Linear and monocyclic endothelin antagonists

Abstract

Novel linear and monocyclic antagonists of endothelin are described, as well as methods for the preparation and pharmaceutical compositions of the same, which are useful in controlling hypertension, myocardial infarction, metabolic, endocrinological, and neurological disorders, congestive heart failure, endotoxic shock, subarachnoid hemorrhage, arrhythmias, asthma, acute renal failure, preeclampsia, and diabetes.

Description

BACKGROUND OF THE INVENTION

The present invention relates to novel linear and monocyclic antagonists of endothelin useful as pharmaceutical agents, to methods for their production, to pharmaceutical compositions which include these compounds and a pharmaceutically acceptable carrier, and to pharmaceutical methods of treatment. More particularly, the novel compounds of the present invention are antagonists of endothelin useful in controlling hypertension, myocardial infarction, metabolic, endocrinological, and neurological disorders, congestive heart failure, endotoxic shock, subarachnoid hemorrhage, arrhythmias, asthma, acute renal failure, preeclampsia, and diabetes.

Endothelin-1 (ET-1), a potent vasoconstrictor, is a 21 amino acid bicyclic peptide that was first isolated from cultured porcine aortic endothelial cells. Endothelin-1, is one of a family of structurally similar bicyclic peptides which include; ET-2, ET-3, vasoactive intestinal contractor (VIC), and the sarafotoxins (SRTXs). The unique bicyclic structure and corresponding arrangement of the disulfide bridges of ET-1, which are the same for the endothelins, VIC, and the sarafotoxins, has led to significant speculation as to the importance of the resulting induced secondary structure to receptor binding and functional activity. ET-1 analogues with incorrect disulfide pairings exhibit at least 100-fold less vasoconstrictor activity. The flexible C-terminal hexapeptide of ET-1 has been shown to be important for binding to the ET receptor and functional activity in selected tissues. Additionally, the C-terminal amino acid (Trp-21) has a critical role in binding and vasoconstrictor activity, since ET[1-20] exhibits approximately 1000-fold less functional activity.

Cody, W. L., et al, Abstract, Second International Conference on Endothelin, Tsukuba, Japan, Dec. 9, 1990, and Johansen, N. L., et al, Peptides 1990, Proceedings of the Twenty First European Peptide Symposium, edited by Giralt, E. and Andreu, D., pages 680-681, Escom Science Publishers B.V. (1990) disclosed various monocyclic analogs of ET-1, none of which exhibited any functional vasoconstricting activity.

However, we have surprisingly and unexpectedly found that a series of linear and monocyclic analogs of ET-1 are antagonists of endothelin.

SUMMARY OF THE INVENTION

Accordingly, the present invention is a compound of Formula I

AA.sup.1 --R--R.sup.1 --R.sup.2 --AA.sup.1a --AA.sup.2 --AA.sup.4 --AA.sup.3 --AA.sup.5 Seq ID No: 1 wherein AA.sup.1 is ##STR1## wherein X is hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, or heteroaryl, n is zero or an integer of 1, 2, 3, 4, 5, or 6, R.sup.3 is --S--R.sup.4, wherein R.sup.4 is as defined hereinafter, ##STR2## wherein R.sup.4 is as defined hereinafter, --C(CH.sub.3).sub.2 --S--R.sup.4, wherein R.sup.4 is hydrogen, alkyl, cycloalkyl, aryl, heteroaryl, or R.sup.4 is absent when AA.sup.1 is covalently linked to AA.sup.1a through a disulfide bridge, ##STR3## wherein X and X.sup.1 are each the same or different and each is as defined above for X or X.sup.1 is Asp-Lys Glu and n and R.sup.3 are as defined above, ##STR4## wherein X, n, and R.sup.3 are as defined above, or ##STR5## wherein X, n, and R.sup.3 are as defined above; R is absent or is one to four amino acids selected from the group consisting of: Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, His, Ile, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr, Val, 3Hyp, 4Hyp, Hcy, Nva, Nle, Orn, Abu, Ahe, Acp, Aoc, Apn, Chx, Cit, HomoPhe, 1-Nal, 2-Nal, Pen, Pgl, Pyr, Tic, Tyr(OMe), Tyr(OEt), and Trp(For); R.sup.1 is absent or is ##STR6## wherein m is an integer of 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; R.sup.2 is absent or is one to three amino acids selected from the group consisting of: Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, His, Ile, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr, Val, HomoPhe, 1-Nal, 2-Nal, Pgl, Pyr, Tic, Tyr(OMe), Tyr(OEt), and Trp(For) and provided that at least one amino acid is selected from the group consisting of: His, Phe, Trp, Tyr, HomoPhe, 1-Nal, 2-Nal, Pgl, Pyr, Tic, Tyr(OMe), Tyr(OEt), and Trp(For); AA.sup.1a is ##STR7## wherein R.sup.3 and n are as defined above; AA.sup.2 is absent or is ##STR8## wherein R.sup.5 is aryl or heteroaryl, and n is as defined above; AA.sup.3 is absent or is ##STR9## wherein R.sup.6 is hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, --OR.sup.7, wherein R.sup.7 is defined hereinafter, --CO.sub.2 --R.sup.7 wherein R.sup.7 is defined hereinafter, ##STR10## wherein R.sup.7 and R.sup.7' are each the same or different and each is hydrogen, alkyl, cycloalkyl, or heterocycloalkyl, and n is as defined above; AA.sup.4 is absent or is ##STR11## wherein R.sup.6 and n are as defined above; AA.sup.5 is ##STR12## wherein R.sup.8 is absent or is alkyl, alkenyl, alkynyl, cycloalkyl, or heterocycloalkyl, R.sup.9 is --CO.sub.2 H, or --CH.sub.2 OH and R.sup.5 and n are as defined above, ##STR13## wherein R.sup.8 and R.sup.9 are as defined above, ##STR14## wherein R.sup.8 and R.sup.9 are as defined above, ##STR15## wherein R.sup.8 and R.sup.9 are as defined above, ##STR16## wherein R.sup.8 and R.sup.9 are as defined above, ##STR17## wherein R.sup.10 is

