in aprotic

solvent to yield ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-thiazeto-[3,2-a]-quinoline-3-carboxylate (formula ill)
b) hydrolysis of ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-
thiazeto-[3,2-a]-quinoline-3-carboxylate (formula III) in the presence of a base
and a suitable solvent to yield 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-
[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylicacid (formula IV);
c) condensation of 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]-thiazeto-
[3,2-a]-quinoline-3-carboxylic acid (formula IV) with 4-bromomethyl-5-methyl-
1,3-dioxol-2-one [DMDO-Br] (formula V) in presence of a base and a suitable
solvent to yield prulifloxacin (formula I); and

15

d) crystallization of prulifloxacin from an aprotic solvents or mixtures thereof. The overall reaction is as depicted below in Scheme 1.


Formula

Formula

COOH
COOH
0
oh
M
H3C CH2Br Formula V

Formula IV

Formula I [PRULIFLOXACIN]
Preferably the reaction in step (a) is carried out in an aprotic solvent selected from N.N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), NpN-dimethyl acetamide (DMA) and mixtures thereof. In a preferred embodiment, the solvent is N.N-dimethyl formamide and the reaction is performed at a temperature ranging from about 25°C to about 70°C, more preferably from about 45°C to about 55°C.
Preferably the reaction in step (b) is carried out in water or an alcoholic solvent selected from methanol, ethanol, isopropanol, n-butanol and tert-butanol. More preferably, the reaction is carried out in water and in the presence of a base selected from an alkaline metal hydroxide (for example sodium hydroxide or potassium hydroxide); an alkaline metal carbonate (for example sodium carbonate or potassium carbonate); and an alkaline metal bicarbonate (for example sodium

bicarbonate or potassium bicarbonate). Preferably, the alkaline metal hydroxide is potassium hydroxide. Suitably, the reaction is carried out at a temperature ranging from about 25°C to the reflux temperature of the solvent used. A preferred temperature range is from about 75° to about 85°C.
The reaction of step (c) is carried out in a solvent selected from acetonitrile, acetone, THF, methylene chloride, toluene, DMF, DMSO and DMA. In a preferred embodiment the solvent is acetonitrile or acetone. The reaction is carried out in the presence of a base selected from sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, triethylamine and diisopropylethylamine. In a preferred embodiment the base is diisopropylethylamine.
The reaction is preferably performed at a temperature ranging from about 0°C to about 50°C. Preferably, the reaction is performed at a temperature ranging from about 20°C to about 40°C to obtain prulifioxacin.
The prulifioxacin is crystallized in step (d), suitably from acetonitrile. The -crystallization may be carried out in accordance with a process disclosed in the prior art or in accordance with a process as disclosed herein. The crystallized prulifioxacin may be in the form of Type I, II, III or a mixture of forms. The crystallized prulifioxacin may be further used as the starting material for the preparation of any form of prulifioxacin, as well as for the preparation of the new polymorphic Form A. This further process is a recrystallization process.
After crystallization with acetonitrile, the prulifioxacin may be recrystallized using DMF or DMSO. Preferably, the reaction is performed in DMF at a temperature ranging from about 30°C to about 100°C, more preferably from about 80°C to about 85°C to yield prulifioxacin. In this embodiment, the prulifioxacin is in the form of Type I crystals. Advantageously, the Type I crystals are stable and reproducible.

After crystallization with acetonitrile, the prulifloxacin may be recrystallized using the process described above for the preparation of Form A. In other words, the prulifloxacin crystallized from acetonitrile may be used as the starting material in the process for preparing Form A as described above.
The process of the present invention may be used as a method for purifiying any form of prulifloxacin, as well as for the preparation of the new polymorphic Form A.
The novel polymorph of prulifloxacin obtained according to the present invention is substantially free from other forms of prulifloxacin. In an embodiment, "substantially free" from other forms of prulifloxacin shall be understood to mean that the polymorphs of prulifloxacin contain less than 10%, preferably less than 5%, of any other forms of prulifloxacin and less than 1% of other impurities, water or solvates.
Prulifloxacin obtained according to the present invention has been found to be effective for treating infectious diarrhea, including travelers' diarrhea, which can be caused by a broad range of bacteria. Pharmaceutical compositions according to the present invention comprise Form A prulifloxacin as an active ingredient together with one or more pharmaceutical^ acceptable carriers, excipients or diluents. Any conventional technique may be used for the preparation of pharmaceutical compositions according to the invention.
The nature of the invention, its object and advantages are explained hereunder in greater detail in relation to non-limiting exemplary embodiments.
Example 1
Preparation of ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-thiazeto-[3,2-a]-quinoline-3-carboxylate (formula III)

