Medicinal chemistry in the development of societies. Biodiversity and natural products.

This document has been elaborated by the IUPAC Medicinal Chemistry section and is backed by a large number of scientists, many of whom have had direct involvement and whose names appear at the end of the article. This work discusses the role that the discovery of new medicinal agents has in the development of societies as well as in the conservation of biodiversity in terms of work carried out on natural products. Also included are several recommendations for countries which are presently in search of their own scientific and technological development in medicinal agents. The IUPAC Medicinal Chemistry section would appreciate the collaboration of the scientific societies in every country to aid in the diffusion of this document.

Analysis of extemporaneous alprostadil formulaions.

Twenty-two extemporaneous alprostadil (PGE1) injection solutions samples from five different suppliers and three Caverject (Pharmacia and Upjohn, Inc., Bridgewater, NJ) samples from three different lots, all intended for the clinical treatment of erectile dysfunction, were analyzed to determine PGE1 concentration, assess formation of the PGE1 aqueous breakdown product (PGA1), define pH and assess active microbial contamination. High-pressure liquid chromatography (HPLC), pH meter and cell culture techniques were used to conduct the analyses. Of the 22 extemporaneously formulated samples, six showed PGE1 concentrations 10% greater than their listed amounts and seven showed PGA1 weight fractions corresponding to at least 1.5% of the total prostaglandidn content. It should be noted that no standard has been published in the United States Pharmacopeia/National Formulary for this preparation as of this date. All samples were within the pH range 4.5 to 6.0. Four samples tested positive for active microbial contamination. In adition, nearly all the extemporaneously formulated samples contained what appeared to be benzyl alcohol, and about one half had at least two other undefined peaks within their HPLC chromatograms. In contrast, all three Caverject samples were within +/- 7.5% of their listed PGE1 concentrations while showing PGA1 prostaglandins weight fractions of less 0.6%, all were within the pH range 4.0 to 4.5 and all tested negative for active microbial contamination. Chromatograms of the Caverject samples also diplayed peaks consistent with the presence of benzyl alcohol but did not exhibit addtional undefined peaks. The results suggest that significant variations in PGE1 concentration and in PGA1 formation, accompanied by the possibility of microbial contamination, can occur as a result of the extemporaneous formulation and subsequent transfer of this type of product as a premixed solution intended for treating erectile dysfunction.

Stability of Magnesium Sulfate in 0.9% Sodium Chloride and Lactated Ringers solutions.

Storage stability test were peformed on two extemporaneous formulation alternatives to the commercially available magnesium sulfate injection solutions that are in 5% dextrose or in water. Preparations of the commercial water for injection formulation and two alternative formulations in lactated Ringers and in 0.9% sodium chloride were stored at room temperature in glass bottles and in polyvinyl chloride bags over a three-month period. Solutions were monitored for gross precipitation and for changes in magnesium, sulfur and calcium levels as measured by elemental analysis using atomic absorption spectroscopy and inductively coupled plasma atomic emission spectroscopy. The results demonstrate no consistent decreases in measured elemental concentrations or gross signs of precipitation for any formulation tested.

Hydroxamic acids as potent inhibitors of endothelin-converting enzyme from human bronchiolar smooth muscle.

Hydroxamic acids 6a-h, derived from malonyl amino acids, and 25a-d, derived from succinyl amino acids, were synthesized as inhibitors of human bronchiolar smooth muscle endothelin-converting enzyme (HBSM ECE). Several unexpected side reactions were discovered, particularly in the synthesis of hydroxamates derived from succinates. In vitro evaluation against human bronchiolar ECE revealed that in all cases hydroxamates derived from malonate were more potent than hydroxamates derived from succinate. Isopropyl and isobutyl P1' side chains were suitable; omission of the P1' side chain seriously diminished potency. In the P2' position, several amino acids gave potent malonate-derived hydroxamate inhibitors (6b, d-h, IC50 = 0.2-6.8 nM), and beta-Ala provided an extremely potent inhibitor (6c, IC50 = 0.01 nM). C-terminus carboxylates are much more potent ECE inhibitors than the corresponding amides. Most of the hydroxamates were also potent inhibitors of thermolysin and neutral endopeptidase (NEP); however, the P2' beta-Ala derivative 6c uniquely inhibited HBSM ECE much more potently than NEP.

(Imidazolylphenyl)pyrrol-2-one inhibitors of cardiac cAMP phosphodiesterase.

