Rett syndrome (RTT) is a rare genetic neurodevelopmental disorder caused primarily by mutations in the MECP2 gene, leading to progressive impairments in motor function, communication, and behavior following an initial period of apparently typical development. Currently, there are no treatments that change the course of the disease, and clinical care is largely focused on managing symptoms. Loss of MeCP2 function has been associated with increased activity of the LINE-1 (L1) retroelement, which may contribute to neuroinflammation and cellular stress in the brain. Lamivudine, a nucleoside reverse transcriptase inhibitor widely used in antiviral therapy, can inhibit L1 reverse transcription and has shown beneficial effects in preclinical models of RTT, including reductions in inflammatory and oxidative stress markers and improvements in neurological and behavioral outcomes. This study aims to evaluate the safety and potential clinical and biological effects of lamivudine in individuals with Rett syndrome using a before-and-after treatment design. Participants will receive oral lamivudine and will undergo clinical assessments and laboratory testing before and after the treatment period to evaluate changes in symptom severity, functional status, quality of life, seizure activity, and biomarkers related to inflammation and neurodevelopment. Biological samples will also be collected to support translational laboratory studies aimed at improving understanding of disease mechanisms and treatment response in RTT. Results from this study may help determine whether lamivudine is a safe and promising therapeutic option and may guide future clinical research in this population.

Rett syndrome (RTT) is a rare genetic neurodevelopmental disorder caused primarily by loss-of-function mutations in the MECP2 gene, leading to progressive impairments in motor, cognitive, and communicative function after an initial period of apparently typical development. Currently, there are no medications that change the course of the disease, and treatment is mainly focused on managing symptoms.

Functional MeCP2 represses the activity of the LINE-1 (L1) retroelement, and loss of this repression may result in increased L1 accumulation in glial cells in the brain, promoting genomic instability, oxidative stress, and neuroinflammation associated with RTT pathophysiology.

Lamivudine inhibits L1 reverse transcription and has shown beneficial effects in preclinical neuronal culture and MeCP2-deficient mouse models, including reduced inflammatory and oxidative stress markers, improved neurological and behavioral outcomes, and increased survival. In this study, each participant is evaluated before and after receiving treatment, allowing changes to be assessed within the same individual over time. Following screening and baseline assessments, participants enter a treatment phase during which oral lamivudine is administered for a defined period. Longitudinal clinical, laboratory, and safety evaluations are conducted at scheduled visits, followed by a post-treatment follow-up period to assess the persistence of effects.

A 24-week intervention period was selected as a feasible and acceptable duration for participants and caregivers and as an appropriate timeframe for an initial open-label evaluation of therapeutic potential. This duration is expected to provide sufficient exposure to lamivudine to allow detection of short-term clinical and biological changes suggested by preclinical studies, while also enabling systematic assessment of safety and tolerability in this population.

Participants may continue their usual medical treatments, including medications for seizures and other supportive therapies, as long as these treatments remain stable during the study, unless a change is medically necessary.

Caregivers receive standardized instructions on medication administration, and adherence is monitored throughout the treatment phase through caregiver reporting and visit-based verification to ensure accurate assessment of treatment exposure.

Clinical assessments employ validated clinician and caregiver-reported instruments to evaluate symptom severity, functional status, quality of life, and seizure activity, capturing changes across motor, behavioral, and daily functioning domains relevant to RTT. Safety is monitored continuously through adverse event surveillance, routine clinical evaluations, and laboratory testing, with predefined criteria for treatment interruption if clinically indicated. Venous blood samples are collected at baseline and during treatment to evaluate systemic biomarkers related to oxidative stress, inflammation, and neurodevelopment. These measures complement clinical outcomes and explore potential mechanistic effects of lamivudine treatment.

In addition to the biomarkers reported as outcome measures, a panel of exploratory, hypothesis-generating biomarkers will be analyzed, including markers of oxidative stress, neuroinflammation and neurodevelopment, mitochondrial and energy metabolism, and nutritional and micronutrient status. These analyses are not considered clinical endpoints and are not reported as outcome measures.

As a translational research component, skin punch biopsies from a subset of participants are used to establish a dermal fibroblast biobank, enabling the generation of patient-derived induced pluripotent stem cells and brain organoids. These models will be used in exploratory laboratory studies to investigate cellular and molecular mechanisms in RTT and responses to lamivudine, without being considered clinical endpoints. Based on preclinical evidence and the proposed mechanism of action, lamivudine treatment is expected to be safe and well-tolerated in individuals with Rett syndrome. Clinically, it may lead to improvements in motor function, social engagement, and overall symptom severity, while reducing seizure frequency and enhancing quality of life. Biologically, lamivudine is anticipated to modulate oxidative stress and inflammatory pathways, potentially restoring a more balanced neurodevelopmental environment. The combined clinical and laboratory observations from this study will provide critical insights into the therapeutic potential of lamivudine and inform future research directions in RTT.

Inclusion Criteria:

• Clinical and molecular diagnosis of Rett syndrome;
• Age 2 years or older at the time of enrollment;
• Ability to swallow liquid medication;
• Stable clinical condition, as determined by the study investigator;
• Availability of a parent or legal guardian able to provide informed consent and comply with study procedures;
• Willingness of the participant and/or legal guardian to comply with study visits and assessments.

Exclusion Criteria:

• Known hypersensitivity or contraindication to lamivudine;
• Severe hepatic or renal impairment that, in the investigator's judgment, would preclude safe participation;
• Use of investigational drugs or participation in another clinical trial within a defined washout period before enrollment;
• Presence of any medical condition or acute illness that could interfere with study participation or outcome assessment, as determined by the investigator;
• Inability to undergo blood collection or skin biopsy procedures required for the study;
• Any condition that, in the opinion of the investigator, would place the participant at undue risk or compromise adherence to the study protocol.