iMagemHTT: FIH Evaluation of Novel Mutant Huntingtin PET Radioligands [¹¹C]CHDI-00485180-R and [¹¹C]CHDI-00485626

This is a FIH (first-in-human) adaptive PET (Positron Emission Tomography) imaging study to explore the binding and kinetic properties of two potential mutant huntingtin (mHTT) radioligands; \[¹¹C\]CHDI-00485180-R and \[¹¹C\]CHDI-00485626. The binding characteristics of these radioligands will be evaluated first in young (\< 35 years of age) healthy control (HC) participants (Phase 1a) and subsequently in young HCs and Huntington's disease gene-expansion carriers (HDGECs) with Stage II HD (Phase 1b). Subsequent phases will continue to explore the radioligands' binding characteristics in HDGECs with Stage II HD (Phase 2a), Stage I HD (Phase 2b) and pre-manifest HDGECs (Phase 2c) and their age matched healthy controls. All phases are cross-sectional and will include comparisons between HCs and HDGECs. Test-retest (TRT) evaluations will be done during Phase 2a, with the option of including further test-retests in Phases 2b, and 2c after review of data by the iMagemHTT Executive Committee. TRT is only applicable to HDGEC participants. There are four planned interim analyses at which either radioligand may be dropped if its characteristics are shown to be suboptimal. If successful, the study will establish \[¹¹C\]CHDI-00485180-R and/or \[¹¹C\]CHDI-00485626 as fit for further development as drug development tools to measure mHTT levels in HDGECs. This development is intended to demonstrate the value of PET imaging with these radioligands as a disease progression biomarker, predictive biomarker, pharmacodynamic biomarker, and eventually as an efficacy biomarker. All HDGEC participants will be invited to provide an optional cerebrospinal fluid (CSF) sample that will be collected after the imaging visits are complete. These samples will be processed to evaluate CSF mHTT levels and other potential biomarkers and to explore potential relationships between soluble CSF mHTT levels and mHTT binding identified by \[¹¹C\]CHDI-00485180-R and/or \[¹¹C\]CHDI-00485626 PET imaging. P