Showing papers in "Journal of Medicinal Chemistry in 1992"

The calanolides, a novel HIV-inhibitory class of coumarin derivatives from the tropical rainforest tree, Calophyllum lanigerum.

Abstract: Eight new coumarin compounds (1-8) were isolated by anti-HIV bioassay-guided fractionation of an extract of Calophyllum lanigerum. The structures of calanolide A (1), 12-acetoxycalanolide A (2), 12-methoxycalanolide A (3), calanolide B (4), 12-methoxycalanolide B (5), calanolide C (6) and related derivatives 7 and 8 were solved by extensive spectroscopic analyses, particularly HMQC, HMBC, and difference NOE NMR experiments. The absolute stereochemistry of calanolide A (1) and calanolide B (4) was established by a modified Mosher's method. Calanolides A (1) and B (4) were completely protective against HIV-1 replication and cytopathicity (EC50 values of 0.1 microM and 0.4 microM, respectively), but were inactive against HIV-2. Some of the related compounds also showed evidence of anti-HIV-1 activity. Studies with purified bacterial recombinant reverse transcriptases (RT) revealed that the calanolides are HIV-1 specific RT inhibitors. Moreover, calanolide A was active not only against the AZT-resistant G-9106 strain of HIV-1 but also against the pyridinone-resistant A17 strain. This was of particular interest since the A17 virus is highly resistant to previously known HIV-1 specific, non-nucleoside RT inhibitors (e.g., TIBO; BI-RG-587; L693,593) which comprise a structurally diverse but apparently common pharmacologic class. The calanolides represent a substantial departure from the known class and therefore provide a novel new anti-HIV chemotype for drug development.

Adenosine Receptors: Pharmacology, Structure–Activity Relationships, and Therapeutic Potential

Abstract: In the 10 years since Daly reviewed1 the potential of adenosine receptors as drug targets, considerable advances have been made in the area of purinergic receptor related research such that there is little doubt remaining that adenosine, as well as adenosine 5′-triphosphate (ATP) and related nucleotides, functions as both neurohumoral agents and autacoids regulating the process of cell to cell communication.2 The techniques of molecular pharmacology have been extensively used to delineate purinergic receptor function, resulting in the identification of several receptor subclasses that subserve discrete physiological functions (Table I).2 And more recently, the two major classes of adenosine receptors, the A1 and A2, have been cloned,3,4 offering the potential to model the receptor–ligand interaction from the receptor side.5 Table I Purinoceptor Subtypes On the ligand front, structure–activity relationships (SAR) studies (Figure 1) for derivatives of adenosine (1), as agonists, and of theophylline (2), as antagonists, have revealed selective agents,6–8 and potent and selective A1- and A2-receptor agonists are now available. Newer antagonist ligands include a large number of 8-substituted xanthine derivatives, some of them over 10000-fold more potent than the parent compound 2, as well as numerous classes of non-xanthine heterocyclic compounds8 described in further detail below. Figure 1 The structures of adenosine (1) and theophylline (2), showing the effects of structural modifications at various sites on receptor binding. The exceptional progress in the preclinical area, both chemical and biological, has not however been paralleled in the clinic. Very few adenosine agonists and antagonists have entered clinical trials and none of these, to the authors’ knowledge, have been successful.7 The only approved compound known to produce its therapeutic actions via a direct interaction with adenosine receptors is adenosine itself, used for the treatment of supraventricular tachycardia (SVT),9 a use designated by the U.S. Food and Drug Administration in their coveted 1A category, indicating a drug for major unmet medical need. Additional potential uses for adenosine include cardiac imaging,10 in cardioplegic solutions11 to delay the onset of ischemic contractions, and as a cardioprotectant in postischemic reperfusion.12 While caffeine and theophylline represent prototypic, albeit weak, adenosine antagonists, second generation forms of these compounds with improved antagonist activity for use as cardiotonics, cognition enhancers or antiasthmatics have not been forthcoming despite considerable chemical effort.8 The reasons for the limited progress in adenosine therapeutics are several-fold and include the ubiquity of action of adenosine (and ATP) on a variety of diverse tissue systems, a paucity of receptor selective ligands that are orally bioavailable and soluble, lack of knowledge of disease states involving a purinergic etiology, and probably most importantly, a failure to target adenosine agents in terms of unmet therapeutic need.13 Thus, agonists have been routinely targeted toward hypertension, an area where these agents have probable CNS and renal side effects and compare unfavorably with the many excellent and efficacious antihypertensive agents currently available in the clinic.13 In the present perspective, advances in knowledge related to adenosine function at the molecular level will be reviewed together with information on the structure–activity relationships for a number of pharmacophore series interacting with adenosine receptors. Therapeutic areas where improved adenosine ligands may represent potentially important therapeutic agents will also be indicated.

