# Patent

## Title
Method for the purification of prostaglandins

## Abstract

The present invention provides a method for the purification of a prostaglandin by supercritical fluid chromatography, said method comprising the use of a stationary phase and a mobile phase comprising carbon dioxide, provided that when the stationary phase is unmodified silica gel, the prostaglandin is not luprostiol. The invention also provides prostaglandins obtainable by the method.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to U.S. Provisional Application No. 61/219,166, filed Jun. 22, 2009, and incorporated herein by reference in its entirety for all purposes.


FIELD OF THE INVENTION
The present invention provides a method for the purification of prostaglandins. In particular, the present invention provides a method for the purification of prostaglandins by supercritical fluid chromatography (SFC).
BACKGROUND OF THE RELATED ART
Prostaglandins are active pharmaceutical ingredients (APIs) and isomerically and chemically pure prostaglandins are required for formulation into drug products. However, the purification of prostaglandins is challenging due to the similar chemical properties of many prostaglandin isomers, as well as their related impurities.
G. H. Brunner et al. (Supercritical Fluids, 653-668, E. Kiran and J. M. H. Levelt Sengers (eds.), Kluwer Academic Publishers, 1994) describes the preparative supercritical fluid chromatography (SFC) separation of Reprodin isomers (i.e. luprostiol isomers). However, the isomers are poorly separated and the purity of the heart cut fraction is only 80%. This method therefore cannot be used to obtain a commercial product as the purity does not meet ICH quality. The authors acknowledge that SFC is disadvantageous in comparison to liquid chromatography.
BRIEF SUMMARY OF THE INVENTION
The present inventors, however, have overcome the problems associated with the prior art procedure to provide an alternative process for the purification of prostaglandins using SFC.
Accordingly, the present invention provides a method for the purification of a prostaglandin by supercritical fluid chromatography, said method comprising the use of a stationary phase and a mobile phase comprising carbon dioxide, provided that when the stationary phase is unmodified silica gel, the prostaglandin is not luprostiol. For example, the invention includes a method for purifying a crude prostaglandin, comprising injecting the crude prostaglandin onto a column comprising a stationary phase, eluting the crude prostaglandin through the column using a mobile phase comprising supercritical carbon dioxide, and collecting a fraction comprising a purified prostaglandin.



DESCRIPTION OF THE FIGURES
FIG. 1 illustrates a chromatogram of a SFC purification injection of crude latanoprost, obtained as described in Example 1.
FIG. 2 sets out the structures of latanoprost isomers.
FIG. 3 illustrates a chromatogram of a SFC purification injection of crude latanoprost, obtained as described in Example 2.



DETAILED DESCRIPTION OF THE INVENTION
By “purification”, it is meant the method produces a prostaglandin which is chemically and/or isomerically pure. A chemically pure compound is one where the compound is essentially free from related compounds, chemical starting materials, chemical intermediates and chemical degradants. An isomerically pure compound is one where the compound is essentially free from known related compounds of the same chemical molecular formula that are different in chemical structure.
In one embodiment, the purified prostaglandin is at least about 99% chemically pure, preferably at least about 99.5% chemically pure and more preferably at least about 99.8% chemically pure.
In another embodiment, the purified prostaglandin is at least 99% isomerically pure, preferably at least about 99.5% isomerically pure and more preferably at least about 99.9% isomerically pure.
The method of the present invention may be utilised to purify a prostaglandin on an analytical or preparative scale. By “analytical”, we mean a scale of providing at least about 0.1 mg of purified prostaglandin, preferably about 1 mg of purified prostaglandin, in a reasonable timeframe, i.e. less than a day. By “preparative”, we mean a scale of providing at least about 1 mg of purified prostaglandin, preferably about 0.1 g of purified prostaglandin and most preferably about 1 g of purified prostaglandin, in a reasonable timeframe, i.e. less than a day.
Preferably, the prostaglandin is a compound of formula (I), a compound of formula (II), a compound of formula (III) or a compound of formula (IV):




wherein,

V is C or O;
W is




X is CONR12R13 or CO2R12;
Y is




Z is C1-20-alkyl, C2-20-alkynyl, —O—(C6-20-aryl) or —(C1-20-alkyl)-(C6-20-aryl), wherein the aryl group is optionally substituted with one to three substituents selected from the group consisting of C1-20-alkyl, halo and C(halo)3;
R1 and R2 are independently H or OH, or R1 and R2 together form




R3 and R4 are independently H or OH;
R5 and R6 are independently H or OH, or R5 and R6 together form




