NO773013L - PROCEDURES FOR THE PREPARATION OF METHIONIN-ENKEFALIN DERIVATIVES - Google Patents

Description

The present invention relates to a method for the production of new pentapeptides with the formula:

H-Ty r -X-Gly -Phe -Met -Y

where X is a D-amino acid selected from the group consisting of D-alanine, D-leucine, D-isoleucine, D-valine, D-phenylalanine, D-tyrosine, D-tryptophan, D-serine, D-threonine, D- methionine, D-glutamic acid, D-glutamine, D-aspartic acid, D-asparagine, D-lysine or D-arginine;

and Y is NH 3 or OH and the pharmaceutically acceptable salts thereof.

Although there are a number of analgesic agents currently used to relieve mild to severe pain, research continues to improve analgesic agents due to the numerous problems associated with the currently available agents. "Aspirin" and related salicylates are considered to be non-narcotic analgesic agents useful in relieving mild to moderate pain in addition to their utility as anti-inflammatory and antipyretic agents. However, ingestion of salicylic acid and related salicylates can lead to epigastralgia, nausea and vomiting. This widely used group of non-narcotic analgesics can also cause stomach ulcers and even bleeding, both in animal and human experiments. Worsening of peptic ulcer symptoms and erosive gout arthritis have all been reported in patients on high-dose therapy, i.e. arthritis patients. Acetylsalicylic acid is also one of the most common causes of drug poisoning in young children and is potentially seriously toxic if used incorrectly.

Acetaminophen is also considered to be a non-narcotic analgesic useful in the treatment of mild pain associated with simple headaches, common muscle pain, etc. Although acetaminophen is particularly useful for patients who cannot take "Aspirin",

i.e. ulcer patients, its use is contraindicated in individuals who have shown a sensitivity to it. Besides their shortcomings

in light of their potential side effects, the mild non-narcotic analgesics are not sufficiently strong to

relieve the severe pain associated with surgery, cancer and the like.

Unfortunately, the strong analgesics capable of relieving such severe pain are also narcotics and their use carries the risk of causing physical and sometimes psychological dependence. There are so far no funds that are. effective against severe pain that is completely free of this risk.

There is thus a strong need for improved analgesic agents for the treatment of mild as well as severe pain. The present invention provides such means.

In addition to the need for improved analgesic agents, there is also a need for improved psychotropic agents to replace or provide an alternative to current therapy. Procedure The compounds exhibit, in addition to their analgesic activity, also anti-depressant activity. Their usefulness as analgesics is thus increased as many patients suffering from pain also exhibit various stages of anxiety and depression.

A recently identified pentapeptide, methionine-enkephalin, has the structure H-Tyr-Gly-Gly-Phe-Met-OH [see Hughes et al., "Nature", 258;577 (1975)]. This peptide is found in many areas of the brain where it seems to act as a neurotransmitter or neuromodulator in a central pain suppressive system. The natural peptide binds stereospecifically to partially purified homeopiate receptor sites [see e.g. Bradbury et al., Nature, 2 60, 793 (1976)], is highly active in bioassays of opiate activity, but exhibits only weak and short-lasting analgesic activity when injected directly into the brain of rats, [see e.g. Belluzzi et al., Nature, 260, 625

(1976)].

It has unexpectedly been shown that when methionine-enkephalin is substituted in the 2-position with a D-amino acid, strong analgesic agents are obtained. Besides their analgesic activity, the compounds exhibit excellent anti-depressant activity.

The present invention relates to a process for the production of new pent apeptides, and more specifically to the production of new methionine-enkepha1in derivatives which are useful as analgesic and anti-depressant agents,<q>g starting materials which are

useful in the production of the new pentapeptides.

Specifically, the new pentapeptides are methionine-enkephalin which is substituted in the 2-position with a D-amino acid and the corresponding amides.

