Imaging Mutant Huntingtin Aggregates: Development of a Potential PET Ligand

Abstract

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.

A novel imaging ligand as a biomarker for mutant huntingtin-lowering in Huntington’s disease

“…When administered early, prior to the appearance of pathology, AAV-ZFP prevents mHTT inclusion and extranuclear aggregation, and the diminution of the signal detected by [ 11 C]CHDI-180R can be explained by the extent of agent distribution and neuronal transduction (in our case, about 40% of the striatum). While we do not yet have a full understanding of the various species of mHTT that constitute the binding site(s) for CHDI-180, based on recombinant, cell, and tissue protein studies 8,9 , we know this ligand can bind oligomerized and some forms of fibrillar mHTT but not to monomeric soluble HTT.…”

“…In zQ175DN and Q140 mouse models, mHTT accumulates in all brain structures 8,13,21,40 and no suitable reference region for relative quantification could be identified. Hence absolute quantification for [ 11 C]CHDI-180R was performed to calculate the total volume of distribution based on image-derived input function (VT (IDIF)) as a non-invasive surrogate of the VT.…”

“…To examine the in vivo kinetic properties of [ 11 C]CHDI-180R 8 as a PET ligand, we selected the zQ175DN model because it displays a moderately slow disease onset with hallmark of mHTTaggregates increasing from 3 to 12 months 21 . We performed in vivo microPET studies in 9month-old zQ175DN het and wt mice for characterization of its pharmacokinetic properties and monitored its stability in the brain and plasma (Extended Data Fig.…”

“…While the distribution and pharmacological activity of these therapeutics have been extensively evaluated in nonhuman primates and mHTT-expressing transgenic minipigs 7 it is unclear whether we can expect a similar distribution in the larger human brain.A key milestone was reached when the Ionis/Roche Phase 1/2a trial 4 (www.clinicaltrials.gov, identifier NCT02519036) showed for the first time sustained dose-and time-dependent decreases in CSF levels of mHTT, demonstrating pharmacological activity in the human CNS.Regrettably, this finding has not led to clinical benefit in the recently-terminated Phase 3 tominersen trial, and analyses are underway to understand the safety issues identified, which led to a worsening of disease symptoms., How the reduction of mHTT in CSF after delivery of ASOs via lumbar puncture and AAVs delivered into brain parenchyma 5,7 relates to lowering in affected circuits in the brain is unclear. To evaluate regional pharmacological effects of candidate therapeutics targeting mHTT, we sought to develop a non-invasive imaging agent specific for aggregated mHTT that could give insight into the timing, durability, and regional therapeutic effects of administered drugs 8,9 . As all current therapeutic agents in development 10 target either HTT or HTT transcriptional or posttranscriptional processes, quantification of mHTT protein offers a good indicator of the extent of HTT lowering and of the biodistribution of the agents.…”

“…Regrettably, this finding has not led to clinical benefit in the recently-terminated Phase 3 tominersen trial, and analyses are underway to understand the safety issues identified, which led to a worsening of disease symptoms., How the reduction of mHTT in CSF after delivery of ASOs via lumbar puncture and AAVs delivered into brain parenchyma 5,7 relates to lowering in affected circuits in the brain is unclear. To evaluate regional pharmacological effects of candidate therapeutics targeting mHTT, we sought to develop a non-invasive imaging agent specific for aggregated mHTT that could give insight into the timing, durability, and regional therapeutic effects of administered drugs 8,9 . As all current therapeutic agents in development 10 target either HTT or HTT transcriptional or posttranscriptional processes, quantification of mHTT protein offers a good indicator of the extent of HTT lowering and of the biodistribution of the agents.…”