hydrogen,

alkyl,

formyl, or

acetyl,

R.sup.8 and R.sup.9 are as defined above, or ##STR18## wherein R.sup.8, R.sup.9, and R.sup.10 are as defined above, and stereochemistry at CH or C in AA.sup.5 is L; and with the exclusion of the compound of formula ##STR19## or a pharmaceutically acceptable salt thereof.

Elevated levels of endothelin have been postulated to be involved in a number of pathophysiological states including diseases associated with the cardiovascular system as well as various matabolic, neurological, and endocrinological disorders. As antagonists of endothelin, the compounds of Formula I are useful in the treatment of hypertension, myocardial infarction, metabolic, endocrinological and neurological disorders, congestive heart failure, endotoxic shock, subarachnoid hemorrhage, arrhythmias, asthma, acute renal failure, preeclampsia, and diabetes.

A still further embodiment of the present invention is a pharmaceutical composition for administering an effective amount of a compound of Formula I in unit dosage form in the treatment methods mentioned above.

Finally, the present invention is directed to methods for production of a compound of Formula I.

DETAILED DESCRIPTION OF THE INVENTION

In the compounds of Formula I, the term "alkyl" means a straight or branched hydrocarbon radical having from 1 to 12 carbon atoms and includes, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, undecyl, dodecyl, and the like.

The term "alkenyl" means a straight or branched unsaturated hydrocarbon radical having from 2 to 12 carbon atoms and includes, for example, ethenyl, 2-propenyl, 1-butenyl, 2-butenyl, 1-pentenyl, 2-pentenyl, 3-methyl-3-butenyl, 1-hexenyl, 2-hexenyl, 3-hexenyl, 3-heptenyl, 1-octenyl, 1-nonenyl, 1-decenyl, 1-undecenyl, 1-dodecenyl, and the like.

The term "alkynyl" means a straight or branched triple bonded unsaturated hydrocarbon radical having from 2 to 12 carbon atoms and includes, for example, ethynyl, 2-propynyl, 1-butynyl, 2-butynyl, 3-butynyl, 1-pentynyl, 3-pentynyl, 1-hexynyl, 2-hexynyl, 3-hexynyl, 3-heptynyl, 1-octynyl, 2-octynyl, 1-nonynyl, 2-nonynyl, 3-nonynyl, 4-nonynyl, 1-decynyl, 2-decynyl, 2-undecynyl, 3-undecynyl, 3-dodecynyl, and the like.

The term "cycloalkyl" means a saturated hydrocarbon ring which contains from 3 to 12 carbon atoms, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, adamantyl, and the like.

The terms "alkoxy" and "thioalkoxy" are O-alkyl or S-alkyl as defined above for alkyl.

The term "aryl" means an aromatic radical which is a phenyl group, a benzyl group, a naphthyl group, a pyrenyl group, an anthracenyl group, or a fluorenyl group, unsubstituted or substituted by 1 to 4 substituents selected from alkyl as defined above, alkoxy as defined above, thioalkoxy as defined above, hydroxy, thiol, nitro, halogen, amino, ##STR20## wherein alkyl is as defined above, ##STR21## wherein alkyl is as defined above, ##STR22## wherein alkyl is as defined above, or aryl.

The term "heteroaryl" means a heteroaromatic radical which is 2- or 3-thienyl, 2- or 3-furanyl, 2-or 3-pyrrolyl, 2-, 4-, or 5-imidazolyl, 3-, 4-, or 5-pyrazolyl, 2-, 4-, or 5-thiazolyl, 3-, 4-, or 5-isothiazolyl, 2-, 4-, or 5-oxazolyl, 3- , 4-, or 5-isoxazolyl, 3- or 5-1,2,4-triazolyl, 4- or 5-1,2,3 triazolyl, tetrazolyl, 2-, 3-, or 4-pyridinyl, 3-, 4-, or 5-pyridazinyl, 2-pyrazinyl, 2-, 4-, or 5-pyrimidinyl, 2-, 3-, 4-, 5-, 6-, 7-, or 8-quinolinyl, 1-, 3- , 4- , 5-, 6-, 7-, or 8-isoquinolinyl, 2-, 3-, 4-, 5-, 6-, or 7-indolyl, 2-, 3-, 4-, 5-, 6-, or 7-benzo[b]thienyl, or 2-, 4-, 5-, 6-, or 7-benzoxazolyl, 2-, 4-, 5-, 6-, or 7-benzimidazolyl, 2-, 4- , 5-, 6- , or 7-benzothiazolyl, unsubstituted or substituted by 1 to 2 substituents selected from alkyl as defined above, aryl as defined above, alkoxy as defined above, thioalkoxy as defined above, hydroxy, thiol, nitro, halogen, formyl, amino, ##STR23## wherein alkyl is as defined above, ##STR24## wherein alkyl is as defined above, ##STR25## wherein alkyl is as defined above or phenyl.

The term "heterocycloalkyl" means 2- or 3-tetrahydrothieno, 2-or 3-tetrahydrofurano, 2-or 3-pyrrolidino, 2-, 4-, or 5-thiazolidino, 2-, 4-, or 5-oxazolidino, 2- , 3-, or 4-piperidino, N-morpholinyl or N-thiamorpholinyl.