The method compounds have the formula:

where X is a D-amino acid selected from the group consisting of D-alanine, D-leucine, D-isoleucine, D-valine, D-phenylalanine, D-tyrosine, D-tryptophan, D-serine, D-threolrtin, D-methionine , D-glutamic acid, D-glutamine, D-aspartic acid, D-asparagine, D-lysine or D-arginine; and Y is NH 3 or OH and the pharmaceutically acceptable salts thereof.

All chiral amino acid residues listed here are in the native or L-configuration unless otherwise specifically stated.

The term "pharmaceutically acceptable salts" is used herein to denote the non-toxic alkali metal, alkaline earth metal and ammonium salts commonly used in the pharmaceutical industry including the sodium, potassium, lithium, calcium, magnesium, barium and ammonium salts which are produced by known methods. The term also includes non-toxic acid addition salts which are usually prepared by reacting the process compounds with a suitable organic or inorganic acid. Representative salts include the hydrochloride, hydrobromide, sulfate, bisulfate, acetate, oxalate, valerate, oleate, laurate, borate,

the benzoate, the lactate, the phosphate, the tosylate, the citrate, the maleate, the fumarate, the succinate, the tartrate, the napsylate and the like.'

The invention also encompasses intermediates of the formula: where R is amino, hydroxy or a derivatized insoluble polystyrene resin carrier represented by the formulas: R<2> is a protecting group for the phenolic hydroxyl group of tyrosine selected from the group consisting of tetrahydropyranyl. t-butyl, trityl, benzoyl, 2,4~dichlorobenzy1, benzyloxycarbonyl1 and 2-bromo-benzyloxycarbonyl (2-Br-Z). The latter is preferred. R<2> can also be a hydrogen atom which means that there is no protecting group on the phenolic. hydroxyl function. RJ is a protecting group which would also be used in the solid phase synthesis of the peptides (I) selected from the group consisting of the acyl type of protecting groups, aromatic urethane type of protecting groups, cycloalkylurethane protecting groups, thiourethan type of protecting groups, alkyl type of protecting groups groups, trialkylsilane groups or aliphatic urethane protecting groups. X is a D-amino acid as indicated for formula I.

When X is a D-amino acid with a functional side chain, the following protecting groups would come into play: D-tyrosxn; same as R 2 stated above. D-serxn, D-threonxn; a protecting group for the alcoholic hydroxyl functions which is preferably a benzyl group. D-glutamic acid, D-aspartic acid; . a protecting group for the carboxylic acid functions which is preferably benzyl or t-butyl. D-lysine; a protecting group for the amino function which is preferably either benzyloxycarbonyl or 2-chlorobenzyloxycarbonyl. D-Arginine; a protecting group for the guanidino function which is preferably either tosyl or nitro.

The term "acyl type of protecting groups" refers to groups exemplified by, but not limited to, formyl, trifluoroacetyl, tosyl, nitrosulfenyl, and the like.

The term "aromatic urethane-type protecting groups" is represented by groups such as benzyloxycarbonyl, p-methoxybenzyloxycarbonyl, p-biphenyl-isopropyloxycarbonyl, 2,5-dimethoxyphenyl-isopropyloxycarbonyl and the like.

The term "cycloalkyl urethane protecting group'.' is used here to denote groups such as cyclopentyloxycarbonyl, adamantyloxycarbonyl, cyclohexylcarbonyl1, isoboronyloxycarbonyl1, etc.

"Urethane type of protecting groups" include but are not limited to groups such as phenylthiocarbonyl.

The "alkyl type of protecting groups" are those commonly used in the art, such as trityl.

"Trialkylsilane groups" include compounds such as trimethylsilane, triethylsilane, tributylsilane and the like.

The preferred protecting groups, the "aliphatic urethane protecting groups", include t-butyloxycarbonyl, diisopropyloxycarbonyl, isopropyloxycarbonyl, allyloxycarbonyl and the like.

The polystyrene resin carrier is preferably a copolymer of styrene with approx. 1 - 2% divinylbenzene as a cross-linking agent, which makes the polystyrene polymer completely insoluble in most organic solvents. In formula III, 0 denotes phenyl.

