Novel peptides, and antidementia agents containing the same

Abstract: Disclosed is a novel peptide having one of the formu­ lae: (A and B are the amino acids: wherein if A is D- or L-Pro, B is Har or Cit; if A is D-Pro, B is D-Arg, and if B is D- or L-Arg, A is Sar, Pip, Aze or Arg) Asn-A-L-(D-)Pro-Arg-(Gly) n (A is Ser, Thr or Ala, n is 1 or 0) A-Ser-Pip-Arg (A is Pro-Asn-, Asn- or Pro-) A- ys-Pro-Arg-B (A is cyclopentylcarbonyl, Pro or pGlu; B is Gly or β-Ala, W is a hydrogen atom or a group having the formula: H- ys-OH) or a peptide having the formula: (A- ys-Pro-Arg-B)₂ wherein A and B have the same meanings as mentioned above, respectively pGlu-Asn-Ser-A-B-(Gly) n (A is Aze, D- or L-Pro, Pip or Sar, B is D- or L-Arg, Cit, Har, Lys or Orn, n is 1 or 0) and Pro-(Asn) m -Ser-L-(D-)Pro-Arg-(Gly) n (m and n are independently 0 or 1) their functional derivatives, and pharmaceutically accept­ able salts thereof.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to novel peptides having a nootropic effect and being useful as medicines, particularly as antidementia agents. Description of Prior Art Vasopressin has been previously known as a compound having a nootropic effect, i.e., intelligence developing effect. Recently, it has been reported that peptides seemingly corresponding to a vasopressin fragment, for example, those having the following formulae: have such a nootropic effect as that of vasopressin in Science , 221, pp.1310-1312 (1983) and Brain Research , 371, 17(1986). Further, there is also reported that a peptide having the following formula: has a nootropic effect in Japanese Patent Provisional Publication No.59(1984)-93036. SUMMARY OF THE INVENTION It is an object of the present invention to provide new peptide derivatives which are superior in the nootropic effect to the known vasopressin as well as to the known peptides corresponding to vasopressin fragments. The invention provides in one aspect a peptide having the formula (I): Asn-A-L-(D-)Pro-Arg-(Gly) n (I) wherein A is Ser, Thr or Ala, and n is 1 or 0, a functional derivative thereof, and a pharmaceutically acceptable salt thereof. The invention provides in another aspect a peptide having the formula (II): A-Ser-Pip-Arg (II) wherein A is Pro-Asn-, Asn- or Pro-, a functional derivative thereof, and a pharmaceutically acceptable salt thereof. The invention further provides in a further aspect a peptide having the formula (III): pGlu-Asn-Ser-A-B-(Gly) n (III) wherein A is Aze, D-or L-Pro, Pip or Sar, B represents D- or L-Arg, Cit, Har, Lys or Orn, n is 1 or 0, a functional derivative thereof, and a pharmaceutically acceptable salt thereof. The invention provides in a yet further aspect a peptide having the formula (IV): Pro-(Asn) m -Ser-L-(D-)Pro-Arg-(Gly) n (IV) wherein m or n are independently 0 or 1, a functional derivative thereof, and a pharmaceutically acceptable salt thereof. The invention also provides an antidementia agent containing, as a pharmaceutically active component, an effective amount of a peptide according to any of the above formulae (I) to (IV), a functional derivative thereof or a pharmaceutically acceptable salt thereof, and a pharmaceutically acceptable carrier or diluent. The above-mentioned peptides, their functional derivatives, and their pharmaceutically acceptable salts show a prominent nootropic effect in passive avoidance tests using rats, and are prominently effective as active components of pharmaceutical agents for prevention or treatment of senile dementia (Alzheimer's dementia), cerebrovascular dementia and other dementia diseases. DETAILED DESCRIPTION OF THE INVENTION The peptides and peptide derivatives having the above formulae of the present invention are compounds of which amino acid sequences are different from those of the aforementioned known peptides. The expression "functional derivative " of the peptides in the present specification means the following derivatives: a) N-acyl derivatives having N-acyl group(s); wherein the N-acyl group is derived from an aliphatic carboxylic acid having 1 to 6 carbon atoms, preferably one derived from acetic acid; the N-acyl group can be expressed by -NHCOR (wherein R is an alkyl group having 1-5 carbon atoms), b) derivatives having groups in the form of amides, or monoalkyl or dialkyl substituted-amides having alkyl chain(s) of 1 to 6 carbon atoms; which can be expressed by -CONH₂, -CONHR, and -CONR₂ (wherein R is an alkyl group having 1-6 carbon atoms), or c) derivatives having groups in the form of esters derived from alcohol having 1 to 18 carbon atoms, preferably those derived from an aliphatic alcohol having 1 to 6 carbon atoms; which can be expressed by -COOR (wherein R is an alkyl group having carbon 1-18 atoms, preferably 1-6 carbons). As examples of pharmaceutically acceptable salts of the peptides and the peptide derivatives of the invention, acid addition salts and basic salts such as alkali metal salts and ammonium salts can be mentioned. Examples of such acid addition salts include salts of inorganic acids (e.g., hydrochloric acid, sulfuric acid and phosphoric acid) or of organic acids (e.g., acetic acid, propionic acid, citric acid, tartaric acid, maleic acid, oxalic acid and methanesulfonic acid). Examples of basic salts include sodium salt, potassium salt, and triethylamine salts. In the specification, amino acids, peptides, protecting groups and solvents are described by abbreviations commonly used in the field of chemistry, or abbreviations recommended by the IUPAC-IUB Commission on Biochemical Nomenclature. For example, the following symbols are used in the specification. The amino acids should be construed to be of the L-type, unless specific description with respect to optical configuration is given. β-Ala : β-alanine Arg : arginine Ala : alanine Asn : asparagine Aze : azetidine-2-carboxylic acid Cit : citrulline Cys : cysteine Gly : glycine Har : homoarginine