Dihydropyrimidine calcium channel blockers. 4. Basic 3-substituted-4-aryl-1,4-dihydropyrimidine-5-carboxylic acid esters. Potent antihypertensive agents.

Abstract: We have examined a series of novel dihydropyrimidine calcium channel blockers that contain a basic group attached to either C5 or N3 of the heterocyclic ring. Structure-activity studies show that a 1-(phenylmethyl)-4-piperidinyl carbamate moiety at N3 and sulfur at C2 are optimal for vasorelaxant activity in vitro and impart potent and long-acting antihypertensive activity in vivo. One of these compounds (11) was identified as a lead, and the individual enantiomers 12a (R) and 12b (S) were synthesized. Two key steps of the synthesis were (1) the efficient separation of the diastereomeric ureido derivatives 29a/29b and (2) the high-yield transformation of 2-methoxy intermediates 30a/30b to the (p-methoxybenzyl)thio intermediates 31a/31b. Chirality was demonstrated to be a significant determinant of biological activity, with the dihydropyridine receptor recognizing the enamino ester moiety (12a) but not the carbamate moiety (12b). Dihydropyrimidine 12a is equipotent to nifedipine and amlodipine in vitro. In the spontaneously hypertensive rat, dihydropyrimidine 12a is both more potent and longer acting than nifedipine and compares most favorably with the long-acting dihydropyridine derivative amlodipine. Dihydropyrimidine 12a has the potential advantage of being a single enantiomer.

Modeling of G-protein-coupled receptors: application to dopamine, adrenaline, serotonin, acetylcholine, and mammalian opsin receptors.

Abstract: Hydropathicity analysis of 39 G-protein-coupled receptors (GPCR) reveals seven hydrophobic stretches corresponding to membrane spanning alpha-helices. The alignment of the primary sequences shows a high degree of homology in the GPCR transmembrane regions. 3D models of 39 GPCRs were generated using the refined model of bacteriorhodopsin as a template. Five cationic neurotransmitter receptors (serotonergic 5-HT2, dopaminergic D2, muscarinic m2, adrenergic alpha 2 and beta 2 receptors) were taken as prototypes and studied in detail. The 3D models of the cationic neurotransmitter receptors, together with their primary structure comparison, indicate that the agonist binding site is located near the extracellular face of the receptor and involves residues of the membrane-spanning helices 3, 4, 5, 6, and 7. The binding site consists of a negatively-charged Asp located at the middle of transmembrane helix 3 and a hydrophobic pocket containing conserved aromatic residues on helices 4, 5, 6, and 7. To define the precise receptor-ligand interactions, the natural neurotransmitters were docked into the binding sites. Residues responsible for the affinity, selectivity, and eventually stereospecificity of dopamine, adrenaline, noradrenaline, serotonin, and acetylcholine for their receptors were identified. The ligands are involved in electrostatic interactions as well as hydrogen bonds and specific hydrophobic aromatic interactions. All the GPCRs possess invariant hinge residues, which might be responsible for a conformational change during agonist binding and therefore influence dissociation and association of G-proteins to the receptors. The role of hydrophobic interactions and hydrogen bonds in the conformational change of the receptors, modulating the coupling to the G-protein, is discussed with regard to these residues. The models are in agreement with published data obtained from mutagenesis and labeling studies and represent important working hypotheses to direct future mutagenesis studies. They also enable structure-activity relationship studies and more rational drug design. The 3D models of other G-protein-coupled receptors have been generated in a similar way.

