Evaluation of Safety and Efficacy of Genetically Modified Autologous CD34+ Hematopoietic Stem Cells in Pediatric Mucopolysaccharidosis Type I Hurler Patients

The primary objective of this study is to evaluate the **safety** and tolerability of autologous CD34+ cell enriched fraction containing hematopoietic stem cells (HSC) transduced with a lentiviral vector encoding the IDUA gene in pediatric patients with Mucopolysaccharidosis Type I, Hurler variant (MPS IH). This evaluation is conducted following a myeloablative and lymphoablative conditioning regimen. The clinical relevance of this objective lies in its potential to provide a novel therapeutic approach for MPS IH, a severe metabolic disorder, by assessing the feasibility and safety of gene therapy using genetically modified autologous cells.

The clinical trial focuses on pediatric patients diagnosed with **Mucopolysaccharidosis type I Hurler** (MPS IH). The study population includes both male and female subjects, ranging in age from 28 days to 11 years. Participants are required to have biochemically and molecularly confirmed MPS IH and must demonstrate a Lansky Index greater than 80%, indicating a relatively stable general health status. The trial targets a vulnerable population, as it involves children with a rare genetic disorder. Participants must have an indication for hematopoietic stem cell transplantation (HSCT) and lack a suitable HLA-matched sibling or cord blood donor, unless their country does not offer unrelated donor cord blood transplantation. Adequate cardiac, renal, hepatic, and pulmonary functions are necessary for inclusion. The sponsor has not provided information regarding the total number of participants in the study.

The clinical trial is designed to evaluate the **safety** and efficacy of autologous hematopoietic stem and progenitor cells genetically modified with the IDUA lentiviral vector for the treatment of patients with **Mucopolysaccharidosis Type I, Hurler variant**. This is a Phase I/II study, characterized by a randomized, double-blind, and controlled design. The trial is expected to run from May 14, 2018, to December 21, 2027, with participant involvement lasting up to eight years, depending on individual response and safety outcomes.

Participants will undergo a series of study visits, beginning with an inclusion (screening) visit to confirm eligibility based on criteria such as age, biochemical and molecular diagnosis, and the absence of a suitable donor. Following successful screening, participants will receive a myeloablative and lymphoablative conditioning regimen before the administration of the investigational product. The primary endpoints include overall survival, hematological engraftment by day 45 post-injection, and the safety of the autologous CD34+ cells transduced with the lentiviral vector. Safety will be assessed through adverse event recording and monitoring for replication-competent lentivirus and insertional mutagenesis over the study duration.

Follow-up visits will occur at regular intervals to monitor IDUA activity in blood and overall safety and tolerability. The end-of-study visit will conclude the participant's involvement, provided there are no adverse events necessitating early termination. Conditions for early withdrawal include significant adverse reactions, non-compliance with study procedures, or withdrawal of consent. The trial aims to provide comprehensive data on the therapeutic potential and safety profile of this advanced therapy in a pediatric population.

The clinical trial involves the administration of several **experimental medications** and **non-experimental treatments**. The primary investigational product is **OTL-203**, a dispersion for infusion containing **autologous CD34+ haematopoietic stem and progenitor cells** genetically modified with the lentiviral vector IDUA LV, encoding for the alpha-L-iduronidase cDNA. This advanced therapy is administered intravenously. The role of OTL-203 in the trial is to evaluate its safety and efficacy in patients with Mucopolysaccharidosis Type I, Hurler variant.

**Lenograstim** is used as an auxiliary treatment in the trial. It is a protein-based medication administered intravenously. The pharmaceutical form is denoted as PHF675. Lenograstim is utilized to support the mobilization of stem cells in the participants.

**Plerixafor**, a chemical compound, is administered subcutaneously. It is provided in the pharmaceutical form PHF00230MIG and serves as an auxiliary treatment to enhance the mobilization of hematopoietic stem cells.

**Busulfan** is another auxiliary chemical treatment used in the trial. It is administered intravenously in the form PHF00230MIG. Busulfan is part of the conditioning regimen to prepare patients for the infusion of genetically modified stem cells.

**Fludarabine** is administered intravenously as part of the conditioning regimen. It is a chemical compound provided in the pharmaceutical form PHF675. Fludarabine aids in the lymphoablative conditioning of patients.

**Rituximab** is a protein-based auxiliary treatment administered intravenously. It is provided in the pharmaceutical form PHF00230MIG. Rituximab is used to deplete B-cells as part of the conditioning regimen.

Participant compliance with the dosing schedules is monitored throughout the trial to ensure adherence to the treatment protocol. The trial aims to assess the safety and tolerability of the investigational and auxiliary treatments in pediatric patients with Mucopolysaccharidosis Type I, Hurler variant.

The efficacy of the investigational treatment in the clinical trial will be assessed using several primary endpoints. These include overall survival and the achievement of hematological engraftment by day 45 following the administration of the Advanced Therapy Investigational Medicinal Product (ATIMP). Hematological engraftment is defined as the first occurrence of three consecutive days with a neutrophil count greater than 500/mm³ and platelet count exceeding 20,000/mm³ without platelet transfusion for seven consecutive days. Additionally, the safety of administering autologous hematopoietic stem cells (HSC) transduced with the lentiviral vector encoding the **IDUA** gene will be evaluated. This will be measured by short-term tolerability within 24 hours post-injection, the absence of Replication Competent Lentivirus (RCL) over a period of up to eight years, and the absence of malignancy or abnormal clonal proliferation due to insertional mutagenesis over the same duration. Overall safety and tolerability will also be assessed through the recording of adverse events. Furthermore, the activity of **IDUA** in blood, measured via dried blood spots (DBS), will be evaluated to reach supraphysiologic levels at one year post-treatment.