Subpicomolar diphenyleneiodonium inhibits microglial NADPH oxidase with high specificity and shows great potential as a therapeutic agent for neurodegenerative diseases

Activation of microglial NADPH oxidase (NOX2) plays a critical role in mediating neuroinflammation, which is closely linked with the pathogenesis of a variety of neurodegenerative diseases, including Parkinson's disease (PD). The inhibition of NOX2‐generated superoxide has become an effective strategy for developing disease‐modifying therapies for PD. However, the lack of specific and potent NOX2 inhibitors has hampered the progress of this approach. Diphenyleneiodonium (DPI) is a widely used, long‐acting NOX2 inhibitor. However, due to its non‐specificity for NOX2 and high cytotoxicity at standard doses (µM), DPI has been precluded from human studies. In this study, using ultra‐low doses of DPI, we aimed to: (1) investigate whether these problems could be circumvented and (2) determine whether ultra‐low doses of DPI were able to preserve its utility as a potent NOX2 inhibitor. We found that DPI at subpicomolar concentrations (10^?14 and 10^?13 M) displays no toxicity in primary midbrain neuron‐glia cultures. More importantly, we observed that subpicomolar DPI inhibited phorbol myristate acetate (PMA)‐induced activation of NOX2. The same concentrations of DPI did not inhibit the activities of a series of flavoprotein‐containing enzymes. Furthermore, potent neuroprotective efficacy was demonstrated in a post‐treatment study. When subpicomolar DPI was added to neuron‐glia cultures pretreated with lipopolysaccharide, 1‐methyl‐4‐phenylpyridinium or rotenone, it potently protected the dopaminergic neurons. In summary, DPI's unique combination of high specificity toward NOX2, low cytotoxicity and potent neuroprotective efficacy in post‐treatment regimens suggests that subpicomolar DPI may be an ideal candidate for further animal studies and potential clinical trials. GLIA 2014;62:2034–2043