Lys : lysine Orn : ornithine pGlu : pyroglutamic acid Pip : pipecolic acid Pro : proline Sar : sarcosine Ser : serine Thr : threonine Boc : t-butoxycarbonyl Z : benzyloxycarbonyl Fmoc : 9-fluorenylmethoxycabonyl Bu t : t-butyl Mbs : p-methoxybenzenesulfonyl MBzl : p-methoxybenzyl Acm : acetamidomethyl Scm : S-carbomethoxysulfenyl Mtr : 4-methoxy-2,3,6-trimethylbenzenesulfonyl NO₂ : nitro Bzl : benzyl OBzl : benzyl ester OSu : N-hydroxysuccinimide ester DCC : N,N'-dicyclohexylcarbodiimide DCUrea: N,N'-dicyclohexylurea DIC : N,N'-diisopropylcarbodiimide HOBt : 1-hydroxybenzotriazole Et₃N : triethylamine Trt : trityl NMM : N-methylmorpholine TFA : trifluoroacetic acid MSA : methanesulfonic acid AcOEt : ethyl acetate AcOH : acetic acid THF : tetrahydrofuran DMF : N,N-dimethylformamide MeOH : methanol The compounds of the present invention can be prepared by the methods conventionally employed in peptide chemistry. For example, the peptides can be prepared by those processes described in Schröder and Lübke, The Peptides , Vol.1, Academic Press, New York, 1965, and Nobuo Izumiya et al., Fundamental and Experiment of Peptide Synthesis , Maruzen, Tokyo, 1985, and can be prepared by either the solution synthesis or the solid synthesis. Examples of the methods for formation of the peptide bonds include aside method, acid chloride method, symmetrical anhydride method, mixed anhydride method, carbodiimide method, carbodiimido-additive method, activated ester method, carbonyldiimidazole method, oxidation-reduction method, and the one employing a Woodward reagent K. Before carrying out the coupling reaction, carboxyl group, amino group, guanidino group, hydroxyl group, mercapto group and the like which do not participate in the reaction can be protected, and those which participate in the coupling reaction can be activated, both by the methods well known in the art. Examples of the protecting groups for the carboxyl group include ester-forming groups such as methyl, ethyl, benzyl, p-nitrobenzyl, t-butyl and cyclohexyl. Examples of the protecting groups for the amino group include benzyloxycarbonyl, t-butoxycarbonyl, isobornyloxycarbonyl, and 9-fluorenylmethyloxycarbonyl. Examples of the protecting groups for the guanidino group include nitro, benzyloxycarbonyl, tosyl, p-methoxybenzenesulfonyl and 4-methoxy-2,3,6-trimethylbenzenesulfonyl. Examples of the protecting groups for the hydroxyl group include t-butyl, benzyl, tetrahydropyranyl and acetyl. Examples of the protecting groups for the mercapto group include trityl, acetamidomethyl, benzyl, p-methoxybenzyl and 3-nitro-2-pyridinesulfenyl. Examples of the activated forms of carboxyl group include symmetrical anhydride, azide and active ester (ester with alcohol e.g., pentachlorophenol, 2,4-dinitrophenol, cyanomethyl alcohol, p-nitrophenol, N-hydroxy-5-norbornene-2,3-dicarboryimide, N-hydroxysuccinimide N-hydroxyphthalimide and 1-hydroxybenzotriazol). An example of the activated amino group is amide phosphate. The reaction is generally carried out in a solvent such as chloroform, dichloromethane, ethyl acetate, N,N-dimethylformamide, dimethylsulfoxide, pyridine, dioxane, tetrahydrofuran, water, methanol and mixture of these solvents. The reaction temperature may be in the range of approx. - 30 °C to 50 °C, which is generally employed for the reaction. The reactions for removing the protecting group of the peptide of the invention may differ depending on the kind of the protecting group, but it should be the one which is able to release the protecting group without giving any influence to the peptide bonding. The protecting group can be removed by acid treatment, for example, treatment with hydrogen chloride, hydrogen bromide, hydrogen fluoride, methanesulfonic acid, trifluoromethanesulfonic acid, trifluoroacetic acid and mixture of these acids. Further, the reduction with sodium metal in liquid ammonia or catalytic reduction over palladium-carbon can be employed. In performing the reaction for removing the protecting group by the above acid treatment, addition of cation scavenger such as anisole, phenol and thioanisole is advantageously adopted. After the reaction is complete, the prepared peptide of the present invention can be recovered by the conventional process for purification of peptides, for example, extraction, partition, reprecipitation, recrystallization or column chromatography. Further, the peptides of the present invention can be converted into their functional derivatives or their pharmaceutically acceptable salts as described above in the conventional manner. The compounds of the invention show a strong nootropic effect in passive avoidance tests using rats as described hereinafter. The peptides and peptide derivatives of the present invention are effective to the following diseases, and be employed for prevention or treatment thereof; senile dementia (Alzheimer's dementia), cerebrovascular dementia, and dementia based on Alzheimer's disease, Pick's disease, Huntington's disease, Creutzfeldt-Jakob disease, Parkinson's disease and cerebellar myelic denatured disease. The peptides of the invention have an extremely low toxicity, and cause no death even by administration at an extremely higher dose than its effective dose. The peptides of the invention may be administrated in the form of the peptides of the above-mentioned formulae, their functional derivatives or salt thereof. Irrespective of their form, the dose expressed as the amount of the peptides of the above-mentioned formulae is preferably in the range of 0.1 ng/day to 1 mg/day. In the case of parenteral administration and nasal administration, the dose preferably is in the range of 0.1 ng/day to 100 µg/day. In the case of oral administration and rectal administration, it is preferable that the dose should be 10 to 100 times that for parenteral administration. The peptides of the invention are mainly administered