Synthesis, Optimization, and Evaluation of Thiophilic Peptidomimetics as Novel AD Therapies

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that threatens to reach epidemic proportions in the near future. Current AD therapies provide temporary symptomatic relief, but eventually become ineffective due to the presence of significant neurodegeneration. In this work, we evaluated two novel approaches utilizing thiophilic peptidomimetics for potential use as AD therapeutics. Calpain is a calcium dependent cysteine protease that becomes hyperactivated in the presence of excess intracellular calcium. Excessive calpain activity contributes to severe cellular damage and synaptic transmission impairment in Alzheimer's disease (AD). Three successive generations of novel epoxide peptiomimetic calpain inhibitors were synthesized and examined using multiple in vitro enzyme inhibition assays. Selected inhibitors were submitted for evaluation of efficacy and safety. Selected inhibitors were capable of restoring synaptic function in the form of synaptic function in the presence of Aβ in ex vivo experiments. Additionally, our inhibitors were also capable of ameliorating defects in contextual fear memory and reference memory in behavioral models using APP/PS1 transgenic AD mice. The inhibitors were well tolerated and possessed negligible in vivo toxicity following chronic administration in murine models. Nitric oxide (NO) is essential for normal physiological brain function. NO-mimetics have been shown to be neuroprotective and cognition enhancing in the past. Furoxans (1, 2, 5 oxadiazole-N-oxides) are thiol-activated NO-mimetics that have not hitherto been studied in the CNS. A library of furoxans was synthesized, and the reactivity of selected furoxans was examined by LC-MS/MS. Some furoxans were found to have procognitive and neuroprotective activity. This activity was attributed to an NO related mechanism of action. The novel observation of furoxan activity of potential therapeutic use in the CNS warrants further studies. In summary, epoxide incorporating molecules were found to be safe and effective in vivo and calpain inhibition is hypothesized as being a primary mechanism responsible for cognition enhancement. The novel approach of designing attenuated furoxans as thiophilic peptidomimetics for the CNS also gave promising results, as some furoxans were found to be neuroprotective and procognitive. Epoxide based calpain inhibitors and attenuated furoxans are exciting prospects in AD drug discovery and further preclinical investigation of both chemical classes is warranted.