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Lentiviral gene therapy with busulfan conditioning safe, effective in infants with SCID-X1
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Infants with newly diagnosed X-linked severe combined immunodeficiency, or SCID-X1, demonstrated multilineage engraftment of transduced cells after receiving lentiviral vector gene therapy and nonmyeloablative targeted busulfan conditioning, according to study findings published in The New England Journal of Medicine.
Results showed the treatment had a favorable overall safety profile and led to functional T-cell and B-cell reconstitution and normalization of natural killer cell counts.
“These patients are toddlers now, who are responding to vaccinations and have immune systems to make all immune cells they need for protection from infections as they explore the world and live normal lives. This is a first for patients with SCID-X1,” Ewelina Mamcarz, MD, assistant faculty member in the department of bone marrow transplantation and cellular therapy at St. Jude Children’s Research Hospital, said in a press release.
SCID is caused by a mutation in the interleukin-2 receptor subunit gamma (IL2RG) gene. The current standard of care for SCID-X1 — also known as “bubble boy disease,” as the patients are born with no immune system and must undergo protective precautions — is hematopoietic stem cell transplantation with a tissue-matched sibling donor. However, more than 80% of these patients lack an appropriate donor and must rely on blood stem cells from other donors, a procedure that has a higher risk for adverse events and lower likelihood of cure.
The gene therapy involved collecting patients' bone marrow, then using a vector virus to insert a correct copy of the IL2RG gene into the patients’ blood stem cell DNA. The cells were then frozen and underwent quality testing.
The vector is unique in that it contains insulators to block activation of genes adjacent to where IL2RG is inserted to prevent the gene therapy from inadvertently causing leukemia by switching on an oncogene.
Also, researchers used a stable producer cell line and cryopreservation to streamline vector production and gene therapy treatment, which would enable expanded treatment access and facilitate commercializing production.
“Previous studies of gene therapy showed that first-generation gamma-retroviral vectors restored T-cell immunity but resulted in vector-induced leukemia. Second-generation gamma-retroviral vectors had an improved safety profile but did not restore humoral immunity or durable [natural killer] cell production when used without conditioning chemotherapy,” the researchers wrote. “Our new lentiviral vector gene therapy, combined with nonmyeloablative busulfan conditioning, has been successful in restoring immunity in five patients [aged] 7 to 23 years in whom a previous allogeneic HSCT for SCID-X1 had failed; two of the five patients gained normal B-cell function and freedom from immune globulin infusions.”
In the dual-center, phase 1/phase 2 safety and efficacy study, Mamcarz and colleagues evaluated eight consecutive infants (median age, 3.5 months) with newly diagnosed SCID-X1 who did not have a matched sibling donor for HSCT. The patients — including four treated at St. Jude Children’s Research Hospital and four at UCSF Benioff Children’s Hospital — all received stem cells between September 2016 and March 2018.
The researchers administered one to two daily doses of IV busulfan to target a cumulative area under the curve of 22 mg an hour per liter. They used a population-based pharmacokinetic model to customize the initial dose based on the weight and age of the patient.
“We found that the addition of very low doses of busulfan based on a model developed at UCSF increased engraftment of gene-corrected stem cells in the bone marrow without causing the side effects associated with standard doses," Mort Cowan, MD, professor of pediatrics and principal investigator of the trial at UCSF, said in the release.
Evaluation of the safety, feasibility and efficacy of lentiviral gene transfer in these patients served as the study’s primary endpoint.
Median follow-up was 16.4 months (range, 6-24).
The researchers observed no unanticipated side effects associated with bone marrow harvest, busulfan conditioning or cell infusion.
Seven infants showed normalization of CD3-positive, CD4-positive and naive CD4-positive T cells and natural killer cells within 3 to 4 months of infusion. This coincided with vector marking in T cells, B cells, natural killer cells, myeloid cells and bone marrow progenitors.
One infant initially had an inadequate T-cell count but developed T cells after a boost of gene-corrected cells without busulfan conditioning.
All infants demonstrated clearance of previous infections and continued to experience normal growth. Seven of the eight infants showed normalization of immunoglobin M levels, and four of them ceased IV immune globulin supplementation. Three of the four infants responded to vaccines.
“While longer follow-up is needed to assess any late effects of treatment, these results suggest most patients treated with this gene therapy will develop a complete durable immune response without side effects,” Cowan said.
The researchers conducted vector insertion-site assessment in seven infants, all of whom demonstrated polyclonal patterns without clonal dominance.
“We found that the combination of a lentiviral vector construct and nonmyeloablative targeted busulfan conditioning in infants with newly diagnosed SCID-X1 resulted in multilineage engraftment of gene-marked cells, with prompt and durable normalization of T-cell immunity and high-level gene marking in B cells and [natural killer] cells, with normalization of IgM levels and the development of normal antibody responses,” the researchers wrote. “It is hoped that durable, complete adaptive immunity will be achieved in the majority of patients over time.” – by Jennifer Byrne
Disclosures: Mamcarz reports no relevant financial disclosures. Cowan reports grants from California Institute of Regenerative Medicine and the NIH NIAID during the conduct of the study; as well as personal fees from Bluebird Bio, Exogen Bio, UpToDate and Homology Medicines. Please see the study for all other authors’ relevant financial disclosures.
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