R7 and R8 are independently H, OH, halo or C1-20-alkyl;
R9 and R10 are independently H or C1-20-alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from CONR12R13, CO2R12 or CO2−M+;
R11 is C1-20-alkyl or —O—(C1-20-alkyl), wherein the alkyl group is optionally substituted with one or more substituents selected from CONR12R13, CO2R12 or CO2−M+;
R12 and R13 are independently selected from the group consisting of H and C1-20 alkyl;
M+ is a counter cation; and
Halo is fluorine, chlorine, bromine or iodine. When V═C, it is understood that the carbon atom bears two hydrogen atoms, one or both of which may be substituted with a substituent group which may be independently selected from halo and C1-20-alkyl, for example.
“Alkyl” refers to linear, branched or cyclic saturated hydrocarbon structures having, unless otherwise indicated, 1 to 20 carbon atoms, more preferably 1 to 15 carbon atoms and most preferably 1 to 10 carbon atoms. Examples of alkyl groups are methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, i-butyl, t-butyl, n-pentyl, n-hexyl and cyclohexyl. When an alkyl group having a specific number of carbons atoms is named, it is intended that all geometric isomers of that alkyl group are encompassed. For example, “butyl” includes n-butyl, i-butyl, t-butyl and cyclobutyl.
“Alkynyl” refers to linear or branched hydrocarbon structures having at least one




group and, unless otherwise indicated, 2 to 20 carbon atoms, more preferably 2 to 15 carbon atoms and most preferably 2 to 10 carbon atoms. Examples of alkynyl groups are ethynyl, propynyl, n-butynyl, isobutynyl and hexynyl. When an alkynyl group having a specific number of carbon atoms is named, it is intended that all geometric isomers of that alkynyl group are encompassed. For example “butynyl” includes n-butynyl and isobutynyl.

“Aryl” refers to an aromatic hydrocarbon structure having, unless otherwise indicated, 6 to 20 carbon atoms, more preferably 6 to 15 carbon atoms and most preferably 6 to 10 carbon atoms. Examples of aryl groups are phenyl and naphthyl.
The term “halo” whether alone or as part of another group refers to a halogen, for example, a fluorine, chlorine, bromine or iodine atom.
M+ is a counter cation of —CO2− i.e. —CO2−M+ is a carboxylic acid salt and preferably a pharmaceutically acceptable carboxylic acid salt. Preferably M+ is a metal ion e.g. an alkali metal ion, such as K+ or Na+.
Preferably, W is




X is preferably CONH(C1-10-alkyl), CO2(C1-10alkyl) or CO2H. More preferably, X is selected from the group consisting of CONHEt, CO2Me, CO2iPr and CO2H.
Preferably, Z is C1-10-alkyl, C2-10-alkynyl, —O—(C6-10-aryl) or —(C1-10-alkyl)-(C6-10-aryl), wherein the aryl group is optionally substituted with one to three substituents selected from the group consisting of Cl and —CF3. More preferably, Z is —(CH2)5CH3, —(CH2)3CH3, —CH2-Ph,




R1 and R2 may independently be H or OH. Preferably, when one of R1 and R2 is H, the other of R1 and R2 is OH. More preferably, when one of R1 and R2 is H and the other of R1 and R2 is OH, R1, R2 and the carbon atom to which they are attached have the following stereochemistry:




R3 and R4 are independently H or OH. Preferably, when one of R3 and R4 is H, the other of R3 and R4 is OH. More preferably, R3, R4 and the carbon atom to which they are attached have the following stereochemistry:




R5 and R6 may independently be H or OH. In one embodiment, when one of R5 and R6 is H, the other of R5 and R6 is OH. More preferably, when one of R5 and R6 is H and the other of R5 and R6 is OH, R5, R6 and the carbon atom to which they are attached have the following stereochemistry:




When R5 and R6 together form




and one of R3 and R4 is OH and the other of R3 and R4 is H, it is possible for a tautomer to be produced i.e. a hemiacetal. It is envisaged that such tautomers are encompassed with the scope of the present invention. A tautomeric equilibrium is exemplified by the prostaglandin lubiprostone:





In another embodiment, R5 and R6 are both H.
Preferably, R7 and R8 are independently selected from the group consisting of H, OH, F or CH3. In one embodiment, R7 and R8 are both H. In another embodiment, R7 and R8 are both F. In yet another embodiment, one of R7 and R8 is CH3 and the other of R7 and R8 is H or OH.
Preferably, R9 and R10 are independently H or C1-10-alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from CO2H or CO2−M+. More preferably, one of R9 and R10 is H and the other of R9 and R10 is —(CH2)3CO2H or —(CH2)3CO2−Na+.
Preferably, R11 is C1-10-alkyl or —O—(C1-10-alkyl), wherein the alkyl group is optionally substituted with one or more substituents selected from CO2H or CO2−M+. More preferably, R11 is —(CH2)3CO2H or —O—CH2—CO2H.
In a preferred embodiment, the prostaglandin is selected from the group consisting of:










