The purpose of the study is to assess the long-term efficacy and safety of add-on pramipexole for treatment of patients with anhedonic depression.

The heterogeneity of depression suggests that several different neurocircuits and pathophysiological mechanisms are involved. Anhedonia - the inability to experience pleasure from, or the lack of motivation to carry out, usually enjoyable activities - is a promising endophenotype within the depression spectrum, with a distinct pathophysiology involving dopaminergic mesolimbic projections. Anhedonia is common in depression and associated with treatment resistance. Pramipexole, an agonist to the dopamine -receptor 3, is an established treatment of Parkinson's disease. Based on its mechanism of action, pramipexole might be efficacious in a subtype of depression characterized by anhedonia and lack of motivation - symptoms linked to dopaminergic hypofunction.

The aim of this open label follow-up study is to test the long-term efficacy and tolerability of add-on pramipexole in anhedonic depression. This is a continuation study of an RCT (EudraCT# 2022-001563-26) in which patients are randomized to either pramipexole or placebo for 9 weeks. After completion of the RCT, patients will be offered to participate in the current trial. Approximately 50% of the patients have been treated with pramipexole and 50% with placebo within the frames of the RCT. Patients randomized to pramipexole in the RCT will continue on their current dose and patients randomized to placebo will start pramipexole using the same dosing schedule as in the RCT. A total of 80 research subjects with unipolar depression, bipolar disorder in depressive phase, or dysthymia will be offered participation in the study after completing the RCT given that they fulfill all the inclusion criteria, and none of the exclusion criteria.

Subjects have study visits once a month during 6 months, or more often if needed based on side effects. Symptom severity, side effects and ecological momentary assessments are recorded at each study visit and dose titration schedule is modified as needed. Between study visits, research subjects may contact study personnel and the investigators can quickly arrange for an additional study visit if needed.

After the study, the decision to continue with pramipexole outside of the study or discontinue with pramipexole will be based on patient preference in combination with a willingness of the patient's regular physician to take over the treatment outside of the study.

Inclusion Criteria:

• Previous participation in RCT testing the short-term efficacy of pramipexole vs placebo (EudraCT# 2022-001563-26).
• Study participants randomized to pramipexole in the RCT who wish to continue with their treatment can enrol in the study.
• Study participants randomized to placebo in the RCT who continue to fulfil the inclusion criteria (and none of the exclusion criteria) after the RCT can enrol in the study.
• The research subject has given informed consent to participate in the study.

Additional inclusion criterion for patients receiving placebo during the RCT

• Anhedonia symptoms: 3 or 4 points on ≥ 3 items of the Snaith-Hamilton Pleasure Scale (SHAPS-C). This has been adopted in previous studies as a definition of "clinically significant anhedonia".

Exclusion Criteria:

• Pregnancy, breastfeeding or planned pregnancy (if female).
• High suicide risk according to the overall clinical assessment of the research physician.
• Ongoing substance abuse (within 6 months).
• Diagnosis of current psychosis.
• Known diagnosis of Emotionally Unstable Personality Disorder.
• Treatment under LPT.
• History of impulse control disorder (including current binge-eating disorder) or a current ADHD diagnosis with hyperactivity.
• Diagnosis of intellectual disability, dementia, or other circumstance that makes it difficult to understand the meaning of participating in the trial and give informed consent.
• Diagnosis of renal failure or severe cardiovascular disease (specifically symptomatic heart failure NYHA Class II). Blood samples from RCT are sufficient to rule this out.
• Recently started psychotherapy (within 6 weeks) or planning to start such treatment during participation in the trial.
• Ongoing ECT, ketamine or rTMS treatment, excluding maintenance ECT, ketamine or rTMS (Maintenance treatment is defined as the use of ECT/ketamine/rTMS for a period exceeding 3 months after a series of ECT/ketamine/rTMS treatment in order to prevent the onset of a new episode).
• Other medical conditions or other concomitant drug treatment (see section 14.5) which, in the opinion of the investigators, may affect the evaluability of the trial or conditions that increase trial risk. For example, Parkinson's disease, hepatic insufficiency, ongoing cancer not in remission for more than one year, gastric bypass surgery that affects the absorption of extended release tablets.
• Known or suspected allergy to any active substance or excipient in the medicinal product included in the trial.
• Participation in other treatment studies.
• Other reason, as assessed by the investigator, that prevents the research subject's participation, such as the risk that the research subject is unable to complete the trial (non-compliance).

Not masked.