ABT-702 (4-amino-5-(3-bromophenyl)-7-(6-morpholino-pyridin-3-yl)pyrido[2,3-d]pyrimidine), a novel orally effective adenosine kinase inhibitor with analgesic and anti-inflammatory properties: I. In vitro characterization and acute antinociceptive effects in the mouse

Adenosine (ADO) is an inhibitory neuromodulator that can increase nocicepti ve thresholds in response to noxious stimulation. Inhibition of the ADO-met abolizing enzyme adenosine kinase (AK) increases extracellular ADO concentr ations at sites of tissue trauma and AK inhibitors may have therapeutic pot ential as analgesic and anti-inflammatory agents. ABT-702 is a novel and po tent (IC50 = 1.7 nM) non-nucleoside AK inhibitor that has several orders of magnitude selectivity over other sites of ADO interaction (A(1), A(2A), A( 3) receptors, ADO transporter, and ADO deaminase). ABT-702 was 1300- to 770 0-fold selective for AK compared with a number of other neurotransmitter an d peptide receptors, ion channel proteins, neurotransmitter/nucleoside reup take sites, and enzymes, including cycloxygenases-1 and -2. ABT-702 was equ ipotent (IC50 = 1.5 +/- 0.3 nM) in inhibiting native human AK (placenta), t wo human recombinant isoforms (AK(long) and AK(short)), and AK from monkey, dog, rat, and mouse brain. Kinetic studies revealed that AK inhibition by ABT-702 was competitive with respect to ADO and noncompetitive with respect to MgATP(2-). AK inhibition by ABT-702 was demonstrated to be reversible a fter 4 h of dialysis. ABT-702 is orally active and fully efficacious in red ucing acute somatic nociception (ED50 = 8 mu mol/kg i.p.; 65 mu mol/kg p.o. ) in the mouse hot-plate assay. ABT-702 also dose dependently reduced nocic eption in the phenyl-p-quinone-induced abdominal constriction assay. The an tinociceptive effects of ABT-702 in the hotplate assay were blocked by the nonselective ADO receptor antagonist theophylline, and by the A(1)-selectiv e antagonist cyclopentyltheophylline (10 mg/kg i.p.), but not by a peripher ally selective ADO receptor antagonist 8-(p-sulfophenyl)-theophylline (50 m g/kg i.p.), by the A(2A)-selective antagonist 3,7-dimethyl-1- propargylxant hine (1 mg/kg i.p.) or the opioid antagonist naloxone (5 mg/kg i.p.). Thus, ABT-702 is a novel and potent non-nucleoside AK inhibitor that effectively reduces acute thermal nociception in the mouse by a nonopioid, non-nonster oidal anti-inflammatory drug, ADO A(1) receptor-mediated mechanism.