Alzheimer's disease (AD) is characterized by the deposition of various amyloid β (Aβ) aggregates forming amyloid in the brain. Evidence from a variety of studies has established that the oligomeric species of Aβ (AβO) carries the greatest toxicity, triggering a variety of downstream effects resulting in neurotoxicity and cognitive deficits 
. A major impediment to the development of effective anti-Aβ compounds for AD therapy is that essentially 100% of large-molecule drugs and >98% of small-molecule drugs fail to cross the blood-brain barrier (BBB) 
. Recently 
, we explored a series of compounds based on a highly rigid tricyclic fluorene ring that were developed as amyloid imaging agents 
. These compounds contain a tertiary amine electron-donating group attached to one aromatic ring and display excellent pharmacokinetics properties and brain bioavailability. In that work, we reported on the ability of two fluorene compounds to disrupt AβO assemblies and reduce Aβ toxicity 
. These compounds (K01-162 and K01-186) were identified based on their ability to block cell death secondary to intracellular AβO production. Both fluorene compounds bind and destabilize AβO, and are capable of penetrating the brain and reducing the cerebral amyloid burden in APP transgenic mice. Fluorenes therefore have a potential use in AD therapy by targeting AβO toxicity at both intraneuronal and extracellular sites 
In AD, accumulating evidence points to oxidative stress as the ultimate downstream component of Aβ-induced toxicity 
. For example, Aβ increases NMDA receptor activation, and one of the newer drugs for the treatment of AD (Memantine) targets NMDA receptors in order to block glutamate excitotoxicity. Among other pathways, over-stimulation of NMDA receptors activates phospholipase A, leading to elevated arachidonic acid levels, which in turn generates oxygen free radicals and further activation of phospholipases 
. Thus the excitotoxicity involves a feedback loop that ultimately leads to neuronal self-digestion via increased Ca2+
levels, protein breakdown, free radical formation and lipid peroxidation 
. As shown previously 
, the anti-amyloid fluorenes have antioxidant properties. Furthermore, because nitroxides such as the pyrroline species can cycle within a redox cascade via a relatively stable non-damaging N-oxyl (nitroxyl) radical intermediate 
, compounds carrying this moiety are likely to have the added potential for decreasing oxidative stress and attenuating the damage caused by reactive oxygen species.
In this study, we apply electron paramagnetic resonance (EPR) spectroscopy to a novel fluorene compound containing a pyrroline nitroxide. This spin-labeled fluorene (SLF) exerts similar potency in AβO disruption and protection against AβO-induced toxicity, while also having superior free radical scavenging compared to the model fluorene compounds. Furthermore, the nitroxide moiety provides an intrinsic reporter group that can be probed by EPR spectroscopy, which may provide a sensitive diagnostic tool for in vivo
detection of Aβ plaques in patients with AD 
. Thus, in addition to its potential as a novel bifunctional candidate to address AβO toxicity, the SLF compound may also help as a diagnostic and research tool in elucidating fluorene mechanism of action.