Analgesic and anti-inflammatory effects of A-286501, a novel orally active adenosine kinase inhibitor

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. N7-((1′ R,2′ S,3′ R,4′ S)-2′,3′-dihydroxy-4′-amino-cyclopentyl)-4-amino-5-bromo-pyrrolo[2,3-a]pyrimidine (A-286501) is a novel and potent ( IC 50=0.47 nM) carbocyclic nucleoside AK inhibitor that has no significant activity ( IC 50>100 μM) at other sites of ADO interaction (A 1, A 2A, A 3 receptors, ADO transporter, and ADO deaminase) or other (IC 50 values>10 μM) neurotransmitter and peptide receptors, ion channel proteins, neurotransmitter reuptake sites and enzymes, including cyclooxygenases-1 and -2. A-286501 showed equivalent potency to inhibit AK from several mammalian species and kinetic studies revealed that A-286501 was a reversible and competitive inhibitor with respect to ADO and non-competitive with respect to MgATP 2−. A-286501 was orally effective to reduce nociception in animal models of acute (thermal), inflammatory (formalin and carrageenan), and neuropathic (L5/L6 nerve ligation and streptozotocin-induced diabetic) pain. A-286501 was particularly potent ( ED 50=1 μmol/ kg , p.o.) to reduce carrageenan-induced inflammatory thermal hyperalgesia as compared to its analgesic actions in other pain models (acute and neuropathic) and its ability to alter hemodynamic function and motor performance. A-286501 was also effective to reduce carrageenan-induced paw edema and myeloperoxidase activity, a measure of neutrophil influx ( ED 50=10 μmol/ kg , p.o.), in the injured paw. The anti-nociceptive effects of A-286501 in the L5/L6 nerve injury model of neuropathic pain ( ED 50=20 μmol/ kg , p.o.) were not blocked by the opioid antagonist naloxone, but were blocked by the ADO receptor antagonist, theophylline. Following repeated administration, A-286501 showed less potential to produce tolerance as compared to morphine. Thus, A-286501 is a structurally novel AK inhibitor that effectively attenuates nociception by a non-opioid, non-non-steroidal anti-inflammatory drug ADO, receptor mediated mechanism.