Inf. J. Peptide Protein Res. 45, 1995. 508-513 Printed in Belgium - all righrs reserved

Copyright 0 Munksgaard 1995 INTERNATIONAL JOURNAL OF

PEPTIDE ti PROTEIN RESEARCH ISSN 0367-5377

Synthesis of potent agonists of Substance P by replacement of Metl1 with Glu(OBz1) and N-terminal glutamine with Glp of the C-terminal hexapeptide and heptapeptide of Substance P

GEORGE STAVROPOULOS I , KOSTAS KARAGIANNIS ', STAVROS ANAGNOSTIDES

Department of Chemistry, University of Patras, Patras, Greece. and Department of Bioiogicai Chemistry, Department of

ITZHAK MINISTROUSKI z, ZVI SELINGER2 and MICHAEL CHOREV

Pharmaceutical Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel

Received 27 May, accepted for publication 17 November 1994

The analogues [Glp6,Glu(OBzl)"]SP~6-ll) and [Glp5,Glu(OBzl)11]SP~5-1 1) of the C-terminal hexapeptide and heptapeptide of Substance P have been synthesized by conventional solution methods. In each analogue the N-terminal glutamine has been replaced by pyroglutamic acid, while the COOCH~C6Hs ester group has replaced the SCH3 group of the Met" side chain. The in vitro activity of both analogues has been determined on three biological preparations: guinea pig ileum (GPI), rat vas deferens (RVD) and rat portal vein (RPV). The results showed that both analogues are highly potent and selective agonists on GPI through the NK-1 receptor. They are more potent than SP itself, with 1.54 and 1.25 respective values of relative potency on GPI. Their selectivity has been studied by utilizing atropine-treated guinea pig ileum (GPI + At). The analogues showed low activity on RVD and RPV tissues, which represent NK-2 and NK-3 monoreceptor assay, respectively. 0 Munksgaard 1995.

Key words: guinea pig ileum; glutamine replacement; methionine replacement; NK-I receptor; Substance P agonist; Sub- stance P C-terminal analogues

Substance P (SP) is a naturally occurring and well studied undecapeptide amide: H-Arg-Pro-Lys-Pro- Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2. It is widely dis- tributed in the central and peripheral nervous system and, together with the decapeptides neurokinin A (NKA) and neurokinin B (NKB), constitutes the family of mammalian tachykinins. They have related primary structures and display high homology at the C-terminal pentapeptide amide: H - Phe- X -Gly -Leu- Met-NH2, where X = Phe in the case of SP and X = Val in the case

Abbreviations used are in accordance with rules of the IUPAC-IUB Joint Commission on Biochemical Nomenclature in Eur. J . Biochem. (1984) 138,9-37 and J . Biol. Chem. (1989) 264,663-673. Additional abbreviations used herein are as follows: Boc, ten-butyloxycarbonyl; Bzl, benzyl; DCC, dicyclohexylcarbodiimide; Glp, pyroglutamic acid; HOBt, I-hydroxybenzotriazole; NMM, N-methylmorpholine; ONp, p-nitrophenyl ester; TFA, trifluoroacetic acid; TLC, thin-layer chromatography. The nomenclature for receptors NK- I, NK-2 and NK-3 is in accordance with the recommendations of the committee from the Substance P and Neurokinins Symposium (Montreal, Canada, 1986).

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of NKA and NKB (1-3). Tachykinins exert a wide spectrum of similar biological actions (4, 5). Also SP is reported to have a role in a variety of important dis- eases (6, 7). Their receptors are designated as NK-1, NK-2 and NK-3 for SP, NKA and NKB, respectively. This classification is based on the relative order of po- tency in various bioassays of tachykinins and their frag- ments. Receptor NK-1 exists in both the central and peripheral nervous systems, while NK-2 and NK-3 are found to predominate in the peripheral and in the cen- tral nervous systems, respectively (2, 8, 9).

Further elucidation of the biological actions of SP requires the synthesis of new agonists capable of spas- mogenic activity mediated through one of the three re- ceptors. Structure-activity studies have shown that the biological activity of SP depends mainly on the se- quence of the C-terminal region of the molecule. The C-terminal hexapeptide a i d e sP6-11 and some of its synthetic analogues are the minimal peptide fragments of SP which retain substantial biological SP-like ago- nist activity and in several pharmacological prepara- tions it is more active than the entire undecapeptide (10, 1 1). Other structure-activity relationship studies on SP

Potent agonists of Substance P

analogues modified at the C-terminus by residues which may lead to NK-1 receptor subtype selectivity.