S^-difluoro-l-methyW-oxo^H-Il^l-thiazeto-IS^-al-quinoline-S-carboxylic acid ethyl ester of formula (II) (100 gms, 0.321 moles) was stirred in 500 ml of DMF at room temperature. Piperazine (76 gms, 0.882 moles) was added at room temperature and stirred for 10 minutes. The temperature was slowly raised to 50-55°C and the reaction mass was stirred at 50-55°C for 5 hours. After completion of the reaction, the reaction mass was cooled to 25-30°C and stirred for 2 hours. The reaction mass was further chilled to 10-15°C and stirred for 2 hours. The precipitated solid was filtered, washed of chilled DMF (2 x 50 ml). The solid was slurry washed with water (300 ml), filtered, washed with water ( 2 x 100 ml) and dried under vacuum at 70-75°C to yield the title compound [90 gms, 74 % yield, 95% HPLC purity].
Example 2
Preparation of 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3Hhiazeto-[3,2-a]-quinoline-3-carboxylic acid (formula IV)
Ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-thiazeto-[3,2-a]-quinoline-3-carboxylate (100 gms, 0.265 moles) was stirred in water (600 ml) at 25-30°C. To this potassium hydroxide solution (50 gms of potassium hydroxide flakes is dissolved in 200 ml of water) was added and the reaction mass was heated to 80-85°C. The contents were stirred for 1 hour and after completion of reaction, the reaction mass was cooled to 25-30°C. The pH of the reaction mass was adjusted to 6.5-7.0 using 1:1 aqueous acetic acid solution. The contents were stirred at room temperature for 1 hour. The precipitated solid was filtered, washed with water (2 x 100 ml). The solid was slurried in methanol (300 ml) for 1 hour at 25-30°C, filtered, washed with methanol (2 x 50 ml) and dried under vacuum at 70-75°C to yield the title compound [90 gms, 97% yield, 96% HPLC purity].
Example 3
Preparation of prulifloxacin

To a solution of 4-(chloromethyl)-5-methyM,3-dioxol-2-one (55 gms, 0.371 moles) in 50 ml of DMF at 25-30°C, sodium bromide (77 gms, 0.748 moles) was added and the reaction mass was slowly heated to 40-45°C. The contents were stirred at 40-45°C for 2 hours, acetone ( 500 ml) was added at 40-45°C and stirred for 3 hours. The reaction mass was filtered over hyflo, and the bed washed with acetone (100 ml). The solvent was completely distilled off under vacuum below 45°C to yield 4-(bromomethyl)-5-methyl-1,3-dioxol-2-one (formula V).
To a solution of 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylic acid of formula IV (100 gms, 0.286 moles) in 4.0 It of acetonitrile, DIPEA (70 ml, 0.402 moles)) was added at room temperature, stirred for 10 minutes. The reaction mass was cooled to 10-15°C and a solution of 4-(bromomethyl)-5-methyl-1,3-dioxol-2-one (formula V) in 500 ml of acetonitrile was slowly added at 10-15°C over a period of 1 hour. The contents were stirred at 25-30°C for 20 hour, filtered over hyflo, and the bed washed with 200 ml of acetonitrile. The solvent was distilled off completely under vacuum below 50°C. Acetonitrile (100 ml) was added at 50°C and the contents were stirred for 30-60 minutes. The reaction mass was slowly chilled to 0-5°C and the precipitated solid was filtered, washed with acetonitrile (25 ml) and dried to yield 65 gms of prulifloxacin.
Example 4
Preparation of Type I crystals of prulifloxacin
Prulifloxacin (65 gms) was added to 200 ml of DMF at 25-30°C and heated to 80-85°C for 1 hour. The mixture was then slowly cooled to 25-30°C, stirred for 2 hours, chilled to 0-5°C for 2 hours. The precipitated solid was filtered and dried under vacuum at 70-75°C to yield Type I crystals of prulifloxacin (55 gms, 99.5 % HPLC purity).
Example 5

