For infants with newly diagnosed X-linked severe combined immunodeficiency (SCID-X1), lentiviral gene therapy and targeted busulfan conditioning successfully induced multilineage engraftment of transduced cells, researchers reported.
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By 3-4 months after infusion, seven of eight patients had normal numbers of CD3+, CD4+, and naive CD4+ T cells; normal counts of natural killer (NK) cells; and vector marking of T cells, B cells, NK cells, myeloid cells, and bone marrow progenitors, Ewelina Mamcarz, MD, of St. Jude Children’s Research Hospital in Memphis, and her associates reported in the New England Journal of Medicine.
The eighth infant at first lacked a sufficient T-cell response but responded to a boost of gene-corrected cells without busulfan conditioning.
By 6-12 months after infusion, IgM levels also had normalized in seven of the eight infants and showed polyclonal patterns without clonal dominance, according to the investigators. Among four infants who were able to stop intravenous immunoglobulin therapy, three responded to vaccinations with tetanus, diphtheria, pertussis, polio, and pneumococcal polysaccharide. Such restoration of humoral immunity “has not been achieved in previously reported trials of gene therapy for infants with newly diagnosed SCID-X1,” wrote the investigators of this dual-center, phase 1/2 study.
X-linked severe combined immunodeficiency – “bubble boy disease” – is characterized by a lack of T cells, NK cells, and B cells, and is caused by mutations in IL2RG. Some 80% of affected infants have no matched sibling donor for hematopoietic stem cell transplantation, and transplantation from other donors can produce an inadequate response and graft-versus-host disease. Prior attempts at gene therapy with gamma-retroviral vectors had led to vector-induced leukemia or had failed to induce humoral immunity or normal NK cell production.
“Our new lentiviral vector gene therapy combined with nonmyeloablative busulfan conditioning has been successful in restoring immunity in five patients 7-23 years of age in whom a previous allogeneic hematopoietic stem cell transplantation for SCID-X1 had failed,” the investigators wrote. “We hypothesized that the combination of this lentiviral vector and low-exposure busulfan administered by means of pharmacokinetic dose targeting would be safe and effective as the primary treatment in infants with newly diagnosed SCID-X1.”
Their protocol included one to two daily intravenous doses of busulfan, targeting a cumulative area under the curve of 22 mg per hr/L. They calculated the first dose by weight and age using a population-based pharmacokinetic model and adjusted the second dose based on first-dose pharmacokinetics.
After a median of 16.4 months, all infants continued to grow normally and cleared previous infections, and there were no unanticipated side effects from bone marrow harvest, busulfan conditioning, or cell infusion.
“It is hoped that durable, complete adaptive immunity will be achieved in the majority of the patients over time,” the researchers wrote.
They continue to follow the patients to assess therapeutic safety, immune durability, and persistence of the transferred gene in hematopoietic and immune cells.
Study funders included the American Lebanese Syrian Associated Charities, the National Institutes of Health, the California Institute of Regenerative Medicine, and the Assisi Foundation of Memphis. St. Jude Children’s Research Hospital has licensed the gene therapy and partnered with Mustang Bio to develop and commercialize it. Dr. Mamcarz reported receiving grant support from the study funders.
SOURCE: Mamcarz E et al. N Engl J Med. 2019; 380:1525-34.