Structure-activity relationships of a series of 2-amino-4-thiazole-containing renin inhibitors.

Abstract: A series of renin inhibitors was synthesized that contained a 2-amino-4-thiazolyl moiety at the P2 position. These derivatives are potent inhibitors of monkey renin in vitro and are selective in that they only weakly inhibit the closely related aspartic proteinase, bovine cathepsin D. Four compounds exhibited oral blood pressure lowering activity in high-renin normotensive monkeys. One of these compounds, 22 (PD 134672), was selected for further evaluation in renal hypertensive monkeys, on the basis of its superior efficacy and duration of action in the in vitro assays and the normotensive primate model.

Hypocholesterolemic activity of synthetic and natural tocotrienols.

Abstract: Tocotrienols are farnesylated benzopyran natural products that exhibit hypocholesterolemic activity in vitro and in vivo. The mechanism of their hypolipidemic action involves posttranscriptional suppression of HMG-CoA reductase by a process distinct from other known inhibitors of cholesterol biosynthesis. An efficient synthetic route to tocotrienols and their isolation from palm oil distillate using an improved procedure is presented. gamma-Tocotrienol exhibits a 30-fold greater activity toward cholesterol biosynthesis inhibition compared to alpha-tocotrienol in HepG2 cells in vitro. The synthetic (racemic) and natural (chiral) tocotrienols exhibit nearly identical cholesterol biosynthesis inhibition and HMG-CoA reductase suppression properties as demonstrated in vitro and in vivo.

Synthesis and evaluation of some water-soluble prodrugs and derivatives of taxol with antitumor activity.

Abstract: The synthesis and evaluation of some 2'- and 7-amino acid derivatives of taxol (1) are reported. Reaction of taxol with N-protected amino acids gave 2'-N-protected amino acid esters of taxol. However, deprotection of the amino group and subsequent isolation of products were complex and only successful when formic acid was used to deprotect a t-BOC protecting group. Esterification of taxol using N,N-dialkylated amino acids gave 2'-amino acid esters of taxol, 2'-(N,N-dimethylglycyl)taxol (4) and 2'-[3-(N,N-diethylamino)propionyl]taxol as its methanesulfonic acid salt (5b), in good yield. The 7-derivatives, 7-(N,N-dimethylglycyl)taxol (9) and 7-L-alanyltaxol (12), were prepared by two alternate methods. In the first approach, the 2'-hydroxyl group was protected using the [(2,2,2-trichloro-ethyl)oxy]carbonyl, or troc, protecting group followed by the esterification of the 7-hydroxyl and subsequent deprotection of the amino and troc groups. In the second approach, taxol was allowed to react with more than 2 molar equiv of the N-protected amino acids or N,N-dialkylated amino acids to give 2',7-diamino acid esters of taxol. For the protected amino acids, the deprotection of the amino group followed by removal of the 2'-substituent gave the 7-amino acid esters of taxol. The methanesulfonic acid salts of both 2'- and 7-amino acid esters showed improved solubility ranging from 2 to greater than 10 mg/mL. The 7-derivatives were effective in promoting microtubule assembly in vitro while 2'-derivatives showed little in vitro activity. The derivatives 2'-(N,N-dimethylglycyl)taxol (4) and 2'-[3-(N,N-diethylamino)propionyl]taxol (5) inhibited proliferation of B16 melanoma cells to an extent similar to that of taxol, while the other derivatives were about 50% as cytotoxic. In a mammary tumor screen, 2'-[3-(N,N-diethylamino)propionyl]taxol showed the greatest antitumor activity compared to the other analogues. The lower activities of the 7-derivatives in inhibiting tumor growth and melanoma cell proliferation (although they were almost as active as taxol in inducing microtubule assembly in vitro) may be due to differences in drug uptake by the cells. The similar cytotoxic and antitumor activities of the 2'-analogues and taxol can be explained by their conversion to taxol or an active taxol metabolite. Therefore, the 2'-analogues appear to behave as prodrugs and have the potential to be developed as chemotherapeutic agents.

Conformationally restrained analogues of pravadoline: nanomolar potent, enantioselective, (aminoalkyl)indole agonists of the cannabinoid receptor.

