Discovery and evaluation of a series of 3-acylindole imidazopyridine platelet-activating factor antagonists.

Studies conducted with the goal of discovering a second-generation platelet-activating factor (PAF) antagonist have identified a novel class of potent and orally active antagonists which have high aqueous solubility and long duration of action in animal models. The compounds arose from the combination of the lipophilic indole portion of Abbott's first-generation PAF antagonist ABT-299 (2) with the methylimidazopyridine heterocycle moiety of British Biotechnology's BB-882 (1) and possess the positive attributes of both of these clinical candidates. Structure-activity relationship (SAR) studies indicated that modification of the indole and benzoyl spacer of lead compound 7b gave analogues that were more potent, longer-lived, and bioavailable and resulted in the identification of 1-(N, N-dimethylcarbamoyl)-4-ethynyl-3-[3-fluoro-4-[(1H-2-methylimidazo[4,5-c] pyrid-1-yl)methyl]benzoyl]indole hydrochloride (ABT-491, 22 m.HCl) which has been evaluated extensively and is currently in clinical development.

The role of platelet-activating factor (PAF) and the efficacy of ABT-491, a highly potent and selective PAF antagonist, in experimental allergic rhinitis.

Platelet-activating factor (PAF) may be an important mediator of allergic rhinitis. In the present study we evaluated the effectiveness of a recently described PAF antagonist (ABT-491) in rat and guinea pig models of allergic rhinitis. PAF, when perfused through the nasal passages of anesthetized Brown Norway rats, provoked an acute increase, measured as dye leakage, in nasal vascular permeability evident within 15 min after exposure to PAF. ABT-491, given orally 1 hr before PAF challenge, inhibited the response in a dose-related manner (ED50 = 0.3 mg/kg). Intranasal perfusion with ovalbumin in rats sensitized to the antigen 18 to 21 days before challenge also induced an increase in vascular permeability. The antigen-induced leakage was inhibited a maximum of 74% (P < or = .001) by pretreatment with ABT-491 (3 mg/kg p.o.). An antihistamine (mepyramine, 10 mg/kg i.p.), a serotonin antagonist (methysergide) and a 5-lipoxygenase inhibitor (A-79175) also exhibited efficacy in this model (56%, 87% and 65% inhibition, respectively). Nearly complete inhibition (93%, P < or = .001) of the response was achieved by coadministration of ABT-491 and methysergide. In guinea pigs intranasal administration of PAF resulted in increased airway resistance that was inhibited in a dose-dependent manner by oral administration of ABT-491 (ED50 = 1 mg/kg). Antigen-induced nasal airway resistance, triggered by exposure of sensitized animals to aerosolized ovalbumin, was also inhibited by ABT-491 (maximum inhibition 64%, P < or = .05, 10 mg/kg p.o.). The effectiveness of the antagonist was increased to 80% protection by coadministration with either an antihistamine or a 5-lipoxygenase inhibitor, agents which were separately insignificant in blocking the response to antigen. These results suggest a therapeutic utility for ABT-491, perhaps in combination with other anti-inflammatory agents, in the treatment of allergic rhinitis.

Aryl ketones as novel replacements for the C-terminal amide bond of succinyl hydroxamate MMP inhibitors.

A series of succinyl hydroxamate MMP inhibitors were prepared incorporating an aryl amino ketone moiety in place of the more typical C-terminal amino acid amides. Compounds of the C-terminal ketone series displayed potent inhibition of MMPs. Several compounds of the series were shown to be orally bioavailable.

The design, synthesis, and structure-activity relationships of a series of macrocyclic MMP inhibitors.

A series of succinate-derived hydroxamic acids incorporating a macrocyclic ring were designed, synthesized, and evaluated as inhibitors of matrix metalloproteinases. The inhibitors were designed based on the published X-ray crystal structure of batimastat (1) complexed with human neutrophil collagenase (MMP-8). The synthesized compounds were shown to inhibit selected MMPs in vitro with low nanomolar potency.

Ex vivo inhibition of beta-thromboglobulin release following administration to man of ABT-299, a novel prodrug of a potent platelet activating factor antagonist.

