Autologous ex vivo hematopoietic stem cell gene therapy is particularly relevant in lysosomal storage diseases (LSD) as it offers the prospect of both a safe transplant, as observed in immune deficiency and hematologic illness, and an effective transplant, since it delivers greater enzyme levels to host tissues than is possible in allogeneic transplant. We report early data from such an approach in an allogeneic transplant refractory LSD, Mucopolysaccharidosis type IIIA (MPSIIIA, Sanfilippo syndrome).

Background: MPSIIIA is a LSD caused by mutations in the SGSH gene leading to a deficiency of the enzyme N-sulfoglucosamine sulfohydrolase. As a result there is accumulation of heparan sulfate, with clinical manifestations of developmental delay, regression of previously acquired skills, hyperactivity, seizures and progressive cognitive decline leading to an early death at the end of the second decade of life. Unlike some other LSDs, MPSIIIA is unresponsive to allogeneic stem cell transplant (Hoogerbrugge et al., The Lancet 1995; Sivakumur & Wraith, Journal of inherited metabolic disease 1999), with the donor cells unable to deliver enough enzyme for clinically meaningful cross-correction.

Significant pre-clinical work undertaken at the University of Manchester led to the design of the lentiviral vector containing the SGSH gene and a CD11b (myeloid) promoter. In murine studies MPSIIIA mice underwent stem cell mobilization and collection and stem cells were transduced with the lentiviral vector ex vivo. The mice received myeloablative busulfan before being infused with the autologous transduced stem cells. Enzyme expression in the brain was high with normalised heparan sulfate and improvement in the behavior of the mice (Sergijenko et al., Molecular Therapy 2013). Transduction and transplant of human CD34+ stem cells to humanized NSG mice demonstrated stable engraftment with no evidence of viral shedding or transformational potential (Ellison et al., Molecular Therapy-Methods & Clinical Development 2019), further adding to the safety profile and the translation of this work to the clinic.

Study Design and Methods: This is a phase I/II safety and tolerability study. It is open-label and aims to recruit up to 5 patients. Patients enrolled into the trial are between the age of 3 and 24 months, have confirmed classical MPSIIIA (either by known genotypes, somatic features or family history) and have preserved neurocognitive function (DQ ≥80) before commencing the trial.

Patients undergo stem cell mobilization and peripheral collection of CD34+ cells. The SGSH gene under the CD11b promoter is introduced by the lentiviral vector. Patients then receive myeloablative busulfan before infusion of the autologous transduced stem cells. Follow up takes place over 3 years. The primary end point is to assess the safety and tolerability of the transduced stem cell product. The primary efficacy endpoint is SGSH activity in leukocytes at 12 months. Several secondary and exploratory end points are also to be reported including neurocognitive outcomes.

Current trial status: We report the preliminary data of the first treated patient recruited to the trial including the mobilization, transplant and sustained engraftment of gene-modified cells by vector copy number. Supra-physiological enzyme expression in multiple lineages - 150 fold increase above median in leukocytes and 200 fold increase above control in myeloid lineage - and substrate reduction in plasma, CSF and urine - reduced in urine to normal range by 3 months - has been observed.

Disclosures

Bigger:Orchard Therapeutics: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees, Research Funding. Thrasher:Orchard Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Equity ownership; Rocket Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Generation bio: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Equity ownership; 4Bio Capital: Consultancy, Membership on an entity's Board of Directors or advisory committees. Jones:Orchard Therapeutics: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees.

Author notes

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Asterisk with author names denotes non-ASH members.