A Series of d-Amino Acid Oxidase Inhibitors Specifically Prevents and Reverses Formalin-Induced Tonic Pain in Rats

Abstract

We have found that mutation of D-amino acid oxidase (DAO) diminished formalin-induced tonic pain. The present research further studied the analgesic effects of a series of DAO inhibitors in this model. 5-Chlorobenzo[d]isoxazol-3-ol (CBIO), 4H-thieno[3,2-b]pyrrole-5-carboxylic acid (compound 8), 5-methylpyrazole-3-carboxylic acid (AS057278), sodium benzoate, and 4-nitro-3-pyrazole carboxylic acid (NPCA) inhibited rat spinal cord-derived DAO activity in a concentration-dependent manner, with maximal inhibition of 100% and potency rank of CBIO Ͼ compound 8 Ͼ AS057278 Ͼ sodium benzoate Ͼ NPCA. In rats, intrathecal injections of CBIO, compound 8, AS057278, and sodium benzoate but not NPCA specifically prevented formalin-induced tonic pain but not acute nociception, with the same potency order as in the DAO activity assay. The highly potent analgesia of DAO inhibitors was evidenced by CBIO, which prevented 50% pain at 0.06 g, approximately 5-fold the potency of morphine. CBIO given after formalin challenge also reversed the established pain state to the same degree as prevention. The antihyperalgesic potencies of these DAO inhibitors were highly correlated to their inhibitions of spinal DAO activity. Maximum inhibition of pain by these compounds was approximately 60%, comparable with that of the N-methyl-D-aspartic acid receptor antagonist (ϩ)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), suggesting that a larger portion of formalin-induced tonic pain is "DAO-sensitive," whereas the remaining 40% of tonic pain and acute nociception is "DAO-insensitive." These findings, combined with our previous DAO gene mutation and induction results, indicate spinal DAO mediates both induction and maintenance of formalin-induced tonic pain and further validate spinal DAO as a novel and efficacious target molecule for the treatment of chronic pain.

Pharmacokinetics of Oral d-Serine in d-Amino Acid Oxidase Knockout Mice

“…6-Chlorobenzo[d]isoxazol-3-ol (CBIO) is a potent competitive inhibitor of DAAO and has a K i value of 100 nM for porcine DAAO (Ferraris et al, 2008). Although its toxicity profile has not been fully established, CBIO has been tested in both mice and rats as treatment for pain and has resulted in no apparent toxicity (Gong et al, 2011;Lu et al, 2012). In another study, oral coadministration of CBIO with D-serine enhanced oral bioavailability of D-serine and its levels in the prefrontal cortex (Ferraris et al, 2008).…”

Pharmacodynamic Effects of a d-Amino Acid Oxidase Inhibitor Indicate a Spinal Site of Action in Rat Models of Neuropathic Pain

“…These observations, although counterintuitive to an understanding of the efficacy of NMDA receptor antagonists in models of pain, have also been described using sodium benzoate in neuropathic models (Zhao et al, 2010). In addition, others (Zhao et al, 2010;Gong et al, 2011Gong et al, , 2012Chen et al, 2012) reported on the effects of DAAO inhibition in a formalin model of tonic pain.…”

“…discovery program (Fang et al, 2007(Fang et al, , 2011Dorsey et al, 2008;Heffernan et al, 2009). In similar work, Gong et al (2011) described the close correlation between in vitro DAAO inhibitory potencies and intrathecal doses required for efficacy of structurally diverse DAAO inhibitors on formalin-induced tonic pain consistent with a downregulation of spinal DAAO blocked formalin-induced tonic pain (Chen et al, 2012). An alternative to the NMDA receptor mechanism, still consistent with DAAO inhibition, is the possibility that D-serine itself might be acting via a non-NMDA receptor target.…”

“…The molecular mechanism of DAAO inhibition in neuropathic pain models may relate to the activation of NMDA receptors via endogenously produced and released D-serine. Although an initial study reporting enhanced formalin-induced tonic pain-related behavior in DAAO mutant mice suggested that the activity of DAAO may contribute to the transmission of ongoing pain via central sensitization and D-serine (Wake et al, 2001), further investigations (Zhao, et al 2008(Zhao, et al , 2010Gong et al, 2011) have demonstrated efficacy of DAAO inhibitors in models of tonic and chronic pain. The latter may seem incongruous with the efficacy of NMDA receptor antagonists in pain models (Fisher et al, 2000) and the role of NMDA receptor activation in central sensitization (Bennett, 2000).…”

D-Serine in Neuropsychiatric Disorders: New Advances

“…Benzoic acid and its salts, including sodium benzoate, exist in many plants and are widely used as food preservatives [190]. Sodium benzoate also acts as a DAAO inhibitor and has favorable effects in NMDAR-based models such as pain relief [191,192] and glial cell death [193]. The potential molecular mechanisms of action of sodium benzoate remain to be determined.…”