In this paper we report the synthesis and biological activityofthe analogues [Glp6,Glu(OBzl)11]SP6-11 and [ GlpS,Glu(OBzl)' 'ISPs-11, in which we combine modi- fication at the N-terminus by Glp and at the C-terminus by Glp(Bz1). These analogues were tested in guinea pig ileum (GPI) in the presence of atropine, in rat vas deferens (RVD) and in rat portal vein (RPV), known to represent NK-1, NK-2 and NK-3 receptor subtype specific assays.

or its C-terminal analogues have shown that methionine at position 11 of the molecule is an important factor associated with SP activity and/or selectivity (12). Re- placement of Met" by neutral lipophilic amino acids (e.g. Val, Leu, Phe) yielded analogues with a lower potency than SP (13, 14), while replacement of the SCH3 group of Met" in [Orn6]SP6-ll by charged groups reduces the biological activity of the analogue at NK-1 receptor in GPI and at NK-2 receptor in rat colon (1 5).

When Met' ' was replaced by glutamate y-esters the resulting analogues showed that the nature of the group attached at the y-position of Glu is a determin- ing factor for activity. Thus the analogue [Om6, Gl~(OBu~)~~]sP6-11 (16) is more active at NK-1 than the SP-OMe, which is a highly potent and selec- tive NK-1 agonist (17). The analogue [Om6, Asp(OB~l)~~]SPs-l l (18) is full agonist at NK-1 recep- tor in GPI with lower potency as compared to the analogue [Orn6,Glu(OBz1)"]SP6-~l (15). These results indicate that not only is the nature of the side group of the residue 11 in SP important but also its position in the side chain may be a determining factor for the bio- logical activity. Another potent NK- 1 selective agonist is the analogue [Glu(OBzl)11]SP6-ll(19), in which only the side chain of Met" has been modified, and the analogue was shown to retain about 70% of SP con- tractile activity. In general, all these results prove that replacement of Met" side chain by lipophilic and bulky groups is tolerable by the guinea pig ileum yielding ana- logues with significant affinity at NK-1 receptor.

Other studies concerning the C-terminal heptapep- tide amide SP5-11 have also shown that this fragment retains considerable biological activity in various prepa- rations (2,lO). However, modifications of the C-terminal carboxamide in the analogue [Nlell]SP5-11 resulted in an important loss of activity (20). In recent studies we have shown that replacement of Metk1 by Glu(OBz1) yielded the heptapeptide [Glu(OBzl)ll]SPs-ll, equi- potent with SP at the NK-1 receptor subtype (19).

Obviously such replacements of the Met" side chain, like those described above, facilitate the synthetic pro- cedure providing the advantage of avoiding sulfoxide formation, which is a common side reaction during the synthesis of Met-containing peptides in either liquid or solid-phase peptide synthesis (2 1).

In many cases, a Glp residue at the N-terminus of SP-derived C-terminal hexa- and heptapeptides is found not only in active analogues such as [Glp6]sP6-ll and [Glp5]SP5-11 (10) but also in highly potent and selec- tive ones such as [Glp6,Pro9]SP6-ll (l), an NK-1 se- lective analogue, and [ Glp6,N-MePhe8]SP6-1l (22) and [ Glp6,N-MePhes,Aib9] SP6-11 (23), analogues selective toward the NK-3 receptor subtype. Incorporation of Glp may provide peptides which are resistant towards non-specific aminopeptidases and therefore contribute toward prolonged activity (24). Taken together, we de- cided to include the Glp residue at the N-terminus in

RESULTS AND DISCUSSION

The syntheses of C-terminal sequences of [Glp6, Gl~(OBzl)~~]sP6- l l and [ Glp5,Glu(OBzl)1 llSP5-1 I analogues were achieved by using the DCC/HOBt frag- ment condensation method, in which the dipeptide amide H-Leu-Glu(OBz1)-NHz was coupled with the tetrapeptide Glp-Phe-Phe-Gly-OH and the pentapep- tide Glp-Gln-Phe-Phe-Gly-OH, respectively. The frag- ments have been synthesized by a stepwise elongation from the C-terminus using the mixed anhydride method for all other couplings and Boc-Gln-ONp for the syn- thesis of the intermediate fragment Boc-Gln-Phe-Phe- Gly-OBzl. Catalytic hydrogenation of benzyl ester over

Glp Phe Phe Gly Leu GlufOBzl)

I I :k.A::; I I A

H

H HZ

H H2

H

Glp Gin Phe Phe Gly Leu Glu(OBz1)

SCHEME 1 Synthetic route of C-terminal hexapeptide (A) and heptapeptide (8) analogues of SP. Reagents and conditions: (i) iso-BuOCOC/NMM, 3-min activation at 0°C. (ii) TFA/Anisole. (iii) DCC/HOBt, DMF. (iv) Hz, 10% Pd/C, DMF. (v) Active ester coupling.

