Rett Syndrome (RTT) is a rare neurodevelopmental disorder caused by an MECP2 gene mutation on the X chromosome, primarily affecting females. It causes progressive motor and cognitive decline, loss of speech, repetitive hand movements, breathing issues, seizures, and sleep problems. Given RTT's association with reduced monoamine levels, antidepressants like mirtazapine (MTZ) may help.Preclinical studies in MeCP2-mutant mice and early adult RTT trials showed that MTZ improved respiratory, motor, and neurological function, sleep, and mood, prompting this pediatric and young adult study. The MirtaRett trial is a multicenter, open-label, single-arm, phase II study enrolling 54 female RTT patients (ages 5-40), divided into groups of 18 (5-10, 11-17, 18-40 years). It aims to evaluate MTZ's safety and efficacy for mood, sleep, and motor symptoms, particularly hand control. Other ares of investigation include autonomic function, behavior, caregiver burden, clinical severity, and neuronal plasticity and metabolic biomarkers. Patients will receive escalating doses of MTZ oral solution: initial low doses (3.75-15 mg/day) for two weeks, followed by optimal doses (7.5-30 mg/day) for six months. Safety, tolerability, and symptoms will be monitored over 10 months (3-month screening, 6-month treatment, 1-month follow-up). The study is conducted at four Italian RTT-specialized hospitals, led by the University of Trieste. Partner sites are in Italy, specifically at the hospitals in Milan, Genova, Siena, and Messina.

Rett Syndrome (RTT) is a rare neurodevelopmental disease caused by a genetic mutation in the MECP2 gene located on the X chromosome and therefore particularly affects female subjects. It is characterised by an altered development of the nervous system leading to learning problems and delayed development of motor and cognitive skills. RTT is a progressive degenerative condition and there may be a worsening over time of the typical symptoms, including motor and cognitive impairments, loss of speech, repetitive hand movements, breathing abnormalities, gastrointestinal issues, mood disturbances, seizures and sleep problems. Given the reduced monoamine levels (serotonin, noradrenaline, dopamine) found in RTT, antidepressants -which modulates these neurotransmitters- may alleviate symptoms. Accordingly, mirtazapine (MTZ) was investigated, a noradrenergic and specific serotonergic antidepressant (NaSSA) for its potential benefits in RTT. Preclinical studies in MeCP2-mutant mice showed that MTZ improved respiratory, motor, and neurological function. In early human trials in RTT adults treated with MTZ for up to 5 years, reported benefits in sleep, mood, and social interactions, prompting this study in paediatric and young adult populations. Therefore, considering these preliminary results in humans, the hypothesis was put forward that MTZ could be beneficial also in RTT children.

MirtaRett is a multicenter, open-label, single-arm, phase II trial that will enroll 54 female RTT patients (ages 5-40), divided into three groups of 18 patients each (5-10 years, 11-17 years and 18-40 years). The overall goal of MirtaRett is to evaluate safety and efficacy of MTZ in the treatment of mood, sleep quality, motor symptoms in particular hand control, in Rett syndrome children and adults. Other areas that will be investigated include autonomic functions, behavior, caregiver burden and overall clinical severity along with neuroplasticity and metabolism biomarkers. Antidepressant drugs are typically administered using a scalar dosing approach, starting with low doses and gradually increasing them. Thus, patients will receive escalating doses of an oral solution of mirtazapine: initial low doses (3.75-15 mg/day) for two weeks, followed by optimal doses (7.5-30 mg/day) for six months. Safety, tolerability, and symptom changes will be monitored over 10 months (3-month screening, 6-month treatment, 1-month follow-up).

The study will be conducted across four Italian RTT-specialized hospitals, led by the University of Trieste.

• Coordinating centre - Department of Life Sciences, University of Trieste. Trieste, Italy.
• Partner 1. Epilepsy Center - Unit of Child Neurology, Hospital Santi Carlo Paolo and Department of Health Sciences, University of Milan, Milan, Italy.
• Partner 2. Unit of Child Neurology, Giannina Gaslini Institute, Genova, Italy.
• Partner 3. Pediatric Unit, Department of Woman and Child - Polyclinic Santa Maria alle Scotte. Siena, Italy.
• Partner 4. Department of Human Pathology in Adults and Children "Gaetano Barresi", University Polyclinic G. Martino, University of Messina. Messina, Italy.

INCLUSION CRITERIA

• 1. Female aged 5 to 39 years inclusive, at the time of signing the informed consent.

• 2. Girls of childbearing age negative to pregnancy test;

• 3. Body weight > 10 kg. and within the expected range for RTT, based on age and height.

• 4. Diagnosis of RTT based on consensus clinical criteria (Neul, 2010) and a confirmed mutation in MECP2 gene.

• 5. Breathing dysfunction (at least one of the following): period apnoea, intermittent hyperventilation, breath holding spells, air swallowing, forced expulsion of air and /or saliva.

• 6. Ten episodes or more/day of breathing dysfunction during wakefulness in the week prior to the screening visit (parents report).

  7. Stable medication regimen for 4 weeks prior to beginning the study (if receiving services - physical, occupational, or speech therapy - subjects must be on a stable regimen of these services for 3 months prior to beginning the study).

• 8. Female patients of childbearing potential must use a highly effective contraceptive method such as combined hormonal contraception (containing estrogen and progestin) associated with ovulation suppression (oral, intravaginal, transdermal); progestin-only hormonal contraception associated with ovulation suppression (oral, injectable, implantable); intrauterine device; hormone-releasing intrauterine system. Sexual abstinence is considered a highly effective contraceptive method if it aligns with the individual's usual lifestyle. Female patients of childbearing potential are to use adequate contraception as recommended by their Health Care Provider.

• 9. Written consent signed by parent/legal guardian/representative prior to screening visit

• 10. Patient is cooperative, willing to complete the study, and capable of doing so with assistance of a caregiver.

• 11. Caregiver is able to understand the instructions and fully participate.

EXCLUSION CRITERIA

Participants are excluded from the study if any of the following criteria apply:

• Patient is participating to another investigational clinical trial.
• Hypersensitivity to MTZ or any of the other ingredients of Mirtapil®.
• Clinically significant (as determined by the investigator) cardiovascular, respiratory, gastrointestinal, renal, hepatic, haematological pathologies or other pathologies, in addition to those directly related to RTT. In particular, patients with the following parameters will be excluded: leucocyte count is < 4000/mm2; neutrophil count is < 2000/mm3; hyponatremia (< 125 mmol/L); renal dysfunction (creatinine > 2 X ULN), hepatic dysfunction (AST, ALT, bilirubin > 2 X ULN); or if severe diabetes mellitus is present.
• QTcF interval on the ECG greater than 450 msec
• Surgery planned during the study.
• Severe diabetes mellitus (hyperglycaemia with values above 250/300 mg/dL);
• Pregnancy, breastfeeding.
• Evidence of clinically significant malnutrition with BMI (or BMI) (kg/m2) <
• Patients who manifested prior suicidal ideation.

The Clinical Trial will be carried out in Italy only. Individual clinical data are therefore strictly protected by the Italian laws and regulations and cannot be shared in accordance withthe "Regolamento Generale sulla Protezione dei Dati (GDPR)" together with the "Codice Privacy" (Decreto Legislativo 196/2003), and its modifications under the "Decreto Legislativo 101/2018".