"Halogen" is fluorine, chlorine, bromine or iodine.

The following table provides a list of abbreviations and definitions thereof used in the present invention.

TABLE______________________________________Abbreviation*______________________________________ Amino AcidAla AlanineArg ArginineAsn AsparagineAsp Aspartic acidCys CysteineGlu Glutamic acidGln GlutamineGly GlycineHis HistidineIle IsoleucineLeu LeucineLys LysineMet MethioninePhe PhenylalaninePro ProlineSer SerineThr ThreonineTrp TryptophanTyr TyrosineVal Valine Modified and Unusual Amino Acid3Hyp 3-Hydroxyproline4Hyp 4-HydroxyprolineHcy HomocysteineNva NorvalineNle NorleucineOrn OrnithineAbu 4-Aminobutyric acidAhe 7-Aminoheptanoic acidAcp 6-Aminocaproic acidAoc 8-Aminooctanoic acidApn 5-Aminopentanoic acidChx Cyclohexylalanine (Hexahydrophenyl- alanine)Cit CitrullineHomoPhe 2-Amino-5-phenylpentanoic acid (Homophenylalanine)1-Nal 3-(1'-Naphthyl)alanine2-Nal 3-(2'-Naphthyl)alaninePen PenicillaminePgl PhenylglycinePyr 2-amino-3-(3-pyridyl)-propanoic acid (3-Pyridylalanine)Tic 1,2,3,4-Tetrahydro-3- isoquinolinecarboxylic acidTyr(OMe) O-Methyl-tyrosineTyr(OEt) O-Ethyl-tyrosineTrp(For) N.sup.in -Formyltryptophan Mercapto AcidsMaa Mercaptoacetic acidMba 4-Mercaptobutyric acidMpa 3-Mercaptopropionic acid Protecting GroupAc AcetylAda 1-Adamantyl acetic acidAdoc AdamantyloxycarbonylBzl BenzylMeBzl 4-MethylbenzylZ Benzyloxycarbonyl2-Br--Z ortho-Bromobenzyloxycarbonyl2-Cl--Z ortho-ChlorobenzyloxycarbonylBom BenzyloxymethylBoc tertiary ButyloxycarbonylDnp 2,4-DinitrophenylFor FormylFmoc 9-FluorenylmethyloxycarbonylNO.sub.2 NitroTos 4-Toluenesulfonyl (tosyl)Trt Triphenylmethyl (trityl) Solvents and ReagentsHOAc Acetic acidCH.sub.3 CN AcetonitrileDCM DichloromethaneDCC N,N' -DicyclohexylcarbodiimideDIEA N,N-DiisopropylethylamineDMF N,N'-DimethylformamideHCl Hydrochloric acidHF Hydrofluoric acidHOBt 1-HydroxybenzotriazoleKOH Potassium hydroxideTFA Trifluoroacetic acidMBHA Resin Methylbenzhydrylamine resinPAM Resin 4-(Oxymethyl)-phenylacetamidomethyl resin______________________________________ *If the optical activity of the amino acid is other than L(S), the amino acid or abbreviation is preceded by the appropriate configuration D(R) or DL(RS).

The compounds of Formula I are capable of further forming both pharmaceutically acceptable acid addition and/or base salts. All of these forms are within the scope of the present invention.

Pharmaceutically acceptable acid addition salts of the compounds of Formula I include salts derived from nontoxic inorganic acids such as hydrochloric, nitric, phosphoric, sulfuric, hydrobromic, hydroiodic, hydrofluoric, phosphorous, and the like, as well as the salts derived from nontoxic organic acids, such as aliphatic mono- and dicarboxylic acids, phenyl-substituted alkanoic acids, hydroxy alkanoic acids, alkanedioic acids, aromatic acids, aliphatic and aromatic sulfonic acids, etc. Such salts thus include sulfate, pyrosulfate, bisulfate, sulfite, bisulfite, nitrate, phosphate, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate, chloride, bromide, iodide, acetate, trifluoroacetate, propionate, caprylate, isobutyrate, oxalate, malonate, succinate, suberate, sebacate, fumarate, maleate, mandelate, benzoate, chlorobenzoate, methylbenzoate, dinitrobenzoate, phthalate, benzenesulfonate, toluenesulfonate, phenylacetate, citrate, lactate, maleate, tartrate, methanesulfonate, and the like. Also contemplated are salts of amino acids such as arginate and the like and gluconate, galacturonate, n-methyl glucamine (see, for example, Berge, S. M., et al, "Pharmaceutical Salts," Journal of Pharmaceutical Science. 66, pp. 1-19 (1977)).

The acid addition salts of said basic compounds are prepared by contacting the free base form with a sufficient amount of the desired acid to produce the salt in the conventional manner. Preferably a peptide of Formula I can be converted to an acidic salt by treating with an aqueous solution of the desired base, such that the resulting pH is less than 4. The solution can be passed through a C18 cartridge to absorb the peptide, washed with copious amounts of water, the peptide eluted with a polar organic solvent such as, for example, methanol, acetonitrile, and the like, and isolated by concentrating under reduced pressure followed by lyophilization. The free base form may be regenerated by contacting the salt form with a base and isolating the free base in the conventional manner or as above. The free base forms differ from their respective salt forms somewhat in certain physical properties such as solubility in polar solvents, but otherwise the salts are equivalent to their respective free base for purposes of the present invention.

