IMPROVED PROCESS FOR PREPARATION OF OCTREOTIDE BY SOLUTION PHASE PEPTIDE SYNTHESIS

FIELD OF INVENTION

The present invention relates to a novel processes for the preparation of pharmaceutical grade octreotide (1) using classical solution phase peptide synthesis in high yield and purity.

BACKGROUND OF THE INVENTION

Octreotide is a cyclic octapeptide; it is highly potent and pharmacologically selective analog of somatostatin. It possesses excellent biological activity both in vitro and in vivo fPless J., Metabolisim, 41, 5-6, (1992). Chemically, octreotide is D-phenylalanyl-L-Cysteinyl- L- phenylalanyl -D-tryptophyl - L-lysyl - threonyl - N- [2-hydroxy - 1-(hydroxymethyl)propyl]- L-Cysteinamide cyclic (2-7)-disulfide (1). The presence of a D- phenylalanine in the N-terminal and an amino alcohol in the C-terminal end, along with the D- tryptophan residue and the disulfide bridge, make the molecule very resistant to metabolic degradation.

Octreotide inhibits growth hormone for a long period and is therefore indicated for acromegaly to control and reduce the plasma level of growth hormone. It is also indicated for the symptomatic treatment of patients with metastatic carcinoid tumors and vasoactive intestinal peptide tumors.


Three letter code formula

H - (D) Phe - Cys-Phe - (D) - Trp - Lys - Thr - Cys - Thr – OL Octreotide-1

Conventional synthesis of octreotide may be divided two main types, direct solid phase peptide synthesis and solution-phase peptide synthesis.

Direct Solid phase synthesis comprises attachment of a C-terminal amino acid to resin, and step by step elongation of the peptide chain, with pre activated amino acid. Several patents describe solid phase synthesis of octreotide, for example EP 953,577; US 5889146 disclose synthesis using 2-chlorotrityl resin and Fmoc-butyl protection.

The other patents US 6346601, KR 2009074316, WO 20081087794, CN 1837232, CN 1699404, CN 1810829, CA 2511711, CN 1569890, TW 519545, US 6476186, WO 2002081499 and several publications Tetrahedron Letters (1998), 39(13) 1783-1784, Tetrahedron Letters 1997, 38,883, Journal of medicinal chemistry 1994, 37, 3749
disclose the processes for the preparation of octreotide using solid phase peptide synthesis.

All the solid phase synthetic schemes involve use of expensive resins and Fmoc/tbu protected amino acids in 3 to 4 fold excess, necessitating complex purification procedures to separate the product from the impurities. These short comings render these processes unsuitable for large scale industrial production of the product.

Hybrid synthesis involving a part of solid phase synthesis and solution phase synthesis are described in patents CA2458084/WO2005087794.

Solution phase synthesis comprises condensation of amino acids in solution. Several segments/blocks containing from 2 to 5 amino acids synthesized independently, followed by condensation of these segments/blocks to each other in the required sequence. The advantage of this type of process is that it is inexpensive than the solid phase synthesis and this method is also more suitable for large scale production.

The first octreotide solution phase synthesis is described in patents US 4395403, EP 029579. In these patents thiol group of cysteines are protected by methoxybenzyl group. To deblock methoxybenzyl group boron tris trifluoroacetate in trifluoroacetic acid is used. This reagent damages tryptophan moiety present.

WO 2003/097668, WO 2007/ 110765 describe solution phase synthesis. In these patents thiol group of cysteines are protected by acetamidomethyl (Acm) groups and iodine was used for deprotection of acetamidomethyl group and followed by oxidative cyclization. These patents apply 3+3=6+2=8 strategy. Basically the strategy involves coupling two tripeptide segments Boc- D - Phe - Cys (Acm) - Phe - OMe and Z-D-Trp-Lys(Boc)-Thr-OMe to yield hexapeptide Boc- D - Phe - Cys (Acm) - Phe - D-Trp-Lys(Boc)-Thr-OMe, further hexapeptide segment is coupled with dipeptide H-Cys(Acm)-Thr-OMe/OL to
yield octapeptide Boc- D - Phe - Cys (Acm) - Phe - D-Trp-Lys(Boc)-Thr-Cys(Acm)-Thr-OMe/OL. The major drawback of the procedures described in these patents is racemization of phenylalanine methyl ester present in Boc- D - Phe - Cys (Acm) - Phe -OMe. In addition the processes involve purification of segments by preparative HPLC, which increases cost of production.