parenterally (e.g., intravenous or hypodermic injection, intracerebroventricular or intraspinal administration, nasal administration and rectal administration). They can be also administered orally depending on the case. The peptides of the invention can be incorporated into pharmaceutical compositions in the form of injection liquids, suppositories, powders, collunaria, granules and tablets. The peptides of the invention can be preserved as a physiological saline solution or can be freeze-dried in an ampoule after addition of mannitol or sorbitol, to be melted when it is used for administration. Examples of the invention are set forth hereinafter. In each example, the eluents used for thin-layer chromatography were as follows. As for the solid phase, TLC Plate Silica Gel 60F₂₅₄ by Merck Co., Ltd. was used. Rf¹: choroform-methanol-acetic acid-water (80:20:2.5:5) lower layer Rf²: choroform-methanol-water (70:30:5) Rf³: n-butanol-acetic acid-water (2:1:1) Further, purification by high-performance liquid chromatography was carried out using the following materials: Column: µBondapak C₁₈ 1.9 x 15 cm Mobile phase: A) 0.05 % TFA, B) acetonitrile The preparation of peptides in the solid phase was carried out by the process in which the peptide chain was extended step by step in the following manner using 1 g (0.24 mmol NH₂/g) of 2,4-dimethoxybenzhydrylamine resin [ J. Org. Chem ., 52(1987), 1197]. Coupling Process reagent, solvent period(minute) x times 1. DMF 1 x 3 2. Fmoc-amino acid + HOBt + DIC (each of them are same amount.)-DMF 120 3. DMF 1 x 3 4. iso-propanol 1 x 3 5. CH₂Cl₂ 1 x 3 N α -Deprotection Process reagent, solvent period(minute) x times 6. DMF 1 x 3 7. 20 % piperidine-DMF 1 8. 20 % piperidine-DMF 20 9. DMF 1 x 3 10. CH₂Cl₂ 1 x 3 The coupling reaction was confirm by Kayser test. If necessary, the above steps 1-5 were repeated. Example 1 H-Asn-Ser-Pro-Arg-OH acetate (1) Boc-Pro-Arg(NO₂)-OBzl To a solution of 15 g of H-Arg(NO₂)-OBzl in 250 ml of THF, 15 g of Boc-Pro-OSu was added under chilling with ice, followed by stirring for 18 hours at room temperature. After THF was distilled off, the residue was dissolved in AcOEt. The AcOEt solution was washed successively with dilute HCl, saturated aqueous NaHCO₃ solution and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off. The residue was dissolved in CHCl₃-MeOH, and purified by silica-gel column chromatography to obtain the desired compound as an oily product. Yield: 22 g R f ¹ : 0.61 , R f ² : 0.77 [α] D : -37.1° (c=1.0, DMF) (2) Boc-Ser(Bzl)-Pro-Arg(NO₂)-OBzl 22 g of Boc-Pro-Arg(NO₂)-OBzl was placed in 110 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the solvent was distilled off. After drying under reduced pressure, the residue was dissolved in 150 ml of DMF. To the solution, 9 ml of Et₃N, 12.8 g of Boc-Ser(Bzl)-OH, 10 g of HOBt and 9.4 g of DCC were added under chilling with ice, followed by stirring for 18 hours at room temperature. DCUrea was removed by filtration, and DMF was distilled off. The residue was dissolved in AcOEt. Then the AcOEt solution was washed successively with dilute HCl, saturated aqueous NaHCO₃ solution and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off, and the residue was treated with AcOEt-ether to give the desired compound as a crystalline product. Yield: 21 g M.P. : 80 - 82 °C R f ¹ : 0.67 , R f ² : 0.83 [α] D : -30.8° (c=1.0, DMF) (3) Z-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl 4.0 g of Boc-Ser(Bzl)-Pro-Arg(NO₂)-OBzl was placed in 15 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and the solvent was distilled off. To the residue, 2-butanol - CH₂Cl₂ (5 : 1 v/v) and saturated aqueous NaHCO₃ solution was added. The organic portion was collected and washed with saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and the residue was dissolved in 50 ml of DMF. To the solution, 1.55 g of Z-Asn-OH, 1.3 g of HOBt and 1.3 g of DCC were added under chilling with ice. After stirring for 18 hours at room temperature, DCUrea was removed by filtration, and DMF was distilled off. The residue was dissolved in 2-butanol - CH₂Cl₂ (5 : 1 v/v). Then the solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl saturated with NaCl and saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and the residue was treated with ether to give the desired compound as a crystalline product. Yield: 4.5 g M.P. : 205 - 209 °C (decomposed) R f ¹ : 0.55 , R f ² : 0.72 [α] D : -25.1° (c=1.0, DMF) (4) H-Asn-Ser-Pro-Arg-OH acetate A solution of 150 mg of Z-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl in 20 ml of 80 % acetic acid was stirred for 18 hours in a stream of hydrogen gas in the presence of 10 % palladium carbon. The palladium-carbon was removed by filtration, and the solvent was distilled off. The residue was dissolved in water, then freeze-dried. Then the resulting product was purified by high-performance liquid chromatography at 12 ml/min.(flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 82 mg R f ³ : 0.15 [α] D : -70.4° (c=0.5, water) FAB mass spectrum (M+1) : 473 Example 2 H-Asn-Thr-Pro-Arg-OH acetate (1) Boc-Thr-Pro-Arg(NO₂)-OBzl 9.6 g of Boc-Pro-Arg(NO₂)-OBzl was placed in 50 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and the solvent was distilled off. After drying under reduced pressure, the residue was dissolved in 100 ml of DMF. To the solution, 4 ml of Et₃N and 6 g of Boc-Thr-OSu were added under chilling with ice, followed by stirring for 18 hours at room temperature. DMF was distilled off and the residue was dissolved in AcOEt. Then the AcOEt solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off, and the residue was purified with CHCl₃-acetone by silica-gel column chromatography, then was treated with ether to give the desired compound as a crystalline product. Yield: 7.1 g M.P. : 86 - 91 °C R f ¹ : 0.59 , R f ² : 0.77 [α] D : -37.5° (c=1.0, DMF) (2) Z-Asn-Thr-Pro-Arg(NO₂)-OBzl