Preferably, the prostaglandin is latanoprost.
Alternatively, if the stationary phase is other than unmodified silica gel, the prostaglandin can be luprostiol:




In one embodiment, the stationary phase is a chiral stationary phase. Preferably, the chiral stationary phase is a derivatised amylose or cellulose polymer or other polysaccharide which is coated or immobilized on silica. More preferably, the chiral stationary phase is selected from the group consisting of Chiralcel OD-H, ChiralPak AS-H, ChiralPak IC, ChiralPak AD-H, Chiralcel OJ-H and Chiralcel OK (products available from Chiral Technologies Inc, and Daicel Chemical Industries, Ltd.). More preferably, the chiral stationary phase is ChiralPak AD-H. For example, the amylose or cellulose polymer may be derivatised with one or more carbamate groups, especially aryl-containing carbamate groups such as 3,5-dimethylphenylcarbamate, (S)-alpha-methylbenzylcarbamate, 4-chlorophenyl carbamate, 4-methylphenylcarbamate, phenyl carbamate, 3-chloro-4-methylphenylcarbamate, 5-chloro-2-methylphenylcarbamate or the like, and/or one or more ester groups, such as acetate, benzoate (e.g., 4-methyl benzoate), cinnamate, or the like.
In another embodiment, the stationary phase is a non-chiral stationary phase. Preferably, the non-chiral stationary phase is selected from the group consisting of Princeton Diol, 4-ethyl pyridine, 2-ethyl pyridine and pyridine urea.
Preferably, the mobile phase further comprises at least one modifier. The modifier can be any suitable liquid solvent. A suitable modifier may be selected from the group consisting of at least one alcohol, acetonitrile, ethyl acetate, methylene chloride and a combination thereof. Preferably, the at least one alcohol is selected from the group consisting of methanol, ethanol, propanol, isopropanol and a combination thereof. It is desirable that the modifier is compatible with the stationary phase. For example, ethyl acetate and methylene chloride cannot be used with a ChiralPak AD column as they will destroy the column.
Carbon dioxide is easily removed and so, the purified prostaglandin can be provided as a solution with the modifier as a solvent. It may therefore be desirable to select a modifier in which the prostaglandin is soluble.
Suitably, the at least one modifier is present in a quantity from (i.e., of at least) about 1% v/v or about 1% w/w to the supercritical carbon dioxide. More preferably, the at least one modifier is present in a quantity from (i.e., of at least) about 5% v/v or about 5% w/w to the supercritical carbon dioxide. The ratio of modifier to carbon dioxide can be varied during the chromatographic process.
Suitable chromatographic apparatus is well known to the skilled person. It is preferred to use an apparatus that is suitable for Supercritical Fluid Chromatography such as the Thar Investigator SFC or Novasep Supersep 20/30 SFC. The crude feed containing the crude prostaglandin is periodically injected into the apparatus wherein the mobile phase flows through the stationary phase which is located in a column. After detection at the column outlet, the purified fractions of the feed are directed to different traps. The carbon dioxide is removed from the purified fractions and is preferably recycled. Detection at the column outlet can be conducted by measuring UV absorption at an appropriate wavelength.
On an analytical scale, the column diameter is suitably from about 2 mm to about 7 mm, preferably about 4.6 mm. The column length is suitably from about 5 cm to about 50 cm, preferably about 25 cm.
On a preparative scale, the column diameter is suitably from about 10 mm to about 200 mm, preferably about 21 mm. The column length is suitably from about 5 cm to about 50 cm, preferably about 25 cm.
The process is suitably carried out at temperatures from about 5° C. to about 45° C., preferably about 25° C. to about 35° C., and at elevated pressures e.g. from about 80 bar to 300 bar, preferably about 100 bar to 150 bar. Typical flow rates depend upon the diameter of the column and may vary from e.g. 1 mL/min to about 5 kg/min.
In another aspect, the present invention provides a prostaglandin obtainable by a method as described above.
The following Examples are illustrative but not limiting of the invention.
Example 1
Analytical Scale
SFC Preparatory Parameters:

    
    