In selecting a particular side chain protecting group for use in the synthesis of the peptides of formula I, several conditions must be met: (a) the protecting group must be stable towards the reagent and under the reaction conditions chosen to remove the α-amino protecting group group in each step of the synthesis; (b) the protecting group must retain its protective properties and not be chemically modified; and (c) the side chain protecting group must. could be removed at the end of the solid-phase synthesis under reaction conditions that will not. change the pept id chain.

The pentapeptides of formulas (I) and (II) are prepared using standard solid phase methods. The synthesis is initiated from the C-terminus of the peptide using an α-amino protected resin. A suitable starting material can be produced, e.g. by attaching an α-amino protected methionine to a chloro-methylated resin, a hydroxymethyl resin or a benzhydrylamine resin. One such chloromethylated resin is sold under the trade name "Bio-beads SX-1" by Bio Rad'Laboratories, Richmond, California, and the preparation of the hydroxymethyl resin is described by

Bodonszky et al., Chem. Ind. (London) _38, 1597 (1966). Benzhydryl- . the amine resin is. described by Pietta and Marshall, Chem. Commun. , 650 (1970) and is commercially available from Beckman Instrument, Palo Alto, California.

In the preparation of the process compounds, α-amino-protected methionine is connected to the chloromethylated resin by means of e.g. cesium bicarbonate catalyst, according to the method described by Gisin: Heiv. Chim. Acta, 5_6, 1476 (1973)- After the first coupling, the α-amino protecting group is removed by a choice of reagents including trifluoroacetic acid or hydrochloric acid solutions in organic solvents at room temperature. After removal of the α-amino protecting group, the remaining protected amino acids are coupled stepwise in the desired order to obtain the compounds of formula II. Each protected amino acid is generally reacted in a 3-fold excess using a suitable carboxyl group activator such as dicyclohexylcarbodiimide in solution in e.g. methylene chloride-dimethylformamide mixtures..

After the desired amino acid sequence is completed, the desired peptide is removed from the resin support by treatment with a reagent such as hydrogen fluoride which not only cleaves the peptide from the resin, but also cleaves off any remaining side chain protecting groups. When the chloromethylated resin is used, hydrogen fluoride treatment leads to the formation of the free peptide acids of formula I (Y=OH). When the benzhydrylamine resin is used, hydrogen fluoride treatment leads directly to the free peptide amides of formula I (Y=NH 2 ). Alternatively, when the chloromethylated resin is used,

the side-chain-protected peptide can be cleaved by treating the peptide resin with ammonia, whereby the desired side-chain-protected amide is obtained. Side chain protection is then removed in the usual way by treatment with hydrogen fluoride, whereby the amides of formula I are obtained.

The solid phase method discussed above is well known and is substantially described by J. M. Stewart, "Solid Phase Peptide Synthesis:

(Freeman and Co., San Francisco, 1969).

The compounds with formula. I are useful as analgesic and anti-depressant agents when administered to mammalian hosts in doses from 0.001 to 10 mg/kg body weight daily, preferably in divided doses. The compounds are preferably administered parenterally, i.e. by intravenous, intraperitoneal, intramuscular or subcutaneous administration. The compounds can also be administered by various other routes including oral or sublingual or by vaginal, rectal or nasal route of administration.

The analgesic activity of the compounds of formula I was determined in the rat tail flick test as described by D'Amour and Smith, J. Pharmac. Exp. Ther., 72, 74 (1941). The currently preferred analgesic process compound according to the invention is D-Ala 2-methionine-enkephalin (X=D-alanine; Y=OH) which is considerably stronger than methionine-enkephalin as well as morphine.

The anti-depressant activity was determined in the DOPA potency test described by Everett, Proe. Ist Int. Symp. Ant i-depressant Drugs, Excerp. With. Int. Cong. Looking. No. 122, l64 (19-66).