The desired compound was prepared from 3.0 g of Boc-Thr-Pro-Arg(NO₂)-OBzl, 15 ml of 4 N HCl-AcOEt, 1.2 g of Z-Asn-OH, 1.2 g of HOBt and 1.1 g of DCC in the same manner as in Example 1-(3). Yield: 2.4 g M.P. : 184 - 186 °C R f ¹ : 0.39 , R f ² : 0.62 [α] D : -30.2° (c=1.0, DMF) (3) H-Asn-Thr-Pro-Arg-OH acetate 150 mg of Z-Asn-Thr-Pro-Arg(NO₂)-OBzl was reduced in the presence of palladium-carbon in the same manner as in Example 1-(4). The resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 104 mg R f ³ : 0.17 [α] D : -58.5° (c=0.5, water) FAB mass spectrum (M+1) : 487 Example 3 H-Asn-Ala-Pro-Arg-OH acetate (1) Boc-Ala-Pro-Arg(NO₂)-OBzl The desired compound was prepared from 28.9 g of Boc-Pro-Arg(NO₂)-OBzl, 150 ml of 4 N HCl-AcOEt, 8 ml of Et₃N and 16.3 g of Boc-Ala-OSu in the same manner as in Example 2-(1). Yield: 25.0 g M.P. : 83 - 85 °C R f ¹ : 0.61 , R f ² : 0.77 [α] D : -54.2° (c=1.0, DMF) (2) Z-Asn-Ala-Pro-Arg(NO₂)-OBzl The desired compound was prepared from 2.9 g of Boc-Ala-Pro-Arg(NO₂)-OBzl, 15 ml of 4 N HCl-AcOEt, 1.4 g of Z-Asn-OH, 1.0 g of HOBt and 1.1 g of DCC in the same manner as in Example 1-(3). Yield: 2.8 g M.P. : 128 - 130 °C R f ¹ : 0.45 , R f ² : 0.65 [α] D : -36.8° (c=1.0, DMF) (3) H-Asn-Ala-Pro-Arg-OH acetate 150 mg of Z-Asn-Ala-Pro-Arg(NO₂)-OBzl was reduced in the presence of palladium-carbon in the same manner as in Example 1-(4). The resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 100 mg R f ³ : 0.14 [α] D : -78.5° (c=0.5, water) FAB mass spectrum (M+1) : 457 Example 4 H-Asn-Ser-D-Pro-Arg-OH acetate (1) Boc-D-Pro-Arg(NO₂)-OBzl The desired compound was prepared as an oily product from 9 g of H-Arg(NO₂)-OBzl and 9 g of Boc-D-Pro-OSu in the same manner as in Example 1-(1). Yield: 13 g R f ¹ : 0.64 , R f ² : 0.76 [α] D : +9.6° (c=1.0, DMF) (2) Boc-Ser(Bzl)-D-Pro-Arg(NO₂)-OBzl The desired compound was prepared as an oily product from 12 g of Boc-D-Pro-Arg(NO₂)-OBzl, 60 ml of 4 N HCl-AcOEt, 3.3 ml of Et₃N, 7 g of Boc-Ser(Bzl)-OH, 4.2 g of HOBt and 5.1 g of DCC in the same manner as in Example 1-(2). Yield: 10 g R f ¹ : 0.71 , R f ² : 0.80 [α] D : +7.4° (c=1.0, DMF) (3) Z-Asn-Ser(Bzl)-D-Pro-Arg(NO₂)-OBzl The desired compound was prepared from 3.0 g of Boc-Ser(Bzl)-D-Pro-Arg(NO₂)-OBzl, 10 ml of 4 N HCl-AcOEt, 1.2 g of Z-Asn-OH, 0.9 g of HOBt and 0.95 g of DCC in the same manner as in Example 1-(3). Yield: 2.5 g M.P. : 89 - 92 °C R f ¹ : 0.59 , R f ² : 0.72 [α] D : +14.8° (c=1.0, DMF) (4) H-Asn-Ser-D-Pro-Arg-OH acetate 100 mg of Z-Asn-Ser(Bzl)-D-Pro-Arg(NO₂)-OBzl was reduced in the presence of palladium-carbon in the same manner as in Example 1-(4). The resulting product was purified by high-performance liquid chromatography at 12 ml/min.(flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 48 mg R f ³ : 0.13 [α] D : +25.3° (c=0.5, water) FAB mass spectrum (M+1) : 473 Example 5 H-Asn-Ser-Pro-Arg-Gly-NH₂ acetate Fmoc-Gly-resin was prepared from 1 g of 2,4-dimethoxybenzhydrylamine resin, 214 mg of Fmoc-Gly-OH, 110 mg of HOBt and 0.12 ml of DIC by the above described coupling process. Then the protecting group was removed by N α -deprotection process to obtain H-Gly-resin. The coupling and N α -deprotection processes were repeated in the same manner to prepare H-Asn-Ser(Bu t )-Pro-Arg(Mtr)-Gly-resin. After drying, the resin was stirred in TFA-anisole (10-1 ml) for 4 hours at room temperature. The resin was removed by filtration and was washed with TFA. After the TFA solution was placed for 2 hours at room temperature, TFA was distilled off. To the residue, ether-water was added, and the aqueous portion was collected and was subjected to Dowex 1x2 (acetate type) treatment and freeze-dried. Then the resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 56 mg R f ³ : 0.11 [α] D : -78.8° (c=0.5, water) FAB mass spectrum (M+1) : 529 Example 6 H-Asn-Ser-Pip-Arg-OH acetate (1) Boc-Pip-Arg(NO₂)-OBzl To a solution of 10.6 g of H-Arg(NO₂)-OBzl in 100 ml of DMF, 7.1 g of Boc-Pip-OH, 7.1 g of HOBt and 6.7 g of DCC were added under chilling with ice. The mixture was stirred for 18 hours at room temperature. The produced DCUrea was removed by filtration, and DMF was distilled off. The residue was dissolved in AcOEt. The resulting solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off, and the residue was purified with CHCl₃-acetone by silica-gel column chromatography to obtain the desired compound as an oily product. Yield: 5.5 g R f ¹ : 0.68 , R f ² : 0.80 [α] D : -24.0° (c=1.0, DMF) (2) Boc-Ser(Bzl)-Pip-Arg(NO₂)-OBzl 5.2 g of Boc-Pip-Arg(NO₂)-OBzl was placed in 25 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the solvent was distilled off. The residue was dried under reduced pressure and then dissolved in 50 ml of DMF. To the resulting solution, 1.4 ml of Et₃N and 3.9 g of Boc-Ser(Bzl)-OSu were added under chilling with ice, and then stirred for 18 hours at room temperature. DMF was distilled off and the residue was dissolved in AcOEt. The resulting solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off, and the residue was purified with CHCl₃-acetone by silica-gel column chromatography to obtain the desired compound as an oily product. Yield: 2.8 g R f ¹ : 0.74 , R f ² : 0.86 [α] D : -42.6° (c=1.0, DMF) (3) Boc-Asn-Ser(Bzl)-Pip-Arg(NO₂)-OBzl 1.6 g of Boc-Ser(Bzl)-Pip-Arg(NO₂)-OBzl was placed in 6 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the solvent was distilled off. To the residue, 2-butanol - CH₂Cl₂ (5 : 1 v/v) and saturated aqueous NaHCO₃ solution were added. The organic portion was collected and then the solution was washed with saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and then the residue was dissolved in 20 ml of DMF. To the resulting solution, 0.53 g of Boc-Asn-OH, 0.53 g of HOBt and 0.52 g of DCC were added under chilling with ice. The resulting solution was stirred for 18 hours at room temperature, and then DCUrea was removed by filtration and