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 βt 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

Figure 1 illustrates a chromatogram of a SFC purification injection of crude latanoprost, obtained as described in Example 1.

Figure 2 sets out the structures of latanoprost isomers. Figure 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 isomeπcally 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 isomeπcally 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 O. lmg of purified prostaglandin, preferably about lmg of purified prostaglandin, in a reasonable timeframe, i.e. less than a day. By "preparative", we mean a scale of providing at least about lmg of purified prostaglandin, preferably about O. lg of purified prostaglandin and most preferably about Ig 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 ,s X=Λ , y^≠ or X^V ;

X is CONR ₁₂ Ri ₃ or CO ₂ Ri ₂ :

Y ,s *W , X^/ or ¹ V^ _;

Z is Ci ₂ o-alkyl, C _{2 20} -alkynyl, -0-(C _{5 2} o-aryl) or -(Ci ₂ o-alkyl)-(C _{6 20} -aryl), wherein the aryl group is optionally substituted with one to three substituents selected from the group consisting of Ci ₂₀ -alkyl, halo and C(halo) ₃ ;

Ri and R ₂ are independently H or OH, or R ₁ and R ₂ together form < ;

R ₃ and R ₄ are independently H or OH;

R ₅ and R ₅ are independently H or OH, or R ₅ and R ₅ together form ^< ,

R ₇ and R ₈ are independently H, OH, halo or C _{1 20} -alkyl;

Rg and Ri ₀ are independently H or C _{1 20} -alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from CONR ₁₂ R ₁₃ , CO ₂ Ri ₂ or CO ₂ M ⁺ ;

R ₁₁ is Ci ₂₀ -alkyl or -0-(C _{1 20} -alkyl), wherein the alkyl group is optionally substituted with one or more substituents selected from CONRi ₂ Ri ₃ , CO ₂ Ri ₂ or CO ₂ M ⁺ ;

Ri ₂ and R _i3 are independently selected from the group consisting of H and Ci 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 Ci _-2 o-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, t-butyl, t-butyl and cyclobutyl.

"Alkynyl" refers to linear or branched hydrocarbon structures having at least one

-\ <-c ^{= ~} c ~\ <- 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 aikyny! group are encompassed. For example "butyrsy!" 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 -CO ₂ i.e -CO ₂ 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 A=/" ¹ - _O r ^^^V

X is preferably CONH(C _{1 10} -alkyl), CO ₂ (C _{1 10} -alkyl) or CO ₂ H. More preferably, X is selected from the group consisting of CONHEt, CO ₂ Me, CO ₂ ¹ Pr and CO ₂ H.

Preferably, Z is C _{1 10} -alkyl, C _{2 10} -alkynyl, -0-(C _{6 10} -aryl) or -(C-, ₁₀ -alkyl)-(C _{β 10} -aryl), wherein the aryl group is optionally substituted with one to three substituents selected from the group consisting of Cl and -CF ₃ . More preferably, Z is -(CH ₂ ) ₅ CH ₃ , -(CH ₂ ) ₃ CH ₃ , -CH ₂ -Ph, ₁ Ri and R ₂ may independently be H or OH. Preferably, when one of R ₁ and R ₂ is H, the other of R ₁ and R ₂ is OH. More preferably, when one of R ₁ and R ₂ is H and the other of Ri and R ₂ is OH, R ₁ , R ₂ and the carbon atom to which they are attached have the following stereochemistry:

HO

R ₃ and R ₄ are independently H or OH. Preferably, when one of R ₃ and R ₄ is H, the other of R ₃ and R ₄ is OH. More preferably, R ₃ , R ₄ and the carbon atom to which they are attached have the following stereochemistry: V V

HO

R ₅ and R ₆ may independently be H or OH. In one embodiment, when one of R ₅ and R ₆ is H, the other of R ₅ and R ₆ is OH. More preferably, when one of R ₅ and R ₆ is H and the other of R ₅ and R ₆ is OH, R ₅ , R ₅ and the carbon atom to which they are attached have the following stereochemistry:

OH

When R ₅ and R ₆ together form < , and one of R ₃ and R ₄ is OH and the other of R ₃ and R ₄ 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:

Tautomer I

In another embodiment, R ₅ and R ₆ are both H.

