Blockade of mGluR1 receptor results in analgesia and disruption of motor and cognitive performances: effects of A-841720, a novel non-competitive mGluR1 receptor antagonist.

BACKGROUND AND PURPOSE: To further assess the clinical potential of the blockade of metabotropic glutamate receptors (mGluR1) for the treatment of pain. EXPERIMENTAL APPROACH: We characterized the effects of A-841720, a novel, potent and non-competitive mGluR1 antagonist in models of pain and of motor and cognitive function. KEY RESULTS: At recombinant human and native rat mGluR1 receptors, A-841720 inhibited agonist-induced calcium mobilization, with IC50 values of 10.7+/-3.9 and 1.0 +/- 0.2 nM, respectively, while showing selectivity over other mGluR receptors, in addition to other neurotransmitter receptors, ion channels, and transporters. Intraperitoneal injection of A-841720 potently reduced complete Freund's adjuvant-induced inflammatory pain (ED50 = 23 micromol kg(-1)) and monoiodoacetate-induced joint pain (ED50 = 43 micromol kg(-1)). A-841720 also decreased mechanical allodynia observed in both the sciatic nerve chronic constriction injury and L5-L6 spinal nerve ligation (SNL) models of neuropathic pain (ED50 = 28 and 27 micromol kg(-1), respectively). Electrophysiological studies demonstrated that systemic administration of A-841720 in SNL animals significantly reduced evoked firing in spinal wide dynamic range neurons. Significant motor side effects were observed at analgesic doses and A-841720 also impaired cognitive function in the Y-maze and the Water Maze tests. CONCLUSIONS AND IMPLICATIONS: The analgesic effects of a selective mGluR1 receptor antagonist are associated with motor and cognitive side effects. The lack of separation between efficacy and side effects in pre-clinical models indicates that mGluR1 antagonism may not provide an adequate therapeutic window for the development of such antagonists as novel analgesic agents in humans.

Hydroxamic acid inhibitors of 5-lipoxygenase: quantitative structure-activity relationships.

An evaluation of the quantitative structure-activity relationships (QSAR) for more than 100 hydroxamic acids revealed that the primary physicochemical feature influencing the in vitro 5-lipoxygenase inhibitory potencies of these compounds is the hydrophobicity of the molecule. A significant correlation was observed between the octanol-water partition coefficient of the substituent attached to the carbonyl of the hydroxamate and in vitro inhibitory activity. This correlation held for hydroxamic acids of diverse structure and with potencies spanning 4 orders of magnitude. Although the hydrophobicity may be packaged in a variety of structural ways and still correlate with potency, the QSAR study revealed two major exceptions. Specifically, the hydrophobicity of portions of compounds in the immediate vicinity of the hydroxamic acid functionality does not appear to contribute to increased inhibition and the hydrophobicity of fragments beyond approximately 12 A from the hydroxamate do not influence potency. The QSAR study also demonstrated that inhibitory activity was enhanced when there was an alkyl group on the hydroxamate nitrogen, when electron-withdrawing substituents were present and when the hydroxamate was conjugated to an aromatic system. These observations provide a simple description of the lipoxygenase-hydroxamic acid binding site.

Structure-activity analysis of a class of orally active hydroxamic acid inhibitors of leukotriene biosynthesis.

The nature of the carbonyl and nitrogen substituents of hydroxamic acids has a major influence on the biological profile of these compounds. Hydroxamates with small groups such as methyl appended to the carbonyl and relatively large nitrogen substituents generally have longer duration in vivo, produce greater plasma concentrations, and often are more potent inhibitors of in vivo leukotriene biosynthesis than hydroxamic acids with the opposite arrangement. The structure-activity relationships that describe in vitro 5-lipoxygenase inhibitory activity and in vivo leukotriene biosynthesis inhibitory potency for a group of these hydroxamic acids were investigated. While most of the compounds examined were potent in vitro inhibitors of 5-lipoxygenase, their in vivo potencies varied widely. This discrepancy was usually attributable to differences in bioavailability. Substitution patterns are described that produce potent, orally active inhibitors of leukotriene biosynthesis.