Seven 3-alkyl-4-aryl-1,5-dihydro-2H-pyrrol-2-ones were prepared as potential inhibitors of cardiac cAMP phosphodiesterase (PDE). The design of these compounds made use of rolipram, a known inhibitor of the brain cAMP PDE isozyme, as a lead structure and was guided by a model which describes the features required for potent inhibition of the cardiac isozyme. Syntheses for the new compounds are described, together with the results of theoretical and crystallographic studies aimed toward ascertaining their three-dimensional structures. The activities of these compounds as inhibitors of the cardiac and brain cAMP PDE isozymes and their positive inotropic activity in ferret papillary muscle are also reported. Selected compounds were further examined in an in vivo hemodynamic model. One compound 1,5-dihydro-4-[4-(1H-imidazol-1- yl)phenyl]-3-methyl-2H-pyrrol-2-one, was identified as a potent and selective positive inotropic agent and inhibitor of cardiac cAMP PDE.

Cardiotonic agents. 7. Prodrug derivatives of 4-ethyl-1,3-dihydro- 5-[4-(2-methyl-1H-imidazol-1-yl)benzoyl]-2H-imidazol-2-one.

The cardiotonic agent 4-ethyl-1,3-dihydro-5-4-(2-methyl-1H-imidazol-1-yl)benzoyl]-2H- imidazol-2-one (1) was found to have low bioavailability when administered orally to rats and dogs. A series of N-acyl derivatives, an underutilized prodrug of acidic NH compounds, has been synthesized and tested for their ability to improve the oral bioavailability of 1. Reaction of the monosodium salt of 1 with various anhydrides afforded the N-1 monoacylimidazolones with surprisingly high regioselectivity. In addition to the prodrugs, acylation of 1 with propionic or phenylacetic anhydride led to the novel 3H-pyrrolo[1,2-c]imidazole-3,5(2H)-diones 6. The prodrugs showed a significant increase in the partition coefficients with a minor decrease in the aqueous solubility. The benzoyl derivative 4b exhibited the highest stability in both pH 1.5 and 7.4 buffer solutions. Further evaluation of 4b showed rapid conversion to 1 in canine plasma (t1/2 = 38 min), and human plasma (t1/2 = 10 min). Oral studies indicated that the bioavailability of 4b was increased to greater than 75% (compared to less than 20% for 1), and hemodynamic studies demonstrated that the selective inotropic profile of 1 was retained.

Novel compounds possessing potent cAMP and cGMP phosphodiesterase inhibitory activity. Synthesis and cardiovascular effects of a series of imidazo[1,2-a]quinoxalinones and imidazo[1,5-a]quinoxalinones and their aza analogues.

A series of novel imidazoquinoxalinones and their aza analogues were prepared by the cyclization of o-amino(1H-imidazol-1-yl)aryls and heteroaryls with carbonyldiimidazole. The compounds were screened for inhibition of Type I and Type IV phosphodiesterases (PDE's) and evaluated for their vasorelaxant and positive inotropic activities in vitro. In general, compounds having potent PDE inhibitory activity also possessed good inotropic and vasodilator activity, although linear correlations between these activities could not be established.

Synthesis and biological activity of angiotensin II analogues containing a Val-His replacement, Val psi[CH(CONH2)NH]His.

The dipeptide mimic Val psi[CH(CONH2)NH]His (4) was incorporated into angiotensin II (AII) analogues to provide an octapeptide saralasin derivative (29) as well as tetrapeptide analogue 19. Three C-terminal tetrapeptides (21, 25, and 28) were also prepared. All compounds were tested for their ability to displace 3H-AII from rabbit adrenal gland homogenate and as antagonists of AII and AI on guinea pig ileum. The octapeptide analogue 29 was 700 times less active than the parent peptide 30. All the C-terminal fragments 19, 21, 25, and 28 have no measurable AII antagonist activity. Of the four tetrapeptide fragments, only 21 showed any appreciable binding activity.

A topographical model for the c-AMP phosphodiesterase III active site.

With this minireview, concepts about how c-AMP and various inhibitor molecules interact with the phosphodiesterases seem to have come full-circle. It will be proposed and elaborated herein that an understanding of SAR for the newest, "second generation" PDE inhibitors is best accomplished by adopting a model that supposes that these compounds are transition state inhibitors. The analysis finds an interesting parallel with early studies where it was recognized that c-AMP adopts a trigonal bipyramid transition state during hydrolysis. The dynamic interaction of ligands with the phosphodiesterase enzymes will also be made evident when simple algebraic expressions are shown to be inadequate for predicting inhibitor potencies. The latter are apparently complicated by cooperative or synergistic relationships that occur among the various binding sites within the receptor. Finally, implications that can be derived from certain topographical features of the model are discussed relative to a range of potential therapeutic indications.