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10% Pd/C yielded the corresponding peptide-acids (Scheme 1). The pyroglutamic acid was used unpro- tected and activated with isobutyl chloroformate to couple with H-Phe-Phe-Gly-OBz1 and H-Gln-Phe-Phe- Gly-OBzl. The crude analogues were purified by gel filtration on Sephadex LH-20. Their purity was checked by TLC and RP-HPLC as described under Experimen- tal Procedures.

The in v i m agonist activity of these analogues was tested in the following smooth muscle organ bath as- says; isolated guinea pig ileum, rat vas deferens and rat portal vein. The GPI is a convenient model system in which functional assays of tachykinins elicit contrac- tion both by direct action on the NK-1 receptor of the smooth muscle and indirectly via the NK-3 neuronal receptor. The selectivity of the analogues for different subtype receptors in GPI has been studied by perform- ing experiments in the presence of the muscarinic blocker atropine to eliminate indirect effects from ac- tivation of neuronal NK-3 receptor (25).

The ECso (nM) values for the synthesized [Glp6, Gl~(OBzl)~~]SP6-11 and [Glp5,Glu(OBzl)11]SP5-ll analogues and for SP, Eledoisin and Senktide are sum- marized in Table 1. Both analogues are highly potent agonists in the GPI. They are more potent than SP itself, and their potencies for stimulation of the mus- cular receptor remain substantially unaffected after treatment of the tissue with atropine, suggesting that their activity is mediated by the NK-1 receptor subtype. Comparison of these analogues with closely related ones reported by us recently (19) shows that the substitution of the N-terminal Gln6 in [Glu(OBzl)ll]- sP6-11 and Gln5 in [Gl~(OBzl)~~]SP5-11 by Glp re- sulted in analogues exerting higher contractile activity in GPI. Indeed the analogue [Glp6,Glu(OBzl)11]SP6-ll acts with an ECso value of 0.65 nM as compared with 1.5 nM of [ G ~ U ( O B Z ~ ) ~ ~ ] S P ~ - ~ ~ and 1 nM of SP. Simi- larly the analogue [ Glp5,Glu(OBzl)11]SP~-~ I acts with an EC50 value of 0.8 AM as compared with 2.3 nM for

[Gl~(OBzl)~~]SP5-11 and 1 nM for SP, respectively (Table 1).

In the RVD assay (NK-2 receptor) and RPV assay (NK-3 receptor) the reported analogues were much less potent than in the GPI assay (NK-1 receptor). In the RVD assay SP seems to be 220-fold more active and eledoisin 80-fold more than the analogues, while in the RPV assay SP is 50-fold and Senktide 2500-fold more active. In general the analogues [Glp6, Glu(0Bzl)' ] SP6-1 I and [ Glp5,Glu(OBzl)11]SP~-~1 are very weak agonists in the assays representing NK-2 and NK-3 receptors.

The relative potency of the analogues on the con- traction through the NK-1 receptor is listed in Table 2, while the relative potency of SP is set as 1. The ana- logue [Glp6,Glu(OBzl)' l] sP6-11 is the more active pep- tide on GPI either in the absence (1.54) or in the pres- ence (2.50) of atropine. This latter value indicates that this hexapeptide is the more effective analogue acting mainly through the NK-1 receptor.

It is known from our previous studies on sP6-11- related analogues that the replacement of the SCH3 group of MetL1 by lipophilic and bulky groups, like COOC(CH3)3 (16) and COOCH2C6H5 (19) which ren- der more lipophilic peptide derivatives, does not reduce the activity of the prepared analogues. On the other hand, analogues containing an aromatic ring at the side chain of the amino acid at position 11 have higher activity than analogues lacking in such a ring (18, 26). In the present study, by combining the replacement of SCH3 by COOCH2C6H5 with simultaneous substitu- tion of the N-terminal amino acid by pyroglutamic acid, we succeeded in synthesizing the C-terminal SP hexapeptide and heptapeptide analogues [ Glp6,- G ~ ( O B Z ~ ) " ] S P ~ - I L and [Glp5,Glu(OBzl)11]SP~-1~ re- spectively. These analogues are potent and selective agonists on GPI acting through an NK-1 receptor sub- type. The hexapeptide was shown to be more active than the heptapeptide on GPI, while there was no dis-

