Structure-activity relationships associated with 3,4,5-triphenyl-1H-pyrazole-1-nonanoic acid, a nonprostanoid prostacyclin mimetic

A series of phenylated pyrazoloalkanoic acid derivatives were synthesized and evaluated as inhibitors of ADP-induced human platelet aggregation. 3,4,5-Triphenyl-1H-pyrazole-1-nonanoic acid (8d), with an IC50 of 0.4 microM, was the most potent inhibitor identified in this study. Biochemical studies determined that 8d increased intraplatelet cAMP accumulation and stimulated platelet membrane-bound adenylate cyclase in a concentration-dependent fashion. Displacement of [3H]iloprost by 8d from platelet membranes indicated that the platelet prostacyclin (PGI2) receptor is the locus of biological action. Structure-activity studies demonstrated that the minimum structural requirements for binding to the platelet PGI2 receptor and inhibition of ADP-induced platelet aggregation within this series are a vicinally diphenylated pyrazole substituted with an omega-alkanoic acid side chain eight or nine atoms long. Potency depended upon both side-chain length and its topological relationship with the two phenyl rings.

Structure-activity relationship studies of a bisbenzimidazole-based, Zn(2+)-dependent inhibitor of HCV NS3 serine protease

A survey of isosteric replacements of the phosphonoalanine side chain coupled with a process of conformational constraint of a bisbenzimidazole-based, Zn(2+)-dependent inhibitor of hepatitis C virus (HCV) NS3 serine protease resulted in the identification of novel series of active compounds with extended side chains. However, Zn(2+)-dependent HCV NS3 inhibition was relatively insensitive to the structural variations examined but dependent on the presence of negatively charged functionality. This result was interpreted in the context of an initial electrostatic interaction between protease and inhibitor that is subsequently consolidated by Zn(2+), with binding facilitated by the featureless active site and proximal regions of the HCV NS3 protein.

An approach to the identification of potent inhibitors of influenza virus fusion using parallel synthesis methodology

Structure-activity studies associated with the salicylic acid-derived inhibitor of influenza fusion, BMY-27709, were examined using a parallel synthesis approach. This SAR survey led to the discovery of potent influenza inhibitory activity in a series of aromatic amides and thioamides derived from 1,3,3-trimethyl-5-hydroxycyclohexylmethylamine. Select compounds were characterized as inhibitors of the H1 subtype of influenza A viruses that act by preventing the pH-induced fusion process, thereby blocking viral entry into host cells. In a plaque-reduction assay, the most potent inhibitors displayed EC(50) values of 0.02-0.14 microg/mL.

Targeting acute ischemic stroke with a calcium-sensitive opener of maxi-K potassium channels

During ischemic stroke, neurons at risk are exposed to pathologically high levels of intracellular calcium (Ca++), initiating a fatal biochemical cascade. To protect these neurons, we have developed openers of large-conductance, Ca++-activated (maxi-K or BK) potassium channels, thereby augmenting an endogenous mechanism for regulating Ca++ entry and membrane potential. The novel fluoro-oxindoles BMS-204352 and racemic compound 1 are potent, effective and uniquely Ca++-sensitive openers of maxi-K channels. In rat models of permanent large-vessel stroke, BMS-204352 provided significant levels of cortical neuroprotection when administered two hours after the onset of occlusion, but had no effects on blood pressure or cerebral blood flow. This novel approach may restrict Ca++ entry in neurons at risk while having minimal side effects.

Salicylamide inhibitors of influenza virus fusion

Structural variation of the quinolizidine heterocycle of the influenza fusion inhibitor BMY-27709 was examined by several topological dissections in order to illuminate the critical features of the ring system. This exercise resulted in the identification of a series of synthetically more accessible decahydroquinolines that retained the structural elements of BMY-27709 important for antiviral activity. The 2-methyl-cis-decahydroquinoline 6f was the most potent influenza inhibitor identified that demonstrated an EC50 of 90 ng/mL in a plaque reduction assay.

Novel quinolizidine salicylamide influenza fusion inhibitors

A novel series of quinolizidine salicylamides was synthesized as specific inhibitors of the H1 subtype of influenza A viruses. These inhibitors inhibit the pH-induced fusion process, thereby blocking viral entry into host cells. Compound 16 was the most active inhibitor in this series with an EC50 of 0.25 microg/mL in plaque reduction assay. The synthesis and the SAR of these compounds are discussed.

