Magenta Therapeutics Announces Additional Preliminary Positive Results from Ongoing Phase 2 Clinical Trial of MGTA-145 and Plerixafor in Patients with Multiple Myeloma at the American Society of Clinical Oncology (ASCO) Annual Meeting

– Preliminary results from the initial cohort of 10 patients were previously reported on May 12 in a published abstract at the European Hematology Association (EHA) Congress –

– The cumulative data as of the data cut-off date show that all patients (15/15) treated with MGTA-145 and plerixafor met the primary endpoint of sufficient stem cell mobilization and collection for transplant –

– All patients (12/12) transplanted with MGTA-145 and plerixafor to date have successfully engrafted; six patients have completed day-100 follow up with demonstrated durable engraftment –

– These results will also be presented at the EHA Congress, June 9-17, 2021 –

Magenta Therapeutics (Nasdaq: MGTA), a clinical-stage biotechnology company developing novel medicines to bring the curative power of stem cell transplants to more patients, today announced additional positive results from a Phase 2 clinical trial of MGTA-145 and plerixafor in patients with multiple myeloma at the American Society of Clinical Oncology (ASCO) Annual Meeting, being held virtually June 4-8, 2021.

“We are very pleased to see continued favorable results for MGTA-145 and plerixafor for stem cell mobilization and collection in patients with multiple myeloma,” said Jason Gardner, D.Phil., President and Chief Executive Officer, Magenta Therapeutics. “These results build on those previously disclosed from this study and the Phase 1 trials to further demonstrate MGTA-145 and plerixafor’s potential as a rapid, reliable, well-tolerated approach to stem cell mobilization and collection, which has positive implications for patients and donors.”

Additional Results – MGTA-145 Multiple Myeloma Phase 2 Clinical Trial

The investigator-initiated, 25-patient Phase 2 clinical trial is designed to evaluate the ability of MGTA-145, in combination with plerixafor, to mobilize and collect hematopoietic stem cells for autologous stem cell transplant in patients with multiple myeloma. This study is led by Surbhi Sidana, M.D., Assistant Professor of Medicine in the Division of Blood and Marrow Transplantation and Cellular Therapy at Stanford University School of Medicine.

Previously reported results from this trial were announced on May 12, 2021, in a published abstract for the European Hematology Association (EHA) Congress and provided preliminary data from the initial cohort of 10 patients.

Summary of cumulative results through data cut-off date:

• All patients (15/15) have met the primary endpoint of mobilization and collection of 2 million CD34⁺ stem cells per kg in up to two days. Twelve of 15 patients achieved the primary endpoint in a single day of dosing and collection.
• The median number of total CD34⁺ stem cells collected on day 1 and 2 (if needed) was 6.3 million per kg. Current standard of care with G-CSF-based regimens require a minimum of five days of dosing to initiate stem cell collection.
• All transplanted patients to date (12/12) successfully engrafted, with median recovery of neutrophils after 12.5 days and platelets after 18 days, which are within transplant expectations in multiple myeloma.
• Six patients have completed day-100 follow up, with demonstrated durable engraftment indicative of a successful transplant.
• The collected CD34⁺ stem cells contain a high percentage (40%) of CD34+CD90+, a stem cell population associated with multi-lineage, long-term engraftment, an amount substantially greater than historically observed with G-CSF-mobilized cells.
• The regimen of MGTA-145 and plerixafor was well tolerated. Acute, transient, MGTA-145-related grade 1 bone or musculoskeletal pain was observed in 40% of patients shortly after MGTA-145 infusion, resolving within 10 minutes for all patients, and none required pain medication or other intervention.

As indicated previously, this trial has broad and clinically representative inclusion criteria and includes patients that represent the general transplant-eligible population of patients with multiple myeloma. Patients enrolled in this trial included those patients with risk factors that could impact stem cell mobilization and collection, such as myeloma-directed therapies that are known to impact stem cell collection, previous malignancy treated with chemotherapy and/or radiation, and other co-morbid conditions. Mobilization agents may be less effective in patients with multiple risk factors. Final clinical data from this trial are anticipated by the end of 2021. MGTA-145 is also being evaluated for its ability to mobilize and collect stem cells from donors for allogenic transplant in patients with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) in a Phase 2 trial, and an additional Phase 2 study is planned to initiate in patients with sickle cell disease in the second half of 2021.

ASCO Poster Presentation

Title: Phase 2 Study of MGTA-145 + Plerixafor for Rapid and Reliable Hematopoietic Stem Cell (HSC) Mobilization for Autologous Stem Cell Transplant in Multiple Myeloma (Abstract #8023)

Author: Surbhi Sidana, M.D., Assistant Professor of Medicine in the Division of Blood and Marrow Transplantation and Cellular Therapy, Stanford University School of Medicine

Poster Session: Hematologic Malignancies – Plasma Cell Dyscrasia

Date/Time: All e-posters are now available in the ASCO Annual Meeting virtual platform

These results will also be presented as an encore at the EHA Virtual Congress, available via the conference’s virtual platform on Friday, June 11 at 3:00am EDT / 9:00am CEST.

About Magenta Therapeutics

Magenta Therapeutics is a clinical-stage biotechnology company developing medicines to bring the curative power of stem cell transplant to more patients with blood cancers, genetic diseases and autoimmune diseases. Magenta is combining leadership in stem cell biology and biotherapeutics development with clinical and regulatory expertise, a unique business model and broad networks in the stem cell transplant community to revolutionize immune reset for more patients.

Magenta is based in Cambridge, Mass. For more information, please visit www.magentatx.com.

Follow Magenta on Twitter: @magentatx.

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