TABLE 1 ECm /.Ml values for SP , Eledoisin. Senkiide and synthetic peptides"

Analogue GPI GPI +At RVD RPV 1 PM NK2 NK3

NK1+ NK3 NK 1

SP 1 x 10-9 3 ~ 1 0 - ~ 2.3 x 10-7 1 x 10-6 [ Glu(OBz1)' IISP6-i I 1.5 x 1.5x10- ' > 5 x 2 5 x

[ G l p h , G I ~ ( O B ~ l ) i l ] S P ~ ~ ~ I 0.65 x 10- 1.2 x 10- > 6 ~ 1 0 - ~ > 5 ~ 1 0 - ~ [Glp5 ,Gl~(OB~l ) ' 1 ]SP~-~ I 0.8 x 10- 2.3 x 10- > 5 ~ 1 0 - ~ > 5 x Eledoisin 6 x lo-' Senktide 2 x 10-8

a Results are mean values calculated from at least three experiments (standard error t 2 5 % ) .

[GIU(OBZI)"]SP~-~I 2.3 x 10- 2.3 x 10 ~ > 3 . 5 ~ 10-5 > i x i 0 - 5

Data from ref. 19.

510

Potent agonists of Substance P

60 F254 (Merck) aluminium plates and the following solvent systems were used: (A) CHC13/CH3OH (6: l),

CH3OH/CH3COOH (85:10:5). The spots on TLC plates were detected: (a) by UV light at 254 nm; (b) by spray solution of 0.3% ninhydrin in 1-butanol and heating at 100 "C for 5 min; (c) by chlorination fol- lowed by spray with a mixture of 1 % starch- 1 % KI (1: 1 v/v). The elemental analyses of the intermediate peptides were within 0.40% of the calculated values. Amino acid analysis was performed on a Beckman model 120 C amino acid analyzer using a three-buffer column system. Peptide samples of 1 mg were hydro- lyzed with 6 M hydrochloric acid at 110 "C for 24 h in evacuated and sealed tubes. Retention times (tR) of analogues were measured by RP-HPLC using a Waters 991 HPLC system equipped with a Nucleosil C8 col- umn 125 x 4 mm, 7 pm with the following solvent sys- tems: (A)0.08% TFA in water, (B)0.08% TFA in CH3CN. Elution with linear gradient 30% (B)-80% (B) for 30 min, UV detection at 214 nm and flow rate 1 mL/min. The Boc-protected amino acid derivatives and intermediate peptides were deprotected with TFA- anisole in the proportion 9:l v/v.

(B) BuOH/CH3COOH/H20 (4: 1: 1) and (C) CHCl3/

TABLE 2 Relative potenciesa of analogues for the contraction of GPI

Peptide GPI GPI+At 1 pM

SP 1.0 1 .o

[GIU(OBZI)~']SP~-~~ 0.43 1.30 [Glp6,Glu(OBzl)'L]SP6-~ 1 1.54 2.50 [ Glp5,Cl~(OB~l)' ] SP5-11 1.25 1.30

[Glu(OBzl)" ]sP6-11 0.67 2.00

a Relative potency (SP = 1) is defined as the ratio of EC5o of sub- stance P to the EC50 of peptides tested. Data from ref. 19. The results show clearly that the synthesized peptides are more potent agonists on GPI than SP.

tinction between them on RVD and RPV biological assays.

In conclusion, the results indicate that methionine in position 11 of SP can be replaced by y-benzyl glutamate and that not only the lipophilic character and the aro- maticity of this ester in the side chain but also the stability of the N-terminal Glp may contribute to the increased activity of the peptide analogues. It is re- markable that these analogues, as well as the one which has already appeared in the literature (16), are the only examples in which replacement of methionine in Sub- stance P C-terminal fragments by amino acid deriva- tives which do not contain sulfur at the &position of their side chain led to more potent analogues on GPI than the parent compound SP. Taken together, these results suggest that the benzene ring located at an ap- propriate place in the side chain of the amino acid at position 11 in SP can fulfil the role of the Met l1 side chain for activation of the NK-1 receptor. It also appears from the result on NK-2 and NK-3 receptors that there are specific requirements, possibly different for each receptor subtype, associated with the side chain of the amino acid at position 11, which in the case of Met" are determined by the SCH3 groups. Finally, the results prove that simple but appropriate modifications at the side chain of Met", which facilitate the synthetic procedure by avoiding sulfoxide formation, combined with substitution of the N-terminal Gln, yield analogues

SPS-1 1 which on GPI are more potent agonists than the corresponding fragments SP6-{1 and sP5-11 and the intact SP. Furthermore, their activity is not affected when the guinea pig ileum tissue is treated with atro- pine, which suggests selectivity for the NK-1 receptor.