Oxidative cyclization of protected or non-protected sulfhydryl groups with formation of disulfide structures is usually carried out as the final synthetic step, the reason being substantial thermal and chemical ability of the disulfide linkage. The oxidation of open-chain peptide containing free and / or certain types of protected sulfhydryl groups with iodine in methanol or acetic acid, acetic acid/water reported. Iodine, however, is not without drawback as cyclization agent. For instance, tryptophan moieties present in peptide substrates are at risk of being iodinated, making the balance between full conversion of starting materials and minimizing side reactions a delicate one, which, in turn impact product purity. In the present invention this aspect has been rightfully tackled by not opting for iodine route for oxidative cyclization. The present invention hydrogen peroxide used for oxidative cyclization.

Another complicating factor in known routes of synthesis is the possibility of interaction between the desired cyclic disulfide and inorganic sulfur compounds used for reducing excess iodine at the end of the reaction, such as sodium dithionite, sodium thiosulfate, and such reducing agents interact with the disulfide linkage. As the process of present invention has avoided use of iodine, the resulting product has high purity and virtually devoid of formation of any impurities.

The present application describes a novel chemistry involving 2+4=6+2=8 strategy for preparation of octreotide. In this strategy no racemized impurities are produced and the processes do not involve any HPLC purification of intermediates.

SUMMARY OF THE INVENTION;

This invention describe a process for obtaining octreotide or a pharmaceutically acceptable salt in commercial scale quantities by solution phase chemistry methods using mild reagents to yield high purity octreotide. The process includes the following

1. Thiol groups of cysteine are protected by triphenyl methyl (Trityl) groups. Treatment of the Cys (Tit) containing linear novel octapeptide (segment-ABC, 20) of the invention with TFA /water/ TIS mixture in one step removes the Trt and Boc groups.

Simultaneously to give non-cyclized octreotide, which is further oxidized with hydrogen peroxide to give cyclized octreotide (1) with 75 -85% yield

The novel protected octapeptide alcohol (Segment-ABC, 20) of present invention is prepared from Fmoc dipeptide alcohoj (Segment C, 17) and hexapeptide acid (Segment AB2,19).

The novel hexapeptide (segment-ABl, 18) is prepared by condensation of two protected novel segments, dipeptide (4) and tetra peptide (13) which on hydrolysis with sodium hydroxide afforded novel hexapeptide acid (19).

ABBREVIATIONS

Fmoc = Flourenylmethoxycarbonyl Trt = Triphenyl methyl (Trityl) Z = Benzyloxycarbonyl tbu =tert-Butyl THF = Tetrahydrofuran DMF = N, N- Dimethylformamide NMM =N-methylmorpholine TEA = Triethylamine Bzl = Benzyl
BTFA = Boron - tris - trifluoroacetate TFA = Trifluoroacetic acid EDT = Ethanedithiol TIS =Triisopropylsilane HBTU = O-Benzotriazole-N, N, N', N'-tetramethyl-uronium-hexafluoro-phosphate DCI = N, N- diisopropylcarbodiimide HOBt = 1 -Hydroxybenzotriazole DCM = Dichloromethane

SYNTHETIC SCHEME OF OCTREOTIDE ACETATE

Segment - A

Formula (1) comprises preparation of appropriate peptide fragments using the standard process of peptide chemistry, known to the practitioners in the art. Thus amino functions of amino acids are protected with protecting groups like Boc, Z, Fmoc and the carboxyl functions of amino acids are protected with alkyl like methyl N-osu or arylphenol like nitrophenol.

The condensation of the carboxyl group of the amino protected amino acid is by using active ester method. The ester could be N-hydroxysuccinimide or aromatic phenol like nitro phenol. The reaction is carried out by dissolving the carboxyl protected amino acid in polar aprotic solvent like, dimethylformamide, tetrahydrofuran, acetonitrile, in presence of tertiary amines such triethylamine, diisopropylethylamine, N-methymorpholine, at temperature of 10 to 40°C for 2-24 hours.

Alternately the condensation of the carboxyl group of the amino protected amino acid is typically carried out by dissolving the appropriately protected amino acids in equimolar quantities in a solvent like dimethylformamide, tetrahydrofuran, dichloromethane and adding a condensing agent such as HBTU, DCI, HOBt in equimolar quantity at temperature of 0-10°C and stirring at 5 to 40°C for 2 to 20 hours. The protected group of amino /carboxyl is removed by the use of appropriate reagent known in the art.

