Major depressive disorder (MDD) is one of the world's greatest contributor to the global burden of disease and MDD affects around 17% of the Swiss population (Tomonaga et al. 2013). It is a chronic condition and can cause the affected person to suffer greatly and function poorly at work, at school and in the family. More than 1'000 suicides were recorded in Switzerland in 2014, about 90% of these fatalities were related to depression or other psychiatric problems. Suicide is the second leading cause of death in individuals 15-24 years of age (Insel \& Charney 2003).

Current pharmacotherapies, including monoaminergic-acting antidepressants, require prolonged administration (weeks if not months) for clinical improvement. This lag time, as well as a high non-response rate, emphasizes the need for better and faster-acting antidepressant medications. However, psychopharmacological research has largely failed to produce novel and more efficacious treatment options for MDD since decades. Advanced pharmaceutical antidepressants should ideally facilitate the psychotherapeutic process for patients, reduce the time onset of antidepressant efficacy, and prime neuroplastic adaptations relevant to symptom improvement. Such novel therapeutics are much needed and would address this detrimental public health problem, particularly in treatment-resistant patients.

Early clinical studies using the psychotropic compound psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine) as an adjunct in psychotherapy reported a significant improvement of clinical symptoms in depression and anxiety disorder (Leuner 1961, 1981). Psilocybin is the main psychoactive principle of the group of hallucinogenic fungi (Hofmann 1968), commonly known as magic mushrooms, and acts as partial agonist at cortical and sub-cortical serotonin 5-HT2A and 5-HT1A receptors. At moderate doses, psilocybin produces a dream-like state of consciousness (Kraehenmann et al. 2016) characterized by perceptual alterations, enhanced mood, facilitated autobiographic memory recollection, and a change of perspective on the self (Leuner 1981; Studerus et al. 2011). Recent clinical studies applying placebo-controlled designs support and extend these early findings by showing that a single dose of psilocybin leads to a fast and sustained reduction in anxiety and depression as well as an improvement of quality of life in advanced cancer patients (Griffiths 2015, Grob et al. 2011). Furthermore, a recent open-label feasibility study showed rapid-onset, sustained symptom improvements over 3 weeks in a small sample of treatment-resistant depressed patients following two psilocybin treatment sessions (Carhart-Harris et al. 2016). Accumulating evidence from pharmacological and neuroimaging studies suggests that psilocybin may produce its antidepressant effects via activation of 5-HT2A receptors located in prefrontal-limbic structures that are also implicated in the pathophysiology of depression (Kraehenmann \& Vollenweider et al. 2015; Vollenweider und Kometer 2010; Disner et al. 2011). In addition, molecular studies suggest that the enduring symptom improvement after a single dose of psilocybin may be mediated through downstream effects on the glutamate system and a subsequent activation of neuroplastic factors such as brain-derived neurotrophic factor (BDNF) (Catlow et al. 2013, Barre et al. 2016).

The present clinical trial aims at investigating the putative antidepressant effects of a single moderate dose of psilocybin (0.215 mg/kg) in patients suffering from MDD by applying a randomized, double-blind, placebo-controlled design. The specific aims of this project are: 1. To investigate whether psilocybin in combination with short-term focused psychotherapy will reduce core symptoms in patients with MDD. 2. Using functional magnetic resonance imaging (fMRI) to longitudinally assess whether a single dose of psilocybin will post-acutely change the negative emotion processing bias in patients with MDD and whether the change in emotion processing bias will predict subsequent symptom improvement. In addition, the investigators will analyze whether psilocybin will lead to sustained changes in functional neuronal network connectivity (FC), e.g. in amygdala-prefrontal FC. 3. To investigate whether psilocybin will increase BDNF plasma concentration and whether the change in BDNF is related to changes in fMRI markers and the subsequent mood improvement.

Recent reviews indicate that impaired neuroplasticity is at the core of the pathophysiology moods and stress-related disorders. Current available antidepressants have been developed with the aim of providing symptom relief rather than targeting neuroplastic impairments. In contrast to this, the present proposal builds on promising new findings that single dose of psilocybin, presumably via a 5-HT2A receptor driven glutamatergic mechanism, leads to a rapid enhancement in neuronal resilience and a to a change in the function of neuronal networks underlying depressive symptoms and behavior. Targeting neuroplasticity with such novel approaches appears to be important for reversing cognitive schemata and emotion processing biases, fostering enduring improvements in mood and cognitive flexibility (Krystal et al. 2009).

Expected value: this is the first randomized, double-blind, placebo-controlled clinical trial (RCT) of psilocybin treatment in MDD. Using state-of-the art behavioral, neuroimaging, and neuroplasticity methodology, the results of this study will help elucidate urgently needed new treatment mechanisms in MDD. Should it turn out that a single moderate dose of psilocybin vs. placebo in conjunction with psychotherapy may rapidly and sustainedly reduce depressive symptoms, this will be a major breakthrough in finding a novel and fast acting treatment strategy in depressed patients. Therefore, the results of this study will have high impact on the field of pharmacological research into novel antidepressant medication.