Mustang Bio Announces Updated Interim Data on X-Linked Severe Combined Immunodeficiency Treatment with Lentiviral Vector Gene Therapy Support Upcoming Multicenter Pivotal Phase 2 Trial for MB-107

All 23 treated patients are alive at 2.6-year median follow-up without evidence of malignant transformation

Data representing largest cohort of infants with XSCID treated with gene therapy presented at the 25^th Annual Meeting of the American Society of Gene & Cell Therapy

Excerpt from the Press Release:

WORCESTER, Mass., May 19, 2022 (GLOBE NEWSWIRE) — Mustang Bio, Inc. (“Mustang”) (NASDAQ: MBIO), a clinical-stage biopharmaceutical company focused on translating today’s medical breakthroughs in cell and gene therapies into potential cures for hematologic cancers, solid tumors and rare genetic diseases, today announced that interim Phase 1/2 data on treatment with the same lentiviral vector used in MB-107, Mustang’s lentiviral gene therapy for X-linked severe combined immunodeficiency (“XSCID”), also known as bubble boy disease, in newly diagnosed infants under the age of two, support plans to initiate a multicenter pivotal Phase 2 trial for MB-107 under the Company’s Investigational New Drug (“IND”) application in the second half of this year.

Representing the largest cohort of infants with XSCID treated with gene therapy, the data include 23 infants with XSCID treated with the lentiviral vector at a median age of 3 months (range: 2-14 months) with a median follow-up of 2.6 years (range: 4 months to 5.6 years). Transduced autologous bone marrow CD34+ cells were generated for all patients with a median vector copy number (VCN) of 0.81/cell (range: 0.16-1.81), and a median CD34+ cell dose of 9.61×10⁶/kg (range 4.40-22.45). Prior to the infusion of cells, patients received busulfan targeted to a cumulative area-under-the-curve (cAUC) of 22 mg*hr/L. The treatment was well tolerated, and all patients experienced complete hematopoietic recovery. Severe adverse events occurred in three patients (two patients with pancytopenia and hemolytic anemia and one patient with delayed neutrophil engraftment), and all resolved. Seventeen patients had active infections prior to therapy, and in all cases these infections cleared. In addition, 15 patients have so far been able to discontinue intravenous immunoglobulin, and to date 14 patients have been successfully immunized. All patients are currently alive with stable vector marking in all cell lineages and without evidence of malignant transformation.

“We are encouraged by the interim results of this ongoing trial, and our post-treatment follow-up shows that patients continue to do very well,” said Ewelina Mamcarz, M.D., Department of Bone Marrow Transplantation and Cellular Therapy, St. Jude Children’s Research Hospital (“St. Jude”). “The therapy appears to be safe and effective, and the patients in the trial to date have developed functional immune systems with no evidence of abnormal cell division. We plan to continue monitoring these patients for ongoing safety and sustained results, given the limited number of studies where patients have received gene therapy at such a young age.”

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