Neuroprotection by small molecule activators of the nerve growth factor receptor

There is a great deal of interest in neurotrophin therapy to prevent neuronal degeneration. However, the blood-brain barrier presents a major hurdle in the use of peptide therapeutics. The goal of this study was to identify small molecule, cell-permeable nerve growth factor (NGF) activators. Combinatorial libraries of asterriquinones (>300) and mono-indolyl-quinones (>60) were screened using a 96-well enzyme-linked immunosorbent assay that detects phosphorylated TrkA, the NGF receptor. The libraries were also screened for dose-dependent cytotoxicity. From these screens, we generated quantitative structure-activity relationship models for activity and toxicity, and then we selected two compounds, 2-(6-chloro-1H-indol-3-yl)-5-(2-cyclopropyl-1H-indol-3-yl)-3,6-dihydroxy-[1,4]benzoquinone (1H5) and 2,5-dimethoxy-3-(7-fluoro-1H-indol-3-yl)-[1,4]-benzoquinone (5E5), for further study based on high activity and low toxicity. Compound 1H5 (30 microM) is an asterriquinone that is a moderate TrkA activator (50% the activity of 100 ng/ml NGF), and it shows little toxicity at concentrations up to 100 microM. 1H5 can protect differentiated PC12 neurons from apoptotic cell death induced by NGF withdrawal. Compound 5E5 (30 microM) is a mono-indolyl-quinone that is a very strong activator of TrkA (>200% the activity of 100 ng/ml NGF), and it is nontoxic at concentrations up to 10 microM. Activation of TrkA can be detected at 1 microM 5E5, and 3 to 10 microM 5E5 activates TrkA and extracellular signal-regulated kinase as strongly as a maximal dose of NGF (100 ng/ml). A combination of a low dose of 5E5 (1 microM) with a submaximal dose of NGF (10 ng/ml) promotes neuronal differentiation of PC12 cells. These compounds represent a new class of TrkA activators that could have potential utility in the treatment of neurodegenerative diseases.