Pharmaceutically acceptable base addition salts are formed with metals or amines, such as alkali and alkaline earth metals or organic amines. Examples of metals used as cations are sodium, potassium, magnesium, calcium, and the like. Examples of suitable amines are N,N'-dibenzylethylenediamine, chloroprocaine, choline, diethanolamine, dicyclohexylamine, ethylenediamine, N-methylglucamine, and procaine (see, for example, Berge, S. M., et al., "Pharmaceutical Salts," Journal of Pharmaceutical Science, 66, pp. 1-19 (1977)).

The base addition salts of said acidic compounds are prepared by contacting the free acid form with a sufficient amount of the desired base to produce the salt in the conventional manner. Preferably, a peptide of Formula I can be converted to a base salt by treating with an aqueous solution of the desired base, such that the resulting pH is greater than 9. The solution can be passed through a C18 cartridge to absorb the peptide, washed with copious amounts of water, the peptide eluted with a polar organic solvent such as, for example, methanol, acetonitrile and the like, and isolated by concentrating under reduced pressure followed by lyophilization. The free acid form may be regenerated by contacting the salt form with an acid and isolating the free acid in the conventional manner or as above. The free acid forms differ from their respective salt forms somewhat in certain physical properties such as solubility in polar solvents, but otherwise the salts are equivalent to their respective free acid for purposes of the present invention.

Certain of the compounds of the present invention can exist in unsolvated forms as well as solvated forms, including hydrated forms. In general, the solvated forms, including hydrated are equivalent to unsolvated forms and are intended to be encompassed within the scope of the present invention.

Certain of the compounds of the present invention possess one or more chiral centers and each center may exist in the R(D) or S(L) configuration. The present invention includes all enantiomeric and epimeric forms as well as the appropriate mixtures thereof.

A preferred compound of Formula I is one wherein

AA.sup.1 is ##STR26## wherein X is hydrogen, alkyl, or cycloalkyl, n is zero or an integer of 1, 2, 3, 4, 5, or 6, R.sup.3 is --S--R.sup.4 wherein R.sup.4 is as defined hereinafter, ##STR27## wherein R.sup.4 is as defined hereinafter, --C(CH.sub.3).sub.2 --S--R.sup.4, wherein R.sup.4 is hydrogen, alkyl or R.sup.4 is absent when AA.sup.1 is covalently linked to AA.sup.1a through a disulfide bridge, ##STR28## wherein X and X.sup.1 are each the same or different and each is as defined above for X or X.sup.1 is Asp-Lys-Glu and R.sup.3 and n are as defined above or ##STR29## wherein X, n, and R.sup.3 are as defined above; R is absent or is one to four amino acids selected from the group consisting of: Ala, Asn, Asp, Gln, Glu, Ile, Leu, Lys, 3Hyp, 4Hyp, Nle, Orn, Ser, Thr, and Val; AA.sup.1a is selected from the group consisting of: Cys, Hcy, and Pen which may or may not be covalently linked to AA.sup.1 through a disulfide bridge; AA.sup.2 is absent or is selected from the group consisting of: His, Phe, Tyr, Trp, Trp(For), Tyr(OMe), Tyr(OEt), Pyr, 2-Nal, and 1-Nal; AA.sup.3 is absent or is selected from the group consisting of: Ala, Asn, Gln, Gly, Ile, Leu, Met, Val, Nva, Nle, Chx, and Abu; AA.sup.4 is absent or is selected from the group consisting of: Asn, Asp, Gln, Glu, and Cit; AA.sup.5 is selected from the group consisting of: Trp, Tyr, Trp(For), 2-Nal, and 1-Nal

A more preferred compound of Formula I is one wherein

AA.sup.1 is selected from the group consisting of: Asp-Lys-Glu-Cys, Cys, Hcy, Pen, Maa, Mba, and Mpa; R is R.sup.a --R.sup.b --R.sup.c --R.sup.b wherein R.sup.a is absent or is selected from the group consisting of: 3Hyp, 4Hyp, Ser, and Thr, R.sup.b is absent or is selected from the group consisting of: Asn, Asp, Gln, and Glu, R.sup.c is absent or is selected from the group consisting of: Ile, Leu, Lys, Nle, Orn, and Val.

Particularly valuable are: ##STR30##

The compounds of Formula I are valuable antagonists of endothelin. The test employed indicate that compounds of Formula I possess endothelin antagonist activity. Thus, the compounds of Formula I were tested for their ability to inhibit [.sup.125 I]-ET-1([.sup.125 I]-Endothelin-1) binding in a receptor assay. The binding of the compounds of Formula I is determined by incubation (37.degree. C., 2 hours) of a compound of Formula I with [.sup.125 I]-ET-1 and the tissue (rat heart ventricle (10 .mu.g)) in 50 mM Tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) (pH 7.4), 5 mM ethylenediamine tetraacetic acid (EDTA), 2 mM ethylene glycol bis(.beta.-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA), 100 .mu.M phenylmethylsulfonyl fluoride (PMSF), and 100 .mu.M bacitracin containing protease inhibitors (total volume of 0.5 mL). IC.sub.50 values are calculated by weighing nonlinear regression curve-fitting to the mass-action (Langmuir) equation. The functional activity of compounds of Formula I is determined in Rat-1 cells by measuring intracellular levels of second messengers. Thus, cells are prelabeled with [.sup.3 H]-inositol and endothelin-stimulated accumulation of total [.sup.3 H]-inositol phosphates in the presence of Li.sup.+ is monitored using anion exchange chromatography as described by Muldoon, L. L., et al, Journal of Biological Chemistry, Volume 264, pages 8529-8536 (1989) and Dudley, D. T., et al, Molecular Pharmacology, Volume 38, pages 370-377 (1990). Antagonist activity is assessed as the ability of added compounds to reduce endothelin stimulated inositol phosphate accumulation. The data in the table show the endothelin antagonist activity of representative compounds of Formula I.