(R)-(+)-N-[3-[5-[(4-fluorophenyl)methyl]-2-thienyl]-1-methyl- 2-propynyl]-N-hydroxyurea (ABT-761), a second-generation 5-lipoxygenase inhibitor.

Structure-activity optimization of inhibitory potency and duration of action of N-hydroxyurea containing 5-lipoxygenase inhibitors was conducted. The lipophilic heteroaryl template and the link group connecting the template to the N-hydroxyurea pharmacophore were modified. Inhibition of 5-lipoxygenase was evaluated in vitro in a human whole blood assay. An in vitro assay using liver microsomes from monkey was used to evaluate congeners for comparative rates of glucuronidation. (3-Heteroaryl-1-methyl-2-propynyl)-N-hydroxyureas were found to be more resistant to in vitro glucuronidation. The promising inhibitor N-[3-[5-(4-fluorophenoxy)-2-furyl]-1-methyl-2-propynyl]-N- hydroxyurea (6) was found to have stereoselective glucuronidation in monkey and man. The R enantiomer 7 provided longer duration of inhibition as evaluated by an ex vivo whole blood assay. Further optimization of the lipophilic template led to the discovery of (R)-(+)-N-[3-[5-[(4-fluorophenyl)methyl]-2- thienyl]-1-methyl-2-propynyl]-N-hydroxyurea (11) with more effective and prolonged inhibition of leukotriene biosynthesis than zileuton (1) and 7 in monkey and man. The optimized 5-lipoxygenase inhibitor 11 was selected for development as an investigational drug for leukotriene-mediated disorders.

Discovery of 4-amino-5-(3-bromophenyl)-7-(6-morpholino-pyridin-3-yl)pyrido[2,3-d]pyrimidine, an orally active, non-nucleoside adenosine kinase inhibitor..

Adenosine (ADO) is an endogenous homeostatic inhibitory neuromodulator that reduces cellular excitability at sites of tissue injury and inflammation. Inhibition of adenosine kinase (AK), the primary metabolic enzyme for ADO, selectively increases ADO concentrations at sites of tissue trauma and enhances the analgesic and antiinflammatory actions of ADO. Optimization of the high-throughput screening lead, 4-amino-7-aryl-substituted pteridine (5) (AK IC(50) = 440 nM), led to the identification of compound 21 (4-amino-5-(3-bromophenyl)-7-(6-morpholino-pyridin-3-yl)pyrido [2,3-d]pyrimidine, ABT-702), a novel, potent (AK IC(50) = 1.7 nM) non-nucleoside AK inhibitor with oral activity in animal models of pain and inflammation.

Heteroarylmethoxyphenylalkoxyiminoalkylcarboxylic acids as leukotriene biosynthesis inhibitors.

A novel series of heteroarylmethoxyphenylalkoxyiminoalkylcarboxylic acids was studied as leukotriene biosynthesis inhibitors. A hypothesis of structure-activity optimization by insertion of an oxime moiety was investigated using REV-5901 as a starting point. A systematic structure-activity optimization showed that the spatial arrangement and stereochemistry of the oxime insertion unit proved to be important for inhibitory activity. The promising lead, S-(E)-11, inhibited LTB(4) biosynthesis in the intact human neutrophil with IC(50) of 8 nM and had superior oral activity in vivo, in a rat pleurisy model (ED(50) = 0.14 mg/kg) and rat anaphylaxis model (ED(50) = 0.13 mg/kg). In a model of lung inflammation, S-(E)-11 blocked LTE(4) biosynthesis (ED(50) of 0.1 mg/kg) and eosinophil influx (ED(50) of 0.2 mg/kg). S-(E)-11 (A-93178) was selected for further preclinical evaluation.

Symmetrical bis(heteroarylmethoxyphenyl)alkylcarboxylic acids as inhibitors of leukotriene biosynthesis.