Solid-phase synthesis of benzisothiazolones as serine protease inhibitors

An efficient solid-phase synthesis of benzisothiazolone-1,1-dioxide-based serine protease inhibitors involving alkylation of carboxylic acids with N-(bromomethyl)benzisothiazolone-1,1-dioxide has been developed. An example using this procedure for preparation of a library of human mast cell tryptase inhibitors is described.

pH-dependent changes in photoaffinity labeling patterns of the H1 influenza virus hemagglutinin by using an inhibitor of viral fusion

The hemagglutinin (HA) protein undergoes a low-pH-induced conformational change in the acidic milieu of the endosome, resulting in fusion of viral and cellular membranes. A class of compounds that specifically interact with the HA protein of H1 and H2 subtype viruses and inhibit this conformational change was recently described (G. X. Luo et al., Virology 226:66-76, 1996, and J. Virol. 71:4062-4070, 1997). In this study, purified HA trimers (bromelain-cleaved HA [BHA]) are used to examine the properties and binding characteristics of these inhibitors. Compounds were able to inhibit the low-pH-induced change of isolated trimers, as detected by resistance to digestion with trypsin. Protection from digestion was extremely stable, as BHA-inhibitor complexes could be incubated for 24 h in low pH with almost no change in BHA structure. One inhibitor was prepared as a radiolabeled photoaffinity analog and used to probe for specific drug interactions with the HA protein. Analysis of BHA after photoaffinity analog binding and UV cross-linking revealed that the HA2 subunit of the HA was specifically radiolabeled. Cross-linking of the photoaffinity analog to BHA under neutral (native) pH conditions identified a stretch of amino acids within the alpha-helix of HA2 that interact with the inhibitor. Interestingly, cross-linking of the analog under acidic conditions identified a different region within the HA2 N terminus which interacts with the photoaffinity compound. These attachment sites help to delineate a potential binding pocket and suggest a model whereby the BHA is able to undergo a partial, reversible structural change in the presence of inhibitor compound.

1,2-Benzisothiazol-3-one 1,1-dioxide inhibitors of human mast cell tryptase

A library of compounds were prepared by reacting 2-(bromomethyl)-1, 2-benzisothiazol-3(2H)-one 1,1-dioxide (5) with commercially available carboxylic acids in the presence of potassium carbonate or a tertiary amine base. From this library, (1,1-dioxido-3-oxo-1, 2-benzisothiazol-2(3H)-yl)methyl N-[(phenylmethoxy)carbonyl]-beta-alanate (7b) emerged as a potent inhibitor of human mast cell tryptase (IC50 = 0.85 microM). Extension of the side chain of 7b by two carbons gave (1, 1-dioxido-3-oxo-1,2-benzisothiazol-2(3H)-yl)methyl 5-[[(phenylmethoxy)carbonyl]amino]pentanoate (7d) which was an 8-fold more potent inhibitor (IC50 = 0.1 microM). Further modification of this series produced benzoic acid derivative (1, 1-dioxido-3-oxo-1,2-benzisothiazol-2(3H)-yl)methyl 4-[[(phenylmethoxy)carbonyl]amino]benzoate (7n) which is the most potent inhibitor identified in this series (IC50 = 0.064 microM). These compounds exhibit time-dependent inhibition consistent with mechanism-based inhibition. For 7b, the initial enzyme velocity is not a saturable function of the inhibitor concentration and the initial Ki could not be determined (Ki > 10 microM). The steady-state rate constant, Ki, was determined to be 396 nM. On the other hand, compounds 7d and 7n are time-dependent inhibitors with a saturable initial complex. From these studies, an initial rate constant, Ki, for 7d and 7n was found to be 345 and 465 nM, respectively. The steady-state inhibition constants, Ki, for 7d and 7n were calculated to be 60 and 52 nM, respectively. Compound 7n is a 13-fold more potent inhibitor than 7b, and these kinetic studies indicate that the increase in inhibitory activity is due to an increase in initial affinity toward the enzyme and not an increase in chemical reactivity. These inhibitors generally show high selectivity for tryptase, being 40-fold weaker inhibitors of elastase, being 100-fold weaker against trypsin, and showing no inhibition against thrombin. These compounds are not inhibitors of thrombin, plasmin t-PA, urokinase, and factor Xa (IC50 > 33 microM). In the delayed-type hypersensitivity (DTH) mouse model, a model of skin inflammation, a 5% solution of 7d reduced edema by 69% compared to control animals.

[3-[4-(4,5-Diphenyl-2-oxazolyl)-5-oxazolyl]phenoxy]acetic acid (BMY 45778) is a potent non-prostanoid prostacyclin partial agonist: effects on platelet aggregation, adenylyl cyclase, cAMP levels, protein kinase, and iloprost binding

[3-[4-(4,5-diphenyl-2-oxazolyl)-5-oxazolyl]phenoxy]acetic acid (BMY 45778) inhibits human (IC50 = 35 nM), rabbit (136 nM) and rat (1.3 microM) platelet aggregation. This compound activates adenylyl cyclase (ED50 = 6-10 nM) and stimulates GTPase in human platelet membrane preparations. The potency (EC50) of BMY 45778 stimulating adenylyl cyclase is comparable to iloprost. However, maximal stimulation of GTPase by BMY 45778 is approximately half the iloprost-stimulated activity, and BMY 45778 limits the GTPase stimulation by iloprost suggesting that BMY 45778 is a partial agonist at the IP receptor. BMY 45778 completely prevents [3H]]Iloprost binding to platelet membranes (IC50 = 7 nM). In whole platelets, BMY 45778 causes elevation of platelet cAMP levels (cAMP content doubles at 13 nM) and activation of the cAMP-dependent protein kinase (cAMP-protein kinase ratio is twice basal at 2 nM). BMY 45778 treatment of whole platelets also desensitizes the adenylyl cyclase activation by iloprost. These results indicate that BMY 45778, which is structurally different from prostacyclin and most prostacyclin agonists, acts by stimulating prostacyclin (IP) receptors.

