Characterization of small molecules that lower mutant huntingtin protein as potential therapeutics for Huntington’s disease

SUMMARY Huntington’s disease (HD) is an autosomal dominant, progressive and fatal neurodegenerative disease that effects 200,000 people worldwide. Despite discovery of the gene more than 25 years ago and more than 150 clinical trials, there is still no effective treatment for HD. The development of any therapy that slows, halts, or prevents disease would have a major impact on the patients and their families. HD is caused by the expansion of a CAG repeat in the huntingtin gene (HTT), resulting in an expanded stretch of glutamines in the huntingtin protein. Normal huntingtin protein (HTT) is essential throughout the body and brain to regulate cell physiology including synaptic transmission and neuroprotection, cell division and differentiation, gene transcription and the DNA damage response. In patients with HD, the expanded polyglutamine tract causes mutant HTT (mHTT) to fold abnormally, resulting in aberrant post-translational modifications and cleavage to generate toxic mHTT fragments. The N-terminal mHTT fragments form oligomers that interact with many cellular proteins, disrupting cell function, resulting in increased levels of mHTT and causing mHTT inclusions. Substantial neuronal dysfunction and death occur in striatal medium spiny neurons (MSNs) and the cerebral cortex. Experimental procedures that lower mHTT have reversed disease symptoms in animal models of HD. However, clinical translation of this mechanism of action has stalled and is, in part, hypothesized to be due to nonselective lowering of both the essential HTT as well as mHTT. In addition, some of the drug candidates in clinical trials target the brain exclusively, use therapeutic modalities that require invasive delivery systems and leave the rest of the body untreated. Therefore, an orally delivered, systemically distributed, brain-penetrant therapeutic that selectively eliminates toxic mHTT while sparing the functional forms of HTT to support normal physiology could offer an effective treatment for all HD patients. By applying its expertise in screening, Origami Therapeutics (OT) has identified a chemical scaffold, represented by OR1-113, that prevents mHTT aggregation, and selectively lowers mHTT levels in cell-based assays by enhancing degradation through an autophagy pathway as demonstrated in HD patient-derived fibroblasts, human HD iPSC-derived medium spiny neurons and in vivo in the cortex and striatum of the YAC128 mouse model of HD. Twelve analogues of ORI-113 have been designed. The efficacy and drug-like properties of OR1-113 and 12 analogues will be compared in HD patient iPSC-derived MSNs and in vitro absorption, distribution, metabolism, and excretion studies will provide insight regarding the metabolism and potential interactions of the drug compounds (Aim 1). Pharmacokinetic (PK) and brain exposure profiles of four lead compounds selected from Aim 1 will be determined in mice for oral availability (Aim 2). The top two ranked leads will be evaluated in a combined PK/Pharmacodynamic and safety study in YAC128 mice with oral administration (Aim 3). These studies will identify a lead therapeutic drug candidate that will move into Investigational New Drug Application studies that are required prior to initiation of clinical testing.