Dysregulated proteostasis and intracellular transport contribute to neurodegeneration. HDAC6, a therapeutic target of interest for neurodegenerative diseases, acts at a nexus modulating both proteostasis and intracellular transport. Inhibition of HDAC6 deacetylase activity promotes autophagic clearance of protein aggregates and increases ⍺-tubulin acetylation, thereby enhancing microtubule resiliency and motor protein-microtubule binding, which facilitates intracellular transport and, subsequently, proteostasis. Despite these benefits, advancement of HDAC6 inhibitor therapeutics for neurodegenerative disease has been hindered by inadequate selectivity and CNS-penetrance of first-generation compounds.

Here we characterize a next-generation small molecule HDAC6 inhibitor, EKZ-438, in preclinical models of amyotrophic lateral sclerosis and frontotemporal dementia. We present the pharmacological properties of EKZ-438, which demonstrate high selectivity for HDAC6 (>8,500-fold selectivity for HDAC6 versus all other HDAC6 paralogs), low nanomolar potency (12 nM) for HDAC6, and, importantly, CNS-penetrance (Kp,uu,brain) ≥ 0.55 and high oral bioavailability (F% = 70). In complementary preclinical in vitro and in vivo immunolabeling and live imaging studies we tested the hypothesis that selective inhibition of HDAC6 deacetylase activity is sufficient to improve pathophysiological proteostasis and intracellular transport deficits in animal models of familial and sporadic amyotrophic lateral sclerosis and frontotemporal dementia. Notably, we extended these findings to human induced pluripotent stem cell-derived neuronal cellular models, supporting the relevance of our findings to human disease.

EKZ-438 treatment fully rescued SOD1 (q < 0.0001) and TDP-43 (q < 0.001) proteostasis defects following an excitotoxic glutamate challenge, and increased survival of SOD1G93A and wildtype motor neurons by 59% (q < 0.0001) and 37% (q < 0.01), respectively, demonstrating in vitro neuroprotection. In SOD1G93A mice, EKZ-438 improved axonal transport by 16% (q < 0.05), motor performance by ∼40% (q < 0.05), and decreased plasma neurofilament light chain levels by 35% (q < 0.05), demonstrating in vivo neuroprotection. In a TDP-43 mouse model, EKZ-438 reduced TDP-43 pathology by ∼30% (q < 0.05) and neuroinflammation by ∼26% (q < 0.05) in the brain, supporting HDAC6 inhibition for sporadic amyotrophic lateral sclerosis and frontotemporal dementia. Furthermore, EKZ-438 treatment improved intracellular transport by 39% (q < 0.001), rescued cytoplasmic TDP-43 accumulation by 87% (q < 0.0001), and restored nuclear TDP-43 splicing activity (P < 0.05) in human TARDBP neurons. These mechanistic improvements aligned with nearly complete rescue of human TARDBP and C9orf72 mutant neuron survival (P < 0.0001). We conclude that selective HDAC6 inhibition represents a promising therapeutic approach for potential disease modification in amyotrophic lateral sclerosis and frontotemporal dementia.