OBJECTIVE AND DESIGN: ABT-299 is a prodrug that is converted by serum esterase to a potent platelet activating factor (PAF) antagonist (A-85783). In order to evaluate the pharmacological activity of this antagonist in man the effect of ABT-299 given to healthy volunteers on ex vivo PAF-induced beta-thromboglobulin (beta-TG) release in blood was assessed. SUBJECTS: 37 healthy male volunteers, age 18 to 40 (mean age of 23.6 years) and free of medication, participated in the study. TREATMENT: Subjects were administered intravenously 0.8 mg, 2 mg, or 70 mg doses of ABT-299 (6-7 subjects per group) or placebo (9 subjects, pooled). METHODS: Peripheral blood taken over 12 h after dosing was used for ex vivo beta-TG release and, in the case of the 70 mg dose, measurement of plasma drug concentration. Data were compared by Student's t-test. RESULTS: All three doses produced highly significant inhibition (p < 0.005 compared to predose values) of PAF-induced beta-TG release (units/ml plasma +/- SEM) 12 h after drug administration (54 +/- 14 vs. 405 +/- 51, n = 8; 79 +/- 23 vs. 480 +/- 127, n = 7; 21 +/- 10 vs. 327 +/- 72, n = 6, respectively) whereas there was no significant difference in beta-TG release in the placebo group (449 +/- 90 vs. 307 +/- 49, n = 9). Inhibition was associated with the rapid appearance in plasma of A-85783 and the pyridine N-oxide metabolite of A-85783. Within 2 h, the plasma concentration of the metabolite exceeded that of the parent drug. Both the parent drug and the metabolite exhibited potent in vitro inhibition of PAF-induced beta-TG release (A2 values of 4 and 1 nM respectively). CONCLUSIONS: These studies are the first to illustrate the utility of the beta-TG release assay for assessing ex vivo activity of PAF antagonists. These studies also demonstrate that the administration of ABT-299 to man results in potent, long lasting inhibition of PAF-mediated platelet activation, due in part to the pyridine-N-oxide metabolite, and support the potential therapeutic utility of this prodrug in treating PAF-mediated diseases.

Structure-activity relationships of N-hydroxyurea 5-lipoxygenase inhibitors.

The discovery of second generation N-hydroxyurea 5-lipoxygenase inhibitors was accomplished through the development of a broad structure-activity relationship (SAR) study. This study identified requirements for improving potency and also extending duration by limiting metabolism. Potency could be maintained by the incorporation of heterocyclic templates substituted with selected lipophilic substituents. Duration of inhibition after oral administration was optimized by identification of structural features in the proximity of the N-hydroxyurea which correlated to low in vitro glucuronidation rates. Furthermore, the rate of in vitro glucuronidation was shown to be stereoselective for certain analogs. (R)-N-[3-[5-(4-Fluorophenoxy)-2-furyl]-1-methyl-2-propynyl]-N-hydroxyure a (17c) was identified and selected for clinical development.

Pharmacology of ABT-491, a highly potent platelet-activating factor receptor antagonist.

ABT-491 (4-ethynyl-N, N-dimethyl-3-[3-fluoro-4-[(2-methyl-1H-imidazo-[4,5-c]pyridin-1-yl)methy l]benzoyl]-1H- indole-1-carboxamide hydrochloride) is a novel PAF (platelet-activating factor) receptor antagonist with a K(i) for inhibiting PAF binding to human platelets of 0.6 nM. Binding kinetics of ABT-491 to the PAF receptor is consistent with a relatively slow off-rate of the antagonist when compared to PAF. Inhibition of PAF binding is selective and is correlated with functional antagonism of PAF-mediated cellular responses (Ca2+ mobilization, priming, and degranulation). Administration of ABT-491 in vivo leads to potent inhibition of PAF-induced inflammatory responses (increased vascular permeability, hypotension, and edema) and PAF-induced lethality. Oral potency (ED50) was between 0.03 and 0.4 mg/kg in rat, mouse, and guinea-pig. When administered intravenously in these species, ABT-491 exhibited ED50 values between 0.005 and 0.016 mg/kg. An oral dose of 0.5 mg/kg in rat provided > 50% protection for 8 h against cutaneous PAF challenge. ABT-491 administered orally was also effective in inhibiting lipopolysaccharide-induced hypotension (ED50 = 0.04 mg/kg), gastrointestinal damage (0.05 mg/kg, 79% inhibition), and lethality (1 mg/kg, 85% vs. 57% survival). The potency of this novel antagonist suggests that ABT-491 will be useful in the treatment of PAF-mediated diseases.