Abstract: Pravadoline (1) is an (aminoalkyl)indole analgesic agent which is an inhibitor of cyclooxygenase and, in contrast to other NSAIDs, inhibits neuronally stimulated contractions in mouse vas deferens (MVD) preparations (IC50 = 0.45 microM). A number of conformationally restrained heterocyclic analogues of pravadoline were synthesized in which the morpholinoethyl side chain was tethered to the indole nucleus. Restraining the morpholine diminished the ability of these pravadoline analogues to inhibit prostaglandin synthesis in vitro. In contrast, mouse vas deferens inhibitory activity was enhanced in [2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl] pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(4-methoxyphenyl)methano ne (20). Only the R enantiomer of 20 was active (IC50 = 0.044 microM). An optimal orientation of the morpholine nitrogen for MVD inhibitory activity within the analogues studied was in the lower right quadrant, below the plane defined by the indole ring. A subseries of analogues of 20 and a radioligand of the most potent analogue, (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo [1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)methanone (21) were prepared. Inhibition of radioligand binding in rat cerebellar membranes was observed to correlate with functional activity in mouse vas deferens preparations. Binding studies with this ligand (Win 55212-2) have helped demonstrate that the (aminoalkyl)indole binding site is functionally equivalent with the CP-55,940 cannabinoid binding site. These compounds represent a new class of cannabinoid receptor agonists.

A dideazatetrahydrofolate analog lacking a chiral center at C-6: N-[4-[2-(2-amino-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyrimidin-5yl)ethyl[benzoyl]-L-glutamic acid is an inhibitor of thymidylate synthase

Abstract: N-[4-[2-(2-Amino-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyrimidin-5- yl)ethyl]benzoyl]-L-glutamic acid (15), prepared in five steps from 2-pivaloyl-7-deazaguanine, has been found to be an antitumor agent with its primary site of action at thymidylate synthase rather than purine synthesis. This compound appears to be a promising candidate for clinical evaluation.

Synthesis and evaluation of analogues of (Z)-1-(4-methoxyphenyl)-2-(3,4,5-trimethoxyphenyl)ethene as potential cytotoxic and antimitotic agents.

Abstract: A series of stilbenes has been prepared and tested for cytotoxicity in the five human cancer cell lines A-549 non-small cell lung, MCF-7 breast, HT-29 colon, SKMEL-5 melanoma, and MLM melanoma. The cis stilbenes 6a-f proved to be cytotoxic in all five cell lines, with potencies comparable to that of combretastatin A-4. These cytotoxic compounds were all potent inhibitors of tubulin polymerization. The corresponding trans stilbenes 7b-f were inactive as tubulin polymerization inhibitors and were significantly less cytotoxic in the five cancer cell lines. In the dihydro series, 8b, 8c, and 8f were inactive as tubulin polymerization inhibitors, while 8a, 8d, and 8e were less active than the corresponding cis compounds 6a, 6d, and 6e. The lack of tubulin polymerization inhibitory activity and cytotoxicity displayed by the phenanthrene 23b, which was synthesized as a conformationally rigid analogue of the lead compound 1, indicates that the activity of the stilbenes is not due to a totally planar conformation. Similarly, inactivity of the conformationally restricted analogue 26 suggests that the biologically active conformation of 1a resembles that of the cis alkene 1. Additional inactive compounds prepared include the benzylisoquinoline series 28-32 as well as the protoberberines 38 and 39. Shortening the two-carbon bridge of 1a to a one-carbon bridge in the diphenylmethane 20 resulted in a decrease in cytotoxicity and tubulin polymerization inhibitory activity. Although the corresponding benzophenone 18 was as active as 1a as a tubulin polymerization inhibitor, it was less cytotoxic than 1a, and the benzhydrol 19 was essentially inactive. With the exception of the amide 15c, which displayed low antitubulin activity, all of the phenylcinnamic acid derivatives 14a-c and 15a-f were inactive in the tubulin polymerization inhibition assay. The acid 14b and the ester 15a were cytotoxic in several of the cancer cell cultures in spite of their inactivity as tubulin polymerization inhibitors.

Inhibitors of protein kinase C. 2. Substituted bisindolylmaleimides with improved potency and selectivity.