Effects of channel modulators on cloned large-conductance calcium-activated potassium channels

Through expression of the cloned mouse (mSlo) or human (hSlo) large-conductance (BK) Ca(2+)-activated K+ channel in Xenopus laevis oocytes and HEK 293 cells, we characterized the effects of reported blockers and openers of BK channels to initiate the study of the molecular determinants of BK channel modulation. In oocytes, iberiotoxin and charybdotoxin, peptidyl scorpion toxins, were both equally effective blockers of BK current, although iberiotoxin was significantly more potent than charybdotoxin. The structurally related peptide kaliotoxin was not a potent blocker of BK current. Paxilline, a fungal tremorgenic alkaloid, was an effective but complex blocker of BK current. Tetrandrine, a putative blocker of type II BK channels, and ketamine were relatively ineffective. The putative BK openers NS004 and NS1619, phloretin, niflumic acid, flufenamic acid, and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) increased BK current in oocytes at microM concentrations; many of these produced biphasic concentration-response relationships. Coapplication of representative blockers and openers revealed several patterns of interaction, including competitive and noncompetitive antagonism. NS1619, niflumic acid, and phloretin were tested by using excised inside-out membrane patches from HEK 293 cells and were found to increase the activity of hSlo BK channels and produce a leftward shift in the G/Gmax-versus-voltage relationship of these channels. These results represent the first comprehensive examination of the molecular pharmacology of BK channels.

Active site-directed thrombin inhibitors--II. Studies related to arginine/guanidine bioisosteres

A series of N-arylsulfonylarginine amides was synthesized wherein the guanidine or arginine moiety was isosterically replaced by a number of heterocyclic functionalities. These compounds were evaluated as potential active-site inhibitors of thrombin. Bisamidines 11a-n showed a similar SAR to that of simple arginine compounds. The ex vivo clotting time measurement of 11d after ip dosing showed prolongation of clotting time in rats.

Azetidin-2-one derivatives as inhibitors of thrombin

A series of 3-(3-guanidinopropyl)-azetidin-2-one derivatives was prepared and evaluated as inhibitors of cleavage of synthetic substrates in vitro by the serine proteases thrombin, trypsin and plasmin. The N-unsubstituted, 4-phenethyl derivative 9a demonstrated weak inhibition of these enzymes but acetylation of the beta-lactam N atom afforded 9b, an effective, time-dependent inhibitor of thrombin and a potent inhibitor of plasmin. Variation of the 4-position of the beta-lactam ring was examined in conjunction with different N-substituents to provide a series of potent, time-dependent inhibitors of thrombin. A C-4 substituent was essential for good inhibitory properties and, in general, polar C-4 substituents enhanced the selectivity of inhibition for thrombin compared to plasmin. A trans relationship between the C-4 and C-3 substituents was found to be superior to a cis disposition whilst homologation of the guanidinopropyl side chain to that of a guanidinobutyl moiety reduced activity. Several compounds were effective inhibitors of thrombin-induced clot formation in human plasma in vitro but activity in this assay did not correlate well with inhibition of thrombin-induced cleavage of a synthetic substrate, presumably a consequence of inherent chemical instability and degradation in plasma.

Opening of large-conductance calcium-activated potassium channels by the substituted benzimidazolone NS004

1. We used electrophysiological techniques to examine the effects of 5-trifluoromethyl-1-(5-chloro-2-hydroxyphenyl)-1,3-dihydro-2H-benzimidaz ole- 2-one (NS004) on large-conductance calcium-activated potassium (BK) channels. 2. We used recordings from excised membrane patches (cell-attached and inside-out single-channel configurations) and whole-cell patch-clamp recordings to examine the effects of NS004 on single BK channels and whole-cell outward currents, respectively, in rat GH3 clonal pituitary tumor cells. We also tested NS004 on voltage-clamped BK channels isolated from rat brain plasma membrane preparations and reconstituted into planar lipid bilayers. Finally, we used two-electrode voltage-clamp techniques to study the effects of NS004 on currents expressed in Xenopus laevis oocytes by the recently described Slo BK clone from Drosophila. 3. In GH3 cells and in Xenopus oocytes expressing the Slo gene product NS004 produced an increase in an iberiotoxin- or tetraethylammonium-sensitive whole-cell outward current, respectively. NS004 produced a significant increase in the activity of single GH3 cell BK channels and rat brain BK channels reconstituted into planar lipid bilayers. In both systems this was characterized by an increase in channel mean open time, a decrease in interburst interval, and an apparent increase in channel voltage/calcium sensitivity. 4. These data indicate that NS004 could be useful for investigating the biophysical and molecular properties of BK channels and for determining the functional consequences of the opening of BK channels.