The following examples further illustrate the invention.

. Example 1

Preparation of 0-2-bromobenzyloxycarbonyl-L-tyrosyL-D-alany1-glycy1-L-phenyl la ny1-L-methiony1-0-CHg-ha rpix

1.089 (0.50 mmol) of t-butyloxyca rbo.nyl-methionine-O-CH 2 -resin prepared by the method according to Gisin: Heiv. Chim. Acta., 56, 1476 (1973)j was placed in the reaction vessel of a Beckman Model 990 automatic peptide synthesizer, programmed to perform the following cycle of washes and reactions: (a) methylene chloride;

(ti) 33% trifluoroacetic acid in methylene chloride (2 times each for 2.5 and 25 minutes); (c) methylene chloride; (d) ethanol; (e) chloroform; (f) 10% triethylamine in chloroform (2 times each for 5 minutes);

(g) chloroform; and (h) methylene chloride.

The deprotected resin was then stirred. with 400 mg (1.5 mmol) of t-butyloxycarbonyl (t-Boe)-phenylalanine in methylene chloride, and 1.5 mmol of dicyclohexylcarbodiimide were added. The mixture was stirred at room temperature for 2 hours, and the peptide resin was then washed successively with methylene chloride (3 times), ethanol

(3 times) and methylene chloride (3 times). The attached amino acid was deprotected with 33% trifluoroacetic acid in methylene (2 times for 2.5 and 25 minutes each), and then steps (c)

through (h) carried out as described in the above wash.

1.5 mmol of the following amino acids were then coupled one after the other by the same cycle of steps; t-Boc-Gly; t-Boe-D-Ala; t-Boe-Tyr

(2-Br-Z). The finished pentapeptide resin was washed 3 times with methanol and dried under reduced pressure, whereby 1.04 g of material was obtained.

Example 2

Preparation of L-tyrosy1-D-ala ny1-L-glycy1-L-phenylalany1-L-methionine

Removal of the protecting group and cleavage of the pentapeptide from the resin obtained according to the procedure in Example 1 was carried out by treating 1.04 g of the peptide resin with 20 ml of hydrogen fluoride and 2 ml of anisole at 0°C for 45 minutes. The hydrogen fluoride was removed under reduced pressure, and the anisole was removed by washing with ethyl acetate.

The crude peptide was purified by gel filtration on a column

(2.5 x 95 cm) of "Sephadex G-15" by elution with 0.2 M acetic acid and fractions shown to contain a major peak by UV absorption at 280 nm were combined and evaporated to dryness.

The residual oil was applied to a column (2.5 x 95 cm) of

"Sephadex G-25", pre-equilibrated with the lower phase followed by the upper phase of n-but a noi: acetic acid re: water (4:1:5) solvent system. Elution with the upper phase gave a symmetrical main peak, material from which was evaporated to dryness and lyophilized from dilute acetic acid solution to give a white bulky powder (119 mg); [α]<26>= +40.40° (C = 0.69, 0.1 M HOAc).

The product was homogeneous by thin layer chromatography in four different non-solvent systems on silica gel when 20 µg loadings were applied and visualized by exposure to ninhydrin reagent followed by chlorine/starch potassium diiodide reagent. The following Rf values were obtained: (A) 1-butanol:acetic acid:water (4:1:5 upper phase), 0.52; (B) a hylacetate : pyridine : acetic acid : water (5:5:1:3') 0.81; (C) 2-propanol:1M acetic acid (2:1), 0.69;

(D) 1-butanol : acetic acid-.water-.ethyl acetate (1:1.: 1:1 )• , 0.66.

Example 3

Preparation of 0-2-bromobenzyloxycarbonyl-L-tyrosyl-D-leucyl-L-glycyl-L-phenylalanyl-L-methiony1-0-CH2 resin

Using the conditions described in Example 1, the t-Boc derivatives of Phe, Gly, D-Leu bg Tyr (2-Br-Z) were sequentially coupled to a Met-O-CH resin (0.63 g, 0.50 mmol), The finished, dried pentapeptide resin weighed 1.07 g.