[ Glp6,Glu(OBzl)"]SP6-ll and [ Glp5,Gl~(OB~l) ' 'I-

EXPERIMENTAL PROCEDURES

All amino acids were of the L-configuration. Melting points were determined on a Buchi SMP-20 apparatus and are reported uncorrected. Optical rotations were measured with a Carl Zeiss precision polarimeter (k 0.005). TLC was performed on precoated silica gel

Preparation of Boc-Leu-Glu(OBzl)-NH2. The preparation of this dipeptide amide is completely described in ref. 19.

Preparation of Glp-Phe-Phe-Gly-OBzl. A sample of Boc- Phe-Phe-Gly-OBzl(840 mg, 1.5 mmo1)was treated with TFA-anisole (3 mL) for 1 h at room temperature. The solvent was removed in vacuo and the oily residue was solidified by trituration with dry ether, filtered, washed with dry ether on the filter and dried over P205/KOH. Subsequently, the TFA-salt was dissolved in DMF (5 mL) and neutralized with an equivalent amount of NMM at -10°C. To a solution of Glp (516mg, 4 mmol) in DMF (5 mL), cooled to -10 "C, NMM (4 mmol) and isobutyl chloroformate (4 mmol) were added. After 3 min the neutralized solution of H-Phe- Phe-Gly-OBzl was added into the mixed anhydride of Glp, and the reaction mixture was stirred for 1 h at - 10 " C and for 3 h at room temperature. At the comple- tion of the reaction the solvent was evaporated in vacuo and the residue was treated with chilled 5 % NaHCO3 solution and filtered. The residue on the filter was washed consecutively with water, 10% citric acid so- lution and water. Recrystallization from DMF/water yielded 760 mg (89%); m.p. 205-206 "C; [a12 -18.6 (c 1, DMF); TLC RdA) 0.61, RdB) 0.73.

Preparation of Glp-Phe-Phe-Gly-OH. A sample of Glp- Phe-Phe-Gly-OBzl (400 mg, 0.7 mmol) was dissolved in DMF and hydrogenated over 10% PdjC under at- mospheric pressure for 3 h. The reaction was followed by TLC in the solvent system C. At the end the reac- tion mixture was filtered through celite and the filtrate

511

G. Stavropoulos et al.

was evaporated in vacuo. The residue was dissolved in methanol and precipitated by addition of ether. The resulted peptide acid was collected with filtration 330 mg (98%); m.p. 227 "C dec.; [a]$' -13.7 (c 1, DMF); TLC RdB) 0.61, RdC) 0.20.

Preparation of Boc-Gln-Phe-Phe-Gly-OBzl. A sample of Boc-Phe-Phe-Gly-OBzl (1.26 g, 2.25 mmol) was de- protected as described above. The resulting salt CF3COOH.H-Phe-Phe-Gly-OBzl was dissolved in DMF (8 mL) at 0 "C, neutralized with an equivalent amount of NMM, and Boc-Gln-ONp (992mg, 2.70 mmol, 20% excess) was added. The mixture was stirred for 48 h at room temperature. The solvent was evapo- rated in vucuo and the residue was treated with chilled 5 % NaHCO3 solution repeatedly, filtered and washed on the filter consecutively with water, 10% citric acid solution and water. Recrystallization from MeOH/ water yielded 1.33 g (86%); m.p. 210-212 "C; [ z ] g -11.8 (C 1, DMF); TLC RdA) 0.58, RdB) 0.86.

Preparation of Glp-GIn-Phe-Phe-Gly-OBzI. A sample of Boc-Gln-Phe-Phe-Gly-OBzl (960 mg, 1.4 mmol) was deprotected with TFA-anisole ( 5 mL) as usual and coupled with Glp (5 16 mg, 4 mmol) as described above for Glp-Phe-Phe-Gly-OBzl. Recrystallization of the crude peptide from DMF/ether yielded 860 mg (88%); m.p. 249-250°C; [a]$' -16.7 (c 1, DMF); TLC Rf(A) 0.26, RdB) 0.62.