Cardiotonic agents. 6. Histamine analogues as potential cardiovascular selective H2 agonists.

Twenty-six alkyl and aralkyl histamine analogues were prepared as potential cardiotonic agents. Compounds were designed to allow interaction with a putative secondary aryl binding site at the H2 receptor, the presence of which was inferred from the structure of cyprohepatadine, which is known to have H2-antagonist properties. The compounds were examined for inotropic activity in ferret papillary muscle. Potent inotropic activity was generally found in N-alkyl- and N,N-dialkylimidazole-4-ethanamines, whereas N-(amidoalkyl)imidazole-4-ethanamines and N-alkylimidazole-4-propanamines were at best weakly active. Five compounds were examined in screens designed to assess hemodynamic effects and gastric acid secretion in vivo. Two of these compounds, alpha-(3-phenyl-2-transpropenyl)-1H-imidazole-4-ethanamine and N-heptyl-1H-imidazole-4-ethanamine, showed positive inotropic activity with minimal effects on heart rate and mean arterial pressure in vivo; however, both compounds were found to stimulate gastric acid secretion. These results demonstrate that selectivity between various H2-receptor-mediated activities can be obtained with substituted histamine analogues.

Cardiotonic agents. 5. Fragments from the heterocycle-phenyl-imidazole pharmacophore.

To examine the role of each component in the heterocycle-phenyl-imidazole inotropic pharmacophore, several imidazolone derivatives, an arylimidazole, a substituted 3,4-dihydro-4-oxopyrimidine, and a quinolin-2(1H)-one derivative were prepared as structural fragments or representatives from this relationship. Tests for cardiac inotropic activity in ferret papillary muscle strips (FPM) and for inhibition of crude cAMP phosphodiesterase obtained from canine cardiac tissue suggest that, while all three components contribute significantly toward potent activity (active at less than 1 microM concentrations in FPM), any combination of two components, in approximately a preferred geometry, represents the minimal requirements for weak activity (active at less than 25 microM concentrations). No single component appears to be requisite in an absolute sense.

Heterocyclic analogues of benzamide antiarrhythmic agents.

A series of heterocyclic N-[(diethylamino)alkyl]arenamides related to acecainide was prepared and examined for antiarrhythmic activity. The compounds were synthesized from the corresponding known heterocyclic carboxylic acids or esters by using standard amide formation methods. The effects of the compounds on the electrophysiological properties of canine Purkinje fibers and ventricular muscle strips were determined. Most of the compounds showed effects consistent with weak class I activity. Two compounds, N-[2-(diethylamino)ethyl]-3,4,5-trimethyl-1H-pyrrole-2-carboxamide and N-[2-(diethylamino)ethyl]-1H-indole-2-carboxamide, displayed prolongation of the action potential duration and functional refractory period indicative of modest class III electrophysiological activity. Representative compounds were examined by using molecular modeling techniques. Compounds of differing activity classes displayed qualitatively different electrostatic potential maps.

Cardiotonic agents. 3. A topographical model of the cardiac cAMP phosphodiesterase receptor.

Based on the pharmacophoric relationship heterocycle-phenyl-imidazole (H-P-I) and upon consideration of several potent inhibitors of cardiac cAMP phosphodiesterase, a topographical model of this receptor is proposed. The model consists of two binding sites which interact with H, two steric features, preferential rotation of P away from coplanarity with H, and a binding site for an electron-rich system (I). It is supported by molecular modeling studies and accommodates a variety of inhibitors. It also encompasses the active site of the enzyme and can distinguish cAMP from cGMP as substrates.

Cardiotonic agents. 1. Novel 8-aryl-substituted imidazo[1,2-a]- and -[1,5-a]pyridines and imidazo[1,5-a]pyridinones as potential positive inotropic agents.

Several 8-arylimidazo[1,2-a]pyridines, 8-arylimidazo[1,5-a]pyridines, and 8-arylimidazo[1,5-a]pyridinones were prepared and tested in vitro for potential cardiac inotropic and electrophysiological activity. Selected analogues were further tested in vivo in canine hemodynamic and cardiac electrophysiology models. Compounds having an imidazole substituent consistently showed activity. A pharmacophoric relationship between heterocycle-phenyl-imidazole and positive inotropic activity was noted. The significance of this relationship is discussed.