The basic differences from other procedures already described are;

1. The thiol groups of cysteine is protected by triphenylmethyl (Trt) group

2. The N-terminal hexapeptide (segment -AB2) (19) is synthesized by condensation of di, tetra peptide fragments (segment -A, 4, segment -B, 13) and followed by saponification.

3. The C-terminal dipeptide alcohol is protected with Fmoc group and the peptide condensed with Fmoc-Cys (Trt)-OH (14) and L- threoniol hydrochloride (15).

4. Treatment of the linear octapeptide alcohol (segment-ABC, 20) of the invention with hydrogen peroxide in one step removes the Boc, Trt groups simultaneously and then
cyclization with hydrogen peroxide to get octreotide.

The following are the preferred embodiments as described. As shown in the process Fmoc - Cys (Trt) - Thr - OL (16) is prepared by using Fmoc - Cys (Trt) - OH (14) and H-Thr-OL. HC1 (15) in presence of HBTU and base N-methylmorpholine in DMF at 25°C.

The resulting dipeptide alcohol is treated with 20% diethylamine in dichloromethane at 25°C to give H- Cys (Trt) - Thr - OL (17), which is further condensed with novel protected hexapeptide acid Boc - D Phe -Cys (Trt)-Phe -D -Trp -Lys (Boc)-Thr - OH, (Segment-AB2) (19) in the presence of HBTU in dimethylformarnide at 25°C to obtain the novel protected linear octapeptide alcohol Boc - D- Phe - Cys(Trt) - Phe -D- Trp -Lys (Boc) -Thr - Cys(Trt)- Thr-ol,(Segment-ABC)( 20). Boc and Trityl groups are removed by treatment with trifluoroacetic acid /water/Triisopropylsilane at 25°C to obtain linear octapeptide which is cyclized with hydrogen peroxide in 1% ammonium acetate solution at pH 7-8 to give octreotide (1)

The hexapeptide acid segment AB2 (19) of the present invention is prepared by condensation of appropriately protected dipeptide and tetrapeptide segments A (4) and segment B (13) followed by saponification as shown in the scheme.

The protected dipeptide Boc - D- Phe - Cys (Trt) -OH (segment A, 4) can be prepared by any of the processes known in the art from commercially available raw materials. D-Phenylalanine (2) carrying Boc group for Na" protection and nitrophenol group for carboxyl group as activation is treated with S-Trityl L- cysteine (3) employing TEA as base in equimolar amounts in dimethylformarnide at 25°C to obtain the protected dipeptide(4).

Similarly, for synthesis of tetrapeptide segment B (13), D- tryptophan (5) carrying Z group as Na' protection and succinidimyl group for carboxyl group activation is treated with Boc- L- lysine (6) using sodium bicarbonate as base in tetrahydrofuran at 0-5°C to give protected dipeptide Z-D- Trp -Lys (Boc) - OH (7). The resulting peptide is condensed with L-Thr-OMe.HCl (8) in presence of TEA, and HBTU in
dimethylformamide at 25°C to obtain the protected tripeptide methyl ester Z-D- Tip -Lys (Boc) - Thr-OMe (9). The Z-group is removed from tripeptide ester by hydrogenation over 10% Pd/C and resulting peptide H-D- Trp -Lys (Boc) - Thr-OMe (10) is condensed with L-Phe-OH (11) carrying Fmoc group, using HBTU as condensation agent in dimethylformamide at 25°C to obtain the protected tetrapeptide methyl ester Fmoc-Phe -D- Trp -Lys (Boc) - Thr-OMe (12). Treatment with diethylamine in dichloromethane affords H-Phe -D- Trp -Lys (Boc) - Thr-OMe (segment B), (13).

The process further consists of coupling of two segments AB2 (19) and segment C (17), in the presence of HBTU in an aprotic solvent such as DMF, DMA, NMP, and THF.

After completion of the reaction, the mass is treated with 10% hydrochloric acid solution, the separated solid was filtered, resulting solid material dissolved in ethyl acetate and treated with n-hexane and filtered to get the desired product segment ABC (20).

For removal of Boc and Trityl protection, the product is treated with trifluoroacetic acid in the presence of 0.25% of water/Triisopropylsilane as scavengers. After completion of the reaction, the mixture is concentrated and the residue is treated with ether. The separated product is filtered and suspended in water followed by adjustment to pH 7-8 with bases selected from sodium hydroxide, potassium hydroxide, sodium bicarbonate, sodium carbonate and calcium carbonate. The resultant solution is oxidized with hydrogen peroxide to afford the desired octreotide and further treated with acetic acid to obtain crude octreotide acetate salt. This crude peptide solution is further purified by employing preparative HPLC process to get the product purity of more than 99.0%. The present invention is illustrated by the following examples which are provided merely to exemplify the invention.

