The purpose of this study is to determine the safety and feasibility of performing psilocybin-assisted psychotherapy in patients hospitalized for treatment-resistant depression.

The proposed study will assess the safety, feasibility, preliminary results and neurological aspects of psilocybin-assisted psychotherapy in the treatment of hospitalized patients with treatment-resistant depression.

Screening, after signing the ICF, is done in patients newly admitted to the psychiatric service of Ghent University Hospital or patients referred for hospitalization through the outpatient clinic of the psychiatric service of Ghent University Hospital. Involved patients are hospitalized at the Anxiety, Compulsion and Mood Unit for 8 weeks. They will receive add-on treatment with psilocybin-supported psychotherapy in addition to standard treatment (daily group therapies, mainly non-verbal and activating). This consists of a preparatory phase (4 sessions of 1.5 hours before the first psilocybin session and 1 session of 1.5 hours before the second psilocybin session), the psilocybin sessions themselves (2 sessions, week 3 and week 6) and an integration phase (3 sessions of 1.5 hours after each psilocybin session). All sessions occur with the same 2 therapists throughout the entire course. Therapists are trained to provide experiential and systemic psychotherapeutic interventions.

After hospitalization, weekly outpatient follow-up consultations continue until 12 weeks after the last psilocybin session. Subsequently, patients may still agree to a prospective, observational and naturalistic follow-up until 1 year after the last psilocybin session, during which they will be called monthly by a psychiatrist from the research team to inquire about current mental status and any therapies undertaken in the interim.

During the study, patients are required to fill out questionnaires at regular intervals. Video-recordings will be made of certain parts of the preparation and integration sessions, for analysis with the client experience scale (EXP). The morning of each psilocybin session, female patients must provide a blood sample for a pregnancy test. The morning of each psilocybin session, all patients must provide a urine sample for toxicology screening. EEGs are also taken from patients the day before each psilocybin session, the morning of the first integration session after each psilocybin session and 12 weeks after the last psilocybin session (at the last outpatient consultation). At the start and at the end of the hospitalization phase, a semi-structured interview will be conducted with the patient to gauge expectations on the one hand and experiences on the other.

Throughout the study, the patient's partner will be involved. The partner must complete a number of questionnaires at baseline and throughout the study and will have to be present at 3 EEGs. In addition, the partner will also participate in 1 preparatory session and 2 integration sessions. The partner will also be given the opportunity to spend the night of a psilocybin session with the patient in the hospital (rooming-in). At the start and at the end of the hospitalization phase, a semi-structured interview will be conducted with the partner to gauge expectations on the one hand and experiences on the other.

Inclusion Criteria:

1. Age 18 years or older.

2. Diagnosis of major depressive disorder (single and recurrent episodes) of moderate to severe degree (MADRS score ≥ 20) according to the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), without psychotic features. Included ICD-11 diagnoses are ICD-11: 6A70.1, 6A70.3, 6A71.1, 6A71.3.

3. Subjects have failed to respond to 2 or more antidepressants prescribed at the minimum effective dose for at least 6 weeks. Augmentation with an add-on treatment counts as a second treatment.

4. The subjects are abstinent from alcohol (breathalyzer blood alcohol concentration (BAC) level 0.00) and provide a negative urine drug screen at the dosing day.

5. The female subjects provide a negative pregnancy test at the dosing day.

6. The subjects must be medically stable based on clinical laboratory tests, medical history, vital signs, and 12-lead ECG performed at screening.

   - In 12-lead ECG, QTcF should be ≤ 450 ms for males or ≤ 470 ms for females and PR-interval < 220 ms at screening.

7. A partner is willing to participate in the study (a cohabiting relationship of at least 1 year).

Exclusion Criteria:

1. Currently comorbid or previously diagnosed DSM-5 diagnosis of a

   1. major depressive episode with psychotic features.
   2. psychotic disorder (defined as meeting DSM-5 criteria for any psychotic disorder) on the MINI 7.0, EXCEPT substance/medication induced psychotic disorder where the duration was limited to the acute period of direct intoxication with the substance/medication. Positive diagnoses on the MINI will be subject to confirmation at clinical interview by a psychiatrist.
   3. bipolar disorder (defined as meeting DSM-5 criteria for bipolar type 1 or bipolar type 2) on the MINI 7.0. Positive diagnoses on the MINI will be subject to confirmation at clinical interview by a psychiatrist.
   4. drug or alcohol dependence syndrome (defined as meeting DSM-5 criteria for any dependence syndrome) on the MINI 7.0. Positive diagnoses on the MINI will be subject to confirmation at clinical interview by a psychiatrist.
   5. cluster B personality disorder. Subjects will be screened for the presence of those personality traits by using the NEO-FFI-3.
   6. PTSD (defined as meeting DSM-5 criteria for PTSD) on the MINI 7.0. Positive diagnoses on the MINI will be subject to confirmation at clinical interview by a psychiatrist.

2. A family history (first-degree relative) of psychosis and/or bipolar disorder.

3. Current active suicidal ideations.

4. Depression secondary to other medical conditions.

5. Medical diagnosis incompatible with psilocybin treatment:

   1. Cardiovascular conditions: recent stroke (< 1 year from signing of ICF), recent myocardial infarction (< 1 year from signing of ICF), uncontrolled hypertension (blood pressure > 140/90 mmHg) or clinically significant arrhythmia within 1 year of signing the ICF.
   2. Uncontrolled insulin-dependent diabetes mellitus.
   3. Uncontrolled epilepsy.

6. Biochemical or electrocardiographic abnormalities determined as clinically significant by a medical doctor. PsiHos-D, Version 2.1 dd. 22/02/2024 page 59 of 114 TMP_Protocol AGO CTR_Version 1.0_Effective Date 2022-07-18

7. Current intake of Lithium, Disulfiram, MAOIs or inhibitors of UGT1A9 and 1A10.

8. The subject has received any prior treatment with vagal nerve stimulation, or a deep brain stimulation device.

9. Women of childbearing potential not using adequate contraception (methods that can achieve a failure rate of less than 1%: combined hormonal contraception, progestogen-only hormonal contraception associated with inhibition of ovulation, intrauterine device, intrauterine hormone-releasing system, bilateral tubal occlusion, vasectomized partner, sexual abstinence). A woman is considered of childbearing potential (WOCBP), i.e. fertile, following menarche and until becoming post-menopausal unless permanently sterile. Permanent sterilization methods include hysterectomy, bilateral salpingectomy and bilateral oophorectomy. A postmenopausal state is defined as no menses for 12 months without an alternative medical cause.

10. Pregnant or breast-feeding women.

11. Those unable to give informed consent.

12. Those enrolled in another trial.