Cardiotonic agents. 2. (Imidazolyl)aroylimidazolones, highly potent and selective positive inotropic agents.

A series of 4-alkyl-1,3-dihydro-5-[(1H-imidazolyl)benzoyl]-2H-imidazol-2-ones 9 was synthesized and evaluated in vitro for positive inotropic and cyclic AMP phosphodiesterase inhibitory activity. A wide range of inotropic and enzyme-inhibitory potencies was observed, substitution on the imidazolyl moiety being the major determinant of activity. The 4-ethyl-5-[4-(1H-imidazol-1-yl)benzoyl] congener 9g exhibited the highest potency in vitro. Incorporation of a methyl group at the imidazolyl 2-position gave 9h, which was less potent but remarkably selective in vivo for positive inotropic effects over heart rate and hypotensive effects.

In search of the digitalis replacement.

There are now several new CT drugs undergoing clinical study for the treatment of CHF. While most of these new agents are like digitalis in that they possess significant inotropic activity, they differ from digitalis in that they tend to also have pronounced vasodilator properties. Such compounds, because they are not pure CT agents, should not be regarded as true digitalis replacements. Despite the increased understanding about the cAMP cascade and about cardiac muscle contraction in general, the long search for a specific digitalis replacement cannot be regarded as having been accomplished. It should also be noted that the initial clinical assessment for any of these new compounds will be complicated by the fact that they are tested in only the latter stages of CHF where prognosis is very poor and the possibility of showing significant improvement in mortality is extremely difficult. It will be unfortunate if the potential for positive inotropic agents (pure CT drugs) is compromised by clinical studies that employ mixed inotropic-vasodilator compounds in extremely sick patient populations. The continued use of the digitalis glycosides for such a long period of history implies that at least some subpopulation of CHF patients should derive benefit from therapy with an appropriately selective CT drug. Neither the appropriate drug nor the appropriate specific patient population, both of which are needed to properly address the eventual role of inotropic therapy in CHF, have, as yet, been identified.

Ultra-short-acting beta-adrenergic receptor blocking agents. 3. Ethylenediamine derivatives of (aryloxy)propanolamines having esters on the aryl function.

Various ethylenediamine derivatives have been incorporated into the nitrogen substituent of certain short-acting (aryloxy)propanolamine systems that contain esters on their aryl functions. Although several of these compounds showed durations of action comparable to their prototypes, most of the nitrogen substituents significantly prolonged the duration of beta-adrenergic blockade. Similarly, while one of the compounds showed appreciable cardioselectivity in vitro, generally, little enhancement of cardioselectivity was obtained. A brief discussion of structure-activity relationships observed for the ethylenediamine derivatives is presented.

Ultra-short-acting beta-adrenergic receptor blocking agents. 1. (Aryloxy)propanolamines containing esters in the nitrogen substituent.

In an attempt to produce short-acting beta-adrenergic receptor blocking agents, we prepared several (aryloxy)propanolamines with ester functions incorporated into the nitrogen substituent. Many of these compounds exhibited a short duration of blocking activity after their continuous intravenous infusion for 40 min. However, their durations were found to increase considerably upon longer intravenous infusion.

Ultra-short-acting beta-adrenergic receptor blocking agents. 2. (Aryloxy)propanolamines containing esters on the aryl function.

Several short-acting beta-adrenergic receptor blocking agents have been prepared by incorporating ester functions into the aryl portion of certain (aryloxy)propanolamine systems. In particular, methyl 3-[4-[2-hydroxy-3-(isopropylamino)propoxy]phenyl]propionate hydrochloride (ASL-8052) was found to be a moderately potent, cardioselective compound with a short duration of action when determined in in vivo canine models.

Ultra-short acting beta-blockers: a proposal for the treatment of the critically ill patient.

Beta-blockade is of proven value in the therapy of acute myocardial infarction but, unfortunately, may produce cardiac failure by removal of needed sympathetic support. The long duration of action of available blockers (hours) makes reversal of failure a complicated problem and precludes rapid modification of therapy to match changing autonomic conditions. To improve the safety and efficacy of beta-blockade in this setting we have developed the concept of ultra-short beta-blockade and have identified a novel beta-blocker (ASL-8052) which possesses a duration of action less than 15 minutes. This compound is cardioselective and possesses efficacy in an animal model of acute myocardial infarction. It, therefore, appears to be suitable for rapid attainment of controlled levels of beta-blockade via intravenous infusion and rapid recovery from beta-blockade if required by the clinical situation. The compound should, therefore, be useful for safe therapy in critically ill cardiac patients.