Our pharmaceutical industry has made massive strides for patients suffering from Cancer, Cardiovascular disease, and Type-II diabetes with some medicine outright curing subtypes of these diseases. Unfortunately, this has not been true for neurodegenerative disorders. As our population grows older, especially in the US, Europe, and China, the incidence of diseases like Parkinson’s and Alzheimer’s is rapidly increasing; 6.7M people each year in the US are diagnosed with Alzheimer’s alone. Due to our poor understanding of the molecular and cellular mechanisms underlying neurodegeneration, pharmaceutical drug development is largely a graveyard for these devastating diseases.

Recent studies have associated high iron levels in the substantia nigra with Parkinson’s and high iron levels in the deep gray matter of neocortical regions with Alzheimer’s diseases. At Arpeggio, we study ferroptosis: a form of cell death due to a buildup of toxic free-radicals fueled by high levels of labile iron.  So it’s no surprise to us that recent epidemiological studies have shown that repression of SLC7A11, a transporter that controls the amount of cellular glutathione and thus resistance to ferroptosis, is significantly repressed in patients with Parkinson’s. Between high levels of iron and a poor ferroptosis defense system, it is Arpeggio’s hypothesis that much of neuronal cell death in these neurodegenerative indications occurs through toxic lipid peroxidation and ferroptosis.  

Within the field of Parkinson’s research, groups have been able to restore dopamine function in the substantia nigra of mice afflicted with Parkinson’s with administration of Ferrostatin-1: a lipophilic antioxidant that prevents free-radical buildup and ultimately ferroptosis (Figure 1). Fascinatingly, this mechanism was also proved out by a separate group of researchers using a different model of Parkinson’s. Following administration of a ferroptosis blocker called Triacsin C, these researchers were also able to restore mobility and dopamine function in a Parkinson’s mouse model (Figure 1). Unfortunately due to extremely poor blood-brain-barrier penetrance of both Ferrostatin-1 and Triacsin C, administration of these compounds via intracerebroventricular injection was required; such a route of administration is not suitable for human clinical studies. As well, the potency of these compounds is limited to the 100nM range, making for limited potential efficacy.

Ferroptosis was only discovered in 2012, but it has seen a meteoric rise in scientific interest with over 5,000 scientific publications in 2023. We now know many proteins like ACSL4, SEC24B, NRF2, or FSP1 can be inhibited or activated to prevent ferroptosis. Unfortunately, many of these proteins are poor pharmaceutical drug targets owing to a protein structure that is extremely disordered. To identify small molecules that prevent ferroptosis, we turned to our GRETA^TM HTS screening technology to find selective blockers of the ferroptosis gene expression signature (Figure 2).

To identify new ferroptosis-blocking small molecules, we screened two libraries in microglial cells using the GRETA^TM HTS technology: a library of about 2,500 FDA-approved small molecules and a library of more than 11,000 cysteine-focused covalent small molecules. Following an initial screen of 10uM incubated for 24 hours, we identified that Ferrostatin-1 and Troglitazone (both known ferroptosis blockers) prevent ferroptosis. Fascinatingly, we also discovered that Entacopone (a presumed COMT inhibitor) approved for Parkinson’s disease, also prevents the ferroptosis pathway (Figure 3).

Following our FDA-approved library screen, we discovered 223 completed novel small molecules from our cysteine-focused covalent small molecule library that block the ferroptosis pathway. These 223 small molecules span an extremely diverse range of chemotypes with over half of them being predicted to be blood-brain-barrier. With these molecules in hand, we are now embarking on a lead optimization campaign to make these compounds orally bioavailable, blood-brain-barrier penetrate, and ultimately assess our Ferroptosis hypothesis in preclinical mouse models of Parkinson’s and Alzheimer’s disease.