2-[3-[2-(4,5-Diphenyl-2-oxazolyl) ethyl] phenoxy] acetic acid (BMY 42393): a new, structurally-novel prostacyclin partial agonist: 1). Inhibition of platelet aggregation and mechanism of action

BMY 42393, (2-[3-[2-(4,5-diphenyl-2-oxazolyl)ethyl]phenoxy]acetic acid), is a new prostacyclin partial agonist that inhibited ADP, collagen and thrombin-induced platelet aggregation (IC50 range 0.3 - 2.0 microM). BMY 42393 stimulated platelet adenylate cyclase activity (EC50 = 25 nM), however, the maximal activation was 75-80% of that observed with maximal iloprost or PGE1. Platelets treated with BMY 42393 showed an elevation of cAMP levels and activation of cAMP-dependent protein kinase. BMY 42393 also inhibited thrombin-induced elevation of intracellular free calcium. BMY 42393 competed for radiolabeled iloprost and PGE1 binding to platelet membranes (IC50; 170 nM and 130 nM, respectively); however, it had little effect on radiolabeled PGE2, PGD2, or SQ 29548 binding. These studies indicate that BMY 42393 is a novel platelet aggregation inhibitor which acts by stimulation of platelet prostacyclin receptors to elevate platelet cAMP levels.

2-[3-[2-(4,5-diphenyl-2-oxazolyl) ethyl] phenoxy] acetic acid (BMY 42393): 2). Oral activity and efficacy in animal models of arterial thrombosis

The oral activity and antithrombotic efficacy of BMY 42393 was examined in ex vivo platelet aggregation studies and arterial thrombosis animal models. In a heterologous ex vivo platelet aggregation assay, ADP-induced human platelet aggregation was inhibited when washed human platelets were combined with rat platelet-poor plasma, taken from rats previously orally-dosed with BMY 42393. The IC50 for platelet aggregation inhibition was approximately 10 mg/kg. In a laser-induced thrombosis model, thrombus formation in a revascularized rabbit ear chamber was prevented in a dose-dependent fashion with an ED50 of about 2 mg/kg. A relatively long duration of anti-thrombotic activity was observed in the rabbit ear laser-induced thrombus study and the ex vivo platelet studies. Inhibition of thrombus formation was also demonstrated in a canine model of electrically-induced coronary artery thrombosis. BMY 42393 also prevented cyclic flow reductions in a monkey stenotic renal artery model. These studies indicate that BMY 42393 is orally active and capable of preventing laser and electric current-induced thrombus formation in animal models of arterial thrombosis.

The substituted benzimidazolone NS004 is an opener of the cystic fibrosis chloride channel

Cystic fibrosis is a major inherited disorder involving abnormalities of fluid and electrolyte transport in a number of different organs. Epithelial cells of cystic fibrosis patients have a decreased capacity to secrete chloride in response to cAMP-mobilizing agents because of the mutation of a single gene. The gene product, the cystic fibrosis transmembrane conductance regulator or CFTR, is a chloride channel. The most frequent mutation is a deletion of phenylalanine in position 508 (delta F508-CFTR) that reduces both the expression of the CFTR protein at the cell surface, and the activity of the Cl- channel. This work presents the properties of NS004, a substituted benzimidazolone, which is the first activator of normal and mutant CFTR-associated chloride channels to be described. NS004 activated CFTR and delta F508-CFTR Cl- channels expressed in Xenopus oocytes, and increased 125I efflux (via the Cl- channel) from Vero cells expressing CFTR and delta F508-CFTR. Application of NS004 to the external side of outside-out patches excised from these CFTR- and delta F508-CFTR-expressing cells induced a marked and reversible increase in channel activity.

Non-prostanoid prostacyclin mimetics. 6. Derivatives of 2-[3-[2-(4,5-Diphenyl-2-oxazolyl)ethyl]phenoxy]acetic acid modified beta-to the oxazole ring

2-[3-[2-(4,5-Diphenyl-2-oxazolyl)ethyl]phenoxy]acetic acid, 1, has been described as a non-prostanoid PGI2 mimetic that demonstrates anti-thrombotic properties of long duration in animal models of thrombosis. The effects of substitution and modification of the carbon beta-to the oxazole heterocycle of 1 were examined and equated with the potency of the compounds as inhibitors of ADP-induced human platelet aggregation in vitro. Potency was sensitive to both the size of the substituent and the identity of the beta-atom. The carbamates 13c-e demonstrated IC50's of 0.28-0.36 microM and were significantly more potent than the progenitor 1, IC50 = 1.2 microM. The ethyl carbamate 13c displaced [3H]-iloprost from platelet membranes in a concentration-dependent fashion that was half maximal at 20 nM, which compares with IC50's of 171 nM for 1 and 39 nM or unlabelled iloprost. Carbamate 13c stimulated platelet adenylate cyclase but the maximal effect was less than that observed for PGI2, identifying 13c as a partial agonist at the platelet PGI2 receptor.

