Angiotensin-converting enzyme inhibitory activity of SCH 31846, a new non-sulfhydryl inhibitor

The design and properties of N-carboxyalkyldipeptide inhibitors of angiotensin-converting enzyme

Angiotensin-converting enzyme inhibitors promise to make important therapeutic contributions to the control of hypertension and congestive heart failure. The nonapeptide teprotide was the first of these inhibitors to be tested clinically. It was followed by orally active inhibitors, captopril in 1977 and enalapril in 1980. The latter is representative of a new design for the inhibition of metallopeptidases and is the subject of this review. The best of the N-carboxyalkyldipeptide inhibitors inhibits angiotensin-converting enzyme with a Ki of 7.6 X 10(-11) M. This compound is the most potent competitive inhibitor of a metallopeptidase yet to have been reported. The basis of this high potency is beginning to be understood and in part is considered to involve precisely arranged multiple interactions within the enzyme active site. X-ray crystallography of a thermolysin-inhibitor complex has been achieved. Assuming that similar interactions within the active site of angiotensin-converting enzyme are mechanistically probable, the authors hypothesize the binding of enalaprilat to converting enzyme as shown in Figure 24. Such interactions are consistent with kinetic studies (Section V) with the understanding that binding to the enzyme is not sensitive to the inhibitor's state of NH protonation. The reason for this surprising conclusion has not been established. Perhaps counterbalancing factors are involved in the energetics of binding or there may be compensating adjustments made in the enzyme which permit NH protonated and nonprotonated inhibitor to bind equally well. Figure 24 also summarizes present understanding of the conformation of enalaprilat when bound to angiotensin-converting enzyme. From studies on conformationally defined analogs of enalaprilat, it seems likely that the Ala-Pro segment of enalaprilat binds in a conformation that is close to a minimum energy conformer. This situation no doubt contributes to the potency of enalaprilat, since little binding energy would be needed to induce conformational changes in this part-structure of enalaprilat when it is bound to the enzyme. The phenethyl group of enalaprilat is believed to be near the alpha-hydrogen of the L-Ala residue in the enzyme-inhibitor complex. However, the synthesis of conformationally restricted analogs to establish this point has not yet been reached. The N-carboxyalkylpeptide design was developed from Wolfenden's collected product inhibitors of carboxypeptidase-A. Whether or not N-carboxyalkyldipeptides should be classified as collected product or transition state inhibitors is unclear.(ABSTRACT TRUNCATED AT 400 WORDS)

Cough induced activity of spirapril in rats

1. We examined the effect of spirapril, a potent angiotensin converting enzyme (ACE) inhibitor, on the number of capsaicin-induced coughs in rats and compared with that of enalapril. 2. Chronic treatment with enalapril, at doses of 1 and 3 mg/kg, p.o., significantly and dose-dependently enhanced the number of capsaicin-induced coughs. 3. Chronic treatment with higher dose of spirapril (3 mg/kg, p.o.) also significantly enhanced the number of capsaicin-induced coughs. However, lower dose (1 mg/kg, p.o.) of spirapril had no significant effect on the number of capsaicin-induced coughs. 4. These results suggest that cough induced activity, one of the most serious side effects associated with chronic treatment with ACE inhibitors, of spirapril is relatively lower than that of enalapril.

Newer ACE inhibitors. A look at the future

Available information indicates that about 78 new molecules belonging to the class of angiotensin converting enzyme (ACE) inhibitors are under investigation, and that at least 11 or 12 of the newer ACE inhibitors will be available for clinical use. The newer ACE inhibitors can be classified, according to the zinc ion ligand of ACE, into 3 main chemical classes: sulfhydryl-, carboxyl- and phosphoryl-containing ACE inhibitors. All the newer sulfhydryl-containing ACE inhibitors differ from captopril since they are prodrugs, and among them alacepril and probably moveltipril (altiopril, MC 838) are converted in vivo to captopril. When compared with captopril, they show a slower onset and a longer duration of action, and obviously the same route of elimination. Zofenopril, a prodrug that is converted in vivo to the active diacid, shows a greater potency, a similar peak time and a longer duration of action than captopril and, unlike captopril, partial elimination through the liver. The newer carboxyl-containing ACE inhibitors are prodrugs which are converted in vivo to active diacids. Like enalaprilat, they are excreted via the kidney; the exception is spirapril, which is totally eliminated by the liver. Compared to enalapril, benazepril shows an earlier peak time and a slightly shorter terminal half-life, cilazapril and ramipril have an earlier peak time and even longer terminal half-life, perindopril shows similar peak time and terminal half-life, while delapril, quinapril and spirapril show an earlier peak time and a shorter half-life. The phosphoryl-containing ACE inhibitors belong to a new chemical class. Fosinopril is a prodrug which is converted to the active diacid in vivo, shows a relatively late peak time, a long terminal half-life, and is eliminated partially by the liver. SQ 29852, the only newly developed ACE inhibitor which is not a prodrug, seems to be more effective than captopril, with a much longer lasting effect and elimination through the kidney. When the differences in potency between these drugs are compensated by dosage adjustment, all the newer ACE inhibitors are expected to exert a similar amount of inhibition of circulating ACE, and therefore to inhibit to a similar extent the generation of circulating angiotensin II and the breakdown of bradykinin. Obviously they may differ in timing and the duration of circulating ACE inhibition according to their pharmacokinetic properties. With regard to the possibility that they may stimulate prostaglandin synthesis, it is suggested that this action, which does not seem to be specific to this drug class, plays only a minor role in their antihypertensive action; the hypothesis that the sulfhydryl group exerts an additional stimulating action remains to be proved.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiotensin-converting enzyme inhibition, anti-hypertensive activity and hemodynamic profile of trandolapril (RU 44570)

Trandolapril (RU 44570), a new non-sulfhydryl angiotensin-converting enzyme (ACE) inhibitor chemically related to enalapril, and its diacid (RU 44403), were investigated for their ability to inhibit angiotensin-converting enzyme. Trandolapril attenuated angiotensin I (Ang I)-induced pressor responses following i.v. administration to rats and dogs with ID50 values of 13.1 +/- 1.3 and 21.1 +/- 2.3 micrograms/kg. RU 44403 produced corresponding values of 9.9 +/- 0.7 and 7.2 +/- 2.3 micrograms/kg. Trandolapril (3-300 micrograms/kg) produced a dose-related attenuation of Ang I-induced pressor responses (ID50 30 micrograms/kg) following oral administration to rats. Oral administration of trandolapril (30-1000 micrograms/kg) to dogs inhibited Ang I pressor responses for over 6 h. The depressor action of bradykinin in the rat was potentiated by i.v. trandolapril and RU 44403 with ED50 values of 5.5 +/- 0.8 and 4.9 +/- 0.3 micrograms/kg respectively. Trandolapril was 2.3-10-fold more potent than enalapril in all experiments, depending on species or route of administration. RU 44403 and MK 422 were approximately equipotent, implying that trandolapril was more readily hydrolysed than enalapril. Trandolapril (0.3-30 mg/kg) produced dose-related, long-lasting (greater than 24 h) reductions in blood pressure (BP) in spontaneously hypertensive rats (SHR) following oral administration. The anti-hypertensive effect was potentiated significantly in hydrochlorothiazide-pretreated SHR when the plasma renin activity was increased. Enalapril was 10-fold less potent than trandolapril in reducing BP. The anti-hypertensive action of trandolapril (3 mg/kg) was abolished in SHR that were bilaterally nephrectomized 24 h beforehand, but was maintained in SHR pretreated by indomethacin (5 mg/kg p.o.). Trandolapril (1 mg/kg i.v.) produced a modest and transient reduction in BP in anesthetized dogs. Trandolapril produced dose-related (30-1000 micrograms/kg) reductions in BP, total peripheral resistance and heart work in dogs pretreated with hydrochlorothiazide to increase plasma renin activity.

Preclinical studies on angiotensin converting enzyme inhibitors

The identification of the renin-angiotensin-aldosterone system in the control of blood pressure, and the preclinical development of the angiotensin converting enzyme inhibitors for therapeutic use are reviewed. The properties of these compounds are discussed with respect to their in vitro enzyme inhibitory potency; prevention of the pharmacological effects of angiotensin I; potentiation of those of bradykinin; tissue enzyme inhibition; mechanism of effect on blood pressure both alone and in combination with other antihypertensive agents; and effect on cardiac parameters.

Topical ocular hypotensive effects of the novel angiotensin converting enzyme inhibitor SCH 33861 in conscious rabbits

SCH 33861 is a novel, non-sulfhydryl angiotensin converting enzyme (ACE) inhibitor. Topical administration of the compound to the eye of conscious rabbits was employed to examine actions on intraocular pressure (IOP). Falls in IOP resulted from SCH 33861 (0.001-0.01%) administration. Ocular hypotensive responses were sustained for as long as 24 hrs following a single application of 0.001% SCH 33861. The RSS isomer of SCH 33861, which is 200-fold weaker an ACE inhibitor than SCH 33861, caused only transient falls in IOP at 0.1% concentration. The magnitude of the fall in IOP induced by 0.001% SCH 33861 (4.8 +/- 0.5 mmHg) was comparable to that produced by 0.5% timolol (4.5 +/- 0.3 mmHg). Other ACE inhibitors such as captopril (0.1%) and enalaprilic acid (0.01%) also reduced IOP by 4.0 +/- 0.4 and 4.7 +/- 0.4 mmHg, respectively. These findings indicate that SCH 33861 is 500-fold more potent on a weight basis than is timolol in lowering IOP. No loss of ocular hypotensive activity was observed when SCH 33861 was administered twice daily for 5 days suggesting little, if any, potential for tolerance development. SCH 33861, as well as the other ACE inhibitors, caused neither ocular irritation nor alteration of pupil diameter. These findings suggest that inhibition of ocular ACE may represent an effective means of reducing IOP.

Enalapril. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in hypertension and congestive heart failure

Enalapril maleate is an orally active angiotensin-converting enzyme inhibitor. It lowers peripheral vascular resistance without causing an increase in heart rate. Enalapril 10 to 40 mg/day administered either once or twice daily is effective in lowering blood pressure in all grades of essential and renovascular hypertension, and shows similar efficacy to usual therapeutic dosages of hydrochlorothiazide, beta-blockers (propranolol, atenolol and metoprolol) and captopril. Most patients achieve adequate blood pressure control on enalapril alone or with hydrochlorothiazide. In patients with severe congestive heart failure resistant to conventional therapy, enalapril improves cardiac performance by a reduction in both preload and afterload, and improves clinical status long term. Enalapril appears to be well tolerated, with few serious adverse effects being reported. It does not induce the bradycardia associated with beta-blockers or the adverse effects of diuretics on some laboratory values. In fact, the hypokalaemic effect of hydrochlorothiazide is attenuated by the addition of enalapril. The incidence of the main (but rare) side effects of hypotension in hypovolaemic patients and reduced renal function in certain patients with renovascular hypertension, which are also seen with captopril, might be reduced by careful dosage titration, discontinuation of diuretics, and monitoring of at-risk patients. Thus, enalapril is a particularly worthwhile addition to the antihypertensive armamentarium, as an alternative for treatment of all grades of essential and renovascular hypertension. It also shows promise in the treatment of congestive heart failure.

Acute and subchronic toxicity of a nonsulfhydryl angiotensin-converting enzyme inhibitor

Acute and 1-month toxicity studies with SCH 31846, a nonsulfhydryl anti-hypertensive agent which acts by inhibiting angiotensin-converting enzyme, were initiated to evaluate its toxicity. The oral LD50s in mice and rats were approximately 1.8 and 2.5 g/kg, respectively, while the iv LD50 was approximately 450 mg/kg in mice and 150 mg/kg in rats. Signs of acute toxicity in rats and mice included salivation, hypoactivity, ataxia, prostration, and convulsions. In a 1-month dog study at oral doses of 25, 75, or 150 mg/kg, there was a dose-related increase in emesis between 1 and 2 hr after dosing. Absorption studies showed peak blood concentrations occurring in dogs between 0.3 and 1 hr after dosing. No other noteworthy antemortem changes were observed. In a 1-month rat study at oral doses of 30, 180, or 600 mg/kg, the hematocrit and hemoglobin values of the 600 mg/kg-dosed female rats were slightly but significantly (p less than 0.05) decreased and the blood urea nitrogen was slightly but significantly (p less than 0.05) increased in all SCH 31846-dosed male rats and the 600 mg/kg-dosed female rats. Absorption studies in male rats at doses of 30, 180, and 600 mg/kg indicate that SCH 31846 is well absorbed in rats. The 150 mg/kg-dosed dogs and the 180- and 600 mg/kg-dosed rats had a slight increase in the number of renin-containing granules in the renal juxtaglomerular cells. No other compound-related microscopic changes were observed. These data are similar to data reported for Captopril and suggest that in the dog and rat the toxicity of ACE inhibitors is not dependent upon the presence or absence of a sulfhydryl group.

