Abstract

Adenosine (ADO) is an inhibitory neuromodulator that can increase nociceptive thresholds in response to noxious stimulation. Inhibition of the ADO-metabolizing enzyme adenosine kinase (AK) increases extracellular ADO concentrations at sites of tissue trauma and AK inhibitors may have therapeutic potential as analgesic and anti-inflammatory agents. ABT-702 is a novel and potent (IC₅₀ = 1.7 nM) non-nucleoside AK inhibitor that has several orders of magnitude selectivity over other sites of ADO interaction (A₁, A_2A, A₃ receptors, ADO transporter, and ADO deaminase). ABT-702 was 1300- to 7700-fold selective for AK compared with a number of other neurotransmitter and peptide receptors, ion channel proteins, neurotransmitter/nucleoside reuptake sites, and enzymes, including cycloxygenases-1 and -2. ABT-702 was equipotent (IC₅₀ = 1.5 ± 0.3 nM) in inhibiting native human AK (placenta), two 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 MgATP²⁻. AK inhibition by ABT-702 was demonstrated to be reversible after 4 h of dialysis. ABT-702 is orally active and fully efficacious in reducing acute somatic nociception (ED₅₀ = 8 μmol/kg i.p.; 65 μmol/kg p.o.) in the mouse hot-plate assay. ABT-702 also dose dependently reduced nociception in the phenyl-p-quinone-induced abdominal constriction assay. The antinociceptive effects of ABT-702 in the hot-plate assay were blocked by the nonselective ADO receptor antagonist theophylline, and by the A₁-selective antagonist cyclopentyltheophylline (10 mg/kg i.p.), but not by a peripherally selective ADO receptor antagonist 8-(p-sulfophenyl)-theophylline (50 mg/kg i.p.), by the A_2A-selective antagonist 3,7-dimethyl-1-propargylxanthine (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-nonsteroidal anti-inflammatory drug, ADO A₁ receptor-mediated mechanism.