Abstract: A hypothetical mode of inhibition of protein kinase C (PKC) by the natural product staurosporine has been used as a basis for the design of substituted bisindolylmaleimides with improved potency over the parent compound. Structure-activity relationships were consistent with the interaction of a cationic group in the inhibitor with a carboxylate group in the enzyme, and the most potent compound had a Ki of 3 nM. The inhibitors were competitive with ATP but inhibited cAMP-dependent protein kinase (PKA) only at much higher concentrations despite the extensive sequence homology between the ATP-binding regions of PKA and PKC. Three compounds were evaluated further and found to inhibit a human allogeneic mixed lymphocyte reaction pointing to the potential utility of PKC inhibitors in immunosuppressive therapy. One of these compounds was orally absorbed in the rat and represents an attractive lead in the development of PKC inhibitors as drugs.

A nonpromoting phorbol from the samoan medicinal plant Homalanthus nutans inhibits cell killing by HIV-1

Abstract: Extracts of Homalanthus nutans, a plant used in Samoan herbal medicine, exhibited potent activity in an in vitro, tetrazolium-based assay which detects the inhibition of the cytopathic effects of human immunodeficiency virus (HIV-1). The active constituent was identified as prostratin, a relatively polar 12-deoxyphorbol ester. Noncytotoxic concentrations of prostratin from greater than or equal to 0.1 to greater than 25 microM protected T-lymphoblastoid CEM-SS and C-8166 cells from the killing effects of HIV-1. Cytoprotective concentrations of prostratin greater than or equal to 1 microM essentially stopped virus reproduction in these cell lines, as well as in the human monocytic cell line U937 and in freshly isolated human monocyte/macrophage cultures. Prostratin bound to and activated protein kinase C in vitro in CEM-SS cells and elicited other biochemical effects typical of phorbol esters in C3H10T1/2 cells; however, the compound does not appear to be a tumor promoter. In skin of CD-1 mice, high doses of prostratin induced ornithine decarboxylase only to 25-30% of the levels induced by typical phorbol esters at doses 1/30 or less than that used for prostratin, produced kinetics of edema formation characteristic of the nonpromoting 12-deoxyphorbol 13-phenylacetate, and failed to induce the acute or chronic hyperplasias typically caused by tumor-promoting phorbols at doses of 1/100 or less than that used for prostratin.

Low molecular weight, non-peptide fibrinogen receptor antagonists

Abstract: The tetrapeptide H-Arg-Gly-Asp-Ser-OH (1) (RGDS), representing a recognition sequence of fibrinogen for its platelet receptor GP IIb-IIIa (integrin alpha IIb beta 3), served as lead compound for the development of highly potent and selective fibrinogen receptor antagonists. Replacement of the N-terminal arginine by p-amidinophenylalanine or the Gly moiety by m-aminobenzoic acid led to compounds which are superior to the lead peptide with regard to activity and selectivity for GP IIb-IIIa vs the closely related vitronectin receptor alpha v beta 3. By random screening [(p-amidinobenzenesulfonamido)ethyl]-p-phenoxyacetic acid derivatives have been identified as fibrinogen receptor antagonists. Further structure-activity relationship studies culminated in the preparation of N-[N-[N-(p-amidinobenzoyl)-beta-alanyl]-L-alpha-aspartyl]-3-phenyl-L- alanine (29h, Ro 43-5054) and [[1-[N-(p-amidinobenzoyl)-L-tyrosyl]-4-piperidinyl]oxy]acetic acid (37f, Ro 44-9883), which exhibit very high activity as platelet aggregation inhibitors (IC50s 0.06 and 0.03 microM, respectively, human PRP/ADP) as well as marked selectivity for GP IIb-IIIa vs alpha v beta 3. Since the activity of 37f in dogs declines according to a two-compartment model with an initial phase having a t1/2 of 8 min and a second phase with a t1/2 of 110 min, this compound is a suitable candidate for the development as iv platelet inhibitor.

Inhibitors of protein kinase C. 1. 2,3-Bisarylmaleimides.

