Ex-vivo modification of autologous CD34+ HSPCs using a CD11B-directed lentiviral vector encoding APOEII-tagged human IDS leads to supraphysiological enzyme activity and early biochemical correction of neuronopathic mucopolysaccharidosis type II patients Free

Background: Mucopolysaccharidosis type II (MPSII, Hunter Syndrome) is a rare paediatric X-linked lysosomal storage disease caused by a deficiency in iduronate-2-sulphatase (IDS). In severe cases, this leads to the intracellular accumulation of glycosaminoglycans causing cellular dysfunction and death, which manifests in progressive multi-organ failure, neurodegeneration and early death in the second to third decade of life. Allogeneic transplant is the only disease-modifying therapy for neurological symptoms described, with very limited efficacy. We describe our first-in-man experience of autologous haematopoietic stem cell gene therapy (HSCGT) using a novel ex-vivo lentiviral (LV) platform to correct MPSII as part of a phase I/II clinical trial in Manchester, UK (NCT05665166).

Methods: Following eligibility assessments including independent expert review, our patient underwent CD34+ cell collection via leukapheresis using G-CSF and plerixafor. CD34+ haematopoietic stem cells (HSCs) were then transduced ex-vivo using a lentiviral vector containing the human IDS gene tagged with ApoEII driven by a human CD11b myeloid-specific promoter. Following quality control, gene-modified HSCs were transplanted after busulfan-based conditioning. Engraftment kinetics, biochemical parameters and adverse events were assessed at set time points.

Results: Three patients have been recruited with one product infusion and two CD34+ leukaphereses for product manufacturing performed at time of writing. Our first patient, a 2-year-old boy, is currently over 6 months post HSGCT with full engraftment and excellent biochemical correction.

The Pre-transplant HSC product met release criteria of vector copy number of 2.77 copies per cell, cell viability >70% and CD34+ purity 99.5% with14.94 x 10⁶ cells infused. Platelet engraftment was achieved on day +14, reticulocyte engraftment was achieved on day +17 and neutrophil engraftment was achieved on day +18 post-transplant. Minimal supportive transfusion was required during inpatient admission.

At 1-month post-treatment, enzyme activity analysis shows early supraphysiological activity in plasma and in leucocytes, predominantly of the CD15/myeloid lineage, as well as plasma and bone marrow. CSF IDS activity was detectable by the 1-month time point. There was corresponding substrate reduction with reduction of urinary GAG levels by 1 month. At 3-month post-treatment, there has been sustained supraphysiological IDS activity in plasma and leucocytes, continued improvement in CSF and BM IDS activity and urinary GAG levels have continued to fall to within the normal reference range.

Three serious adverse events were recorded to date. Of these, one was unrelated to HSCGT (central line-related pyrexia) and two had unclear relationship to the HSCGT. A transient dip in both platelet and neutrophil count was noted at day +29 and +43 respectively. After evaluation, no clear aetiology was found, and the patient has responded fully to a brief course of G-CSF administration with no further concerns regarding counts. There is no evidence of oligoclonality or insertional oncogenesis to date.

Discussion: Patient 1 has been treated and shows early and sustained multi-compartment biochemical correction. Treatment of a further 4 patients is planned, and CD34+ leukapheresis for product manufacture has already been performed for 2 patients, and all 5 patients recruited. In conclusion, treatment with this novel autologous HSCGT product is well tolerated and delivers supraphysiological enzyme levels. Further follow-up is required to correlate biochemical improvement with developmental and cognitive outcomes.