DMF was distilled off. The residue was dissolved in AcOEt, and the solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl and saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and the residue was purified with CHCl₃-MeOH by silica-gel column chromatography to obtain the desired compound as an oily product. Yield: 1.0 g R f ¹ : 0.57 , R f ² : 0.74 [α] D : -37.8° (c=1.0, DMF) (4) H-Asn-Ser-Pip-Arg-OH acetate 150 mg of Boc-Asn-Ser(Bzl)-Pip-Arg(NO₂)-OBzl was placed in 0.5 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the solvent was distilled off. To the residue, 20 ml of 80 % acetic acid was added and then the resulting mixture was stirred for 18 hours in a stream of hydrogen gas in the presence of 10 % palladium-carbon. The palladium-carbon was removed by filtration, and the solvent was distilled off. The residue was dissolved in water, and then freeze-dried. Then the resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 63 mg R f ³ : 0.19 [α] D : -46.2° (c=0.5, water) FAB mass spectrum (M+1) : 487 Example 7 H-Pro-Ser-Pip-Arg-OH acetate (1) Z-Pro-Ser(Bzl)-Pip-Arg(NO₂)-OBzl The desired compound was prepared as an oily product from 1.0 g of Boc-Ser(Bzl)-Pip-Arg(NO₂)-OBzl, 5 ml of 4 N HCl-AcOEt, 0.2 ml of Et₃N and 0.49 g of Z-Pro-OSu in the same manner as in Example 6-(1). Yield: 0.9 g R f ¹ : 0.72 , R f ² : 0.82 [α] D : -51.2° (c=1.0, DMF) (2) H-Pro-Ser-Pip-Arg-OH acetate 150 mg of Z-Pro-Ser(Bzl)-Pip-Arg(NO₂)-OBzl was reduced in the presence of palladium-carbon in the same manner as in Example 6-(4). The resulting product was purified by high-performance liquid chromatography at 12 ml/min.(flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 31 mg R f ³ : 0.18 [α] D : -76.5° (c=0.5, water) FAB mass spectrum (M+1) : 470 Example 8 H-Pro-Asn-Ser-Pip-Arg-OH acetate (1) Z-Pro-Asn-Ser(Bzl)-Pip-Arg(NO₂)-OBzl The desired compound was prepared from 0.68 g of Boc-Asn-Ser(Bzl)-Pip-Arg(NO₂)-OBzl, 3 ml of 4 N HCl-AcOEt, 0.14 ml of NMM and 0.32 g of Z-Pro-OSu in the same manner as in Example 6-(2). Yield: 0.7 g M.P. : 95 - 97 °C R f ¹ : 0.57 , R f ² : 0.74 [α] D : -48.2° (c=1.0, DMF) (2) H-Pro-Asn-Ser-Pip-Arg-OH acetate 150 mg of Z-Pro-Asn-Ser(Bzl)-Pip-Arg(NO₂)-OBzl was reduced in the presence of palladium-carbon in the same manner as in Example 6-(4). The resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 60 mg R f ³ : 0.14 [α] D : -76.4° (c=0.5, water) FAB mass spectrum (M+1) : 584 Example 9 pGlu-Asn-Ser-Pro-Arg-Gly-NH₂ acetate Fmoc-Gly-resin was prepared from 1 g of 2,4-dimethoxybenzhydrylamine resin, 214 mg of Fmoc-Gly-OH, 110 mg of HOBt and 0.12 ml of DIC by the above described coupling process. Then the protecting group was removed by N α -deprotection process to obtain H-Gly-resin. The coupling and N α -deprotection processes were repeated in the same manner to prepare H-Asn-Ser(Bu t )-Pro-Arg(Mtr)-Gly-resin, and then the coupling process using pGlu-OH were performed to obtain pGlu-Asn-Ser(Bu t )-Pro-Arg(Mtr)-Gly-resin. After drying, the resin was stirred in TFA-anisole (10-1 ml) for 4 hours at room temperature. The resin was removed by filtration and was washed with TFA. After the TFA solution was placed for 2 hours at room temperature, TFA was distilled off. To the residue, ether-water was added, and the aqueous portion was collected, subjected to Dowex 1x2 (acetate type) treatment and freeze-dried. Then the resulting product was purified by high-performance liquid chromatography at 12 ml/min.(flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 69 mg R f ³ : 0.14 [α] D : -91.8° (c=0.5, water) FAB mass spectrum (M+1) : 640 Example 10 pGlu-Asn-Ser-D-Pro-Arg-Gly-NH₂ acetate pGlu-Asn-Ser(Bu t )-D-Pro-Arg(Mtr)-Gly-resin was prepared from 1 g of 2,4-dimethoxybenzhydrylamine resin in the same manner as in Example 9 , and then TFA treatment, purification by high-performance liquid chromatography, ion exchange treatment and freeze-drying were performed in the same manner as in Example 9 to obtain the desired compound. Yield: 74 mg R f ³ : 0.15 [α] D : -13.9° (c=0.5, water) FAB mass spectrum (M+1) : 640 Example 11 pGlu-Asn-Ser-Pro-D-Arg-Gly-NH₂ acetate pGlu-Asn-Ser(Bu t )-Pro-D-Arg(Mtr)-Gly-resin was prepared from 1 g of 2,4-dimethoxybenzhydrylamine resin in the same manner as in Example 9 , and then TFA treatment, purification by high-performance liquid chromatography ion exchange treatment and freeze-drying were performed in the same manner as in Example 9 to obtain the desired compound. Yield: 49 mg R f ³ : 0.15 [α] D : -67.4° (c=0.5, water) FAB mass spectrum (M+1) : 640 Example 12 pGlu-Asn-Ser-D-Pro-D-Arg-Gly-NH₂ acetate pGlu-Asn-Ser(Bu t )-D-Pro-D-Arg(Mtr)-Gly-resin was prepared from 1 g of 2,4-dimethoxybenzhydrylamine resin in the same manner as in Example 9 , and then TFA treatment, purification by high-performance liquid chromatography, ion exchange treatment and freeze-drying were performed in the same manner as in Example 9 to obtain the desired compound. Yield: 62 mg R f ³ : 0.16 [α] D : +15.3° (c=0.5, water) FAB mass spectrum (M+1) : 640 Example 13 pGlu-Asn-Ser-Pro-Arg-OH (1) Boc-Pro-Arg(NO₂)-OBzl To a solution of 15 g of H-Arg(NO₂)-OBzl in 250 ml of THF, 15 g of Boc-Pro-OSu was added under chilling with ice, followed by stirring for 18 hours at room temperature. After THF was distilled off, the residue was dissolved in AcOEt. The AcOEt solution was washed successively with dilute HCl, saturated aqueous NaHCO₃ solution and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off. The residue was dissolved in CHCl₃-MeOH, and purified by silica-gel column chromatography to obtain the desired compound as an oily product. Yield: 22 g R f ¹ : 0.61 , R f ² : 0.77 [α] D : -37.1° (c=1.0, DMF) (2) Boc-Ser(Bzl)-Pro-Arg(NO₂)-OBzl 22 g of Boc-Pro-Arg(NO₂)-OBzl was placed in 110 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the the solvent was distilled off. After drying under reduced pressure, the residue