Example-1: Preparation of (Boc-D - Phe - Cys (Trt) - OH) (segment-A)

Boc -D - Phe - ONp, 2 (6.5g, 16.8mmoles) was added to a solution of H - Cys (Trt) -OH, 3 (9.5g, 26.1mmoles) and pH adjusted to 7-8 using N-methylmorpholine (2.0mL) in DMF at 10°C. The reaction was stirred at room temperature for 24 hours. Then the reaction mass was treated with 1M hydrochloric acid (300mL) at 0-5°C. The separated pale yellow solid material was filtered and washed with 1M hydrochloric acid (lOOmL) and dried. The dried material was treated with a mixture of isopropyl ether/hexane (2x80mL) to yield compound 4. Yield: 85%, HPLC purity >95%

Example-2: Preparation of Z-D - Trp - Lys (Boc) - OH (segment-Bl)

A solution of Z- D - Trp - Osu, 5 (25.0g, 57.4mmoles) in THF (300mL) was added to L - Lys (Boc) - OH, 6 (21.5g, 87.3mmoles) in water (125mL)containing sodium bicarbonate (7.25g, 86.3mmoles) and stirred at room temperature for 3 hours. The reaction mass was adjusted to pH 7.0 by adding hydrochloric acid (10%). Then the reaction mass was distilled to remove tetrahydrofuran, pH was adjusted to 2-3 and extracted with ethyl acetate (300 mL) and washed with 10% sodium bicarbonate solution (2x200mL), 10% hydrochloric acid solution (2x200mL), and brine (2x200mL), dried with sodium sulfate and evaporated to yield brown oily mass. The mass was treated with hexane and separated solid was filtered, and dried to get product (7) Yield 27.6g (85%).

Example-3: Preparation of Z-D - Trp - Lys (Boc) - Thr - OMe (Segment-B2)

A solution of Z-D - Trp - Lys (Boc) - OH, 7 (30.0g, 53.2mmol) and HOBt (7.2g, 52.9 mmol) in dimethylformamide (80mL) was added to a solution containing L-Thr -OMe.HCl, 8 (12.0g, 70.0mmol) and N-methylmorpholine (7.8mL, 70.9mmol) in dimethylformamide (80mL) at 0-5°C. Then HBTU (20g, 53.4mmol) and N-methylmorpholine (11.7mL, 106.4mmol) was added and stirred at same temperature for 3 hours. The reaction mass was filtered to remove undissolved matter; the filtrate is concentrated under vacuum. The mass was extracted with ethyl acetate (400mL) and washed with 10% hydrochloric acid solution(3x250mL), 10% sodium bicarbonate
solution (2x200mL), brine (2x200mL), dried over sodium sulfate and evaporated to yield light pale yellow mass. The mass was treated with n-hexane (lOOmL) and separated solid material, filtered, dried to get product 9. Yield 31.0g (86%).

Example-4: Preparation of H-D - Trp - Lys (Boc) - Thr - OMe (Segment-B3)

Z-D - Trp - Lys (Boc) - Thr - OMe, 9 (lO.Og) dissolved in methanol (175mL) and acetic acid (4.0mL) was hydrogenated in the presence of 10% palladium on charcoal (l.Og, 50% wet) at temperature 45°C for 6 hours. Then the reaction mass was filtered through celite bed and concentrated under vacuum. The resulting residue was extracted with ethyl acetate (250mL), and washed with 10% sodium bicarbonate solution (2xl50mL), brine (2x200mL), dried on sodium sulfate and evaporated under reduced pressure to get product 10. Yield 6.3g (86%).

Example-5: Preparation of Fmoc -Phe - D - Trp - Lys (Boc) - Thr - OMe, (Segment-B4)
Fmoc - Phe - OH, 11 (14.0g) and HBTU (14.0g, 36mmol) was added to a solution of H-D - Trp - Lys (Boc) - Thr - OMe 10 (20.0g, 36mmol) in dimethylformamide at 25°C. Then NMM (8.0mL, 72.0mmol) was added and the reaction mixture maintained at same temperature for 3 hours. The reaction mass was treated with 300 mL of 1M hydrochloric acid at 0-5°C, separated solid material was filtered and, washed with 10% hydrochloric acid solution (2xl50mL), to get product 12. Yield 26.4g (80%).