Example 4

Preparation of L-tyrosyl-D-leucyl-L-glycyl-L-phenylalanyl-L-methionine

The above compound was prepared by deprotecting and cleaving the desired peptide from the resin support in Example 3 under

the conditions described in Example 2. The crude peptide was purified by gel filtration on a column (2.5 x 95 cm) of "Sephadex G-15" by elution with 2.0 molar acetic acid followed by partition chromatography as described in Example 2. The pentapeptide was obtained as a white bulky powder (138 mg); [α]<26>= +39.96° (C = 1.051 50% HOAc).

The product is homogeneous by thin layer chromatography using the spray reagents and solvent systems described in Example 2: (A), 0.71; (B), 0.79; (C) , o'.70; (D), 0.75. Amino acid analysis. gave: Taurus, 0.99; Leu, 0.99; Gly, 1.00; Phe, 1.00; Met, 0.94-

Example 5

Preparation of 0-2-bromobenzyloxyca rbony1-L-tyrosyl-D-phenylalanyl-L- pheny la lanyl - L- methionyl - 0 - CH? - have rpiks

Using the conditions described under Example 1, t-Boe derivative one of Phe, Gly, D-Phe and Tyr (2-Br-Z) was coupled to 0.63 g (0.50 mmol) of Met-O-CH2 -ha rpiks to form the desired peptide resin, which, when dried, weighed 1.08 g.

Example 6

Preparation of L-tyrosyl-D-phenylalanyl-L-glycyl-L-phenylala ny1-L-methionine

The pentapeptide was deprotected and cleaved from the resin support

in Example 5 under the conditions described in Example 2. The crude peptide was purified by gel filtration on a column (2.5 x 95 cm) of "Sephadex G-15" by elution with .50% acetic acid followed by partition chromatography as described in example 2. The pentapeptide was

obtained as a white voluminous powder (230.m<g>); [a]2°<=>-8.<6>°

(C = 1.04, 50% HOAc).

The product is homogeneous by thin layer chromatography using the spray reagents and solvent systems described in Example 2: (A), 0.65; (B), 0.80; (C), 0.66; (D) , 0.78- Amino acid analysis. gave: Taurus, 0.99; Phe, 1.99; Gly, 1.01; Met, 1.00.

Example 7

Preparation of 0-2-bromobenzyloxycarbonyl-L-tyrosyl-D-a la ny 1-L-phenylalany1-L-methionyl-benzhydrylamine-resin

Using the conditions described under Example 1, the t-Boc derivatives of Met, Phe, Gly, D-Ala and Tyr (2-Br-Z) were coupled to 0.63 g (0.5 mmol) of a benzhydrylamine resin which was purchased from Beckman Instruments, Palo Alto, California. The complete dried pentapeptide resin weighed 1.149-

Example 8

Preparation of L-tyrosyl-D-alanyl-L-glycyl-L-phenylalanyl-L-methionine amide

The pentapeptide amide was deprotected and cleaved from the resin carrier in Example 7 under the conditions described in Example 2. The crude material was purified by gel filtration and partition chromatography as described in Example 2, whereby a voluminous white powder (17 mg) was obtained; [α]<26>= +37.04° (C = 0.27, 0.1 M HOAc).

The product is homogeneous by thin layer chromatography using the spray reagents and solvent systems described in Example 2: (A), 0.56; (B), 0.82; (C), 0.56; (d), 0.62. Amino acid analysis gave: Tyr, 0.99; Ala, 1.04v Gly, 1.00; Phe, 1.00; Met, 0.95; NH .,. 0.95. - The active process compounds can be processed into pharmaceutical preparations which as active ingredient contain at least one of the compounds of formula I together with a pharmaceutical carrier or diluent. The preparations containing the process compounds can be administered orally, parenterally, nasally, vaginally, rectal or sublingual route, and can be worked up into dosage forms suitable for each method of administration.