was dissolved in 150 ml of DMF. To the solution, 9 ml of Et₃N, 12.8 g of Boc-Ser(Bzl)-OH, 10 g of HOBt and 9.4 g of DCC were added under chilling with ice, followed by stirring for 18 hours at room temperature. DCUrea was removed by filtration, and DMF was distilled off. The residue was dissolved in AcOEt. Then the AcOEt solution was washed successively with dilute HCl, saturated aqueous NaHCO₃ solution, dilute HCl and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off, and the residue was treated with AcOEt to give the desired compound as a crystalline product. Yield: 21 g M.P. : 80 - 82 °C R f ¹ : 0.67 , R f ² : 0.83 [α] D : -30.8° (c=1.0, DMF) (3) Z-pGlu-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl 2.3 g of Boc-Ser(Bzl)-Pro-Arg(NO₂)-OBzl was placed in 10 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the solvent was distilled off. To the residue, 2-butanol - CH₂Cl₂ (5 : 1 v/v) and saturated aqueous NaHCO₃ solution were added. The organic portion was collected and washed with saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solution was distilled off, and the residue was dissolved in 20 ml of DMF. To the solution, 1.4 g of Z-pGlu-Asn-OH, 0.7 g of HOBt and 0.8 g of DCC were added under chilling with ice. After stirring for 18 hours at room temperature, DCUrea was removed by filtration, and DMF was distilled off. The residue was dissolved in 2-butanol - CH₂Cl₂ (5 : 1 v/v) and then the solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl saturated with NaCl and saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and the residue was dissolved in CHCl₃-MeOH, and purified by silica-gel column chromatography to obtain the desired compound as a crystalline product. Yield: 1.9 g M.P. : 122 - 125 °C R f ¹ : 0.46 , R f ² : 0.64 [α] D : -38.6° (c=1.0, DMF) (4) pGlu-Asn-Ser-Pro-Arg-OH A solution of 100 mg of Z-pGlu-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl in 20 ml of 80 % acetic acid was stirred for 18 hours in a stream of hydrogen gas in the presence of 10 % palladium-carbon. The palladium-carbon was removed by filtration, and the solvent was distilled off. The residue was dissolved in water, and then freeze-dried. Then the resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 55 mg R f ³ : 0.18 [α] D : -85.5° (c=0.5, water) FAB mass spectrum (M+1) : 584 Example 14 H-Pro-Asn-Ser-Pro-Arg-OH acetate (1) Boc-Pro-Arg(NO₂)-OBzl To a solution of 15 g of H-Arg(NO₂)-OBzl in 250 ml of THF, 15 g of Boc-Pro-OSu was added under chilling with ice, followed by stirring for 18 hours at room temperature. After THF was distilled off, the residue was dissolved in AcOEt. The AcOEt solution was washed successively with dilute HCl, saturated aqueous NaHCO₃ solution and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off. The residue was dissolved in CHCl₃-MeOH, and purified by silica-gel column chromatography to obtain the desired compound as an oily product. Yield: 22 g R f ¹ : 0.61 , R f ² : 0.77 [α] D : -37.1° (c=1.0, DMF) (2) Boc-Ser(Bzl)-Pro-Arg(NO₂)-OBzl 22 g of Boc-Pro-Arg(NO₂)-OBzl was placed in 110 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the the solvent was distilled off. After drying under reduced pressure, the residue was dissolved in 150 ml of DMF. To the solution, 9 ml of Et₃N, 12.8 g of Boc-Ser(Bzl)-OH, 10 g of HOBt and 9.4 g of DCC were added under chilling with ice, followed by stirring for 18 hours at room temperature. DCUrea was removed by filtration, and DMF was distilled off. The residue was dissolved in AcOEt. Then the AcOEt solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl and water, followed by drying over anhydrous Na₂SO₄. AcOEt was distilled off, and the residue was treated with AcOEt-ether to give the desired compound as a crystalline product. Yield: 21 g M.P. : 80 - 82 °C R f ¹ : 0.67 , R f ² : 0.83 [α] D : -30.8° (c=1.0, DMF) (3) Boc-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl 4.0 g of Boc-Ser(Bzl)-Pro-Arg(NO₂)-OBzl was placed in 20 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the solvent was distilled off. To the residue, 2-butanol - CH₂Cl₂ (5 : 1 v/v) and saturated aqueous NaHCO₃ solution was added. The organic portion was collected and washed with saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solution was distilled off, and the residue was dissolved in 60 ml of DMF. To the solution, 1.34 g of Boc-Asn-OH, 1.34 g of HOBt and 1.32 g of DCC were added under chilling with ice. After stirring for 18 hours at room temperature, DCUrea was removed by filtration, and DMF was distilled off. The residue was dissolved in 2-butanol - CH₂Cl₂ (5 : 1 v/v) and then the solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl saturated with NaCl and saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and to the residue was added AcOEt to obtain the desired compound as a crystalline product. Yield: 4.3 g M.P. : 186 - 187 °C R f ¹ : 0.55 , R f ² : 0.73 [α] D : -34.6° (c=1.0, DMF) (4) Z-Pro-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl 3.5 g of Boc-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl was placed in 15 ml of 4 N HCl-AcOEt for 30 min. at room temperature, and then the solvent was distilled off. After drying under reduced pressure, the residue was dissolved in DMF. To the solution, 0.8 ml of NMM and 1.52 g of Z-Pro-OSu were added under chilling with ice, followed by stirring for 18 hours at room temperature. DMF was distilled off. The residue was dissolved in 2-butanol - CH₂Cl₂ (5 : 1 v/v) and then the solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl saturated with NaCl and saturated aqueous NaCl solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and the residue was purified with CHCl₃-methanol by silica-gel column chromatography, then was treated with ether to give the desired compound as a crystalline product. Yield: 3.2 g M.P. : 94 - 96 °C R f ¹ : 0.55 , R f ² : 0.75 [α] D : -45.5° (c=1.0, DMF) (5) H-Pro-Asn-Ser-Pro-Arg-OH acetate A solution of 150 mg of Z-Pro-Asn-Ser(Bzl)-Pro-Arg(NO₂)-OBzl in 20 ml of 80 % acetic acid was stirred for 18 hours in a stream of hydrogen gas in the presence of 10 % palladium-carbon. The palladium-carbon was removed by filtration, and the solvent was distilled off. The residue was dissolved in water, then freeze-dried. Then the resulting product was purified by high-performance liquid chromatography at 12 ml/min.(flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 96 mg R f ³ : 0.10 [α] D : -91.0° (c=0.5, water) FAB mass spectrum (M+1) : 570 Example 15 H-Pro-Ser-Pro-Arg-OH acetate (1) Z-Pro-Ser(Bzl)-Pro-Arg(NO₂)-OBzl The desired compound was prepared from 4.0 g of Boc-Ser(Bzl)-Pro-Arg(NO₂)-OBzl, 20 ml of 4 N HCl-AcOEt, 1 ml of NMM and 2.2 g of Z-Pro-OSu in the same manner as in Example 14-(4). The ether treatment was performed to obtain the desired compound as a crystalline product. Yield: 4.9 g M.P. : 72 - 74 °C R f ¹ : 0.66 , R f ² : 0.82 [α] D : -45.8° (c=1.0, DMF) (2) H-Pro-Ser-Pro-Arg-OH acetate 150 mg of Z-Pro-Ser(Bzl)-Pro-Arg(NO₂)-OBzl was reduced in the presence of palladium-carbon in the same manner as in Example 14-(5). The resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 75 mg R f ³ : 0.13 [α] D : -97.4° (c=0.5, water) FAB mass spectrum (M+1) : 456 Example 16 H-Pro-Ser-Pro-Arg-Gly-NH₂ acetate (1) Boc-Arg(NO₂)-Gly-NH₂ To a solution of 10 g of Boc-Arg(NO₂)-OH in 80 ml of DMF, 3.5 ml of NMM and 3.1 ml of ethyl chlorocarbonate were added under chilling with ice, followed by stirring for 15 minutes. To the resulting solution, a mixture of 3.5 g of H-Gly-NH₂ hydrochloride and 3.5 ml of NMM in 20 ml of DMF was added, and then the resulting mixture was stirred for 3 hours under chilling with ice. DMF was distilled off. The residue was dissolved in 2-butanol - CH₂Cl₂ (5 : 1 v/v) and then the solution was washed successively with saturated aqueous NaHCO₃ solution, dilute HCl saturated with NaCl and saturated NaCl aqueous solution, followed by drying over anhydrous Na₂SO₄. The solvent was distilled off, and to the residue was added AcOEt to obtain the desired compound as a crystalline product. Yield: 7.4 g M.P. : 160 - 162 °C R f ¹ : 0.21 , R f ² : 0.42 [α] D : +3.2° (c=1.0, DMF) (2) Boc-Pro-Arg(NO₂)-Gly-NH₂ The desired compound was prepared from 6.0 g of Boc-Arg(NO₂)-Gly-NH₂, 40 ml of 4 N HCl-AcOEt, 3.4 ml of Et₃N and 5.1 g of Boc-Pro-OSu in the same manner as in Example 14-(4). Yield: 6.5 g M.P. : 109 - 111 °C R f ¹ : 0.23 , R f ² : 0.45 [α] D : -30.5° (c=1.0, DMF) (3) Boc-Ser(Bzl)-Pro-Arg(NO₂)-Gly-NH₂ The desired compound was prepared from 6.0 g of Boc-Pro-Arg(NO₂)-Gly-NH₂, 35 ml of 4 N HCl-AcOEt, 1.8 ml of Et₃N and 5.1 g of Boc-Ser(Bzl)-OSu in the same manner as in Example 14-(4). Yield: 6.7 g M.P. : 109 - 113 °C R f ¹ : 0.32 , R f ² : 0.56 [α] D : -29.2° (c=1.0, DMF) (4) Z-Pro-Ser(Bzl)-Pro-Arg(NO₂)-Gly-NH₂ The desired compound was prepared from 1.0 g of Boc-Ser(Bzl)-Pro-Arg(NO₂)-Gly-NH₂, 10 ml of 4 N HCl-AcOEt, 0.34 ml of NMM and 0.54 g of Z-Pro-OSu in the same manner as in Example 14-(4). Yield: 0.7 g M.P. : 108 - 111 °C R f ¹ : 0.34 , R f ² : 0.56 [α] D : -56.0° (c=1.0, DMF) (5) H-Pro-Ser-Pro-Arg-Gly-NH₂ acetate 150 mg of Z-Pro-Ser(Bzl)-Pro-Arg(NO₂)-Gly-NH₂ was reduced in the presence of palladium-carbon in the same manner as in Example 14-(5). The resulting product was purified by high-performance liquid chromatography at 12 ml/min. (flow rate), 0 to 10 % (B) 20 min. linear gradient (A) (mobile phase), subjected to Dowex 1x2 (acetate type) treatment and freeze-dried to obtain the desired compound. Yield: 105 mg R f ³ : 0.10 [α] D : -96.7° (c=0.5, water) FAB mass spectrum (M+1) : 512 An example of pharmacological test showing the effectiveness of the peptides and the peptide derivatives of the present invention is set forth below. Pharmacological Test: Examination on improvement effect of experimental retrograde amnesia by cycloheximide The effect of peptides and the peptide derivatives of the present invention on memory consolidation was evaluated by conducting one-trial passive avoidance experiment using male Wistar rats in accordance with the method described by Burbach et al ., Science , vol. 221, pp. 1310-1312, 1983. The apparatus consisted of an illuminated room and a dark room, and their floors were made of stainless-steel grid. The rats placed in the illuminated room could freely enter the dark room. Upon entering the dark room the rats received an electro-shock. Retention of passive avoidance behavior to the electro-shock was determined by the measurement of a response latent period, i.e. period required for the rat experienced the electro-shock to reenter the dark room from the time on which the rat was placed in the illuminated room after predetermined intervals. The rats received an electro-shock (0.5 mA) after one hour from the administration of the peptides of the present invention or a physiological saline solution. Immediately after receiving the electro-shock, the rats were treated with 2.7 to 3.0 mg/kg of cycloheximide or the saline solution by subcutaneous injection. At 48 hours after the administration was made, memory retention of the rats were tested. The rats administered with only the physiological saline solution showed the response latent period of approx. 300 seconds, and those rats of control group administered with cycloheximide alone showed the response latent period of approx. 