Huntington's

This is a FIH (first-in-human) study to evaluate the clinical utility of the radioligand \[11C\]CHDI-00491009 as a PET tracer that binds specifically to mutant huntingtin (mHTT) aggregates in Huntington's disease (HD). The study is divided into three cohorts defined by the Huntington's Disease Integrated Staging System (HD-ISS): Cohort 1 - initial tracer validation (3 healthy controls (HCs)); Cohort 2 - target validation and test-retest variability (6 HD-ISS Stage 3 participants and 6 age and biological sex-matched HCs); Cohort 3 - target sensitivity (6 HD-ISS Stage 2 participants and 6 age and biological sex-matched HCs). An interim analysis (IA) will be conducted after the completion of each cohort, followed by a final analysis for the study. In addition to imaging, exploratory biomarkers, including somatic instability index, soluble mHTT and total huntingtin (HTT), will be assessed. All participants with HD (PwHD) will have an additional blood sample drawn at the screening visit to assess the somatic instability index and will also be invited to provide an optional cerebrospinal fluid (CSF) sample for measurement of soluble mHTT and total HTT.