Symmetrical bis(quinolylmethoxyphenyl)alkylcarboxylic acids were investigated as inhibitors of leukotriene biosynthesis and 4, 4-bis(4-(2-quinolylmethoxy)phenyl)pentanoic acid sodium salt (47.Na) met our design parameters for a drug candidate (ABT-080). This compound was readily synthesized in three steps from commercially available diphenolic acid. Against intact human neutrophils, 47.Na inhibited ionophore-stimulated LTB(4) formation with an IC(50) = 20 nM. In zymosan-stimulated mouse peritoneal macrophages producing both LTC(4) and PGE(2), 47.Na showed 9000-fold selectivity for inhibition of LTC(4) (IC(50) = 0.16 nM) over PGE(2) (IC(50) = 1500 nM). Preliminary pharmacokinetic evaluation in rat and cynomolgus monkey demonstrated good oral bioavailability and elimination half-lives of 9 and 5 h, respectively. Pharmacological evaluation of leukotriene inhibition with oral dosing was demonstrated in a rat pleural inflammation model (ED(50) = 3 mg/kg) and a rat peritoneal passive anaphylaxis model (LTB(4), ED(50) = 2.5 mg/kg; LTE(4), ED(50) = 1.0 mg/kg). In a model of airway constriction induced by antigen challenge in actively sensitized guinea pigs, 47.Na dosed orally blocked bronchoconstriction with an ED(50) = 0.4 mg/kg, the most potent activity we have observed for any leukotriene inhibitor in this model. The mode of inhibitory action of 47.Na occurs at the stage of 5-lipoxygenase biosynthesis as it blocks both leukotriene pathways leading to LTB(4) and LTC(4) but not PGH(2) biosynthesis. However, 47.Na does not inhibit 5-lipoxygenase catalysis in a broken cell enzyme assay; therefore it is likely that 47.Na acts as a FLAP inhibitor.

Selective inhibition of ICAM-1 and E-selectin expression in human endothelial cells. 2. Aryl modifications of 4-(aryloxy)thieno[2,3-c]pyridines with fine-tuning at C-2 carbamides.

The elevated expression of cell adhesion molecules (CAMs) on the lumenal surface of vascular endothelial cells is a critical early event in the complex inflammatory process. The adhesive interactions of these CAMs that include E-selectin, ICAM-1, and VCAM-1 with their counter-receptors on leukocytes, such as integrins of the alpha(L)beta(2) family, result in migration of the leukocytes to the site of inflammation and cause tissue injury. Pharmaceutical agents that could suppress the induced expression of one or more of these cell adhesion molecules would provide a novel mechanism to attenuate the inflammatory responses associated with chronic inflammatory diseases. A-205804 (1), a potent and selective inhibitor of the induced expression of E-selectin and ICAM-1 over VCAM-1, was further modified with emphasis at the C-4 and C-2 positions to identify a more potent drug candidate with a good pharmacokinetic profile and physical properties. Replacement of the C-4 sulfur linkage in 1 with an oxygen atom eliminated one of the two major metabolites for this lead molecule. The para-position of the 4-phenoxy group of the thieno[2,3-c]pyridine lead is found to be very critical for a higher in vitro potency and selectivity of E-selectin and ICAM-1 over VCAM-1 expression. This position is presumably close to the solvent-accessible region of the target protein-inhibitor complex. An attempt to install a water-solubilizing group at the para-position of the phenoxy group to increase the aqueous solubility of this lead series through various linkages failed to provide an ideal inhibitor. Only small substituents such as fluorine are tolerated at the meta- and ortho-positions of the 4-phenoxy to retain a good in vitro potency. Bromo, trifluoromethyl, pyrazol-1-yl, and imidazol-1-yl are among the better substituents at the para-position. With fine-tuning at the C-2 position we discovered a series of very potent (IC(50) < 5 nM for ICAM-1) and selective (>200-fold vs VCAM-1) inhibitors with a good pharmacokinetic profile. Demonstrated efficacy in a rat rheumatoid arthritis model and in a mice asthma model with selected compounds is also reported.

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.

Discovery of inhibitors of cell adhesion molecule expression in human endothelial cells. 1. Selective inhibition of ICAM-1 and E-selectin expression.

