Although much has been written in recent years about rational drug design, no drug has been designed de novo, that is, without using a natural substrate or inhibitor or screening lead as a starting point. Instead, as we have seen, medicinal chemists continue to depend upon serendipitous discovery of novel biological activities and novel chemical entities for structures on which to begin work. What rational drug design really means at present is rational drug discovery and rational optimization. These result from the application of modern structural and mechanistic biochemistry, and good synthetic chemistry, to obtain structures with the desired spectrum of biological activities. Traditionally, lead compounds were discovered in plant and animal extracts, and more recently in microorganisms and chemical libraries. These traditional approaches continue, but are augmented by advances in molecular biology, which now provide pure proteins in quantity for screening and structure determination, as well as for characterization by modern biophysical methods. Remarkably, x-ray and NMR methods can now provide the most important information needed to design new drugs, that is, the conformations of ligands bound to target proteins. Approaches to identifying possible ligands based only on the knowledge of the enzyme active site are being developed. Some of these, such as CAVEAT, have been recently reviewed. In spite of these impressive gains, de novo design of new drugs will not be achieved until we learn how to logically build specific inhibitors of a target enzyme knowing only the protein sequence of the enzyme or the amino acid sequence of the messenger substances. We have a long way to go, because by this very rigorous definition, even the successful design of a new nonpeptide drug beginning with enzyme-ligand NMR or x-ray structure constitutes rational optimization. However, as this article has illustrated, we have made great progress. Some of the current and futuristic approaches to drug design are shown in Fig. 8. Development of useful enzyme inhibitors, designed by knowing the enzyme catalytic mechanism or discovered by screening for natural inhibitors, is a very successful rational method. Discovery of receptor antagonists by screening protocols is also productive.(ABSTRACT TRUNCATED AT 400 WORDS)

1. The metabolism and covalent binding of [3H/14C]bromobenzene has been investigated using liver microsomes from untreated and phenobarbital (PB)-pretreated rats. A model has been developed to relate the observed 3H/14C ratios in the covalently bound residues to the type of metabolite (epoxide versus quinone) responsible for their formation. 2. With control microsomes metabolism was linear for 60 minutes, but with PB microsomes the time course showed a short-lived burst of rapid metabolism followed by a long phase with an overall rate comparable to control. With both types of microsomes covalent binding was synchronous with metabolism. 3. The normalized 3H/14C ratios of recovered substrate and water-soluble metabolites was 1.0, whereas that of the covalently bound material was only 0.5. Such extensive loss of tritium implies that a considerable portion of the covalent binding arises from bromobenzene metabolites more highly oxidized than an epoxide (e.g. quinones). 4. The normalized 3H/14C ratios for bromobenzene metabolites covalently bound to liver proteins in vivo (total and microsomal) was the same as with microsomes in vitro (0.5). However, for the lung and kidney the 3H/14C ratios were considerably higher (0.71 and 0.62), indicating that differences between tissues in vivo may be greater than between liver microsomes in vitro and in vivo.

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1 Pahmitaldehyde, olealdehyde and linolealdehyde acetal phosphatidic acids induced rapid shape change and dose-dependent biphasic aggregation of human platelets in platelet-rich plasma; aggregation was reversible at low doses and irreversible at high doses of the acetal phosphatidic acids. The palmitaldehyde congener elicited monophasic dose-dependent aggregation of sheep platelets in platelet-rich plasma. 4 The adenosine diphosphate (ADP) antagonist, 2-methylthio-AMP, and the cyclo-oxygenase inhibitor, aspirin, abolished PGAP-induced second phase aggregation and release in human platelets but did not affect the first, reversible, phase of aggregation. Both the first and second phases of PGAP-induced aggregation were abolished by chlorpromazine, by the phospholipase A2 inhibitor, mepacrine, and by nmolar concentrations of prostaglandin E1 (PGEI); these agents abolished the second, but not the first phase of ADP-induced aggregation.5 The related phospholipids, lecithin, lysolecithin and phosphatidic acid, at < 100 tM, neither induced aggregation of human platelets in platelet-rich plasma, nor modified PGAP-induced aggregation; 1-palmityl lysophosphatidic acid elicited aggregation of human platelets at a threshold concentration of 100 jIM. 6 It is concluded that the acetal phosphatidic acids induce platelet aggregation per se by direct action at the platelet membrane, and that the acetal function is of primary importance in their potent platelet-stimulating activity. Moreover, as the acetal phosphatidic acids are the major components of the smooth muscle-contracting acidic phospholipid tissue extract 'Darmstoff' (Vogt, 1949), their potent platelet-aggregating properties may be of physiological or pathological significance.

The chemical structures of an acidic phospholipid originally isolated from equine intestine by Vogt and of a phospholipid isolated from rabbit kidney medulla by Walaszek are shown by total synthesis to be similar mixtures of 2‐alkyl (and alkenyl)‐4‐hydroxymethyl‐1,3‐dioxolane‐dihydrogen phosphate esters. In these materials, the alkyl residues derive primarily from oleyl, palmityl and linoleyl aldehydes. The smooth muscle contracting activity observed in the natural substances is shown to reside exclusively in the oleyl aldehyde derivative.

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