Preparation of Glp-Gln-Phe-Phe-Gly-OH. This peptide acid was prepared by hydrogenation of the correspond- ing benzyl ester (489 mg, 0.7 mmol) in an analogues way as described for Glp-Phe-Phe-Gly-OH. The yield was 410 mg (96%); m.p. 240 "C; [z ]E -19.2 (c 1, DMF); TLC RdB) 0.42, RdC) 0.05.

Preparation of Glp-Phe-Phe-Gly-Leu-GlufOBzl)-NH2. A sample of Boc-Leu-Glu(OBz1)-NH2 (198 mg, 0.44 mmol) was treated with TFA-anisole (2 mL) for 1 h at room temperature and the resulting salt, after washing with dry ether, was dissolved in DMF (3 mL), neutral- ized with NMM and allowed to react with Glp-Phe- Phe-Gly-OH (192 mg, 0.4 mmol) preactivated at 0 'C for0.5 h with DCC (0.44 mmol) and HOBt (0.70 mmol). The reaction mixture was stirred for 2 h at 0 'C and overnight at room temperature. The precipitated DCU was filtered and the crude product was precipitated from the filtrate with addition of 5% NaHCO3. The product was collected by filtration, washed on the fil- ter with 5 % NaHC03, water, 5 % citric acid and water, and dried over P205. Yield 280 mg (86;~) . Complete purification was achieved by gel filtration on Sephadex LH-20, using as eluent CH30H/DMF 91/9 v/v. The product was lyophilized from a mixture of CH3OH/ H20; m.p. 225 "C; [a]'," -4.5 (c 0.5, DMF); TLC RdA) 0.28, RdB) 0.69, RdC) 0.41; t R 12.2 min. Amino acid analysis: Glu 1.96, Gly 1.06, Leu 1.00, Phe 1.94.

512

Preparation of Glp- Gln-Phe-Phe-Gly-Leu- Glu(0Bzl)- NH2. This heptapeptide analogue was prepared as the hexapeptide analogue by coupling the Glp-Gln-Phe- Phe-Gly-OH (213 mg, 0.35 mmol) with deprotected Boc-Leu-Glu(OBz1)-NHz (173 mg, 0.39 mmol). Yield 280 mg (85%); m.p. 230 "C softening 255 "C dec.;

RdC) 0.13; t R 9.5 min. Amino acid analysis: Glu 2.80, Gly 0.94, Leu 1.00, Phe 2.18.

[ XI$' -3.6 (C 0.3, DMF); TLC RdA) 0.07, RdB) 0.63,

Biological assays. Stock solutions of SP analogues (1 mM) were made in MezSO and were diluted with buffer to the final concentration. The MezSO concen- tration in the organ bath (10 mL) did not exceed 0.2%. All the smooth muscle contraction assays were con- ducted at 34 ' C in continuously oxygenated (95 % 02-5 % COZ) Tyrode's buffer of the following compo- sition (in mM): NaCl 118; KC1 4.7; CaClz 1.8; MgClz 0.5; NaH2P04 1.0; NaHCO3 25 and glucose 10. Con- tractions were recorded isotonically under a resting load of 0.2-0.8 g (for ileum and vas deferens). Peptides were applied at 2-3 min intervals with contact time of less than 30 s (ileum), or at 15-30 min intervals with con- tact time of 1-3 min (vas deferens). All tested peptides produced similar maximal contractions in a given test preparation. Results of each analogue were expressed as a percentage of the maximal contraction elicited by that compound. Relative potencies were calculated from EC5o values (concentration of agonist producing 50 % of the maximal contraction).

Guinea pig ileum (GPI) assay in the absence or pres- ence of atropine (1 p ~ ) was carried out as described previously (1, 25).

In the rat vas deferens assay the muscle was stimu- lated transmurally, using a Grass stimulator with plati- num electrodes, at supramaximal voltage (0.1 Hz, 1 ms, biphasic pulses).

Rat portal vein assay was conducted as described by Mastrangelo et al. (28). Longitudinal muscles were pre- incubated in the organ bath for 60 min prior the first application of analogue. Intervals between two con- secutive applications were 20 min. Contractions were recorded with a Grass isometric transducer for 2-3 min.

1.

2.

3.

4. 5 . 6 .

7.

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Potent agonists of Substance P

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Address:

Dr George Stavropoulos Department of Chemistry University of Patras Patras 26100 Greece

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