GB2114561A - Anti-hypertensive agents - Google Patents

Abstract

Inhibitors of angiotensin converting enzyme which are physiologically acceptable salts of compounds which have the formula: <IMAGE> wherein R is hydrogen, formyl, acetyl, propanoyl, butanoyl, phenylacetyl, phenylpropanoyl, benzoyl, cyclopentanecarbonyl, tert-butyloxycarbonyl, cyclopentanecarbonyl-L-lysyl, pyro-L- glutamyl-L-lysyl, L-lysyl, L-arginyl or pyro-L-glutamyl; A is phenylalanyl, glycyl, alanyl, tryptophyl, tyrosyl, isoleucyl, leucyl, histidyl, valyl, the amino group thereof being in amide linkage with R; R1 is hydrogen or methyl; R2 is L-proline, L-3,4- dehydroproline, D,L-3,4- dehydroproline, L-3-hydroxyproline, L-4-hydroxyproline, L- thiazolidine-4 -carboxylic acid, or L-5-oxo-proline, the imino group thereof being in imide linkage with the adjacent <IMAGE> ; and, n is 0 or 1, such that when n is 0, R1 is methyl are disclosed as useful anti-hypertensive agents.

Description

SPECIFICATION Anti-hypertensive agents This is a continuation-in-part of Serial Nos. 64,897; 64,898; 64,899; 64,900; 64,901; 64,902 and 64,903, all filed August 1979, Serial No. 116,950, filed January 1980 which is a continuation of Serial No. 941,289,filed September 11,1978, and now abandoned, Serial No. 116,951, file January30, 1980, which is a continuation of Serial No. 958,180, filed November 6, 1978, and now abandoned, and Serial No. 121,188,filed March 3,1980, all of which are incorporated herein by reference as though setforth in full.

Antiotensin converting enzyme (peptidyldipeptide hydrolase, hereinafter referred too as ACE) occupies a central role in the physiology of hypertension. The enzyme is capable of converting the decapeptide angiotensin I, having the sequence AspArgValTyrlleHisProPheHisLeu to an octapeptide, angiotensin 11, by removal of the carboxyterminal HisLeu.The symbols for various chemical entities have the meaning given in the following table unless otherwise indicated: Ala = L-alanine Arg = L-arginine Asp = L-aspartic acid Boc = t-butyloxycarbonyl Glu = glutamic acid < Glu = pyro-L-glutamic acid (L- 5 - oxo - proline) Gly = glycine Hip = Hippuric acid (benzoyl-glycine) His = L-histidine lle= L-isoleucine Leu = L-leucine Phe = L-phenylalanine Pro = L-proline A Pro= L-3,4-dehydroproline Ser = L-serine Trp = L-tryptophan Tyr = L-tyrosine Val = L-valine ACE = Angiotensin converting enzyme Hepes = N - 2 - hydroxyethylpiperazine - N' - 2 ethanesulfonic acid Angiotensin I is formed by the action of the enzyme renin, an endopeptidase found in kidney, other tissues and plasma, acting on a serum -2 globulin.

Blood pressure in affected by certain peptides found in the blood. One of these, angiotensin II, is a powerful pressor (blood pressure elevating) agent.

Another, bradykinin, a nonapeptide with the sequence ArgProProGlyPheSerProPheArg is a powerful depressor (blood pressure lowering) agent. In addition to a direct pressor effect, angiotensin 11 stimulates release of aldosterone which tends to elevate blood pressure by causing retention of extracellular salt and fluids. Angiotensin II is found in measurable amount in the blood of normal humans.

However, it is found at elevated concentrations in the blood of patients with renal hypertension.

The level of ACE activity is ordinarily in excess, in both normal and hypertensive humans, of the amount needed to maintain observed levels of angiotensin II. However, it has been found that significant blood pressure lowering is achieved in hypertensive patients by treatment with ACE inhibitors. [Gavras, l., et al., New Engl. J. Med. 291,817 (1974)].

ACE is a peptidyldipeptide hydrolase. It catalyzes the hydrolysis of the penultimate peptide bond at the C-terminal end of a variety of acylated tripeptides and larger polypeptides having an unblocked o(-carboxyl group. The action of ACE results in hydrolytic cleavage of the penultimate peptide bond from the carboxyl-terminal end yielding as reaction products a dipeptide and a remnant.

The reactivity of the enzyme varies markedly depending on the substrate. At least one type of peptide bond, having the nitrogen supplied by proline, is not hydrolyzed at all. The apparent Michaelis constant (Km) varies from substrate to substrate over several orders of magnitude. For general discussion of the kinetic parameters of enzyme catalyzed reactions, see Lehninger, A., Biochemstry, 2nd Ed., Worth Publishers, Inc., New York, 1975, pp.189-195. Many peptides which are called inhibitors of the enzymatic conversion of angiotensin I to angiotensin II are in fact substrates having a lower Km than angiotensin I. Such peptides are more properly termed competitive substrates.

Examples of competive substrates include bradykinin, and the peptide BPP5a (also called SQ20475) from snake venom, whose sequence is < GiuLysTrpAlaPro.

numerouus synthetic peptide derivatives have been shown to be ACE inhibitors by Ondetti, et al. in U.S. patent 3,832,337 issued August 1974.

The role of ACE in the pathogenesis of hypertension has prompted a search for inhibitors of the enzyme that could act as antihypertensive drugs.

See for example U.S. Patents 3,891,616; 3,947,575; 4,052,511 and 4,053,651. A highly effective inhibitor, with high biological activity when orally administered, is D - 3 - mercapto - 2 - methylpropanoyl - L proline, designated SQ14225, disclosed in U.S. Patent 4,046,889 to Ondetti et al., issued September 6, 1977, and in scientific articles by Cushman, D.W. et al., Biochemistry 16,5484(1977), and by Ondetti, M.

et al., Science 196, 441 (1977). The inhibitor SQ14225 reportedly has an so value of 2.3 x 1 0-8M. The 150 value reported by Cushman, et al., supra is the concentration of inhibitor required to produce 50% inhibition of the enzyme under a standard assay system containing substrate at a level substantially above Km. It will be understood that 150 values are directly comparable when all potential factors affect ing the reaction are kept constant. These factors include the source of enzyme, its purity, the substrate used and its concentration, and the composition of the assay buffer.All i50 data reported herein have been performed with the same assay system This print takes account of replacement documents later filed to enable the application to comply with the formal requirements of the Patents Rules 1982.

and same enzyme (human urinary ACE) and with an approximately 1/2 Km level of substrate and are therefore internally consistent. Discrepancies with data obtained by other workers may be observed.

Indeed such discrepancies do exist in the literature, for unknown reasons. See, for example, the 150 values for BPPga reported by Cushman, D.W. et al., Experientia29, 1032(1973) and byDorer, FE., etal., Biochim. Biophys. Acta 429,220 (1976).

The mode of action of SQ14225 has been based upon a model of the active site of ACE developed by analogy with the better known related enzyme, carboxypeptidase A. The active site was postulated to have a cationic site for binding the carboxyl end group of the substrate and a pocket or cleft capable of binding the side chain of the C-terminal amino acid and providing especially tight binding for the heterocyclic ring of a terminal proline residue. A similar pocket for the penultimate amino acid residue was postulated, and the published data suggested a rather stringent steric requirements, since the D-form of the inhibitor was substantially more potent than its stereoisomer or the 3-methyl and unsubstituted analogs.The sulfhydryl group on the Inhibitor, postulated to be bound at the active site near the catalytic centre, was believed to play a central role in activation of the enzyme by combining with the zinc moiety known to be essential for catalytic activity. Substituents on the sulfhydryl, such as a methyl group, and an S-acetyl derivative, substantially reduced potency of the inhibitor. See Cushman, D.W., et al., Biochemistry, supra.

In vitro study of the mechanism by which SQ14225 and its analogs act to inhibit ACE has been somewhat hampered by the instability of these molecules under ambient conditions. For example, it has been observed that a fresh aqueous solution of concentration, e.g, 1 mg per ml of SQ14225 at a pH of about 8 becomes substantially less active upon standing for as little as 30 minutes, and that activity continues to decrease as the solution stands for longer periods. It is believed that this loss in activity is mainly the result of dimerization of SQ14225 occurring at the sulfhydryl end groups, whereby a disulfide is formed which is largely inactive as an inhibitor.Since the free sulfhydryl group is highly reactive and may be readily oxidized to polar acidic moieties such as sulfone and sulfoxide groups, it may also be that the observed in vitro loss of activity of aqueous solutions of SQ14225 on standing is in some part a consequence of one or more such oxidation reactions, with formation of a sulfone or sulfoxide which does not function effectively as an inhibitor for ACE.

Such reports of SQ14225 clinical testing as are currently available, some of which refer to the compound under the name "Captopril", suggest that the product is sufficiently stable in the normal gastric and intestinal environments of most patients to be an effective inhibitorfor ACE when administered orally. It is not yet clear, however, whether there may be a group of patients for which SQ14225 is substantially ineffective. Because of the high reactivity of the free sulfhydryl group, SQ14225 could readily form mixed disulfides with serum, cellular proteins, peptides or other free sulfhydryl group-containing substances in the gastric or intestinal environments, in addition to the possibility for dirnerformation or oxidative degradation reactions.

A mixed disulfide with protein may be antigenic acid, indeed, occasional allergic reactions have been clinically observed. See Gavras, et al., New En gland J. Med. 298,991(1978). Disulfides and oxidative degradation products of SQ14225, if formed, may at best be expected to be largely ineffective as inhibitors. It may be postulated accordingly that dose reponse to SQ14225 may varvwith conditions of administration and among individual patients.

Moreover, in at least some patients, unwanted side effects may occur and maintenance of an effective concentration of the inhibitor in the body may be difficult to control.

Thioester compounds generally are thought to be highly reactive in that the thioester linkage is readily hydrolyzable to a sulfhydryl moiety and a carboxylic moiety. Thioesters are accordingly often used as active ester intermediates for acylation under mild conditions. Such groups as e.g. acetylthio have been used as blocking groups in the above cited Ondetti, et al. patents. Thioester intermediates are also postulated to occur in the biosynthesis of cyclic peptides such as tyrocidin or gramicidin S. See Lipmann, F. in Accounts Chem. Res. 6,361(1973).

Thioestercompounds having potent ACE inhibitory activity and oral effectiveness as anti-hypertensive agents have ben disclosed in copending applicationsSerial No. 116,950,filed January 30,1980, Serial No. 116,951, filed January 1980, and Serial Nos. 064,897 through 064,903, all filed on August 14, 1979. All copending applications are incorporated herein by reference.

Compounds related to SQ14225 have been disclosed by Ondetti, et al., U.S. Patents 4,046,889; 4,052,511; 4,053,651; 4,113,715 and 4,154,840. Of interest are disclosed analogs of SQ14225 having the five-membered heterocyclic ring of proline replaced by a four- or a six-membered ring. The inhibitory potencies of such analogs relative to SQ14225 are not disclosed. Substitution of D-proline for L-proline is reported to drastically reduce inhibitory potency of 3-mercaptopropanoyl amino acids (Cushman, D. W.

et al., supra). The substitution of L - 3,4 - dehydroproline for proline has been studied in several systems.

Substitution of L - 3,4 - A Pro in the 7 position of bradykinin yields a bradykinin derivative which has significantly reduced physiological activity. See Fisher, G. H. et al., Arch Biochem. Biophys 189,81 (1978). On the other hand, substitution of L-3,4-A Pro at the 3,5,8 or 9 position in ACE inhibitor BPPga enhances its inhibitory activity. See Fisher, G. H. et al., FEBS Letters 107, 273 (1979). Incopending application Serial No. 116,951,appiicants found that the compounds having APro, which are disclosed in said application, have high inhibitory potency and anti-hypertensive effectiveness. However, at present, no rationale can be advanced to explain the diversity of observed results following substitution of APro for proline. Similarly, no clear picture has emerged of the effects of other proline derivatives or analogs substituted at various loci on ACE inhibitors.

To date, the effect of the amino acid to the left of the sulfur in the thioester compounds disclosed in ourcopending applications, has not been deter mined. It is thought that this amino acid functions as an additional recognition site for the enzyme. If this is true, it would be expected that a compound with an amino acid here would be a better inhibitor.

Applicants have found that many amino acids, substituted amino acids or analogous compounds can be utilized for A in Formula I below to provide effective anti-hypertensive agents and effective inhibitors of ACE. See copending applications of Ryan, Serial Nos. 145,772; 145,773; and 146,107, all filed on May 2, 1980. Applicants have found that the hydroxyprolines, proline, L-, and D,L-, - 3,4 dehydroproline, thiazolidine - 4 - carboxylic acid, and L - 5 - oxo - proline derivatives are all effective anti-hypertensive agents and have high inhibitory potency for ACE.

In copending applications Serial Nos. 064,897 - 064,903; 115,950; 116,951 and 121,188, applicants found that compounds according to Formula I were effective anti-hypertensive agents and have high inhibitory potency for ACE. At the time of filing these applications, applicants had not examined any salts of these compounds and thus did not know whether the salts would be effective. Applicants have now prepared salts of these compounds and have determined that they are effective anti-hypertensive agents and potent inhibitors of ACE.

Accordingly, the present invention relates to novel inhibitors of ACE which are basic salts of compounds which have the general formula

wherein R is hydrogen, formyl, acetyl, propanoyl, butanoyl, phenylacetyl, phenylpropanoyl, benzoyl, cyclopentanecarbonyl, tert-butyloxycarbonyl, cyclopentanecarbonyl - L - lysyl, pyro - L - glutamyl - L - lysyl, L-lysyl, L-arginyl or pyro - L - giutamyl; A is phenylalanyl, glycyl, alanyl, tryptophyl, tyrosyl, isoleucyl, leucyl, histidyl, or valyl, the amino group thereof being in amide linkage with R; R1 is hydrogen or methyl;; R2 is L-proline, L - 3,4 - dehydroproline, D.L. - 3,4 - dehydroproline, L - 3 - hydroxyproline, L - 4 hydroxyproline, L - thiazolidine - 4 - carboxylic acid, or L - 5 - oxo - proline, the imino group thereof being in imide linkage with the adjacen

n isO to 1, such that when n is 0, R1 is methyl. The amino acid A may exist in any optical form. That is A may be in the L-, D- or D,L-form.

All of the above compounds are inhibitors or ACE and are useful as orally effective anti-hypertensive agents.

The compounds of Formula I form basic salts with various inorganic and organic bases. Such salts include ammonium salts, alkali metal salts, alkaline earth salts, salts with organic bases, salts with amino acids and the like. Examples of alkali metal salts include the sodium or potassium salts. Exam ples of alkaline earth metal salts include the calcium or magnesium salts. Examples of organic base salts include the benzathine, N - methyl - D - glucamine or hydrabamine salts. Examples of amino acid salts include arginine or lysine. The sodium potassium or lysine salts are preferred. Likewise, non-toxic, physiologically acceptable salts are preferred.

The salts are formed by reacting the free acid of the compounds of Formula I with one equivalent of the appropriate base providing the desired cation in a solvent or medium in which the salt is insoluble, or in water and removing the water by freeze drying.

The salts of this invention inhibit the conversion of the decapeptide angiotensin I to angiotensin II and therefore are useful in reducing or relieving angiotensin related hypertension. The action of the enzyme renin on angiotensinogen, a pseudoglobulin in blood plasma, produces angiotensin li. Angiotensin lis converted by angiotensin converting enzyme (ACE) to angiotensin 11. The latter is an active pressor substance which has been imlicated as the causative agent in various forms of hypertension in various mammalian species, e.g. rats and dogs.The compounds of this invention intervene in the anqiotensi- nogen

angiotensin

angiotensin Il sequence by inhibiting angiotensin converting en enzyme and reducing or eliminating the formation of the pressor substance angiotensin 11. Thus by the administration of a composition containing one or a combination of salts of the compounds of Formula I, angiotensin dependent hypertension in the species of mammal suffering therefrom is alleviated. A single dose, or preferably two or four divided daily doses, provided on a basis of about 0.1 to 100 mg.

per kilogram per day, preferably about 1 to 50 mg.

per kilogram per day is appropriate to reduce blood pressure as indicated in the animal model experi ments described by S. L. Engel, T. R. Schaeffer, M. l H.

Waugh and B. Robin, Proc. Soc. Exp. Biol. Med. 143, 483(1973). The substance is preferably administered orally, but paranteral routes such as subcutaneous, intramuscular, intravenous or intraperitoneal can also be employed.

The salts of this invention can be utilized to achievethe reduction of blood pressure byformulating in compositions such as tablets, capsules or elixirs for oral administration or in sterile solutions or suspensions for parenteral administration. About 10 to 600 mug. of a salt or mixture of salts of the compounds of formula I is compounded with a physiologically acceptable vehicle, carrier, excipient, binder, preservative, stabilizer, flavor, etc., in a unit dosage form as called for by accepted pharmaceutical practice. The amount of active substance in these compositions or preparations is such that a suitable dosage in the range indicated is obtained.

Illustrative of the adjutants which may be incorporated in tablets, capsules and the like are the following: a binder such as gum tragacanth, acacia, corn starch or gelatin; an excipient such as dicalcium phosphate; a disintegrating agent such as corn starch, potato starch, alginic acid and the like; a lubricant such as magnesium stearate; a sweetening agent such as sucrose, lactose or saccharin; a flavouring agent such as peppermint, oil of wintergreen or cherry. When the dosage unit form isa capsule, it may contain in addition to materials of the above type a liquid carrier such as a fatty oil. Various other materials may be present as coatings or to otherwise modify the physical form of the dosage unit. For instance, tablets may be coated with shellac, sugar or both.A syrup or elixir may contain the active compound, sucrose as a sweetening agent, methyl and propyl parabens as preservatives, a dye and a flavoring such as cherry or orange flavor.

Sterile compositions for injection can be formulated according to conventional pharmaceutical practice by dissolving or suspending the active substance in a vehicle such as water for injection, a naturally occurring vegetable oil like sesame oil, coconut oil, peanut oil, cottonseed oil, etc., or a synthetic fatty vehicle like ethyl oleate or the like.

Buffers, preservatives, and the like can be incorporated as required.

A more complete appreciation of the invention will be realised by reference to the following specific examples which describe the details of the preparation and operational effectiveness of the foregoing salts. The following examples are not intended to limit the invention disclosed herein except to the extent that limitations are specifically stated or to the extent to which limitations appear in the appended claims.

Example 1 Preparation of the sodium salt of N - [3 - (N - benzoyl - D,L - phenylalanylthio) - 2- D - methyl- pro pano vi] - L - proline NaHCO3 (0.5 mmol of a 1 M solution) was added to 234.3 mg (0.5 mmol) of N - [3 - (Bz - D, L- phenylalanylthio) - 2- D - methyipropanoyl] - L proline in 1 ml of absolute ethanol with stirring.

Solvents were removed with a rotary evaporator at approximately 35"C. Fresh ethanol was added to the oily residue and again solvent was removed with the rotary evaporator. The procedure was repeated once more with absolute alcohol and then twice with benzene. The white residue was dried in a vacuum desiccator over P205. Recrystallization from 95% ethanol and benzene yielded 135 mg (55.1% yield) of white crystals having a decomposition point of 185"-186"C (softens at 153"C). Infrared analysis using a KBr pellet showed zwitterion bands at 1398 and 1600 cam~1 and a thio ester band at 1682 cm~1. Paper electrophoresis at pH 2 and 5 and thin layer chromatography (silica gel plates) using three separate solvent systems showed only one spot detectable under UV light at short wavelength after reaction with phenazine methosulfate reagents.

Elemental analysis: Calc. for C25H27N2SNaO5.2H2O Calc.: C = 57.02; H = 5.93; N = 5.32; S = 6.09 Found: C = 56.37; H = 5.59; N = 5.30; S = 5.99 Example 2 Preparation of the potassium salt of NQ - [3 - (N benzoyl - D,L - phenylalanylthio) -2- D - methyl- propanoyl]-L- proline The potassium salt of the named compound was prepared by following the procedure of Example 1 using 1 M KHCO3 in place of the 1 M NaHCO3.

Recrystallization yielded 70 mg of a white solid having a decomposition point of 1321340C. Infrared analysis, paper electrophoresis, and thin layer chro matography yielded values identical to those of the corresponding sodium salt from Example 1.

Example 3 Preparation of the L-lysine salt of NU - [3 - (N benzoyl - D,L - phenylalanylthio) - 2- D - methyl- propanoyl] - L - proline A solution of 73.1 mg (0.5 mmole) of L-lysine free base in 0.3 ml of deionized water was added drop wise, with stirring, to a solution of 234.5 mg (0.5 mmole) of N - [3 - (N - benzoyl - D,L - phenylalany lthio) - 2 - methylpropanoyli L - proline 1 ml of absolute alcohol. Solvents were removed with a rotary evaporator to yield a white residue. Fresh absolute alcohol was added and again solvent was removed with a rotary evaporator.The procedure was repeated two more times with absolute alcohol -and then twice with benzene. The final yield was 0.292 g (dp 152-154.4"C) after drying in a vacuum desiccator over P205. Recrystallization from absolute alcohol, a few drops of water and benzene gave a yield of 141 mg of a white precipitate having a decomposition point of 162.5 -163.5 C (softened at 1600C). Infrared analysis using a KBr pellet showed zwitterion bands at 1389 and 1620 cm-7, carbonyl of amide band at 1641 cm~1 and a thio ester band at 1678 cm~1. Paper electrophoresis at pH 5 and thin layer chromatography (silica gel plates) using three separate solvent systems showed the presence of both the named compound and lysine.