Solid dosage forms for oral administration include capsules, tablets, pills, powders and granules. In such solid dosage forms, the active compound is mixed with at least one inert diluent such as sucrose, lactose or starch. Such dosage forms can also include other common substances such as, for example, lubricants such as magnesium stearate. In the case of capsules, tablets and pills, the dosage forms may also include buffering agents. Tablets and pills can also be produced with enteric coatings.

Liquid dosage forms for oral administration include pharmaceutically acceptable emulsions, solutions, suspensions, syrups and elixirs containing inert diluents commonly used in the art, such as water. In addition to inert diluents, such preparations can also include auxiliaries, such as wetting agents, emulsifying and suspending agents, sweeteners, flavourings, substances and perfumes.

Preparations containing the process compounds for parenteral administration include sterile aqueous or non-aqueous solutions, suspensions or emulsions. Examples of non-aqueous solvents or media are propylene glycol, polyethylene glycol, vegetable oils such as olive oil, and injectable organic esters such as ethyl oleate. Such dosage forms may also contain excipients such as preservatives, wetting agents. emulsifiers and dispersants. They can be sterilized by e.g. filtering through a bacteria-retaining filter, by incorporating sterilizing agents into the preparations, by irradiating the preparations or by heating the preparations. They can. also produced in the form of sterile, solid preparations that can be dissolved in sterile water, or another sterile injectable medium immediately before use.

Preparations for rectal or vaginal administration are preferably suppositories which, in addition to the active ingredient, may contain excipients such as cocoa butter or a suppository wax.

Preparations for nasal or sublingual administration are also prepared with common aids known in the art.

The dose of active ingredient in the preparations can be varied, but it is however necessary that the amount of active ingredient is such that a suitable dosage form is obtained. The selected dose depends on the desired therapeutic effect, on the method of administration and on the duration of the treatment. Generally, dosage levels of between 0.001 to 10 mg/kg body weight are administered daily to mammals to obtain effective relief of pain or to alleviate depression.

The following examples further illustrate the pharmaceutical preparations.

Example 9

Tablets weighing 200 mg and having the following compositions were produced:

Example 10

Sterile 10 ml ampoules can be prepared containing 10 mg/ml of L-tyrosyl-D-alanyl-L-glycyl-L-phenylalanyl-L-methionine amide,

0.1% sodium bisulphate, 0.7% sodium chloride and 0.5% chlorobutanol as a preservative.

Example 11

Topical aqueous preparations for administration as nasal drops or nasal sprays were prepared containing 1 mg L-tyrosyl-D-leucyl-L-glycyl-L-phenylalanyl-L-methionine, 3.8 mg glycerol, 40 mg sorbital, 0.02 mg benzalkonium chloride and purified water to give 1 ml.

The following compounds are illustrative of further compounds of formula I which are prepared according to the procedures in examples 1-8:

L-tyrosyl-D-isoleucyl-L-glycyl-L-phenylalanyl-L-methionine L-tyrosyl-D-tyrosyl-L-glycyl-L-phenylalanyl-L-methionine L-tyrosyl-D-tryptophyll-L-glycyl-L -phenylalanyl-L-methionine L-tyrosy1-D-seriny1-L-glycyl-L-phenylalanyl-L-methionine L-tyrosy1-D-threonyl-L-glycyl-L-phenylalanyl-L-methionine L-tyrosyl-D- methionyl-L-glycyl-L-phenylalanyl-L-methionine L-tyrosy1-D-glutamyl-L-glycyl-L-phenylalanyl-L-methionine L-tyrosyl-D-aspart yl-L-glycyl-L-phenylalanyl-L -methionine L-tyrosy1-D-asparaginyl-L-glycyl-L-f enylalanyl-L-methionine L-tyrosyl-D-lysyl-L-glycyl-L-f enylalanyl-L-methionine L-tyrosyl-D-arginyl-L-glycyl- L-phenylalanyl-L-methionine L-tyrosyl-D-glutaminyl-L-glycyl-L-phenylalanyl-L-methionine L-tyrosyl-D-isoleucyl-L-glycyl-L-phenylalanyl-L-methionine amide L-tyrosyl-D -tyrosyl-L-glycyl-L-phenylalanyl-L-methionine-amide L-tyrosyl-D-tyropyl-L-glycyl-L-phenylalanyl-L-methionine-amide L-tyrosyl-D-serinyl-L-glycyl-L-phenylalany1 -L-methionine-amide L-tyrosy1-D-threonyl-L-glycyl-L-phenylalanyl-L-methionine-amide L-tyrosy1- D-raethion<y>1-L-<g>l<y>c<y>l-L-phen<y>lalan<y>l-L-methionine-amideL-tyrosyl-D-glutamyl-L-glycyl-L-phenylalanyl -L-methionine amide L-tyrosyl-D-aspart yl-L-glycyl-L-phenylalanyl-L-methionine amide L-tyrosyl-D-asparginyl-L-glycyl-L-phenylalanyl-L-methionine amide L -tyrosyl-D-lysyl-L-glycyl-L-phenylalanyl-L-methionine-amide L-tyrosyl-D-arginyl-L-glycyl-L-phenylalanyl-L-methionine-amide L-tyrosyl-D-glutaminyl-L -glycyl-L-phenylalanyl-L-methionine amide

In accordance with standard peptide nomenclature, abbreviations for chiral amino acid residues are used here as follows:

Tyr - L-tyrosine

Gly - L-glycine

Phe - L-phenylalanine

Met - L-methionine

D-Ala - D-Alanine.

D-Leu - D-leucine

D-Phe - D-phenylalanine

The exceptions from specifying whether the chiral amino acid residue is in the natural (L)-configuration or in the D-configuration are in the amino acid analyzes indicated in examples 2, 4, 6 and 8-

Claims (8)

1. Procedure for the preparation of a compound with the formula: where X is a D-amino acid selected from the group consisting of D-alanine, D-leucine, D-isoleucine, D-valine, D-phenylalanine, D-tyrosine, D-tryptophan, D-serine, D-threonine, D- methionine, D-glutamic acid, D-glutamine, D-aspartic acid, D-asparagine, D-lysine or D-arginine; Y is NH2 or OH, or a pharmaceutically acceptable salt thereof, characterized in that it comprises the preparation of a compound of the formula: where: R1 is OH, NH2 or a derivatized, insoluble polystyrene support with the formula: or -0-CH2~polystyrene- resin support R 2 is a protecting group selected from the group consisting of t-butyl, tetrahydropyranyl, trityl, benzoyl, 2,4-dichlorobenzyl, benzyloxycarbonyl or 2-bromobenzyloxycarboxyl; R 1 is hydrogen or an α-amino acid protecting group; and X is as above, by deprotecting t-butyloxycarbonyl-methnionine-0-CH2 resin in a peptide synthesizer to obtain a deprotected resin which is treated at room temperature with phenylalanine protected with an R2 compound, after deprotection, is coupled with protected L-glycine, followed by deprotection and coupling with a protected D-amino acid selected from the group indicated by X, and by coupling with protected L-tyrosine to obtain the desired pentapeptide resin which is treated with hydrogen fluoride at 0 C to cleave the resin and deprotect the pentapeptide which is then purified. 2. Method according to claim 1, characterized in that the deprotection is carried out with trifluoroacetic acid. 3. Method according to claim 1, characterized in that the deprotection is carried out with hydrogen fluoride. 4. Method according to claims 1-3, characterized by data OHi 5. Process according to claims 1 - 3., characterized by Yer NH^. 6. Method according to claim 4 or 5, characterized in that X is D-alanine. 7. Method according to claim 4 or 5, characterized in that X is D-leucine. 8. Method according to claim 4 or 5, characterized in that X is D-phenylalanine.