Abstract: The design and synthesis of a series of novel inhibitors of protein kinase C (PKC) is described. These 2,3-bisarylmaleimides were derived from the structural lead provided by the indolocarbazoles, staurosporine and K252a. Optimum activity required the imide NH, both carbonyl groups, and the olefinic bond of the maleimide ring. 2,3-Bisindolylmaleimides were the most active, and the potency of these was improved by a chloro substituent at the 5-position of one indole ring (compound 28, IC50 0.11 microM). In a series of (phenylindolyl)maleimides, nitro compound 74 was most active (IC50 0.67 microM). Naphthalene 19 and benzothiophene 21 showed greater than 100-fold selectivity for inhibition of PKC over the closely related cAMP-dependent protein kinase (PKA).

Novel 1,2,4-oxadiazoles and 1,2,4-thiadiazoles as dual 5-lipoxygenase and cyclooxygenase inhibitors.

Abstract: A series of 1,2,4-oxadiazoles and 1,2,4-thiadiazoles containing a 2,6-di-tert-butylphenol substituent were prepared and evaluated as dual inhibitors of 5-lipoxygenase and cyclooxygenase in rat basophilic leukemia (RBL-1) cells. Several of these compounds show oral efficacy in the rat carrageenan footpad edema (CFE) and mycobacterium footpad edema (MFE) antiinflammatory models, without concomitant gastric ulceration. Structure-activity relationships are discussed. The best compounds (ID40 values in MFE of 3-8 mg/kg po) contain guanidine-derived substituents on the heterocyclic ring.

A novel probe for the cannabinoid receptor.

Abstract: The 1,1-dimethylheptyl (DMH) homologue of 7-hydroxy-delta 6-tetrahydrocannabinol (3) is the most potent cannabimimetic substance reported so far. Hydrogenation of 3 leads to a mixture of the epimers of 5'-(1,1-dimethylheptyl)-7-hydroxyhexahydrocannabinol or to either the equatorial (7) or to the axial epimer (8), depending on the catalysts and conditions used. Compound 7 discriminates for delta 1-THC (2) in pigeons (ED50 = 0.002 mg/kg, after 4.5 h), at the potency level of 3, and binds to the cannabinoid receptor with a KD of 45 pM, considerably lower than the Ki of 180 pM measured for compound 3 and the Ki of 2.0 nM measured for CP-55940 (1), a widely employed ligand. Tritiated 7 was used as a novel probe for the cannabinoid receptor.

4-Amido-2-carboxytetrahydroquinolines. Structure-activity relationships for antagonism at the glycine site of the NMDA receptor.

Abstract: trans-2-Carboxy-5,7-dichloro-4-amidotetrahydroquinolines, evolved from the lead 5,7-dichlorokynurenic acid, have been synthesized and tested for in vitro antagonist activity at the glycine site on the N-methyl-D-aspartate (NMDA) receptor. Optimization of the 4-substituent has provided antagonists having nanomolar affinity, including the urea trans-2-carboxy-5,7-dichloro-4[[(phenylamino)carbonyl]amino]-1,2,3, 4-tetrahydroquinoline (35; IC50 = 7.4 nM vs [3H]glycine binding; Kb = 130 nM for block of NMDA responses in the rat cortical slice), which is one of the most potent NMDA antagonists yet found. The absolute stereochemical requirements for binding were found to be 2S,4R, showing that, in common with other glycine-site NMDA receptor ligands, the unnatural configuration at the alpha-amino acid center is required. The preferred conformation of the trans-2,4-disubstituted tetrahydroquinoline system, as shown by X-ray crystallography and 1H NMR studies, places the 2-carboxyl pseudoequatorial and the 4-substituent pseudoaxial. Modifications of the 4-amide show that bulky substituents are tolerated and reveal the critical importance for activity of correct positioning of the carbonyl group. The high affinity of trans-2-carboxy-5,7-dichloro-4-[1-(3-phenyl-2-oxoimidazolidinyl)]- 1,2,3,4-tetrahydroquinoline (55; IC50 = 6 nM) suggests that the Z,Z conformer of the phenyl urea moiety in 35 is recognized by the receptor. Molecular modeling studies show that the 4-carbonyl groups of the kynurenic acids, the tetrahydroquinolines, and related antagonists based on N-(chlorophenyl)glycine, can interact with a single putative H-bond donor on the receptor. The results allow the establishment of a three-dimensional pharmacophore of the glycine receptor antagonist site, incorporating a newly defined bulk tolerance/hydrophobic region.