        Column: Chiral Technologies Chiralpak AD-H, 4.6×250 mm
        Column Temperature: 35° C.
        System Back Pressure: 150 Bar
        Eluent: Carbon Dioxide (CO2) with an alcoholic modifier, Ethanol
        Total Flow Rate: 3 mL/min
        Gradient Co-Solvent Profile: Initial conditions=5% Ethanol: 95% CO2, linearly increase to 45% Ethanol: 55% CO2 in 15 minutes, maintain 45% Ethanol: 55% CO2 for 2 minutes, ramp back to initial conditions and equilibrate for 3 minutes prior to next injection
        Detection: UV at 210 nm
        Sample Preparation: Prepare a 0.1 g/mL solution in Ethanol and thoroughly mix the feed solution to completely dissolve the Latanoprost
        Sample Loading: 5 mg (50 μL of 0.1 g/mL crude solution in Ethanol)
        Latanoprost Collection: Collect the Latanoprost peak (heart cut) from approximately 2% above the baseline from the detected Latanoprost peak front (at  ̃9 minutes in retention time) to the Latanoprost detected peak tail (at  ̃10 minutes in retention time).
        The chromatogram of a SFC purification injection of Latanoprost Crude is provided in FIG. 1.
        Obtained Results using Ethanol as the Co-Solvent and AD-H Stationary Phase:
    
    















Chromatographic Purity - Reported as Measured % Peak Area









Isomeric Content*











Latanoprost

C15S-













Sample
Chemical
Isomeric
C11-beta
C15R-trans
C15S-cis
trans





Crude Feed
95.87%
90.02%
1.11%
3.88%
3.04%
0.05%


(0.01 g/mL)


Heart Cut
99.95%
99.92%
—
—
0.08%
—


(5 mg Loading)





*The structures of the latanoprost isomers are set out in FIG. 2.






Example 2
Preparative Scale
SFC Preparatory Parameters:

    
    
        Column: Chiral Technologies Chiralpak AD-H, 21×250 mm
        Column Temperature: 25° C.
        System Back Pressure: 100 Bar
        Eluent: Carbon Dioxide (CO2) with an alcoholic Co-solvent modifier (specifically Ethanol:Methanol (4:1))
        Total Flow Rate: 50 g/min
        Co-Solvent Profile: Initial conditions=15% Co-solvent: 85% CO2, for 360 seconds, Step to 40% Co-solvent: 60% CO2 at 361 seconds until 720 seconds, step back to initial conditions at 721 seconds until 900 seconds
        Detection: UV at 220 nm
        Crude Solution Preparation: Prepare a 0.3 g/mL solution in Ethanol and thoroughly mix the feed solution to completely dissolve the Latanoprost
        Sample Loading:  ̃0.6 mL crude solution in Ethanol
        Latanoprost Collection: Collect the Latanoprost peak (heart cut) from approximately 30 seconds after peak start the baseline from the detected Latanoprost peak front (at  ̃410 seconds in retention time) to approximately 30 seconds prior to the peak end (at  ̃510 seconds in retention time).
        The chromatogram of a SFC purification injection of Latanoprost Crude is provided in FIG. 3.
        Obtained Results using Ethanol as the Co-Solvent and AD-H Stationary Phase of the collected fractions:
    
    















Chromatographic Purity - Reported as Measured % Peak Area










Latanoprost
Isomeric Content§













Sample
Chemical
Isomeric
C11-beta
C15R-trans
C15S-cis
C15S-trans
















Crude
96.51%
93.07%
0.70%
2.79%
2.15%
0.06%


Feed


Purified
99.81%*
99.98%
—
—
0.02%
—


Heart Cut





*No impurities measured above 0.04% by percent peak area.


§The structures of the latanoprost isomers are set out in FIG. 2.






    
    
        No isomers were detected after 11 months storage at freezer temperature when the purified Latanoprost was analyzed with a typical HPLC method for isomer content determination to the following detection levels:
    
    
















Isomer
Detection Limit









C11-beta
0.03%



C15R-Trans
0.03%



C15S-Trans
0.05%



C15S-Cis
0.03%



Latanoprost
0.03%

## Claims

1. A method for the purification of a prostaglandin by supercritical fluid chromatography, said method comprising using a stationary phase and a mobile phase comprising carbon dioxide, wherein the stationary phase is a non-chiral stationary phase selected from the group consisting of 4 ethyl pyridine, 2-ethyl pyridine and pyridine urea or a chiral stationary phase and the prostaglandin is a compound of formula (I), a compound of formula (II), or a compound of formula (IV):



wherein,
V is C;
W is




X is CONR12R13 or CO2R12;
Y is




Z is C1-20-alkyl, C2-20-alkynyl, —O—(C6-20-aryl) or —(C1-20-alkyl)-(C5-20-aryl), wherein the aryl group is optionally substituted with one to three substituents selected from the group consisting of C1-20-alkyl, halo and C(halo)3;
R1 and R2 are independently H or OH, or R1 and R2 together form