Example-6: Preparation of H -Phe - D - Trp - Lys (Boc) - Thr - OMe, (Segment-B5)

A solution of Fmoc -Phe - D - Trp - Lys (Boc) - Thr - OMe, 12 (25.0g) in 20% diethylamine in dichloromethane (150mL) was stirred at room temperature for 3hours.

Then the solvent was distilled to remove dichloromethane. The residue was triturated with (3x250mL) isopropyl ether and filtered off to get product 13. Yield 20.0g (85%).

Example-7: Preparation of Fmoc -Cys (Trt) -) Thr - OL, (Segment-Cl)

A solution of Fmoc - Cys (Trt) - OH, 14 (20.0g, 34.0mmol) and HOBt (5.2g, 38.09 mmol) in dimethylformamide (lOO.OmL) was added to a solution containing H-Thr - OL. HC1, 15 (7.2g, 60.0mmol) and N-methylmorpholine (7.5mL, 68.0 mmol) in dimethylformamide (70.0mL) at 0-5°C. Then HBTU (14.0g, 36.0mmol) and N-methylmorpholine (9.0mL) was added and stirred at same temperature for 3 hours. The reaction mass was treated with 1M hydrochloric acid (600 mL) at 0-5°C, separated pale yellow solid material was filtered and, washed with 10% hydrochloric acid solution(2x250mL), to get product 16. Yield 20.0g (89%).

Example-8: Preparation of H -Cys (Trt) - Thr - OL, (Segment-C2)

A solution of Fmoc -Cys (Trt) - Thr - OL, 16 (10.0g) in 20% diethylamine in dichloromethane (100mL) was stirred at room temperature for 2 hours and distilled to remove dichloromethane. The crude mass was triturated with (3x250mL) isopropyl ether and filtered to get product 17. Yield 5.7g (85%).

Example-9: Preparation of Boc - D Phe - Cys (Trt)-Phe - D - Trp - Lys (Boc) - Thr - OMe, (Segment-ABl)

The dipeptide 4 (13.0g, 21.3mmol), HBTU (8.3g, 21.8mmol) were added to a solution of a tetrapeptide 13 (20.0g, 21.3mmol) in DMF at room temperature. The pH of reaction mass was adjusted to 7-8 with N-methylmorpholine (4.7mL, 42.6mmol) and stirred for 3 hours. The reaction mass was treated with 1M hydrochloric acid (600mL) at 0-5°C. The separated pale yellow solid material was filtered and washed with 10% hydrochloric acid solution (2x250mL) and dried to get product 18. Yield 26.4g (80%).

Example-10: Preparation of Boc - D- Phe -Cys (Trt)-Phe -D -Trp -Lys (Boc)-Thr - OH, (Segment-AB2)

Sodium hydroxide (3N, 13mL) solution was added to a solution of hexapeptide methyl ester 18 (8.0g) in methanol (lOOmL) and stirred for 4 hours at 0-5°C. The reaction mass was adjusted to pH 7 with hydrochloric acid, the solution was concentrated under reduced pressure, and further acidified to pH 3 with 10% hydrochloric acid and the resulting residue dissolved in ethyl acetate (400mL). The ethyl acetate layer was washed with 10% hydrochloric acid solution (3x250mL), 10% sodium bicarbonate solution (2x200mL), brine (2x200mL), dried over sodium sulfate and evaporated under reduced pressure to get product 19. Yield 6.3g (80%).

Example-11: Preparation of Boc - D- Phe - Cys (Trt)-Phe - D - Trp - Lys (Boc) -Thr - Cys (Trt) - Thr - OL, (Segment-ABC) Dipeptide 17 (6.1 g, 13.6mmol) was dissolved in DMF (100.0mL) and hexapeptide acid 19 (14.0g, llmmol), HBTU (4.6g, 12.1mmol) were added at 0-5°C, then N-methylmorpholine (3.0mL, 24.0mmol) was added and stirred for 3 hours. The reaction mass was treated with 1M hydrochloric acid (600mL) at 0-5°C. The separated pale yellow solid material was filtered. The filtered solid was dissolved in ethyl acetate (250mL) and washed with 10% hydrochloric acid solution (2x250mL), 10% sodium bicarbonate solution (200mL) and distilled to remove ethyl acetate then hexane was added to get product 20. Yield 15.0g (80%).