A critical early event in the inflammatory cascade is the induced expression of cell adhesion molecules on the lumenal surface of vascular endothelial cells. These adhesion molecules include E-selectin, ICAM-1, and VCAM-1, which serve to recruit circulating leukocytes to the site of the inflammation. These adhesive interactions allow the leukocytes to firmly adhere to and cross the vascular endothelium and migrate to the site of tissue injury. Pharmaceutical agents which would prevent the induced expression of one or more of the cell adhesion molecules on the endothelium might be expected to provide a novel mechanism to attenuate the inflammatory responses associated with chronic inflammatory diseases. A thieno[2,3-d]pyrimidine, A-155918, was identified from a whole-cell high-throughput assay for compounds which inhibited the tumor necrosis factor-alpha (TNFalpha)-induced expression of E-selectin, ICAM-1, or VCAM-1 on human vascular endothelial cells. Traditional medicinal chemistry methods were applied to this low-micromolar inhibitor, resulting in the 2,4-disubstituted thieno[2,3-c]pyridine A-205804, a potent and selective lead inhibitor of E-selectin and ICAM-1 expression (IC(50) = 20 and 25 nM, respectively). The relative position of the nitrogen atom in the thienopyridine isomer was shown to be critical for activity, as was a small amide 2-substituent.

Preclinical and clinical activity of zileuton and A-78773.

The importance of leukotrienes as mediators of inflammation and bronchoconstriction was examined with two recently described 5-lipoxygenase inhibitors, zileuton and A-78773. Preclinical evaluation of these two molecules indicates that they are potent, selective, direct, reversible inhibitors of 5-lipoxygenase with activity in a variety of purified cells and in more complex biological systems such as whole blood, lung fragments, and tracheal tissues. In various animals models of inflammation and allergy, the molecules inhibited edema, inflammatory cell influx, and bronchospasm. These observations are consistent with the recent clinical success of zileuton in treating asthma and inflammatory bowel disease. In all preclinical systems tested thus far, A-78773 is more potent and longer acting than zileuton, indicating that the molecule could be even more effective in the clinic than zileuton and that both molecules are useful tools in defining the role of leukotrienes in preclinical and clinical settings.

Stereoselective metabolism of the 5-lipoxygenase inhibitor A-78773.

Based on the knowledge that glucuronidation was a major route of metabolism of the N-hydroxyurea class of 5-lipoxygenase inhibitors, a simple in vitro glucuronidation assay was established using liver microsomes from various species, including man. Compounds that were potent inhibitors of 5-LO and showed a reduced metabolic liability in vitro were then characterized more extensively in experimental animals. This prudent usage of in vitro glucuronidation proved to be highly efficient and was indispensable in the identification of A-78773, a potent new long-acting 5-LO inhibitor. Further studies with liver microsomes revealed that glucuronidation of A-78773 was stereoselective and that the R(+) enantiomer was considerably more resistant to conjugation than the S(-) stereoisomer. Pharmacokinetic studies in experimental animals and humans confirmed the greater metabolic stability of the R(+) enantiomer. A single 400-mg oral dose of A-78773 inhibited ex vivo leukotriene biosynthesis for more than 24 hours. Since 78% of the drug plasma AUC following A-78773 administration was accounted for by the R(+) enantiomer, it is reasonable to assume that the majority of the leukotriene inhibition caused by the racemate is attributable to the R(+) enantiomer, A-79175, particularly at the later times. The equivalent 5-lipoxygenase inhibitory potency coupled with the superior pharmacokinetic profile of the R(+) enantiomer, A79175, compared to the S(-) enantiomer, A-79176, indicate that the development of this compound may be preferable to the racemate A-78773.

Improving the in vivo duration of 5-lipoxygenase inhibitors: application of an in vitro glucuronosyltransferase assay.