R3 and R4 are independently H or OH;
R5 and R6 are independently H or OH, or R5 and R6 together form




R7 and R8 are independently H, OH, halo or C1-20-alkyl; R9 and R10 are independently H or C1-20-alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from CONR12R13, CO2R12, and CO2−-M+; R11 is C1-20-alkyl or —O—(C1-20-alkyl), wherein the alkyl group is optionally substituted with one or more substituents selected from CONR12R13, CO2R12, and CO2−-M+; R12 and R13 are independently selected from the group consisting of H and C1-20 alkyl; M+ is a counter cation; and
Halo is fluorine, chlorine, bromine or iodine.

2. A method according to claim 1, wherein the prostaglandin is selected from the group consisting of:

3. A method according to claim 1, wherein the prostaglandin is:

4. A method according to claim 1, wherein the stationary phase is a chiral stationary phase.

5. A method according to claim 4, wherein the chiral stationary phase is an amylose or cellulose containing polymer which is coated or immobilized on silica.

6. A method according to claim 4, wherein the chiral stationary phase is selected from the group consisting of cellulose-3,5-dimethylphenylcarbamate-on silica, amylose-(S)-alpha-methylbenzylcarbamate on silica, cellulose-3,5-dichlorophenylcarbamate on silica, amylose-3,5-dimethylphenylcarbamate on silica, cellulose-4-methylbenzoate on silica and cellulose-cinnamate on silica.

7. A method according to claim 1, wherein the stationary phase is a non-chiral stationary phase selected from the group consisting of 4-ethyl pyridine, 2-ethyl pyridine and pyridine urea.

8. A method according to claim 1, wherein the mobile phase further comprises at least one modifier.

9. A method according to claim 8, wherein the at least one modifier is selected from the group consisting of at least one alcohol, acetonitrile, ethyl acetate, methylene chloride and a combination thereof.

10. A method according to claim 8, wherein the at least one modifier includes an alcohol selected from the group consisting of methanol, ethanol, propanol, isopropanol and a combination thereof.

11. A method according to claim 8, wherein the at least one modifier is present in a quantity from about 1% v/v to the carbon dioxide.

12. A method according to claim 8, wherein the at least one modifier is present in a quantity from about 1% w/w to the carbon dioxide.

13. A method according to claim 8, wherein the at least one modifier is present in a quantity from about 5% v/v to the carbon dioxide.

14. A method according to claim 8, wherein the at least one modifier is present in a quantity from about 5% w/w to the carbon dioxide.

15. A method according to claim 1, wherein the method provides a purified prostaglandin which is at least 99% chemically pure.

16. A method according to claim 1, wherein the method provides a purified prostaglandin which is at least 99% isomerically pure.

17. A method for the purification of latanoprost by supercritical fluid chromatography, said method comprising the use of a chiral stationary phase and a mobile phase comprising carbon dioxide.

18. A method for purifying a crude prostaglandin, comprising injecting the crude prostaglandin onto a column comprising a stationary phase, eluting the crude prostaglandin through the column using a mobile phase comprising supercritical carbon dioxide, and collecting a fraction comprising a purified prostaglandin, wherein the stationary phase is a non-chiral stationary phase selected from the group consisting of 4-ethyl pyridine, 2-ethyl pyridine and pyridine urea or a chiral stationary phase and the prostaglandin is a compound of formula (I), a compound of formula (II), or a compound of formula (IV):



wherein,
V is C;
W is




X is CONR12R13 or CO2R12;
Y is




Z is C1-20-alkyl, C2-20-alkynyl, —O—(C6-20-aryl) or —(C1-20-alkyl)-(C6-20-aryl), wherein the aryl group is optionally substituted with one to three substituents selected from the group consisting of C1-20-alkyl, halo and C(halo)3;
R1 and R2 are independently H or OH, or R1 and R2 together form




R3 and R4 are independently H or OH;
R5 and R6 are independently H or OH, or R5 and R6 together form




R7 and R8 are independently H, OH, halo or C1-20-alkyl;
R9 and R10 are independently H or C1-20-alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from CONR12R13, CO2R12, and CO2−M+;
R11 is C1-20-alkyl or —O—(C1-20-alkyl), wherein the alkyl group is optionally substituted with one or more substituents selected from CONR12R13, CO2R12, and CO2−-M+;
R12 and R13 are independently selected from the group consisting of H and C1-20 alkyl;
M+ is a counter cation; and
Halo is fluorine, chlorine, bromine or iodine.