Preferably, R ₇ and R ₈ are independently selected from the group consisting of H, OH, F or CH ₃ . In one embodiment, R ₇ and R ₈ are both H. In another embodiment, R ₇ and R ₈ are both F In yet another embodiment, one of R ₇ and R ₈ is CH ₃ and the other of R ₇ and R ₈ is H or OH.

Preferably, R ₉ and R ₁₀ are independently H or Ci i _O -alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from CO ₂ H or CO ₂ M ⁺ . More preferably, one of R ₉ and R ₁₀ is H and the other of R ₉ and R ₁₀ is -(CH ₂ ) ₃ CO ₂ H or - (CHz) ₃ CO ₂ Na ⁺ .

Preferably, Rn is Ci i _O -alkyl or -0-(Ci _l o-alkyl), wherein the alkyl group is optionally substituted with one or more substituents selected from CO ₂ H or CO ₂ M ⁺ . More preferably, R ₁₁ is -(CH ₂ ) ₃ CO ₂ H or -0-CH ₂ -CO ₂ H.

In a preferred embodiment, the prostaglandin is selected from the group consisting of:

Bimatoprost Travoprost

Cloprostenol Unoprotone

Dinoprostone

Misoprostol Alprostadil

Preferably, the prostaglandin is latanoprost.

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

Luprostiol

In one embodiment, the stationary phase is a chiral stationary phase. Preferably, the chiral stationary phase is a deπvatised 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 amylase or cellulose polymer may be deπvatised with one or more carbamate groups, especially aryl- containing carbamate groups such as 3,5-dιmethylphenylcarbamate, (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 2mm to about 7mm, preferably about 4.6mm . The column length is suitably from about 5cm to about 50cm, preferably about 25cm.

On a preparative scale, the column diameter is suitably from about 10mm to about 200mm, preferably about 21mm. The column length is suitably from about 5cm to about 50cm, preferably about 25cm.

The process is suitably carried out at temperatures from about 5 ⁰ C to about 45 ⁰ C, preferably about 25 ⁰ C to about 35 ⁰ 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. lmL/min to about 5kg/mιn.

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:

o Column : Chiral Technologies Chiralpak AD-H, 4.6 x 250mm

o Column Temperature: 35°C

o System Back Pressure ¹ 150Bar

o Eluent: Carbon Dioxide (CO ₂ ) with an alcoholic modifier, Ethanol

o Total Flow Rate: 3mL/mιn

o Gradient Co-Solvent Profile: Initial conditions = 5% Ethanol : 95% CO ₂ , linearly increase to 45% Ethanol : 55% CO ₂ in 15mιnutes, maintain 45% Ethanol : 55% CO ₂ for 2 minutes, ramp back to initial conditions and equilibrate for 3 minutes prior to next injection o Detection : UV at 210nm

o Sample Preparation : Prepare a O. lg/mL solution in Ethanol and thoroughly mix the feed solution to Lυmμieteiy dissolve the Latanoprost

o Sample Loading : 5mg (50μL of O. lg/mL crude solution in Ethanol)

o 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).

o The chromatogram of a SFC purification injection of Latanoprost Crude is

provided in Figure 1.

o Obtained Results using Ethanol as the Co-Solvent and AD-H Stationary Phase :

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

Example 2 - Preparative Scale

SFC Preparatory Parameters:

o Column : Chiral Technologies Chiralpak AD-H, 21 x 250mm

o Column Temperature : 25°C

o System Back Pressure : lOOBar

o Eluent: Carbon Dioxide (CO ₂ ) with an alcoholic Co-solvent modifier (specifically

Ethanol : Methanol (4: 1))

o Total Flow Rate: 50g/mιn

o Co-Solvent Profile: Initial conditions = 15% Co-solvent : 85% CO ₂ , for 360

seconds, Step to 40% Co-solvent : 60% CO ₂ at 361 seconds until 720 seconds, step back to initial conditions at 721 seconds until 900 seconds

o Detection : UV at 220nm

o Crude Solution Preparation : Prepare a 0.3g/mL solution in Ethanol and thoroughly mix the feed solution to completely dissolve the Latanoprost

o Sample Loading : ~0.6mL crude solution in Ethanol o 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).

o The chromatogram of a SFC purification injection of Latanoprost Crude is

provided in Figure 3.

o Obtained Results using Ethanol as the Co-Solvent and AD-H Stationary Phase of the collected fractions:

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

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

o 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: