Design of new antihypertensive drugs: Potent and specific inhibitors of angiotensin-converting enzyme

The similarity of the biologically important enzyme angiotensin-converting enzyme to the structurally characterized digestive enzyme carboxypeptidase A has led us to develop a hypothetical model of the mechanism of binding of substrates to its active site. In this model, a positively charged group on the enzyme forms an ionic bond with the negatively charged carboxyl group of the substrate; a hydrogen bonding group of the enzyme binds with the terminal peptide bond of the substrate, and the tightly bound zinc ion of the enzyme binds to the penultimate (scissile) peptide bond of the substrate. Succinyl-l-proline (SQ 13,745) was synthesized as a potential inhibitor of angiotensin-converting enzyme by analogy to d-2-benzylsuccinic acid, an inhibitor of carboxypeptidase A; it was a moderately potent but specific inhibitor of the enzyme. Structure-activity studies carried out using the hypothetical model as a guide led to the synthesis of d-2-methyl-succinyl-l-proline (SQ 13,-297) and d-2-methylglutaryl-l-proline (SQ 14,-102), more potent inhibitors of the enzyme that were shown to be orally active in rats. Attempts to replace the zinc-binding carboxyl group of these compounds with groups with greater affinity for zinc have led to the synthesis of extremely potent inhibitors such as 3-mercapto-propanoyl-l-proline (SQ 13,863) and d-3-mercapto-2-methylpropanoyl-l-proline (SQ 14,225). The most active compound, SQ 14,225, is a purely competitive inhibitor of angiotensin-converting enzyme with an enzyme-inhibitor dissociation constant (K_i) of 1.7 × 10⁻⁹M. It is an extremely potent and specific inhibitor of angiotensin-converting enzyme and appears to have great potential for the treatment of hypertensive disease.     

Development and design of specific inhibitors of angiotensin-converting enzyme

Captopril is a remarkably effective new antihypertensive drug designed and developed as a potent and specific inhibitor of angiotensin-converting enzyme, a zinc metallopeptidase that participates in the synthesis of a hypertensive peptide, angiotensin II, and in the degradation of a hypotensive peptide, bradykinin. Earlier studies with a snake venom peptide (teprotride or SQ 20881) that could be administered only by injection demonstrated that specific inhibitors of angiotensin-converting enzyme could be highly effective as antihypertensive drugs, and helped to clarify the specificity and mechanism of action of the enzyme. A hypothetical model of the active center of angiotensin-converting enzyme based on its presumed analogy to the well characterized zinc metallopeptidase carboxypeptidase A was used to guide logical sequential improvements of a weakly active prototype inhibitor that led eventually to the highly optimized structure of captopril. The hypothetical working model of the active site of angiotensin-converting enzyme used to develop captopril continues to provide a firm basis for development of new types of specific inhibitors of this biologically important enzyme.     

Hyperimmunoglobulinemia associated with narcotic addiction. Effects of methadone maintenance treatment

Increased serum immunoglobulins were common in narcotic addicts. Immunoglobulin M (IgM) levels were high in 75 per cent of 46 adult addicts and in 65 per cent of 63 adolescent addicts seeking methadone maintenance or detoxification. Isolated hypermacroglobulinemia was found in 56 per cent. During methadone maintenance or abstinence high IgM levels were much less frequent. History of overt hepatitis, manifest liver disease, serum glutamic oxaloacetic transaminase (SGOT) or alkaline phosphatase levels did not correlate with the presence of high serum IgM levels. Serum immunoglobulin G (IgG) was more variably and less frequently increased than IgM in the untreated addicts, but was commonly increased in the patients maintained on methadone. Prospective studies of 21 patients starting methadone maintenance showed a decrease in mean serum IgM during treatment. The incidence of normal IgM levels in these patients rose to 48 per cent after one year compared to 24 per cent before treatment. The pattern of immunoglobulin changes in narcotic addiction is significantly altered during methadone maintenance treatment, perhaps as a result in the reduction of drug abuse.     

Ethanol and methadone in man: A possible drug interaction

Possible opiate-ethanol interaction was studied in five selected stable non-alcoholic chronic methadone-maintained patients. Ethanol and methadone levels were determined during oral methadone, ethanol and combined methadone-ethanol tolerance tests. The data do not suggest a significant acute interaction between ethanol and methadone as reflected by the rates of disappearance in the blood.     