Nonprostanoid prostacyclin mimetics. 4. Derivatives of 2-[3-[2-(4,5-diphenyl-2-oxazolyl)ethyl]phenoxy]acetic acid substituted alpha to the oxazole ring

The 4,5-diphenyloxazole derivatives 2-4 were previously identified as nonprostanoid prostacyclin (PGI2) mimetics. A series of derivatives of 2-4 bearing substitutents at the carbon atom alpha to the oxazole ring were synthesized and evaluated as inhibitors of ADP-induced aggregation of human platelets in vitro. In the unsaturated series, the alpha-carbethoxy derivative 10a, evaluated as an equal mixture of geometrical isomers, inhibited platelet aggregation with an IC50 of 0.36 microM. Evaluation of the individual methyl ester derivatives (E)-9a and (Z)-9a revealed that (E)-9a was 10-fold more potent than (Z)-9a. In the saturated series, the alpha-carbomethoxy-substituted compound 12a inhibited platelet aggregation with an IC50 of 0.08 microM, 15-fold more potent than the unsubstituted prototype 2. The potency of 12a was found to be sensitive to variation of the methoxy moiety. The ethyl (12b) and isopropyl (12d) esters were less effective as were the acid 12e and a series of amides (12f-h). Other substituents introduced at this site of the pharmacophore included P(O)(OEt)2 (25), SCH3 (31a), S(O)CH3 (31b), SO2CH3 (31c), isopropyl (31d), phenyl (31f), and CH2OH (31i). However, none were significantly more potent inhibitors of platelet function than the parent compound 2. The results indicate the presence of a pocket in the PGI2 receptor protein that preferentially recognizes small, polar but uncharged substituents. The structure-activity correlates are suggestive of a hydrogen-bond interaction between a donor moiety on the PGI2 receptor and the methoxycarbonyl functionality of 12a that is sensitive to both the size of the substituent and its stereochemical presentation in this structural class of PGI2 mimetic. The ethyl ester 12b dose-dependently displaced [3H]iloprost from human platelet membranes and stimulated adenylate cyclase. However, the maximal stimulation was less than that recorded for iloprost, indicating that 12b functions as a partial agonist at the PGI2 receptor.

Nonprostanoid prostacyclin mimetics. 5. Structure-activity relationships associated with [3-[4-(4,5-diphenyl-2-oxazolyl)-5- oxazolyl]phenoxy]acetic acid

cis-[3-[2-(4,5-Diphenyl-2-oxazolyl)ethenyl]phenoxy]acetic acid (3) was previously identified as a nonprostanoid prostacyclin (PGI2) mimetic that potently inhibits ADP-induced aggregation of human platelets with an IC50 of 0.18 microM. As part of an effort to further explore structure-activity relationships for this class of platelet inhibitor and to provide additional insight into the nonprostanoid PGI2 mimetic pharmacophore, the effect of constraining the cis-olefin moiety of 3 into various ring systems was examined. Incorporation of the cis-olefin of 3 into either an oxazole (26) or an unsubstituted pyrazole (35) heterocycle provided compounds that are equipotent with progenitor 3. However, the oxazole 11f, which is isomeric with 26, inhibits ADP-induced human platelet aggregation in vitro with an IC50 of 0.027 microM, 6-fold more potent than 3, 26, or 35. These results suggest that the central oxazole ring of 11f is functioning as more than a simple scaffold that provides optimal stereodefinition for interaction with the PGI2 receptor. The nitrogen atom of the central heterocycle of 11f is postulated to engage in hydrogen-bond formation with a donor moiety in the PGI2 receptor protein, an interaction not available to 26 due to the markedly different topology. In support of this contention, the crystal structures of 11f and 26 contain strong intermolecular hydrogen bonds between the carboxylic acid hydrogen atom and the nitrogen atom of the central oxazole ring. Although 11f and 26 are exact isosteres and could, in principle, adopt the same molecular packing arrangement in the solid state, this is not the case, and the intermolecular hydrogen-bonding interactions in 11f and 26 are accommodated by entirely different molecular packing arrangements. Incorporation of the olefin moiety of 3 into a benzene ring provided a compound, 40, over 60-fold weaker with an IC50 of 11.1 microM. The affinities of 11f, 26, 31, 32, and 40 for the human platelet PGI2 receptor, determined by displacement of [3H]iloprost, correlated with inhibition of platelet function. The solid-state structures of 11f, 26, 31, 32, and 40 were determined and revealed that the more potent compounds 11f and 26 adopt a relatively planar overall topography. In contrast, the central phenyl ring and the phenoxy ring of the weakly active compound 40 are rotated by 53 degrees from planarity. The chemical shifts of the protons of the phenoxy rings of 3, 11f, 18, 26, 31, 32, and 40 suggest that in solution 3, 11f, 18, and 26 adopt a planar conformation while 40 does not.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitors of blood platelet cAMP phosphodiesterase. 4. Structural variation of the side-chain terminus of water-soluble 1,3-dihydro-2H-imidazo[4,5-b]quinolin-2-one derivatives

