Mutant huntingtin (mHTT) protein carrying the elongated N-terminal polyglutamine (polyQ) tract misfolds and forms protein aggregates characteristic of Huntington’s disease (HD) pathology. A high-affinity ligand specific for mHTT aggregates could serve as a positron emission tomography (PET) imaging biomarker for HD therapeutic development and disease progression. To identify such compounds with binding affinity for polyQ aggregates, we embarked on systematic structural activity studies; lead optimization of aggregate-binding affinity, unbound fractions in brain, permeability, and low efflux culminated in the discovery of compound 1, which exhibited target engagement in autoradiography (ARG) studies in brain slices from HD mouse models and postmortem human HD samples. PET imaging studies with 11C-labeled 1 in both HD mice and WT nonhuman primates (NHPs) demonstrated that the right-hand-side labeled ligand [11C]-1R (CHDI-180R) is a suitable PET tracer for imaging of mHTT aggregates. [11C]-1R is now being advanced to human trials as a first-in-class HD PET radiotracer.

The expanded polyglutamine-containing mutant huntingtin (mHTT) protein is implicated in neuronal degeneration of medium spiny neurons in Huntington's disease (HD) for which multiple therapeutic approaches are currently being evaluated to eliminate or reduce mHTT. Development of effective and orthogonal biomarkers will ensure accurate assessment of the safety and efficacy of pharmacologic interventions. We have identified and optimized a class of ligands that bind to oligomerized/aggregated mHTT, which is a hallmark in the HD postmortem brain. These ligands are potentially useful imaging biomarkers for HD therapeutic development in both preclinical and clinical settings. We describe here the optimization of the benzo [4,5]imidazo[1,2a]pyrimidine series that show selective binding to mHTT aggregates over Aβ-and/or tau-aggregates associated with Alzheimer's disease pathology. Compound [ 11 C]-2 was selected as a clinical candidate based on its high free fraction in the brain, specific binding in the HD mouse model, and rapid brain uptake/washout in nonhuman primate positron emission tomography imaging studies.

Huntington’s disease (HD) is a dominantly inherited neurodegenerative disorder caused by a CAG trinucleotide expansion in the huntingtin ( HTT ) gene that encodes the pathologic mutant HTT (mHTT) protein with an expanded polyglutamine (polyQ) tract. Whereas several therapeutic programs targeting mHTT expression have advanced to clinical evaluation, methods to visualize mHTT protein species in the living brain are lacking. Here, we demonstrate the development and characterization of a positron emission tomography (PET) imaging radioligand with high affinity and selectivity for mHTT aggregates. This small molecule radiolabeled with 11 C ([ 11 C]CHDI-180R) allowed noninvasive monitoring of mHTT pathology in the brain and could track region- and time-dependent suppression of mHTT in response to therapeutic interventions targeting mHTT expression in a rodent model. We further showed that in these animals, therapeutic agents that lowered mHTT in the striatum had a functional restorative effect that could be measured by preservation of striatal imaging markers, enabling a translational path to assess the functional effect of mHTT lowering.

Therapeutic interventions are being developed for Huntington’s disease (HD), a hallmark of which is mutant huntingtin protein (mHTT) aggregates. Following the advancement to human testing of two [11C]-PET ligands for aggregated mHTT, attributes for further optimization were identified. We replaced the pyridazinone ring of CHDI-180 with a pyrimidine ring and minimized off-target binding using brain homogenate derived from Alzheimer’s disease patients. The major in vivo metabolic pathway via aldehyde oxidase was blocked with a 2-methyl group on the pyrimidine ring. A strategically placed ring-nitrogen on the benzoxazole core ensured high free fraction in the brain without introducing efflux. Replacing a methoxy pendant with a fluoro-ethoxy group and introducing deuterium atoms suppressed oxidative defluorination and accumulation of [18F]-signal in bones. The resulting PET ligand, CHDI-650, shows a rapid brain uptake and washout profile in non-human primates and is now being advanced to human testing.

The development of facile, wide scope synthetic methodologies providing access to fluorinated motifs is important in medic-inal chemistry; for our purposes, we are interested in developing fluorine-tagged compounds to investigate their utility in Huntington’s disease. Here, we describe a novel, operationally simple and mild procedure for the synthesis of mono-fluoromethyl 6,5-heteroaromatic bicycles using 1,3-difluoroacetone (DFA). The scope of the reaction was investigated, and 27 examples synthesized with yields up to 96%.

The development of facile, wide scope synthetic methodologies providing access to fluorinated motifs is important in medic-inal chemistry; for our purposes, we are interested in developing fluorine-tagged compounds to investigate their utility in Huntington’s disease. Here, we describe a novel, operationally simple and mild procedure for the synthesis of mono-fluoromethyl 6,5-heteroaromatic bicycles using 1,3-difluoroacetone (DFA). The scope of the reaction was investigated, and 27 examples synthesized with yields up to 96%.

The development of facile, wide scope synthetic methodologies providing access to fluorinated motifs is important in medic-inal chemistry; for our purposes, we are interested in developing fluorine-tagged compounds to investigate their utility in Huntington’s disease. Here, we describe a novel, operationally simple and mild procedure for the synthesis of mono-fluoromethyl 6,5-heteroaromatic bicycles using 1,3-difluoroacetone (DFA). The scope of the reaction was investigated, and 27 examples synthesized with yields up to 96%.