Persistent increased immunoglobulin M in treated narcotic addition. Association with liver disease and continuing heroin use

The increased serum IgM in treated narcotic addiction was studied using quantitative radial diffusion techniques. Seventy per cent of 68 untreated heroin addicts had serum IgM levels above 880 mg. per deciliter, the upper limit of normal. Only 7 per cent of 15 abstinent patients had high serum IgM, while 36 or 33 per cent of 109 patients maintained on methadone for at least one year had high serum IgM levels. Fourteen of these 36 had known heroin use and 10 others abused other drugs including alcohol, whereas only 2 of the 73 methadone-maintained patients with normal serum IgM had known drug abuse. The association between positive urine spots for morphine and elevated serum IgM levels was significant (p < 0.01). There was a significant relationship between laboratory evidence of liver disease and increased serum IgM levels. Increased serum alkaline phosphatase correlated better (p < 0.01) than the serum glutamic oxaloacetic transaminase (SGOT) (p < 0.05) with high serum IgM levels in methadone-maintained patients. High serum IgM levels in treated addicts are associated with continuing drug abuse and/or with laboratory evidence of continuing mild liver disease.     

A simplified method for purification of the creatine kinase isoenzyme of human brain (BB)

A greatly simplified procedure has been developed for purification in high yields of creatine kinase isoenzyme BB from human brain. The procedure consists of fractional precipitation with ethanol, adsorption chromatography on hydroxylapatite, and fractional precipitation with ammonium sulfate. The essentially homogeneous enzyme obtained may be used as the antigen for radio immunoassay of blood isoenzyme MB of creatine kinase, which specifically increases following myocardial infarction.     

Plasma testosterone in narcotic addiction

Mean plasma testosterone levels in male heroin addicts, meth adone-maintained, former methadone-maintained and abstinent addicts did not differ significantly from that of normal controls. A prospective study before and during 1 year of methadone maintenance treatment showed no change in the mean plasma testosterone levels during treatment; no correlation was observed between plasma testosterone levels and symptoms of sexual disturbances. Some untreated heroin addicts and some methadone-maintained patients had plasma testosterone values below the lower limits of normal. In the methadone-treated group there was no direct methadone dose-testosterone level relationship, although patients receiving 40 mg of methadone or less had significantly higher mean testosterone levels than those receiving more than 40 mg of methadone daily. There was no relationship between serum glutamic oxaloacetic trans-aminase (SGOT), presence or absence of illicit drug use, or plasma luteinizing hormone level and plasma testosterone. A significant relationship between low testosterone levels and recognized alcoholism was evident.     

The major medical sequelae of opioid addiction

Some of the medical complications of narcotic addiction can be directly related to the biochemical properties of the opiates. Amongst these are dependence, overdose, changes in levels of some serum proteins and antibodies, pulmonary edema, and endocrine effects. Many other complications are more related to the lifestyles that addicts adopt, to infections, to consequences of antigen-antibody reactions, and to the consequences of polydrug abuse.Opioids can be taken for years with little apparent damage resulting from the agent per se, providing the amount used is within the person's tolerance. Overdosage, the most threatening consequence of opiate use, occurs when the dose exceeds tolerance. Overdosage is usually an accentuation of the opioid effect: acute sedation leading to stupor, coma, respiratory failure and death, unless adequately treated with a narcotic antagonist [1, 2]. Some “overdose” deaths may result from other mechanisms, such as cardiac arrhythmias, anaphylactic reactions to some antigen(s) in the injection mixture, or to a depressive effect combined with other sedative drugs producing lethal depression [3].     

Abstinence following detoxification and methadone maintenance treatment

During an 11 year overview of methadone treatment, 161 (72 per cent) of 225 patients who completed detoxification were followed up to eight years. Fifty-one (22.6 per cent) of those were classified as stable and narcotic free 2.9 years after detoxification. Of 89 self-selected patients who had undergone a planned, supported, “therapeutic” detoxification, 37 (42 per cent) were classified narcotic-free; whereas many fewer were so judged after other methods of detoxification. Relapse to nonprescribed opioid use, detected in 34 (38.7 per cent) of those traced, was inversely related to time since detoxification. Relapse potential was very low after three years of apparent continuous narcotic-free existence; three years should be a minimal time for successful detoxification. Some subjects had several cycles of methadone treatment and detoxification. Most patients with combined alcohol-methadone dependencies did poorly, whether or not detoxification from opiolds was undertaken. Since the frequency of enduring narcotic-free state was only 9.7 per cent of 522 patients in the treatment sample, detoxification should not be a realistic goal for all patients who enter treatment.