TABLE 1__________________________________________________________________________Biological Activity of Compounds of Formula I Binding Assay in IP (Inositol Phosphate) Rat Heart Ventricle Accumulation,Example IC.sub.50 (.mu.M) or % (Rat Skin Fibroblasts)Number Compound Inhibition IC.sub.50 (.mu.M)__________________________________________________________________________ ##STR31## 6.5 2.22 ##STR32## 2.07/1.17* 5.03 ##STR33## 2.5 8.04 ##STR34## 0.66 9.25 ##STR35## 1.8 306 ##STR36## 100% inhibition @ 10.sup.-57 ##STR37## 3.58 ##STR38## 1.829 ##STR39## 2.8310 ##STR40## 3.211 ##STR41## 0.9412 ##STR42## >10 .mu.M13 ##STR43## 4.8 16.114 ##STR44## 2.6 0.75__________________________________________________________________________ *Two independent test results

General Method for Preparing Compounds of Formula I

The compounds of Formula I may be prepared by solid phase peptide synthesis on a peptide synthesizer, for example, an Applied Biosystems 430A peptide synthesizer using activated esters or anhydrides of N-alpha-Boc protected amino acids, on PAM or MBHA resins. Additionally, the compounds of Formula I may also be prepared by conventional solution peptide synthesis. Amino acid side chains are protected as follows: Bzl(Asp, Glu, Ser), 2-Cl-Z(Lys), 2-Br-Z(Tyr), Bom(His), For(Trp), and MeBzl(Cys). Each peptide resin (1.0 g) is cleaved with 9 mL of HF and 1 mL of anisole or p-cresol as a scavenger (60 minutes, 0.degree. C.). The peptide resin is washed with cyclohexane, extracted with 30% aqueous HOAc, followed by glacial HOAc, concentrated under reduced pressure, and lyophilized. (A peptide containing For(Trp) is dissolved in 0.degree. C., the pH is adjusted to 12.5 with 1N KOH (2 minutes), neutralized with glacial HOAc, desalted on C.sub.18 and lyophilized.) The peptide is dissolved in water (.about.0.2 mg/mL) at pH 8-9 (adjusted and maintained with dilute ammonium hydroxide) and dithiothreitol (.about.100 mg) is added. After 1 hour, the peptide solution is titrated with dilute aqueous potassium ferricyanide (.about.0.01 N) until a yellow solution is obtained and maintained for 2 hours. The pH is adjusted to <4.0 with acetic acid and a weak anion exchange resin is added (50 equivalents). After 1 hour, the resin is filtered and the solution passed through a C.sub.18 cartridge, washed with water and the peptide eluted with acetonitrile with 10-30% H.sub.2 O and 0.1% TFA. The solution is concentrated under reduced pressure and lyophilized. The crude cyclic peptide is purified by preparative reversed phase high performance liquid chromatography (RP-HPLC) on a C.sub.18 column (2.2.times.25.0 cm, 15.0 mL/min) with a linear gradient of 0.1% TFA in water to 0.1% TFA in acetonitrile and lyophilized. The homogeneity and composition of the resulting peptide is verified by RP-HPLC, capillary electrophoresis, thin layer chromatography (TLC), proton nuclear magnetic resonance spectrometry (NMR), and fast atom bombardment mass spectrometry (FAB-MS).

The compounds of the present invention can be prepared and administered in a wide variety of oral and parenteral dosage forms. Thus, the compounds of the present invention can be administered by injection, that is, intravenously, intramuscularly, intracutaneously, subcutaneously, intraduodenally, or intraperitoneally. Also, the compounds of the present invention can be administered by inhalation, for example, intranasally. Additionally, the compounds of the present invention can be administered transdermally. It will be obvious to those skilled in the art that the following dosage forms may comprise as the active component, either a compound of Formula I or a corresponding pharmaceutically acceptable salt of a compound of Formula I.

For preparing pharmaceutical compositions from the compounds of the present invention, pharmaceutically acceptable carriers can be either solid or liquid. Solid form preparations include powders, tablets, pills, capsules, cachets, suppositories, and dispersible granules. A solid carrier can be one or more substances which may also act as diluents, flavoring agents, binders, preservatives, tablet disintegrating agents, or an encapsulating material.

In powders, the carrier is a finely divided solid which is in a mixture with the finely divided active component.

In tablets, the active component is mixed with the carrier having the necessary binding properties in suitable proportions and compacted in the shape and size desired.

The powders and tablets preferably contain from five or ten to about seventy percent of the active compound. Suitable carriers are magnesium carbonate, magnesium stearate, talc, sugar, lactose, pectin, dextrin, starch, gelatin, tragacanth, methylcellulose, sodium carboxymethylcellulose, a low melting wax, cocoa butter, and the like. The term "preparation" is intended to include the formulation of the active compound with encapsulating material as a carrier providing a capsule in which the active component with or without other carriers, is surrounded by a carrier, which is thus in association with it. Similarly, cachets and lozenges are included. Tablets, powders, capsules, pills, cachets, and lozenges can be used as solid dosage forms suitable for oral administration.

For preparing suppositories, a low melting wax, such as a mixture of fatty acid glycerides or cocoa butter, is first melted and the active component is dispersed homogeneously therein, as by stirring. The molten homogenous mixture is then poured into convenient sized molds, allowed to cool, and thereby to solidify.

