The aim of this study is to measure longitudinally the availability of the PDE10A enzyme in HDGECs using the radioligand [18F]MNI-659. The study will be a follow-up, examining HDGECs from the CHDIKI1201/PET-HD-PDE10A (NCT02061722) study from 18 to 28 months after the initial PET measurement.

The HDGECs will perform 2 study visits: Visit 1 (screening) and Visit 2 (PET analysis utilizing the radioligand [18F]MNI-659). There will also be 2 telephone follow ups; one after Visit 1 (telephone follow-up after screening) and one after Visit 2, (telephone follow-up after PET). Study visits and telephone follow-ups will take place during a maximum of 97 days.

Inclusion Criteria:

• Have participated in and completed study CHDIKI1201/PET-HD-PDE10A and are capable of providing informed consent or have a legal representative authorized to give consent on their behalf.
• Otherwise healthy according to medical history, no co-morbidity of psychotic disorders, physical examination, vital signs and laboratory assessments
• Willing to travel to Stockholm (with a companion if requested) for PET examinations
• Suitable physically and psychologically to travel to Stockholm and undergo PET examinations, as judged by the recruiting Investigator with attention given to symptoms of chorea and anxiety
• Willing to comply with the use of adequate contraceptive measures

Exclusion Criteria:

• Any disease, condition or concomitant medications that significantly compromises the function of the body systems and that, in the opinion of the Investigator might interfere with the conduct of the study or the interpretation
• Regular use of any medication prohibited by this protocol (selective or non-selective PDE inhibitors), with the exception for erectile dysfunction medication, which may be temporarily discontinued prior to the PET measurement
• History of anaphylactoid or anaphylactic reactions to any allergen including drugs and contrast media
• History of other neurological condition (including brain surgery, intracranial hematoma, stroke/cerebrovascular disorders, epilepsy), co-morbidity of psychiatric disorders
• Clinically relevant findings in hematological, biochemical testing or 12-lead ECG as determined by the evaluating physician
• Female subjects who are breast-feeding or have a positive pregnancy test at screening or at Visit 2