An in vitro glucuronidation assay was used to optimize a series of N-hydroxyurea-containing 5-lipoxygenase inhibitors for metabolic stability. The glucuronidation of these compounds in cynomolgus monkey microsomes followed Michaelis-Menten kinetics allowing calculation of V(max) and K(M). The V(max) values ranged from 0.02 to 7.9 nmol/min/mg microsomal protein, a 400-fold difference, whereas K(M) ranged from 204 to 2500 microM, only a 12-fold difference. In vitro intrinsic clearance values (CL(int) were calculated for 18 compounds tested in the kinetic assay and compared with the in vivo plasma clearance (CL(p)) calculated from intravenous studies done in cynomolgus monkeys. These initial results suggested a relationship between the in vitro CL(int) and in vivo duration as defined by CL(p). A more rapid in vitro assay was developed in a 96-well format using a single concentration of substrate (100 microM) from which a glucuronidation rate was calculated. The results from this assay for 40 compounds correlated with in vivo plasma clearance (r = 0.57). This more efficient assay was used to test more than 100 compounds and develop structure-metabolism relationships based on metabolic stability and improved duration. The culmination of this effort contributed to the discovery of ABT-761, a 5-lipoxygenase inhibitor with in vivo duration in monkey improved 40-fold over thefirst generation inhibitor. Further studies performed in human liver microsomes demonstrated a similar trend that was corroborated by the 8-fold increase in duration after oral dosing in humans observed with ABT-761.

A-78773: a selective, potent 5-lipoxygenase inhibitor.

The potency and selectivity of A-78773, a newly discovered 5-lipoxygenase inhibitor, were examined. The compound was significantly more potent than zileuton in inhibiting leukotriene formation in cell free lysates and in isolated human neutrophils. A-78773 inhibited a RBL cell lysate 5-lipoxygenase at concentrations 2 orders of magnitude lower than those required to inhibit rabbit reticulocyte 15-lipoxygenase or human platelet 12-lipoxygenase. The compound was also a potent, long lasting, orally active inhibitor of leukotriene formation ex vivo in dogs and in vivo in the rat. In experiments where leukotriene formation was completely inhibited, no increase in eicosanoids from other pathways was observed. A-78773 should prove to be a valuable clinical tool in treating leukotriene mediated diseases.

Optimization of the potency and duration of action of N-hydroxyurea 5-lipoxygenase inhibitors.

As in vitro glucuronidation assay and several biochemical assays were utilized to discover potent new N-hydroxyurea-containing 5-lipoxygenase inhibitors with long durations of action. The best of these, A-78773, is a racemic mixture of two enantiomers. These enantiomers were purified and the R(+)-enantiomer A-79175 was found to be superior to the S(-)-enantiomer with respect to in vitro metabolism and duration of action in the monkey. A-79175 was a potent selective inhibitor of 5-hydroxyeicosatetraenoic acid formation in rat basophilic leukemic homogenates (IC50 = 54 nM) and of calcium ionophore-induced leukotriene B4 (LTB4) formation in purified human polymorphonuclear leukocytes (IC50 = 25 nM) and human whole blood (IC50 = 80 nM). The compound inhibited LT formation in the rat with oral ED50s of 1 to 2 mg/kg. It also was a potent inhibitor of edema and inflammatory cell influx in rats and mice. A-79175 was resistant to glucuronidation and had an elimination half-life of nearly 9 hr in cynomolgus monkeys. A-79175 inhibited ex vivo LTB4 formation by monkeys for extended periods. A single 0.5-mg/kg oral dose gave > 50% inhibition of calcium ionophore-induced LTB4 formation ex vivo for 12 hr. A good correlation was found between the elimination half-life for A-78773 and its enantiomers in cynomolgus monkeys and humans. These data indicate that A-79175 is a promising long-acting agent that should be useful to delineate the importance of LTs in animal and human studies.

Structure-activity studies of 5-substituted pyridopyrimidines as adenosine kinase inhibitors.

The synthesis and SAR of a novel series of non-nucleoside pyridopyrimidine inhibitors of the enzyme adenosine kinase (AK) are described. It was found that pyridopyrimidines with a broad range of medium and large non-polar substituents at the 5-position potently inhibited AK activity. A narrower range of analogues was capable of potently inhibiting adenosine phosphorylation in intact cells indicating an enhanced ability of these analogues to penetrate cell membranes. Potent AK inhibitors were found to effectively reduce nociception in animal models of thermal hyperalgesia and persistent pain.

Pyridopyrimidine analogues as novel adenosine kinase inhibitors.

A novel series of pyridopyrimidine analogues 9 was identified as potent adenosine kinase inhibitors based on the SAR and computational studies. Substitution of the C7 position of the pyridopyrimidino core with C2' substituted pyridino moiety increased the in vivo potency and enhanced oral bioavailability of these adenosine kinase inhibitors.