Liquid form preparations include solutions, suspensions, and emulsions, for example, water or water propylene glycol solutions. For parenteral injection liquid preparations can be formulated in solution in aqueous polyethylene glycol solution.

Aqueous solutions suitable for oral use can be prepared by dissolving the active component in water and adding suitable colorants, flavors, stabilizing and thickening agents as desired.

Aqueous suspensions suitable for oral use can be made by dispersing the finely divided active component in water with viscous material, such as natural or synthetic gums, resins, methylcellulose, sodium carboxymethylcellulose, and other well-known suspending agents.

Also included are solid form preparations which are intended to be converted, shortly before use, to liquid form preparations for oral administration. Such liquid forms include solutions, suspensions, and emulsions. These preparations may contain, in addition to the active component, colorants, flavors, stabilizers, buffers, artificial and natural sweeteners, dispersants, thickeners, solubilizing agents, and the like.

The pharmaceutical preparation is preferably in unit dosage form. In such form the preparation is subdivided into unit doses containing appropriate quantities of the active component. The unit dosage form can be a packaged preparation, the package containing discrete quantities of preparation, such as packeted tablets, capsules, and powders in vials or ampoules. Also, the unit dosage form can be a capsules, tablet, cachet, or lozenge itself, or it can be the appropriate number of any of these in packaged form.

The quantity of active component in a unit dose preparation may be varied or adjusted from 0.1 mg to 100 mg preferably 0.5 mg to 100 mg according to the particular application and the potency of the active component. The composition can, if desired, also contain other compatible therapeutic agents.

In therapeutic use as antagonist of endothelin, the compounds utilized in the pharmaceutical methods of this invention are administered at the initial dosage of about 0.01 mg to about 20 mg per kilogram daily. A daily dose range of about 0.01 mg to about 10 mg per kilogram is preferred. The dosages, however, may be varied depending upon the requirements of the patient, the severity of the condition being treated, and the compound being employed. Determination of the proper dosage for a particular situation is within the skill of the art. Generally, treatment is initiated with smaller dosages which are less than the optimum dose of the compound. Thereafter, the dosage is increased by small increments until the optimum effect under the circumstances is reached. For convenience, the total daily dosage may be divided and administered in portions during the day, if desired.

The following nonlimiting examples illustrate the inventors' preferred methods for preparing the compounds of the invention.

EXAMPLE 1 ##STR45##

The title compound is prepared by standard solid phase synthetic peptide methodology utilizing a Boc/benzyl strategy (Stewart, J. M. and Young, J. D., Solid Phase Peptide Synthesis, Pierce Chemical Co., Rockford, Ill., 1984). All protected amino acids and reagents are obtained from commercial sources and are not further purified. The protected peptide resin is prepared on an Applied Biosystems 430A Peptide Synthesizer, utilizing protocols supplied for a dicyclohexylcarbodiimide mediated coupling scheme (Standard 1.0, Version 1.40). Starting with 0.69 meq of N-.alpha.-Boc-Trp(For)-PAM resin (0.72 meq/g, 0.50 meq of Boc-Trp(For) total) the protected peptide is prepared by the stepwise coupling of the following amino acids (in order of addition): Boc-Cys(4MeBzl); Boc-Val; Boc-Tyr(2-Br-Z); Boc-Phe; Boc-Cys(4Me-Bzl); Boc-His(Dnp); Boc-Leu H.sub.2 O; Boc-Asp(OBzl); Boc-Ile 0.5 H.sub.2 O; and Boc-Ile 0.5 H.sub.2 O. A typical cycle for the coupling of an individual amino acid residue is illustrated below (reproduced from the ABI manual):

All the single couple RV cycles conform to the following pattern:

1) 33% TFA in DCM for 80 seconds 2) 50% TFA in DCM for 18.5 minutes 3) Three DCM washes 4) 10% DIEA in DMF for 1 minute 5) 10% DIEA in DMF for 1 minute 6) Five DMF washes 7) Coupling period 8) five DCM washes

After coupling the last amino acid the protected resin is treated with 10% thiophenol in DMF (1.times.60 min), DMF (2.times.1 min), DCM (2.times.1 min), 50% TFA in DCM (2.times.10 min), DCM (3.times.1 min), 10% DIEA in DMF (2.times.2 min), 10% piperidine in DMF (1.times.120 min, 0.degree. C.), DCM (3.times.1 min), and dried under reduced pressure (1.32 g).

The peptide is liberated from the solid support, and the carboxylate of aspartic acid deprotected by treatment with anhydrous hydrogen fluoride (9.0 mL), anisole (0.5 mL), and dimethyl sulfide (0.5 mL) (60 minutes, 0.degree. C.). After removing the hydrogen fluoride under a stream of nitrogen, the resin is washed with cyclohexane (3.times.20 mL) and extracted with glacial HOAc (3.times.30 mL) and TFA (1.times.20 mL). The aqueous extractions are combined, concentrated under reduced pressure, diluted with H.sub.2 O, and lyophilized (350 mg). The peptide is dissolved in water (3.5 L) and the pH is adjusted to 11 to clarify the solution (1N KOH), readjusted to 7.5 with 10% aqueous HOAc and dithiothreitol (200 mg) is added. After 1 hour, the peptide solution is titrated with dilute aqueous potassium ferricyanide (.about.0.01 N) until a yellow solution is obtained and maintained for 1 hour. The pH is adjusted to <4.0 with acetic acid and a weak anion exchange resin is added (50 equivalents). After 1 hour, the resin is filtered and the solution passed through a C18 cartridge, washed with water, and the peptide eluted with 30% aqueous acetonitrile with 0.1% TFA (100 mL). The solution is concentrated under reduced pressure and lyophilized. The crude peptide is dissolved in 4.0 mL of 50% TFA/H.sub.2 O, filtered through a 0.4 .mu.M syringe filter, and chromatographed on a Vydac 218TP 1022 column (2.2.times.25.0 cm, 15.0 mL/min, A: 0.1% TFA/H.sub.2 O, B: 0.1% TFA/CH.sub.3 CN, Gradient; 0% B for 10 minutes, 10% to 50% B over 120 minutes). Individual fractions are collected and combined based upon analysis by analytical HPLC. The combined fractions are concentrated under reduced pressure (10 mL), diluted with H.sub.2 O (50 mL), and lyophilized (62.0 mg). The homogeneity and structure of the resulting peptide is confirmed by analytical HPLC, capillary zone electrophoresis, Proton Nuclear Magnetic Resonance Spectroscopy (H.sup.1 -NMR) and Fast Atom Bombardment Mass Spectroscopy (FAB-MAS), MH.sup.+ 1410.3.