50 seconds, which revealed retrograde amnesia. The average response latent period of rats administered with each peptide of the present invention was compared with that of the control group. Six to eight rats were used for each group to be tested. The response latent period was measured up to a maximum of 600 seconds. The dose and the effect (the ratio of response latent period of each group to that of the control groups, shown as %) of the peptides obtained in each example are set forth in Table 1. Compound Dose (ng/kg) Effect (%) Example 3 1 460 Example 6 1 235 Example 9 0.1 353 Example 15 0.1 213 As readily apparent from the above experimental results, the peptides and the peptide derivatives of the invention showed superior effect on improving retrograde amnesia. Preparation examples of pharmaceuticals containing the peptide derivatives of the present invention are shown below. Preparation Example 1 (Injection) To 100 ml of distilled water for injection were added 0.1 mg of the peptide derivative obtained in Example 1 and 0.9 g of NaCl to prepare an aqueous solution whose pH was adjusted to 6.0 to 8.0 with NaOH. The solution was filtered under sterile conditions, and the filtrate was filled up into 1 ml ampoule. The ampoule was fused to seal under sterile conditions heating to prepare an agent for injection. With respect to each of the peptides obtained in Examples 6, 9 and 14, the above-described procedure was repeated to prepare agents for injection containing each peptide. Preparation Example 2 (Freeze-Dried Agent) To 100 ml of distilled water for injection were added 5 mg of the peptide derivative obtained in Example 1 and 5 g of D-mannitol to prepare an aqueous solution whose pH was adjusted to 6.0 to 8.0 with a phosphate buffer. The solution was filtered under sterile conditions, and the filtrate was divided into a plurality of 1 ml vials. The divided portions were freeze-dried to prepare a freeze-dried agent for injection. With respect to each of the peptides obtained in Examples 6, 9 and 14, the above-described procedure was repeated to prepare freeze-dried agents for injection containing each peptide. Preparation Example 3 (Collunarium) To 100 ml of a physiological saline solution was added 10 mg of the peptide derivative obtained in Example 1. The pH of the mixture was adjusted to 3.0 to 6.0 with a citric acid buffer to prepare a collunarium which contains 50 µg of the peptide of the invention in a dose of 0.5 ml. With respect to each of the peptides obtained in Examples 6, 9 and 14, the above described proceduce was repeated to prepare collunaria containing each peptide. Preparation Example 4 (Suppository) To 98.5 g of hard fat (triglyceride of saturated fatty acid) was added 0.5 g of egg yolk lecithin. The mixture was melted at temperature of 40 to 45 °C and to the melted mixture was added under stirring a solution of 5 mg of the peptide derivative obtained in Example 1 in 1 g of PEG 400. The resulting dispersion (1 g) was filled into the mold for suppository. The content was removed from the mold after being caked to prepare a suppository. With respect to each of the peptides obtained in Examples 6, 9 and 14, the above-described procedure was repeated to prepare suppositories containing each peptide.

Claims: A peptide having the formula (I): Asn-A-L-(D-)Pro-Arg-(Gly) n (I) wherein A is Ser, Thr or Ala, n is 1 or 0, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. A peptide as claimed in claim 1, the peptide having one of the following formulae: H-Asn-Ser-Pro-Arg-OH H-Asn-Thr-Pro-Arg-OH H-Asn-Ala-Pro-Arg-OH H-Asn-Ser-D-Pro-Arg-OH H-Asn-Ser-Pro-Arg-Gly-NH₂, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. An antidementia agent containing, as a pharmaceutically active component, a peptide according to claim 1 or claim 2, a functional derivative thereof or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. A peptide having the formula (II): A-Ser-Pip-Arg (II) wherein A is Pro-Asn-, Asn- or Pro-, and Pip is pipecolic acid, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. A peptide as claimed in claim 4, the peptide having one of the following formulae: H-Asn-Ser-Pip-Arg-OH H-Pro-Ser-Pip-Arg-OH H-Pro-Asn-Ser-Pip-Arg-OH, wherein Pip is pipecolic acid, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. An antidementia agent containing, as a pharmaceutically active component, a peptide according to claim 4 or claim 5, a functional derivative thereof or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. A peptide having the formula (III): pGlu-Asn-Ser-A-B-(Gly) n (III) wherein A is Aze(azetidine-2-carboxylic acid), D- or L-Pro, Pip (pipecolic acid) or Sar, B represents D- or L-Arg, Cit(citrulline), Har(homoarginine), Lys or Orn, n is 1 or 0, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. A peptide derivative as claimed in claim 7, the peptide having one of the following formulae: pGlu-Asn-Ser-Pro-Arg-Gly-NH₂ pGlu-Asn-Ser-D-Pro-Arg-Gly-NH₂ pGlu-Asn-Ser-Pro-D-Arg-Gly-NH₂ pGlu-Asn-Ser-D-Pro-D-Arg-Gly-NH₂ pGlu-Asn-Ser-Pro-Arg-OH, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. An antidementia agent containing, as a pharmaceutically active component, a peptide according to claim 7 or claim 8, a functional derivative thereof or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. A peptide having the formula (IV): Pro-(Asn) m -Ser-L-(D-)Pro-Arg-(Gly) n (IV) wherein m and n are independently 0 or 1, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. A peptide derivative as claimed in claim 10, the peptide having one of the following formulae: H-Pro-Asn-Ser-Pro-Arg-OH H-Pro-Ser-Pro-Arg-OH H-Pro-Ser-Pro-Arg-Gly-NH₂, a functional derivative thereof, or a pharmaceutically acceptable salt thereof. An antidementia agent containing, as a pharmaceutically active component, a peptide according to claim 10 or claim 11, a functional derivative thereof or a pharmaceutically acceptable salt thereof, together with a pharmaceutically accpetable carrier or diluent.

Inventors: HIRATE KENJI, ISOWA YOSHIKAZU, MASAKI MITSUO, NAKASHIMA YOSHIHARU, SATO YOSHIAKI, UEHARA MASAKI, HIRATE, KENJI, ISOWA, YOSHIKAZU, MASAKI, MITSUO, NAKASHIMA, YOSHIHARU, SATO, YOSHIAKI, UEHARA, MASAKI
Applicants: FUJIREBIO KK, NIPPON CHEMIPHAR CO, FUJIREBIO KABUSHIKI KAISHA, NIPPON CHEMIPHAR CO., LTD.
Application year: 1990
Main classification: C07K5
IPC codes: A61K38, C07K7, C07K5