Mechanistic Validation of Selective Brain-Penetrant HDAC6 Inhibitors as a Disease- Modifying Treatment for Tauopathies including Alzheimer's Disease

PROJECT SUMMARY Significance: Disease-delaying or -preventing therapies are desperately needed for Alzheimer’s disease (AD). Unfortunately, most AD therapeutics in development have focused on amyloid, based on the observed plaques in AD patients. These efforts have been unsuccessful thus far, pointing to the need to develop other promising targets. Tau is emerging as a more hopeful therapeutic target for AD because tau neurofibrillary tangles (NFTs) are not only a hallmark of AD and other tauopathies, but also correlate with cognitive decline more closely than amyloid. In contrast to pharma’s current efforts to target tau through immunotherapy, Eikonizo’s approach uses small molecules to target tau pathology through a completely different mechanism. Hypothesis: We hypothesize that inhibiting the enzyme histone deacetylase 6 (HDAC6) will positively modify tau to prevent tau aggregation. Beyond modifying tau, inhibiting HDAC6 will also promote degradation of HSP90 client proteins, including tau, and improve microtubule function required for mitochondrial and axonal transport – which are also known to be disrupted in AD. Preliminary Data: Eikonizo’s initial lead compound, EKZ-001, is superior to all other reported HDAC6 inhibitors for CNS indications with high potency (IC50 15 nM), high selectivity (andgt;100-fold over related isoforms) and high brain penetration (brain:plasma andgt;8). EKZ-001 demonstrates HDAC6 mechanistic engagement in human neural progenitor cells and critically does not target histone-modifying HDACs, thus avoiding known dose-limiting liabilities of pan-HDAC inhibitor therapeutics. EKZ-001 also lowers tau aggregation in primary mouse neurons exposed to β-amyloid. Eikonizo has proprietary technology to validate target engagement through a companion HDAC6 positron emission tomography (PET) imaging agent and we have demonstrated that EKZ-001 blocks HDAC6 PET signal in vivo in the brain of non-human primates. Eikonizo has synthesized improved, drug-like derivatives of EKZ-001 for testing in cellular and preclinical proof-of-mechanism experiments. Specific Aims: This study will evaluate drug-like lead compounds derived from EKZ-001 in primary neurons and in vivo in a mouse model of AD to demonstrate proof-of-mechanism. Following treatment with EKZ compounds, molecular markers of HDAC6 mechanistic engagement will be confirmed in primary hippocampal neurons (Aim1), and the two top lead compounds will be tested in PS19 mice, in vivo (Aim 2). Successful completion of the proposed Aims will be a major step towards IND-enabling studies of the development candidate for a first- in-human and first-in-class trial of an HDAC6 inhibitor as a therapeutic for tauopathies including AD. Milestones for Progression to Phase II: 1) proof-of-mechanism and 2) favorable PK profile for lead compound to be advanced to IND-enabling studies. [Specific, quantitative targets included in Aims and Research Strategy.]PROJECT NARRATIVE The protein tau is emerging as a hopeful therapeutic target for treating Alzheimer’s disease (AD), and inhibiting the enzyme HDAC6 may be an effective way of targeting tau because this inhibition reduces or prevents tau aggregation and positively alters other mechanisms relevant to AD progression. The proposed project will use Eikonizo’s highly potent, selective, brain-penetrant HDAC6 inhibitor lead compounds to 1) confirm molecular target engagement in primary neurons from a tauopathy mouse model, 2) establish proof-of-concept that the lead compound prevents or alters the markers of tauopathy observed in this mouse model, and 3) identify and optimize the best compound for use in further studies. Completion of this project will de-risk and accelerate the identification and optimization of a drug candidate to enable clinical trials of an HDAC6 inhibitor for AD treatment.