In a process analogous to Example 1 using the appropriate amino acids, the corresponding compounds of Formula I are prepared as follows:

EXAMPLE 2 ##STR46##

EXAMPLE 3 ##STR47##

EXAMPLE 4 ##STR48##

EXAMPLE 5 ##STR49##

EXAMPLE 6 ##STR50##

EXAMPLE 7 ##STR51##

EXAMPLE 8 ##STR52##

EXAMPLE 9 ##STR53##

EXAMPLE 10 ##STR54##

EXAMPLE 11 ##STR55##

EXAMPLE 12 ##STR56##

EXAMPLE 13 ##STR57##

EXAMPLE 14 ##STR58##

__________________________________________________________________________SEQUENCE LISTING(1) GENERAL INFORMATION:(iii) NUMBER OF SEQUENCES: 18(2) INFORMATION FOR SEQ ID NO:1:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 11 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:XaaXaaXaaXaaXaa XaaXaaXaaXaaXaaXaa1510(2) INFORMATION FOR SEQ ID NO:2:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond (B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:CysSerXaaValTyrPheCysHisLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:3:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 11 amino acids(B) TYPE: amino acid (D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..5(xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:CysValTyrPheCysHisLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:4:(i ) SEQUENCE CHARACTERISTICS:(A) LENGTH: 12 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..6(xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:CysSerXaaValPheCysHisLeuAspIleIleTrp1 510(2) INFORMATION FOR SEQ ID NO:5:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 12 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..6(xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:Cys SerValTyrPheCysHisLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:6:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE: (A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:CysSerSerValTyrPheCysHisLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:7:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids (B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:CysSerSerSerValTyrCysHisLeuAspIleIleTrp15 10(2) INFORMATION FOR SEQ ID NO:8:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:CysSerSerSerTyrPheCysHis LeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:9:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond( B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:CysSerXaaValTyrPheCysPheLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:10:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D ) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:CysSerXaaValTyrPheCysHisLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:11: (i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:CysSerXaaValTyrPheCysHisLeuAspIleIleTrp 1510(2) INFORMATION FOR SEQ ID NO:12:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:12: CysSerXaaValTyrPheCysHisLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:13:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 13 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide (ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..7(xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:CysThrXaaValTyrPheCysHisLeuAspIleIleTrp1510(2) INFORMATION FOR SEQ ID NO:14:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 14 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..8(xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:CysSerSerSerValTyrPheCysHisLeuAspIleIleTrp1 510(2) INFORMATION FOR SEQ ID NO:15:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 15 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..9(xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:CysSerAspLys GluValTyrPheCysHisLeuAspIleIleTrp151015(2) INFORMATION FOR SEQ ID NO:16:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 15 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..9(xi) SEQUENCE DESCRIPTION: SEQ ID NO:16:CysSerAspLeuGluValTyrPheCysHisLeuAspIleIleTrp151015(2) INFORMATION FOR SEQ ID NO:17:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 15 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 1..9(xi) SEQUENCE DESCRIPTION: SEQ ID NO:17:CysSerAsnLysGlnValTyrPheCysHisLeuAs pIleIleTrp151015(2) INFORMATION FOR SEQ ID NO:18:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 14 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(ix) FEATURE:(A) NAME/KEY: Disulfide-bond(B) LOCATION: 4..8(xi) SEQUENCE DESCRIPTION: SEQ ID NO:18:AspLysGluCysValTyrPheCysHisLeuAspIleIleTrp1510

Claims

1. A compound of formula I

AA.sup.1 --R--R.sup.1 --R.sup.2 --AA.sup.1a --AA.sup.2 --AA.sup.3 --AA.sup.4 --AA.sup.3 --AA.sup.3 --AA.sup.5 I

wherein

AA.sub.1 is ##STR59## wherein X is

hydrogen,

alkyl,

alkenyl,

alkynyl,

cycloalkyl,

heterocycloalkyl,

aryl, or

heteroaryl,

n is zero or an integer of 1 or 2,

R.sup.3 is --S--R.sup.4, wherein R.sup.4 is as defined hereinafter, ##STR60## wherein R.sup.4 is as defined hereinafter, --C(CH.sub.3).sub.2 --S--R.sup.4, wherein R.sup.4 is covalently linked to AA.sup.1a through a disulfide bridge, ##STR61## wherein X is as defined above, X.sup.1 is hydrogen or Asp-Lys-Glu and n and R.sup.3 are as defined above, ##STR62## wherein X, n, and R.sup.3 are as defined above, or ##STR63## wherein X, n, and R.sup.3 are as defined above; R is absent or is one to four amino acids selected from the group consisting of:

Asn,

Asp,

Glu,

Gln,

Lys,

Ser,

Thr,

Val, and

Orn,

R.sup.1 is absent or is ##STR64## wherein m is an integer of 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; R.sup.2 is absent or is one to three amino acids selected from the group consisting of:

Ala,

Ile,

Leu,

Phe,

Trp,

Tyr,

Val,

HomoPhe,

1-Nal,

2-Nal,

Pgl,

Pyr,

Tic

Tyr(OMe),

Tyr(OEt), and

Trp(FOR) and

provided that at least one amino acid is selected from the group consisting of:

Phe,

Trp,

Tyr,

HomoPhe,

1-Nal,

2-Nal,

Pgl,

Pyr,

Tic,

Tyr(OMe),

Tyr(OEt), and

Trp(For);

AA.sup.1a is ##STR65## wherein R.sup.3 and n are as defined above; AA.sup.2 is ##STR66## wherein R.sup.5 is phenyl, 4-hydroxy phenyl, 4-alkoxy phenyl, or naphthyl or, 3-indolyl unsubstituted or substituted by formyl on the indole nitrogen, and

n is as defined above;

AA.sup.3 is ##STR67## wherein R.sup.6 is

hydrogen,

alkyl,

alkenyl,

alkynyl,

cycloalkyl,

heterocycloalkyl,

--OR.sup.7, wherein R.sup.7 is defined hereinafter,

--CO.sub.2 --R.sup.7 wherein R.sup.7 is defined hereinafter, ##STR68## wherein R.sup.7 and R.sup.7' are each the same or different and each is hydrogen,

alkyl,

cycloalkyl, or

heterocycloalkyl, and

n is as defined above;

AA.sup.4 is ##STR69## wherein R.sup.6 and n are as defined above; AA.sup.5 is ##STR70## wherein R.sup.8 is

alkyl,

alkenyl,

alkynyl,

cycloalkyl, or

heterocycloalkyl,

R.sup.9 is --CO.sub.2 H, or --CH.sub.2 OH and

R.sup.5 and n are as defined above,

stereochemistry at CH or C in AA.sup.5 is L; and with the exclusion of the compound of the formula ##STR71## or a pharmaceutically acceptable salt thereof.

2. A compound according to claim 1, in which

AA.sup.1 is ##STR72## wherein X is

hydrogen,

alkyl, or

cycloalkyl,

n is zero or an integer of 1, or 2

R.sup.3 is --S--R.sup.4 wherein R.sup.4 is as defined hereinafter, ##STR73## wherein R.sup.4 is as defined hereinafter, --C(CH.sub.3).sub.2 --S--R.sup.4, wherein R.sup.4 is covalently linked to AA.sup.1a through a disulfide bridge, ##STR74## wherein X is as defined above X.sup.1 is hydrogen or Asp-Lys-Glu and R.sup.3 and n are as defined above or ##STR75## wherein X, n and R.sup.3 are as defined above; R is absent or is one to four amino acids selected from the group consisting of:

Asn,

Asp,

Gln,

Glu,

Lys,

Orn,

Ser,

Thr, and

Val;

AA.sup.1a is selected from the group consisting of:

Cys,

Hcy, and

Pen

which is covalently linked to AA.sup.1 through a disulfide bridge;

AA.sup.2 is selected from the group consisting of:

Phe,

Tyr,

Trp,

Trp(For),

Tyr(OMe),

Tyr(OEt),

2-Nal, and

1-Nal;

AA.sup.3 is selected from the group consisting of:

Ala,

Asn,

Gln,

Gly,

Ile,

Leu,

Met,

Val,

Nva,

Nle,

CHx, and

Abu;

AA.sup.4 is selected from the group consisting of:

Asn,

Asp,

Gln,

Glu, and

Cit;

AA.sup.5 is selected from the group consisting of:

Trp,

Tyr,

Trp(For),

2-Nal, and

1-Nal.

3. A compound according to claim 2, in which

AA.sup.1 is selected from the group consisting of:

Asp-Lys-Glu-Cys,

Cys,

Hcy,

Pen,

Maa,

Mba, and

Mpa;

R is R.sup.a --R.sup.b --R.sup.c --R.sup.b wherein

R.sup.a is absent or is selected from the group consisting of:

Ser, and

Thr,

R.sup.b is absent or is selected from the group consisting of:

Asn,

Asp,

Gln, and

Glu,

R.sup.c is absent or is selected from the group consisting of:

Lys,

Orn, and

Val.

4. A compound according to claim 3 selected from the group consisting of: ##STR76##

5. A method of inhibiting elevated levels of endothelin comprising administering to a host suffering therefrom a therapeutically effective amount of a compound according to claim 1 in unit dosage form.

6. A pharmaceutical composition adapted for administration as an antagonist of endothelin; as an antihypertensive agent; as an agent for treating metabolic and endocrine disorders; as an agent for treating congestive heart failure and myocardial infarction; as an agent for treating endotoxic shock; a an agent for treating subarachnoid hemorrhage; as an agent for treating arrhythmias; as an agent for treating asthma; as an agent for treating acute renal failure; as an agent for treating preeclampsia; as an agent for treating diabetes; and as an agent for treating neurologic disorders comprising a therapeutically effective amount of a compound according to claim 1 in admixture with a pharmaceutically acceptable excipient, diluent, or carrier.

7. A method of treating hypertension comprising administering to a host suffering therefrom a therapeutically effective amount of a compound according to claim 1 in unit dosage form.