1-(Cyclohexylmethyl)-4-[4-[(2,3-dihydro-2-oxo-1H-imidazo[4,5-b] quinolin-7-yl)oxy]-1-oxobutyl]piperazine (2) was previously identified as a potent, water-soluble inhibitor of human blood platelet cAMP phosphodiesterase and of induced aggregation in vitro that demonstrated effective antithrombotic activity in animal models of thrombosis. Although 2 exhibited 25% oral bioavailability in rats, pharmacokinetic studies conducted in monkeys revealed that the parent compound was less than 5% bioavailable, the result of extensive first-pass biotransformation in the liver. In an effort to identify potent platelet aggregation inhibitors with enhanced metabolic stability, the side-chain amide moiety of 2 was replaced with chemically more stable urea (6a-s), sulfonamide (13a-m), sulfone (19a-r), and tetrazole (23a-s) moieties. Many representatives from each of these structural types effectively combined potent inhibition of ADP-induced human platelet aggregation in vitro with excellent aqueous solubility, and several are superior to 2. Within each series, the N-(cyclohexylmethyl)-, N-(2-ethylbutyl)-, N-benzyl-, and N-(4-fluorobenzyl)-substituted derivatives were evaluated for in vitro metabolic stability by incubating with the S-9 fraction of monkey liver for 2 h, and the extent of biotransformation was compared with that of the prototype 2. The sulfone 19e and the tetrazoles 23e, 23g, 23j, and 23q were significantly more stable than 2 under these conditions, and 19e and 23e were selected for evaluation in vivo. Tetrazole 23e exhibited 72% bioavailability following ip administration to rats compared with 35% bioavailability for 2 and 19e under the same conditions. However, the oral bioavailability of 19e and 23e in the rat was estimated to be only 3%, suggesting that 19e and 23e are less readily absorbed from the gastrointestinal tract than 2.

Nonprostanoid prostacyclin mimetics. 2. 4,5-Diphenyloxazole derivatives

4,5-Diphenyl-2-oxazolenonanoic acid (18b) was synthesized and found to inhibit ADP-induced aggregation of human platelets with an IC50 of 2.5 microM. Acid 18b displaced [3H]iloprost from human platelet membranes in a concentration-dependent fashion, consistent with 18b inhibiting platelet function by acting as a prostacyclin mimetic. By inserting a phenoxy ring into the side-chain moiety of 18b and systematically varying the pattern of substitution and length of the tethers, more potent inhibitors of platelet aggregation were identified. A phenoxy ring inserted centrally in the side chain proved to be the optimal arrangement but significant activity was observed when the aromatic ring was bound directly to the 2 position of the heterocycle. The meta-substituted cis-(ethenylphenoxy)acetic acid 37 is the most potent platelet aggregation inhibitor synthesized as part of this study with an IC50 of 0.18 microM. Acid 37 displaces [3H]iloprost from human platelet membranes with an IC50 of 6 nM. The trans-olefinic isomer of 37 (25p) is 72-fold weaker as an inhibitor of ADP-induced platelet aggregation, but the saturated derivative 25w (BMY 42393) is intermediate in potency. Structure-activity studies using 25w as a template focused on modification of the tethers intervening between the side-chain phenyl ring and the oxazole and carboxylate termini and substitution of the phenyl ring. These studies revealed that biological activity was sensitive to both the identity of the concatenating atoms and the pattern of ring substitution. The structure-activity relationships provide insight into the topographical relationship between the diphenylated oxazole ring and the carboxylic acid terminus that comprise the nonprostanoid prostacyclin mimetic pharmacophore.

Nonprostanoid prostacyclin mimetics. 3. Structural variations of the diphenyl heterocycle moiety

4,5-Diphenyl-2-oxazolenonanoic acid (2) and 2-[3-[2-(4,5-diphenyl-2-oxazolyl)ethyl]phenoxy]acetic acid (3) were previously identified as nonprostanoid prostacyclin (PGI2) mimetics that inhibit ADP-induced aggregation of human platelets in vitro. The effects on biological activity of substitution and structural modification of the 4- and 5-phenyl rings of 3 was examined. Potency showed a marked sensitivity to the introduction of substituents to these aromatic rings and only the bis-4-methyl derivative 9j, IC50 = 0.34 microM, demonstrated enhanced potency compared to the parent structure 3, IC50 = 1.2 microM. Substitution at the ortho or meta positions of the phenyl rings, replacement by thiopheneyl or cyclohexyl moieties, or constraining in a planar phenanthrene system resulted in compounds that were less effective inhibitors of ADP-induced platelet aggregation. In contrast, variation of the heterocycle moiety revealed a much less stringent SAR and many 5- and 6-membered heterocycles were found to effectively substitute for the oxazole ring of 2 and 3. The diphenylmethyl moiety functioned as an effective isostere for 4,5-diphenylated heterocycles since 13aad showed similar platelet inhibitory activity to 3. With the exception of the 3,4,5-triphenylpyrazole derivative 13g, compounds presenting the (m-ethylphenoxy)acetic acid side chain discovered with 3 demonstrated enhanced potency compared to the analogously substituted alkanoic acid derivative. The structure-activity findings led to a refinement of a model of the nonprostanoid PGI2 mimetic pharmacophore.

