The Emerging Landscape of Small-Molecule Therapeutics for the Treatment of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene (HTT). The new insights into HD's cellular and molecular pathways have led to the identification of numerous potent small-molecule therapeutics for HD therapy. The field of HD-targeting small-molecule therapeutics is accelerating, and the approval of these therapeutics to combat HD may be expected in the near future. For instance, preclinical candidates such as naphthyridine-azaquinolone, AN1, AN2, CHDI-00484077, PRE084, EVP4593, and LOC14 have shown promise for further optimization to enter into HD clinical trials. This perspective aims to summarize the advent of small-molecule therapeutics at various stages of clinical development for HD therapy, emphasizing their structure and design, therapeutic effects, and specific mechanisms of action. Further, we have highlighted the key drivers involved in HD pathogenesis to provide insights into the basic principle for designing promising anti-HD therapeutic leads.

Discovery and Optimization of Chromone Derivatives as Novel Selective Phosphodiesterase 10 Inhibitors

Phosphodiesterase 10 (PDE10) inhibitors have received much attention as promising therapeutic agents for central nervous system (CNS) disorders such as schizophrenia and Huntington's disease. Recently, a hit compound 1 with a novel chromone scaffold has shown moderate inhibitory activity against PDE10A (IC₅₀ = 500 nM). Hit-to-lead optimization has resulted in compound 3e with an improved inhibitory activity (IC₅₀ = 6.5 nM), remarkable selectivity (>95-fold over other PDEs), and good metabolic stability (RLM t_1/2 = 105 min) by using an integrated strategy (molecular modeling, chemical synthesis, bioassay, and cocrystal structure). The cocrystal structural information provides insights into the binding pattern of 3e in the PDE10A catalytic domain to highlight the key role of the halogen and hydrogen bonds toward Tyr524 and Tyr693, respectively, thereby resulting in high selectivity against other PDEs. These new observations are of benefit for the rational design of the next generation PDE10 inhibitors for CNS disorders.

Validation of Phosphodiesterase-10 as a Novel Target for Pulmonary Arterial Hypertension via Highly Selective and Subnanomolar Inhibitors

Pulmonary arterial hypertension (PAH) causes pathological increase in pulmonary vascular resistance, leading to right-heart failure and eventual death. Previously, phosphodiesterase-10 (PDE10) was reported to be a promising target for PAH based on the studies with a nonselective PDE inhibitor papaverine, but little progress has been made to confirm the practical application of PDE10 inhibitors. To validate whether PAH is ameliorated by PDE10 inhibition rather than other PDE isoforms, here we report an integrated strategy to discover highly selective PDE10 inhibitors as chemical probes. Structural optimization resulted in a PDE10 inhibitor 2b with subnanomolar affinity and good selectivity of >45 000-fold against other PDEs. The cocrystal structure of the PDE10-2b complex revealed an important H-bond interaction between 2b and Tyr693. Finally, compound 2b significantly decreased the arterial pressure in PAH rats and thus validated the potential of PDE10 as a novel anti-PAH target. These findings suggest that PDE10 inhibition may be a viable treatment option for PAH.

Striatal phosphodiesterase 10A and medial prefrontal cortical thickness in patients with schizophrenia: a PET and MRI study

The enzyme phosphodiesterase 10A (PDE10A) is abundant in striatal medium spiny neurons and has been implicated in the pathophysiology of schizophrenia in animal models and is investigated as a possible new pharmacological treatment target. A reduction of prefrontal cortical thickness is common in schizophrenia, but how this relates to PDE10A expression is unknown. Our study aim was to compare, we believe for the first time, the striatal non-displaceable binding potential (BP_ND) of the new validated PDE10A ligand [¹¹C]Lu AE92686 between patients with schizophrenia and healthy controls. Furthermore, we aimed to assess the correlation of PDE10A BP_ND to cortical thickness. Sixteen healthy male controls and 10 male patients with schizophrenia treated with clozapine, olanzapine or quetiapine were investigated with positron emission tomography (PET) and magnetic resonance imaging (MRI). Striatal binding potential (BP_ND) of [¹¹C]Lu AE92686 was acquired through dynamic PET scans and cortical thickness by structural MRI. Clinical assessments of symptoms and cognitive function were performed and the antipsychotic dosage was recorded. Patients with schizophrenia had a significantly lower BP_ND of [¹¹C]Lu AE92686 in striatum (P=0.003) than healthy controls. The striatal BP_ND significantly correlated to cortical thickness in the medial prefrontal cortex and superior frontal gyrus across patients with schizophrenia and healthy controls. No significant correlation was observed between the BP_ND for [¹¹C]Lu AE92686 in striatum and age, schizophrenia symptoms, antipsychotic dosage, coffee consumption, smoking, duration of illness or cognitive function in the patients. In conclusion, PDE10A may be important for functioning in the striato-cortical interaction and in the pathophysiology of schizophrenia.