Nonsteroidal anti-inflammatory drugs as scaffolds for the design of 5-lipoxygenase inhibitors.

Representative nonsteroidal anti-inflammatory drug (NSAID) cyclooxygenase inhibitors such as ibuprofen, naproxen, and indomethacin were used as orally bioavailable scaffolds to design selective 5-lipoxygenase (5-LO) inhibitors. Replacement of the NSAID carboxylic acid group with a N-hydroxyurea group provided congeners with selective 5-LO inhibitory activity.

Cognitive impairment in spontaneously hypertensive rats: role of central nicotinic receptors. I.

Both human essential hypertension and genetically induced hypertension in rats have been associated with a range of impairments of cognitive ability. The spontaneous hypertensive rat (SHR) previously has been shown to exhibit a decrease in the expression of brain nicotinic acetylcholine receptors, a factor that could play a role in the impaired ability of this strain in the performance of learning and memory-related tasks. The purpose of this study was to help determine whether task impairment by SHR was related to the reduced expression of central nicotinic acetylcholine receptors. Twelve-week-old SHR were tested in two phases of a water maze (spatial memory) task, and their performance was compared with that of two age-matched normotensive strains, Wistar Kyoto (WKY) and Wistar rats. During Phase 1, SHR exhibited significantly increased latencies to locate a hidden platform as compared with either WKY or Wistar rats. During Phase 2 (subsequent series of trials after a 4-day inter-phase period), where rats were required to find a new platform location, SHR again exhibited significantly impaired performance compared to the normotensive strains. In a single trial passive avoidance paradigm, SHR again displayed significantly reduced avoidance behavior as compared with both WKY and Wistar rats. In consecutive coronal sections, the density of [3H]cytisine binding sites was decreased in SHR by up to 25% in about half of the brain regions examined, with the deficits particularly apparent in cephalic regions. The binding of [125I]alpha-bungarotoxin to brain sections also was decreased in SHR; however, only certain brain areas exhibited significant interstrain differences. These alterations in the expression of putative nicotinic receptor subtypes in SHR were not due to changes in the density of cholinergic neurons since there were no interstrain differences in the binding densities for [3H]vesamicol, which labels the vesicular acetylcholine transporter. Moreover, the magnitude of nicotine-stimulated rubidium efflux from cortical and striatal synaptosomes in vitro was significantly reduced in samples derived from SHR as compared with those from normotensive rats. These results are consistent with the possibility that a reduction in the expression of cortical nicotinic receptors in SHR plays a role in this strain's impaired performance of both spatial and non-spatial learning and memory-related tasks.

ABT-299, a novel PAF antagonist, attenuates multiple effects of endotoxemia in conscious rats.

ABT-299, a highly potent and selective platelet activating factor (PAF) antagonist, was found to be effective in rat models of endotoxic shock. ABT-299 inhibited and reversed LPS-induced hypotension (ED50 of .008 mg/kg, intraarterially). When given prior to LPS challenge, ABT-299 (.1 mg/kg, intravenously) completely inhibited LPS-induced intestinal damage for as long as 8 h after the administration of the antagonist. Pretreatment of rats with ABT-299 (5 mg/kg, intravenously over 4 h) prevented by 85-95% symptoms of disseminated intravascular coagulation (DIC) induced by LPS, including thrombocytopenia, prolongation of prothrombin and partial thromboplastin time, decreased serum fibrinogen, and elevation of serum fibrinogen/fibrin degradation products. A .1 mg/kg dose of ABT-299 administered orally or intravenously improved long-term survival to 80% and 90%, respectively, following a lethal dose (LD65) of LPS. ABT-299 (.1 mg/kg) was also effective in preventing hypotension and gastrointestinal damage induced by lipoteichoic acid (LTA), a putative causative agent of shock in Gram-positive infections. These results illustrate the impressive potency and duration of action of ABT-299 and support the putative role of PAF in acute models of endotoxic shock.