Inhibitors of blood platelet cAMP phosphodiesterase. 2. Structure-activity relationships associated with 1,3-dihydro-2H-imidazo[4,5-b]quinolin-2-ones substituted with functionalized side chains

A series of 1,3-dihydro-2H-imidazo[4,5-b]quinolin-2-one derivatives, substituted at the 7-position with functionalized side chains, was synthesized and evaluated as inhibitors of human blood platelet cAMP phosphodiesterase (PDE) as well as ADP- and collagen-induced platelet aggregation, in vitro. Structural modifications focused on variation of the side-chain terminus, side-chain length, and side-chain connecting atom. Functionality incorporated at the side-chain terminus included carboxylic acid, ester and amide, alcohol, acetate, nitrile, tetrazole, and phenyl sulfone moieties. cAMP PDE inhibitory potency varied and was dependent upon the side-chain terminus and its relationship with the heterocyclic nucleus. Methylation at N-1 or N-3 of the heterocycle diminished cAMP PDE inhibitory potency. Several representatives of this structural class demonstrated potent inhibition of ADP- and collagen-induced blood platelet aggregation and were half-maximally effective at low nanomolar concentrations. Amides 13d, 13f, 13h, 13k, 13m, and 13w are substantially more potent than relatively simply substituted compounds. However, platelet inhibitory properties did not always correlate with cAMP PDE inhibition across the series, probably due to variations in membrane permeability. Several compounds inhibited platelet aggregation measured ex vivo following oral administration to rats. Ester 11b, acid 12b, amide 13d, and sulfone 29c protected against thrombus formation in two different animal models following oral dosing and were found to be superior to anagrelide (2) and BMY 20844 (5). However, ester 11b and acid 12b demonstrated a unique pharmacological profile since they did not significantly affect hemodynamic parameters in dogs at doses 100-fold higher than that required for complete prevention of experimentally induced vessel occlusion in a dog model of thrombosis.

Inhibitors of blood platelet cAMP phosphodiesterase. 3. 1,3-Dihydro-2H-imidazo[4,5-b]quinolin-2-one derivatives with enhanced aqueous solubility

Two series of 1,3-dihydro-2H-imidazo[4,5-b]quinolin-2-one derivatives incorporating an additional site for acid salt formation were synthesized and evaluated as inhibitors of human blood platelet cAMP phosphodiesterase (PDE) and ADP-induced platelet aggregation. The objective of this study was to identify compounds that blended potent biological activity with a satisfactory level of aqueous solubility. From a series of 7-aminoimidazo[4,5-b]quinolin-2-ones, biological and physical properties were optimally combined in the 1-piperidinyl derivative 11c. However, this compound offered no significant advantage over earlier studied compounds as an antithrombotic agent in an animal model of small vessel thrombosis. A series of 7-alkoxy alkanoic piperazinamide derivatives, in which the additional basic nitrogen atom was remote from the heterocyclic nucleus and accommodated in a secondary binding region of the cAMP PDE enzyme, demonstrated greater intrinsic cAMP PDE inhibitory activity. Structural modifications of this series focused on variation of the piperazine substituent and side-chain length. The lipophilicity of the N-substituent influenced biological potency and aqueous solubility, with substituents of seven carbon atoms or less generally providing acceptable solubility properties. The N-(cyclohexylmethyl)piperazinamide 21h was identified from this series of compounds as a potent inhibitor of platelet cAMP PDE, IC50 = 0.4 nM, and ADP-induced platelet aggregation, IC50 = 0.51 microM after a 3-min exposure and 0.1 microM after a 15-min exposure of platelet-rich plasma to the drug. Evaluation of 21h and representative analogues in vivo using a rabbit model of small vessel thrombosis revealed significantly greater antithrombotic efficacy compared to that of previously studied compounds with similar intrinsic biological activity measured in vitro but inferior aqueous solubility.

1,3-Dihydro-2H-imidazo[4,5-b]quinolin-2-ones--inhibitors of blood platelet cAMP phosphodiesterase and induced aggregation

A series of 1,3-dihydro-2H-imidazo[4,5-b]quinolin-2-one derivatives was synthesized and evaluated as inhibitors of cAMP hydrolysis by a crude human platelet phosphodiesterase preparation and as inhibitors of ADP- and collagen-induced aggregation of rabbit blood platelets. The parent structure 7a, demonstrated potent inhibitory activity that was enhanced by the introduction of alkyl, alkoxy, or halogen substituents at the 5-, 6-, 7-, and 8-positions. Methylation at N-1 or N-3 produced weaker inhibitors of cAMP PDE and platelet aggregation. 1,3,9,9a-Tetrahydro-2H-imidazo[4,5-b]quinolin-2-ones (6) were found to be equipotent with their fully oxidized congeners (7). On the basis of platelet inhibitory properties in vitro, efficacy at preventing thrombus formation in animal models of thrombosis, and a favorable hemodynamic profile, 1,3-dihydro-7,8-dimethyl-2H- imidazo[4,5-b]quinolin-2-one (7o, BMY 20844) was selected for advancement into toxicological evaluation and clinical trial. An efficient synthesis of 7o is described.