Preparation of prulifloxacin
(55 gms, 0.371 moles) of 4-(chloromethyl)-5-methyl-1,3-dioxol-2-one is taken in 5.0 ml of DMF at 25-30°C. (77 gms, 0.748 moles) of sodium bromide is added and slowly heated the reaction mass to 40-45°C. The contents are stirred at 40-45°C for 2 hours, 500 ml of acetone is added at 40-45°C and stirred for 3 hours. The reaction mass is clarified over hyflo, and the bed washed with 100 ml of acetone to yield a solution of 4-(bromomethyl)-5-methyl-1,3-dioxol-2-one (formula V).
To a solution of 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylic acid of formula IV (100 gms, 0.286 moles) in 3.5 Its of acetone was at room temperature DIPEA (70 ml, 0.402 moles) and stirred for 10 minutes. The reaction mass was cooled to 10-15°C and a solution of 4-(bromomethyl)-5-methyl-1,3-dioxol-2-one (formula V) in acetone was slowly added to the reaction mass at 10-15°C over a period of 1 hour. The contents were further stirred at 25-30°C for 20 hour, filtered over hyflo and the bed washed with 200 ml of acetone. The solvent was distilled off completely under vacuum below 50°C. Acetonitrile (100 ml) was added at 50°C and the contents were stirred for 30-60 minutes. The reaction mass was further chilled to 0-5°C and stirred for 2 hours. The precipitated solid was filteredand dried to yield prulifloxacin.
Example 6
Preparation of Type I crystals of prulifloxacin
Prulifloxacin (65 gms) was added to 200 ml of DMF at 25-30°C and heated to 80-85°C for 1 hour. The mixture was then slowly cooled to 25-30°C, stirred for 2 hours, chilled to 0-5°C for 2 hours. The precipitated solid was filtered and dried under vacuum at 70-75°C to yield Type I crystals of prulifloxacin (52.5 gms, 99.5 % HPLC purity).
Example 7

Preparation of prulifloxacin Form A
Type I crystals of prulifloxacin (25 gms) were dissolved in 150 ml of DMSO at BO¬SS^, and quenched into 375 ml of water at 25-30°C. The contents were stirred at 25-30°C, filtered, and washed with 25 ml of water. The solid was slurried in 125 ml of water at 25-30°C for 1 hour, filtered, washed with 50 ml of water and dried at 25-30°C for 6 hours (22.5 gms, 90 % Yield, 99.5 % HPLC purity).
Example 8
Preparation of prulifloxacin Form A
Type I crystals of prulifloxacin (25 gms) were dissolved in 150 ml of DMSO at 80-85°C, clarified and quenched into 375 ml of water at 25-30°C. The contents were stirred at 25-30°C, filtered, and washed with 25 ml of water. The solid was slurried in 125 ml of 25% aqueous methanol at 25-30°C for 1 hour, filtered, washed with 50 ml 25% aqueous methanol and dried at 25-30°C for 6 hours (22 gms, 88 % Yield, 99.5% HPLC purity).
It will be appreciated that the invention may be modified within the scope of the appended claims.

WE CLAIM:
1. Crystalline Form A of prulifloxacin.
2. Crystalline Form A of prulifloxacin according to claim 1, characterized by having an XRD pattern comprising peaks at 6.2, 13.7, 16.5, 19.8 and 26.8 °20 ± 0.2°29.
3. Crystalline Form A of prulifloxacin according to claim 1 or 2, characterized by having an XRD pattern as shown in Figure 1.
4. Crystalline Form A of prulifloxacin according to any preceding claim, characterized by having an IR spectrum comprising characteristic IR spectra peaks at about 2940 cm"1, 2821 cm"1, 1812 cm'1, 1739 cm"1, 1706 cm"1, 1631 cm'1, 1598 cm"1, 1497 cm"1, 1450 cm"1, 1380 cm"1, 1325 cm"1, 1308 cm"1, 1288 cm"1, 1206 cm"1, 1125 cm"1, 1102 cm"1, 1049 cm"1, 1002 cm'1, 979 cm"1, 937 cm"1, 892 cm"1, 862 cm"1, 822 cm"1 and 804 cm"1 ± 2cm"1.
5. Crystalline Form A of prulifloxacin according to any preceding claim, characterized by having an IR spectrum as shown in Figure 2.
6. A process for preparing crystalline Form A of prulifloxacin, comprising: dissolving prulifloxacin in DMF or DMSO at a temperature ranging from 20°C to the boiling point of the solvent, quenching the solution in water, filtering the solids, suspending the solids in alcohol or water or a mixture thereof for a period of time ranging from 30 minutes to 2 days to crystallize Form A of prulifloxacin.
7. A process according to claim 6, wherein the polar solvent is dimethylsulfoxide.