Properties of ABT-299, a prodrug of A-85783, a highly potent platelet activating factor receptor antagonist.

ABT-299 is an aqueous soluble prodrug that is converted rapidly in vivo to A-85783, a novel, highly potent, specific platelet activating factor (PAF) antagonist. The K, for inhibiting PAF binding to rabbit platelet membranes is 3.9 and 0.3 nM for human platelets. Inhibition is selective and reversible and is correlated with functional antagonism of PAF-mediated cellular responses (calcium mobilization, priming of superoxide generation, aggregation and degranulation). The in vivo generation of A-85783 from ABT-299 leads to potent inhibition of PAF-induced inflammatory responses (increased vascular permeability, hypotension and edema) and PAF-induced lethality. When administered i.v., the potency (ED50) of ABT-299 for inhibiting PAF responses was between 6 to 10 micrograms/kg in the rat and mouse and 100 micrograms/kg in the guinea pig. A dose of 100 micrograms/kg in the rat provided greater than 60% protection for 8 to 16 hr against cutaneous and systemic PAF challenge. This duration was also evidenced by ex vivo inhibition of platelet aggregation in guinea pig and sheep. In addition to being active parenterally, ABT-299 exhibited p.o. activity in the rat and mouse (ED50 = 100 micrograms/kg in both species). Pharmacokinetic studies in the rat revealed that ABT-299 was converted rapidly to A-85783 and, in turn, metabolized to the corresponding pyridine-N-oxide and sulfoxide metabolites. These metabolites exhibited significant potency in vitro and in vivo and thus may contribute to the activity observed after administration of ABT-299.

Attenuation of endotoxin-induced pathophysiology by a new potent PAF receptor antagonist.

The role of platelet-activating factor (PAF) as a mediator of endotoxin-induced pathophysiology has been studied in several animal models with conflicting results. We evaluated the effect of a new, potent, and specific PAF receptor antagonist, ABT-299 (Abbott Laboratories) against endotoxin (lipopolysaccharide; LPS)-induced cardiopulmonary dysfunction in a porcine model. In initial experiments, the potency of ABT-299 was confirmed in vitro by its ability to inhibit PAF-induced porcine platelet aggregation at an IC50 of .047 +/- .01 microM, and in vivo by the ability of low doses (.12 mg/kg + .03 mg/kg/h) to block the cardiopulmonary pathologic response to exogenous PAF infusion. To evaluate the effect of ABT-299 administration during endotoxemia, pigs were randomly assigned to one of three groups: controls (n = 7), LPS (n = 9), or ABT-299 + LPS (n =7). ABT-299 was given at 1.0 mg/kg from -0.5 to 0 h plus .3 mg/kg/h from 0 to 6 h. LPS was given at .5 micrograms/kg/hr from 0 to 6 h. ABT-299 reduced the early LPS-induced fall in cardiac index and stroke volume, pulmonary hypertension and vasoconstriction, bronchoconstriction, and hypoxemia. Administration of LPS resulted in 44% mortality (before 6 h), which was blocked by ABT-299. Results with this antagonist indicate that PAF contributes to endotoxin-induced cardiopulmonary dysfunction in the pig, and is associated with mortality in this model.

N-(acyloxyalkyl)pyridinium salts as soluble prodrugs of a potent platelet activating factor antagonist.

Pyrrolothiazole 4 is a potent antagonist of platelet activating factor-mediated effects in a variety of in vitro and in vivo assays. Despite its positive activity in models of inflammation and septic shock, 4 lacks the aqueous solubility necessary for intravenous administration. This deficit was overcome by conversion of 4 to water-soluble pyridinium prodrugs. A two-step procedure was used to prepare a series of N-(acyloxyalkyl)pyridinium salts, all of which exhibited aqueous solubility of greater than 20 mg/mL. The rate of conversion of these prodrugs to 4 was faster in human plasma than in pH 7 aqueous buffer. This rate difference was shown to be due to serum enzymes since the conversion in plasma was significantly slower in the presence of a serine esterase inhibitor. A strong correlation between prodrug structure and buffer/plasma half-life was established. The N-(acetyloxymethyl)pyridinium prodrug 11 (ABT-299) is currently undergoing clinical evaluation for the treatment of sepsis.