Antithrombotic activity of BMY-43351, a new imidazoquinoline with enhanced aqueous solubility

BMY-43351 is a new broad-spectrum inhibitor of platelet aggregation with greater aqueous solubility than earlier analogs from the imidazoquinoline series. This report compares the antithrombotic activity of BMY-43351 to that of two other imidazoquinolines: BMY-20844, a simply-substituted compound, and BMY-21638, a more potent ether-linked side chain analog. All of these compounds act, at least in part, via inhibition of platelet low-Km cyclic AMP phosphodiesterase. Antithrombotic activity was assessed in the rabbit ear chamber-biolaser preparation, an animal model of small vessel thrombosis, and in the canine coronary artery stenosis-occlusion model of large vessel thrombosis. BMY-43351 was found to be remarkably potent in the biolaser model, with an EDso of 0.074 mg/kg p.o. In comparison, compounds such as aspirin, ticlopidine, sulfinpyrazone, and dipyridamole demonstrate little or no activity at much higher doses, (eg. 100 mg/kg p.o.). Other inhibitors of platelet low Km cyclic AMP phosphodiesterase are active but substantially weaker than BMY-43351. Similarly, in the coronary artery stenosis-occlusion model, BMY-43351 demonstrated impressive activity, significantly inhibiting arterial thrombosis at intraduodenal doses as low as 1 micrograms/kg. The potential use of BMY-43351 as adjunct therapy in thrombolysis was suggested in a series of experiments where this drug was used in combination with a thrombolytic regimen of stretokinase plus heparin. In this experimental setting, time to reperfusion was reduced from 42 +/- 5 minutes to 11 +/- 5 minutes, and reocclusion was totally inhibited.

Imidazoquinoline derivatives: potent inhibitors of platelet cAMP phosphodiesterase which elevate cAMP levels and activate protein kinase in platelets

Compounds containing the imidazoquinoline nucleus are a new class of potent, broad-spectrum inhibitors of platelet aggregation. This report describes studies with a simply-substituted imidazoquinoline (BMY 20844) and several new ether-linked side chain derivatives (BMY 21638 and BMY 43351). These compounds are potent inhibitors of platelet cAMP phosphodiesterase (IC50 values: BMY 20844, 1.3 X 10(-8); BMY 21638, 2 X 10(-10); and BMY 43351, 1 X 10(-10) M, measured using 0.15 microM cAMP) but have little effect on platelet homogenate cGMP phosphodiesterase (IC50 greater than 10(-5) M). Inhibition of different cAMP phosphodiesterase isozymes was tested to determine if the compounds inhibited similar isozymes in other tissues. Rabbit heart cAMP phosphodiesterase isozymes were resolved by ion-exchange chromatography and three peaks of activity were obtained. BMY 20844 inhibited only fraction III (a "cGMP-inhibitable, low Km" cAMP-specific phosphodiesterase) with an IC50 value of 5 X 10(-8) M. These compounds also inhibited canine cardiac sarcoplasmic reticulum membrane-bound "cGMP-inhibitable, low Km" cAMP-specific phosphodiesterase with virtually the same potency as inhibition of cAMP phosphodiesterase in platelet homogenate. In washed platelets these compounds elevated cAMP levels and activated the platelet cAMP dependent protein kinase. Activation of cAMP-dependent protein kinase was determined by cAMP-dependent protein kinase ratio measurements and phosphorylation of intracellular proteins. These studies suggest that this potent new class of agents inhibits platelet phosphodiesterase activity in intact platelets causing an elevation in cAMP levels sufficient to activate the cAMP-dependent protein kinase and stimulate protein phosphorylation. This mechanism is, at least in part, responsible for the ability of these compounds to prevent platelet aggregation and thrombosis in experimental animal models.

Octimibate inhibition of platelet aggregation: stimulation of adenylate cyclase through prostacyclin receptor activation

Octimibate inhibited ADP- and collagen-induced platelet aggregation in human, rabbit and rat platelet-rich plasma. Washed human platelets treated with octimibate had elevated cyclic AMP (cAMP) levels and cAMP-dependent protein kinase activity. When whole platelets were incubated with radiolabeled phosphate, octimibate produced an increase in the phosphorylation of platelet proteins with relative molecular weights of 22, 26, 50 and 80 kilodaltons. This pattern of protein phosphorylation is identical to that observed when the platelets were treated with forskolin, phosphodiesterase inhibitors or other compounds that elevate platelet cAMP levels. Octimibate also inhibited the rise in intracellular Ca++ caused by thrombin, as measured using Fura-2-loaded platelets, which is consistent with octimibate's ability to elevate platelet cAMP levels. When isolated platelet plasma membranes were treated with octimibate, adenylate cyclase activity was stimulated, reaching maximal activation at 1 microM octimibate. (The maximal activation of adenylate cyclase observed with octimibate is 70-75% of that observed with 10 microM PGE1.) This stimulation of platelet adenylate cyclase activity was enhanced by GTP. Octimibate competed for radiolabeled prostaglandin E1 and lloprost binding to isolated platelet membranes at submicromolar concentrations, but did not compete with radiolabeled prostaglandin D2 binding. These studies suggest that octimibate inhibits platelet aggregation by activating platelet adenylate cyclase through stimulation of platelet prostacyclin receptors.