8. A process according to claim 6, wherein the alcoholic solvent is selected from methanol, ethanol, isopropanol, t-butanol and isobutanol.
9. A process according to claim 8, wherein the alcoholic solvent is methanol.
10. A process according to any one of claims 6 to 9, wherein the prulifloxacin starting material is in any polymorphic form or in a mixture of any polymorphic forms.
11. A process according to claim 10, wherein the prulifloxacin starting material is in the form of Type I crystals of prulifloxacin.
12. A process according to claim 11, wherein preparation of Type I crystals of prulifloxacin comprises: dissolving/suspending prulifloxacin in a polar aprotic solvent and isolating the Type I crystals therefrom.
13. A process according to claim 12, wherein the process comprises maintaining the solution/suspension at a temperature ranging from 20°C to the boiling point of the solvent, and cooling the solution to a temperature ranging from about 0°C to about 5°C under stirring for a period of time ranging from 1 hour to 5 hours to crystallize Type I crystals of prulifloxacin.
14. A process according to claim 13, wherein the temperature of the solution/suspension is reduced to about 0°C in the cooling step.
15. A process according to claim 12, 13 or 14, wherein the polar aprotic solvent is selected from tetrahydrofuran, acetone, 1,4-dioxane, dimethylformamide, dimethylsulfoxide and N-methyl-2-pyrolidone.
16. A process according to claim 15, wherein the polar aprotic solvent is DMF.

17. A process according to any preceding claim, wherein the preparation of prulifloxacin starting material comprises:
a) condensation of 5,6-difluoro-1-methyl-4-oxo-4H-[1,3]thiazeto-[3,2-a]-quinoline-3-carboxylic acid ethyl ester (formula II) with piperazine in an aprotic solvent to yield ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-thiazeto-[3,2-a]-quinoline-3~carboxylate (formula III)

b) hydrolysis of ethyl-6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-(1,3)-thiazeto-[3,2-a]-quinoline-3-carboxylate (formula III) in the presence of a base and a solvent to yield 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]-thiazeto-[3,2-a]-
quinoline-3-carboxylic acid (formula IV); and
o
F. /^ A .COOH

Formula IV
c) condensation of 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]-thiazeto-[3,2-a]-quinoline-3-carboxylic acid (formula IV) with 4-bromomethyl-5-methyl-1,3-dioxol-2-one [DMDO-Br] (formula V) in the presence of a base and a solvent to yield prulifloxacin (formula I).


18. A process according claim 17, wherein the aprotic solvent is selected from N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), N,N-dimethyl acetamide (DMA) and mixtures thereof.
19. A process according claim 18, wherein the aprotic solvent is N.N-dimethyl
formamide.
20. A process according any one of claims 17 to 19, wherein the condensation
step (a) is performed at a temperature ranging from about 25°C to about 70°C,
more preferably from about 45°C to about 55°C.
21. A process according to any one of claims 17 to 20, wherein the hydrolysis
step (b) is performed in water or an alcoholic solvent selected from methanol,
ethanol, isopropanol, n-butanol and tert-butanol.
22. A process according claim 21, wherein the solvent is water.

23. A process according to any one of claims 17 to 22, wherein the base is selected from sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate or potassium bicarbonate.
24. A process according any one of claims 17 to 23, wherein the hydrolysis step (b) is performed at a temperature ranging from about 25°C to the reflux temperature of the solvent used, more preferably from about 75°C to about 85°C.
25. A process according to any one of claims 17 to 24, wherein the condensation step (c) is performed in solvent selected from acetonitrile, acetone, THF, methylene chloride, toluene, DMF, DMSO and DMA.
26. A process according to claim 25, wherein the solvent is acetonitrile or acetone.
27. A process according to any one of claims 17 to 26, wherein the condensation step (c) is carried out in the presence of a base selected from sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, triethylamine and diisopropylethylamine, more preferably in diisopropylethylamine.
28. A process according to any one of claims 17 to 27, wherein, following step (c), the prulifloxacin is crystallized in acetonitrile and the crystallized prulifloxacin is in the form of Type I, II, III or a mixture of forms.
29. A process according to claim 28, wherein the crystallized prulifloxacin is used as the starting material for the process according to claim 6.
30. Crystalline Form A of prulifloxacin prepared by a process according to any one of claims 6 to 29.

31. A pharmaceutical composition comprising crystalline Form A of prulifloxacin according to any one of claims 1 to 5 or 30, together with one or more pharmaceutically acceptable excipients.
32. Crystalline Form A of prulifloxacin according to any one of claims 1 to 5 or 30 for use in medicine.
33. Crystalline Form A of prulifloxacin according to any one of claims 1 to 5 or 30, for use in treating infectious diarrhea, including travelers' diarrhea.
34. Use of crystalline Form A of Prulifloxacin according to any one of claims 1 to 5 or 30 for use in the manufacture of medicament for treating infectious diarrhea, including travelers' diarrhea.
35. A method of treating infectious diarrhea, including travelers' diarrhea comprising administering to a patient in need thereof a therapeutically effective amount of crystalline Form A of prulifloxacin according to any one of claims 1 to 5 or 30
36. Crystalline Form A of prulifloxacin substantially as herein described with reference to the examples.
37. A process substantially as herein described with reference to the examples.