Recent developments in the design of angiotensin-converting enzyme inhibitors

Orally-active angiotensin-converting enzyme inhibitors are rapidly establishing themselves in the therapy of hypertension and congestive heart failure. Concerted efforts in a number of laboratories have now led to the discovery or synthesis of an unparalleled variety of potent inhibitors. The manner in which several of these inhibitors bind to ACE is beginning to be understood. It is hoped that some of the insights to be derived from the SAR and structural studies done with ACE inhibitors will be applicable to other enzyme targets as well. The success of ACE inhibitors as pharmacological tools and in the clinic will also quite certainly encourage future efforts to develop new enzyme inhibitor approaches to drug therapy.

New potent inhibitors of angiotensin converting enzyme

Using an earlier model of the favoured orientation of binding functions of angiotensin converting enzyme (ACE) inhibitors, it has been possible to postulate a new, 7,6-bicyclic system, based on hexahydropyridazine, which might be expected to have high potency. Some members of this system which have been synthesised have been shown to be very active ACE inhibitors, in vitro and in vivo.

Mechanism of the pressor response to tetradecapeptide renin substrate in the rat

The synthetic tetradecapeptide renin substrate (TDP; Asp-arg-val-tyr-ile-his-pro-phe-his-leu-leu-val-tyr-ser) has been employed frequently to elucidate the enzymatic action of renin in vitro and, to a lesser extent, in vivo. We assessed the utility of TDP as a renin substrate in vivo using conscious spontaneously hypertensive rats. Intravenous injection of TDP (1 and 3 micrograms/kg) increased diastolic pressure by 45 +r2 and 67 +/- 2 mmHg, respectively. The pressor response to TDP was significantly inhibited by captopril (3 mg/kg, po), indicating its dependence on conversion by ACE to some active molecule. Pressor responses to TDP also were less in animals subjected to bilateral nephrectomy 18-24 hr before study. However, responses to angiotensin I and II also were reduced, implying a non-specific effect of nephrectomy. Intravenous infusion of the renin inhibitor pepstatin (200 micrograms/min) inhibited pressor responses to hog renin by approximately 60%, but did not affect those to TDP. Intravenous infusion of the water soluble renin inhibitor, pepstatinyl-arginine-o-methyl ester (500 micrograms/min), also inhibited pressor responses to renin (approx. 80%) and did not affect those of TDP. Incubation to TDP (5 microM) with rabbit lung ACE resulted in generation of AI that was blocked by captopril (1 microM). These data suggest that TDP is a substrate for ACE and that the increase in blood pressure produced by TDP is due to its sequential cleavage by ACE to AII and can be independent of renin.

Enkephalinase 'A' inhibition by thiorphan: central and peripheral cardiovascular effects

On the basis of the distribution of enkephalins within the central and peripheral nervous systems as well as on responses to their administration, it has been suggested that these peptides participate in the regulation of the circulation. The present series of experiments examined the effects of thiorphan, an inhibitor of enkephalinase A, on cardiovascular responses to intracerebroventricular (i.c.v.) administration of [D-Ala2,Met5]enkephalin (DAME) and its amide and on peripheral interactions with the sympathetic nervous system and vasoactive peptides. Thiorphan (30 micrograms i.c.v.) potentiated the pressor response to i.c.v. DAME and DAMEamide in conscious spontaneously hypertensive rats. Responses to i.c.v. angiotensin I (AI) were unaffected suggesting lack of inhibition of central angiotensin converting enzyme (ACE). Peripheral administration of relatively large doses of thiorphan (30 and 100 mg/kg s.c.) attenuated the pressor response to i.v. AI by 30-40% and enhanced the depressor effect of i.v. bradykinin in anesthetized normotensive rats indicating inhibition of peripheral ACE. Pressor and tachycardic responses to activation of spinal sympathetic outflow were not altered by thiorphan in pithed normotensive rats. Thiorphan itself did not affect baseline blood pressure or heart rate in any of these experiments. In conclusion, inhibition of central enkephalinase A by i.c.v. administration of thiorphan potentiates the pressor response to i.c.v. DAME. The compound inhibits peripheral ACE but has little direct cardiovascular activity in its own right.