Protected linear octapeptide alcohol (10.0g segment-ABC, 20) was dissolved in a cold solution of cocktail mixture (0-5°C) TFA/H20/TIS (9.0:0.5:0.25) (152.4mL) and stirred for 2 hours. The reaction mass was filtered to remove precipitated product, the solution
was concentrated to 3/4 volume under reduced pressure and the remaining solution was triturated with isopropyl ether (250mL). The separated solid was filtered and washed with isopropyl ether (200mL) and the solid dissolved immediately in 5% ammonium acetate solution (5000mL). The pH of the solution was adjusted to pH 7-8 with ammonia, added hydrogen peroxide (l.OmL), and stirred for 3hours.The pH was adjusted to 3-4 with acetic acid (60mL), the product was purified by reverse phase HPLC chromatography to yield octreotide acetate 1. Yield 3.0g (30%)

Example 13: Purification of octreotide

Primary Purification

The above octreotide acetate solution is loaded on to prep C-18 column (50x 250mm, 100A). Fractions with >95%purity is pooled and taken up for secondary purification.

Buffer A: 0.1% ammonium acetate pH=5.5 to 6.0 Buffer B: Acetonitrile

Salt exchange

Fraction with purity greater than 95-98% are pooled and loaded again onto to prep C-18
column (50x 250mm, 100A0). The fraction with more than 99% purity were pooled and
lyophilization.

Buffer A: 0.5% AcOH in water.

Buffer B: 0.5% AcOH in water + acetonitrile (50+50)

We Claim

1. The process for preparing octreotide from novel linear octapeptide alcohol Boc -D-Phe - Cys(Trt)-Phe -D - Trp - Lys (Boc) - Thr - Cys (Trt) - Thr - OL, (20, Segment ABC) and oxidation followed by purification to yield pharmaceutical grade octreotide acetate.

2. The process for preparing octreotide according to claim 1, wherein the novel linear octapeptide alcohol Boc -D- Phe - Cys(Trt)-Phe -D - Trp - Lys (Boc) - Thr - Cys (Trt) - Thr - OL is treated with TFA/H20/TIS and cyclized with H202 to yield octreotide.
3. The process according to claim 1, wherein the novel octapeptide Boc -D- Phe -Cys(Trt)-Phe -D - Trp - Lys (Boc) - Thr - Cys (Trt) - Thr - OL, (20, Segment ABC) is prepared by condensation of hexapeptide Boc -D- Phe - Cys(Trt)-Phe -D - Trp - Lys (Boc) - Thr - OH (19) with dipeptide alcohol H- Cys (Trt) - Thr - OL
(17).

4. The process according to claim 3, wherein hexapeptide Boc -D- Phe - Cys(Trt)-Phe -D - Trp - Lys (Boc) - Thr - OH (19) is prepared by condensing Boc - D -Phe - Cys (Trt) - OH (4) with tetrapeptide H-Phe -D - Trp - Lys (Boc) - Thr -OMe (13) followed by hydrolysis.

5. The process according to claim 3, wherein dipeptide H- Cys (Trt) - Thr - OL (17), was prepared by condensation of Fmoc - Cys (Trt) - OH (14) with L-Thr - OL.HC1 (15) and removing Fmoc group.

6. The process according to claim 4, wherein dipeptide Boc -(D) Phe - Cys(Trt)- OH (4) is prepared by coupling Boc - (D) - Phe - ONp (2) with H - Cys (Trt) - OH (3) through active ester method.

7. The process according to claim 4, wherein tetrapeptide H - Phe -(D) - Trp - Lys (Boc) - Thr - OMe (13) is prepared by coupling Z - (D) -Trp - Osu (5) with H -Lys (Boc) - OH (6) through active ester method, followed by condensation with L-Thr - OMe HC1 (8) removing Z-group and then with Fmoc - Phe -OH (11) followed by removal of Fmoc group.

8. The process according to claim 1, wherein oxidation of thiol groups of cysteine is carried with hydrogen peroxide.

9. The process according to claim 1, wherein octreotide is purified by ion exchange chromatography/affinity chromatography or reverse phase CI8 chromatography.

10. The process according to claim 9, wherein octreotide is purified by reverse phased chromatography to yield pharmaceutical grade.

11. The process according to claim 8, wherein the purity of octreotide is greater than 99%.

12. The process for preparation of octreotide essentially as described in the examples cited.