Novel Nucleoside Reverse Transcriptase Inhibitor Derivatives for Chronic Neuropathic Pain

One in three Americans suffer from chronic pain. The economic burden of chronic pain in the US is significant and estimated to surpass 0.6 trillion dollars per year(Dzau and Pizzo 2014). Particularly problematic is neuropathic pain, a complex chronic state arising from injured and dysfunctional nerve tissue that is poorly responsive to analgesic drugs. A promising target is the pro-inflammatory cytokine IL1-â. Exogenous administration of IL1-â to brain, spinal cord or peripheral nerves lead to the development of thermal/mechanical hyperalgesia and allodynia, while IL-1-â inhibitors inhibit these behavioral signs of neuropathic pain. Moreover, increased levels of endogenous IL1-â are seen in the spinal cord and brain in neuropathic pain states. Caspase-1 activation is critical for cleavage and activation of pro-IL1-â and IL1-â release(Ren and Torres 2009). In turn, the activation of Caspase-1 is mediated by assembly of the multi-protein complex NACHT, LRR and PYD domainscontaining protein 3 (NLRP3) inflammasome (Figure 1). Assembly and activation of the NLRP3 inflammasome is triggered by a number of stimuli, including viruses, bacteria, toxins, cholesterol crystals, ATP-induced activation of a key pain receptor, the purinoceptor P2X7, which is upstream of the NLRP3 inflammasome(Pelegrin and Surprenant 2006, Pelegrin, Barroso-Gutierrez et al. 2008, Skaper, Debetto et al. 2010). P2X7 is an essential component of mechanisms of inflammatory and neuropathic pain models, since P2X7 null mice and mice treated with P2X7 antagonists do not develop neuropathic pain behavior after nerve injury(Chessell, Hatcher et al. 2005). Based on our recent findings that nucleoside reverse transcriptase inhibitors (NRTIs), used in millions of individuals worldwide for the treatment of HIV, inhibit NLRP3 mediated caspase-1 activation via blockade of P2X7 receptor, our overall hypothesis is that NTRIs will effectively treat neuropathic pain(Fowler, Gelfand et al. 2014). However, widely used NRTIs such as d4T (Stavudine/Zerit), azidothymidine (AZT; 3'-azido-2',3'-dideoxythymidine; Retrovir), lamivudine (3TC; 2’3’ dideoxycytidine; Zeffix) and abacavir (ABC; a di-deoxyguanosine analog; Ziagen) inhibit reverse transcriptase and mitochondrial DNA polymerases, leading to peripheral neuropathy and mitochondrial dysfunction(Lewis, Day et al. 2003, Lewis, Kohler et al. 2006). To molecularly circumvent this problem, we synthesized novel chemical derivatives of NRTIs that cannot be phosphorylated intracellularly and do not inhibit reverse transcriptase: 5’ O-methylmodified version of NRTIs (me-d4T, me-AZT, me-3TC, me-ABC). Our preliminary data indicate that these novel NRTI derivatives block P2X7-mediated inhibition of the inflammasome by a mechanism distinct from RT inhibition. In this proposal, I aim to advance these new compounds as treatment strategies for chronic inflammatory and neuropathic pain.Specific Aim 1. Test the hypothesis that 5’ O-methyl-modified NRTIs will inhibit the NLRP3 inflammasome and the P2X7 receptor in microglial, macrophages and neuronal tissues. Using cell culture models (including dissociated lumbar dorsal root ganglion (DRG) neurons and microglia and immortalized neuronal cell lines) we will study the ability of our novel NRTI derivatives to inhibit inflammasome activation triggered by a variety of stimuli, including AluRNA, LPS/ATP inhibition. Western blotting, ELISA and quantitative PCR will be used to visualize and quantify the various NLRP3 inflammasome downstream signaling components (Caspase-1, pIRAK-4, IL-1â and IL-18). Specific Aim 2. Test the hypothesis that 5’ O-methyl-modified NRTIs will prevent and/or reverse hyperalgesia in models of inflammatory and neuropathic pain. We will assess the antihyperalgesic activity of 5’O-Me modifications of d4T, AZT, 3CT and ABC compared to placebo and parent non-modified NRTIs in the following rodent models: tibial/peroneal and sural/peroneal nerve transection. We will correlate efficacy with pharmacokinetics by testing drug absorption, distribution, metabolism, elimination and toxicity in blood and vitreous using state-of-the-art techniques (Mass spectrometry and HPLC of serum and vitreous fluid). Regarding its safety profile, special attention will be given to commonly known adverse effects of NRTIs such as genotoxicity, lactic acidosis, hepatic steatosis, and body fat redistribution.