Cyclic RGD peptide analogues as antiplatelet antithrombotics.

Abstract: Stimulation of platelets activates GPIIbIIIa, the heterodimeric integrin receptor, to bind fibrinogen (Fg), which results in platelet aggregation. GPIIbIIIa/Fg binding inhibitors are potentially suitable for acute use during and after thrombolytic therapy as antithrombotic agents. Incorporation of the tripeptide sequence Arg-Gly-Asp (RGD), a common structural element of many integrin ligands, into cyclic peptides produced a series of peptides of the general structure BrAc-(AA1)-RGD-Cys-OH, which were prepared by solid-phase peptide synthesis. Cyclization was accomplished by reaction of the N-terminal bromoacetyl group with the cysteine sulfhydryl at pH 8 at high dilution, resulting in thioether-bridged cyclic peptides [cyclo-S-Ac-(AA1)-RGD-Cys-OH]. Use of alpha-substituted bromoacetyl groups gave rise to an analogous series of acetyl-substituted thioether-bridged cyclic peptides. Oxidation of the thioethers produced separable diastereomeric sulfoxide-bridged cyclic peptides. After thorough evaluation in a GPIIbIIIa ELISA assay and a platelet aggregation assay, G-4120 (70A; AA1 = D-Tyr; sulfoxide bridge) was selected for further investigation as an antithrombotic agent. G-4120 was equipotent in the platelet aggregation assay to kistrin, a highly potent inhibitor of fibrinogen-mediated platelet aggregation isolated from snake venom (IC50 = 0.15 microM).

Synthesis and antiviral activity of a series of HIV-1 protease inhibitors with functionality tethered to the P1 or P1' phenyl substituents: X-ray crystal structure assisted design.

Abstract: By tethering of a polar hydrophilic group to the P1 or P1' substituent of a Phe-based hydroxyethylene isostere, the antiviral potency of a series of HIV protease inhibitors was improved. The optimum enhancement of anti-HIV activity was observed with the 4-morpholinylethoxy substituent. The substituent effect is consistent with a model derived from inhibitor docked in the crystal structure of the native enzyme. An X-ray crystal structure of the inhibited enzyme determined to 2.25 A verifies the modeling predictions.

Synthesis and anti-HIV activity of 4'-azido- and 4'-methoxynucleosides.

Abstract: A series of nucleosides were synthesized in which the 4'-hydrogen was substituted with either an azido or a methoxy group. The key steps in the syntheses of the 4'-azido analogues were the stereo- and regioselective addition of iodine azide to a 4'-unsaturated nucleoside precursor followed by an oxidatively assisted displacement of the 5'-iodo group. The 4'-methoxynucleosides were made via epoxidation of 4'-unsaturated nucleosides with in suit epoxide opening by methanol. Reaction-mechanism considerations, empirical conformation rules, NMR-based conformational calculations, and NOE experiments suggest that the 4'-azidonucleosides prefer a 3'-endo (N-type) conformation of the furanose moiety. When evaluated for their inhibitory effect on HIV in A3.01 cell culture, all the 4'-azido-2'-deoxy-beta-D-nucleosides exhibited potent activity. IC50's ranged from 0.80 microM for 4'-azido-2'-deoxyuridine (6c) to 0.003 microM for 4'-azido-2'-deoxyguanosine (6e). Cytotoxicity was detected at 50-1500 times the IC50's in this series. The 4'-methoxy-2'-deoxy-beta-D-nucleosides were 2-3 orders of magnitude less active and less toxic than their azido counterparts. Modifications at the 2'- or 3'-position of the 4'-substituted-2'-deoxynucleosides tended to diminish activity. Further evaluation of 4'-azidothymidine (6a) in H9, PBL, and MT-2 cells infected with HIV demonstrated a similar inhibitory profile to that of AZT. However, 4'-azidothymidine (6a) retained its activity against HIV mutants which were resistant to AZT.