Biochemical Engraftment and Clinical Outcomes Following Ex-Vivo Autologous Stem Cell Gene Therapy for Mucopolysaccharidosis Type IIIA

Background: Mucopolysaccharidosis type IIIA (MPSIIIA, Sanfilippo syndrome) is a lysosomal storage disease caused by a gene defect in the SGSH gene, producing the defective enzyme N-sulfoglucosamine sulfohydrolase. Children with MPSIIIA, accumulate the substrate of SGSH, heparan sulphate, in all cells in the body, causing developmental delay, regression of previously acquired skills, hyperactivity, seizures and progressive cognitive decline. The disease is life-limiting, with death occurring typically within the second decade of life. MPSIIIA is unresponsive to allogeneic stem cell transplant, presumed because insufficient enzyme is delivered via cross correction from donor-derived engrafted cells, particularly in the brain.

Autologous ex vivo hematopoietic stem cell gene therapy (HSC-GT) is an experimental treatment for MPSIIIA, with the insertion of a functional copy of the SGSH gene into the patient's own CD34⁺ stem cells prior to transplant, to allow overexpression of the missing SGSH enzyme. In our pre-clinical work, we tested a lentiviral vector containing the SGSH gene driven by the human CD11b (myeloid-specific) promoter in a mouse model of MPSIIIA. This lentiviral vector was used to transduce CD34⁺ stem cells before transplant into fully-myeloablated busulfan-treated animals. Six-months post-transplant, SGSH enzyme was detected in the brain, with normalised behavior and heparan sulfate levels. Transduction and transplant of human CD34⁺ stem cells into humanized NSG mice demonstrated stable engraftment with no evidence of viral shedding or transformational potential, supporting the translation of this work to the clinic.

Study Design: This is a phase I/II open label study (NCT04201405/EudraCT#2019-002051-42) with primary outcomes of safety and tolerability and peripheral expression of SGSH activity in total leukocytes at 12 months. Secondary endpoints include neurocognition with measures including the Bayley Scales of Infant and Toddler Development. Patients will be followed for a minimum of 3 years.

Five patients with severe MPSIIIA, aged 6 to 24 months, have been treated with HSC-GT as part of this trial, completing enrolment. An additional patient was treated off-trial, at 30 months old, on a compassionate use basis. All children underwent stem cell mobilization and peripheral collection of CD34+ cells, followed by transduction with the lentiviral vector and cryopreservation. Patients received myeloablative busulfan condition before infusion of the drug product.

Trial Status and Results: Three trial patients have >18 months follow-up post-transplant, one patient has >12 months and one patient has >9 months, with the off-trial 'special' patient >36 months. Vector copy number of the IMP ranged from 1.19 to 8.91 copies/cell, with a cell dose between 4.3 to 22.7x10⁶ CD34⁺/kg. Engraftment was rapid with median time to neutrophil, platelet and red cell engraftment of 19, 28 and 25 days, respectively. In all patients, engraftment is sustained to date, with supra-physiological levels of SGSH enzyme measurable in leukocytes (38-91-fold above median normal range at one-month post-transplant; see Figure A). Supra-physiological SGSH levels were also rapidly detected in CD15⁺ lineages, plasma and bone marrow. CSF SGSH levels were within or above normal range by 6-months post-transplant. Abnormal heparan sulfate levels at baseline were rapidly reduced in both urine (>90%) and plasma (82%, see Figure B).

Data from the specials patient, continues to show supra-physiological biochemical data and neurocognitive outcomes at 36 months post-transplant indicating an improved phenotype compared to MPSIIIA natural history. Neurocognitive assessments of trial patients continue and early outcomes will be presented suggesting an alteration in the neurologic phenotype of the disease in one patient, whilst three patients are currently within the normal DQ range but require longer follow-up. All trial patients show an improvement in SGSH expression in leukocytes and other lineages at early time points compared to that of the specials patient.

Conclusions: Treatment with ex-vivo autologous HSC-GT is generally well tolerated and delivers supra-physiological levels of enzyme throughout the body.

Wynn:AVROBIO: Other: licence agreement. Holley:Avrobio: Other: licence payment. Thrasher:Orchard Therapeutics: Consultancy; Rocket Pharmaceuticals: Consultancy; 4bio capital: Consultancy; Generation Bio: Consultancy, Current equity holder in publicly-traded company. Booth:GSK: Honoraria; Takeda: Honoraria; Orchard Therapeutics: Consultancy, Honoraria; SOBI: Consultancy, Honoraria; Rocket Pharmaceuticals, Inc.: Consultancy. Rust:Takeda: Other: Travel grants, Speakers Bureau. Jones:AVROBIO: Other: licence agreement, Research Funding. Ellison:AVROBIO: Other: licence payment. Bigger:AVROBIO: Consultancy, Other: Licence agreement, Research Funding; Orchard Therapeutics: Consultancy, Research Funding.