Additional physiologically acceptable salts can be prepared by substituting the appropriate base for the NaHCO3, KHCO3 or lysine in Examples 1-3 and substantially following the procedures described below. Although Examples 1-3 described the preparation of the salts of the compound N - [3 - (N benzoyl - D,L - phenylalanylthio) - 2 - D - methylpropanoyl] - L - proline, it will be understood that physiologically acceptable salts of any compound described by Formula I can be prepared in the same manner.

Example 4 Oral Effectiveness of the sodium salt of N" - [3 - (Na - benzoyl - D,L - phenylalanylthio) - 2- D - methylpropanoyi] - L - proline Rats (150-190 g body weight) were fasted overnight and then anesthetized with intraperitoneal pentobarbital, 50-60 mg/kg. Trachepstomy was performed and the animals were ventilated mechanically. A cannula was inserted into a femoral vein for injection of angiotensin I, and a second cannula was inserted into a common carotid artery for direct measurement of arterial blood pressure. Heparin, 1,000 units, was injected via the femoral vein to prevent coagulation. Blood pressure was measured with a pressure transducer connected to a poly graph. The rats were injected with 400 ng/Kg of angiotensin I in 20 aul of 0.9 9 % NaCI; an amount of angiotensin I sufficient to raise mean arterial blood pressure by 45 mm Hg. After the responsiveness of a given rate of angiotensin I was established, the named compound of 10,umol/kg (drug dissolved in 0.15 ml of H20 plus 10 ul of 1 N NaHCO3), was given via a stomach tube. At timed intervals, the effects of 400 ng/kg of angiotensin Ion mean arterial blood pressure were tested.Results are shown below: Time after Oral Blood Pressure Response to Administration 400 ng/kg of Angiotensin I (Mntes) (S of Control) - 5 100% (45 mm Hg) + 5 82 10 33 15 24 20 27 30 27 40 22 50 24 60 27 90 120 44 150 180 210 71 240 73% Example 5 Intravenous Effectiveness of the potassium salt of N - [3 - (Na - benzoyl - D,L - phenylalanylthio) - 2 D - methylpropanoyll - L - proline The intravenous effectiveness of the named compound was examined by following the procedure described in Example 4 except that 1,umole/Kg of the drug was given intravenously. The results are shown below: Time After IV Blood Pressure Response to Administration 400 ng/kg of Angiotensin I (Minutes) (S of Control) - 5 100% (54 mm Hg) + 5 46 10 37 15 41 20 37 30 44 40 50 50 39 60 46 90 52 120 59S While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims (8)

1. An angiotensin converting enzyme inhibitor comprising physiologically acceptable salts of compounds having the formula

wherein R is hydrogen, formyl, acetyl, propanoyl, butanoyl, phenylacetyl, phenylpropanoyl, benozyl, cyclopentaneca rbonyl, tert-butyloxycarbonyl, cyclopentanecarbonyl - L - lysyl, pyro - L - glutamyl - L lysyl, L-lysyl, L-arginyl or pyro - L - glutamyl; A is phenylalanyl, glycyl, alanyl, tryptophyl, tyrosyl, isoleucyl, leucyl, histidyl or valyl, the amino group thereof being in amide linkage with R; R1 is hydrogen or methyl;; R2 is L-proline, L - 3,4 - dehydroproline, D,L - 3,4 dehydroproline, L - 3 - hydroxyproline, L - 4 hydroxyproline, L - thiazolidine - 4 - carboxylic acid, or L - 5 - oxo - proline, the imino group thereof being in imide linkage with the adjace

n is 0 or 1, such that when n is 0, R1 is methyl

2. The inhibitor of claim 1 wherein said salt is selected from the group comprising sodium, potassium calcium, magnesium, benzathine, hydrabamine, N - methyl - D - glucamine, arginine or lysine.

3. The inhibitor of claim 1 wherein said salt is sodium.

4. The inhibitor of claim 1 wherein said salt is potassium.

5. The inhibitor of claim 1 wherein said salt is lysine.

6. The inhibitor of claim 2,3,4 or 5 wherein R is benzoyl, A is phenylalanyl, R1 is methyl, R2 is L-proline and n is 1.

7. A method for inhibiting angiotensin converting enzyme in vivo comprising administering an effective oral dose of an inhibitor of claim 1.

8. A method for reducing blood pressure in vivo comprising administering an effective oral dose of an inhibitor of claim 1.