Magenta Therapeutics to Present Preclinical Data on E478 Stem Cell Gene Therapy Expansion Program at the 2019 Annual Meeting of the American Society of Gene and Cell Therapy

— Developing novel methods to expand gene-modified stem cells to
achieve higher cell doses —

CAMBRIDGE, Mass.–(BUSINESS WIRE)–Magenta
Therapeutics (NASDAQ:MGTA), a clinical-stage biotechnology company
developing novel medicines to bring the curative power of stem cell
transplant to more patients, today announced the Company will highlight
preclinical data on its E478 program in an oral presentation on
Thursday, May 2, at the annual meeting of the American Society of Gene
and Cell Therapy (ASGCT) in Washington, D.C.

Magenta is developing E478 to achieve high doses of gene-modified stem
cells for better outcomes in patients with genetic disorders, including
sickle cell disease and thalassemia, where viral vector or gene editing
technologies are used to correct stem cells. E478 is a novel and
proprietary small molecule that uses AHR antagonism to expand
gene-modified hematopoietic stem cells during manufacture, then the
corrected cells are infused as the medicine for the patient. Magenta
intends to develop E478 in partnership with gene therapy companies.

“Stem cell gene therapy and genome editing are promising approaches for
treating genetic disorders. However, these therapies are most effective
when a large dose of gene-modified cells is administered by a stem cell
transplant, as cell dose is crucial for patient outcomes. Expansion may
also increase the efficiency of manufacturing these therapies, which has
proved to be a challenge with current approaches,” said Michael Cooke,
Ph.D., Chief Scientific Officer, Magenta Therapeutics. “AHR antagonism
is a clinically validated mechanism for expanding hematopoietic stem
cells, as we have reported in Phase II studies with our MGTA-456 cell
therapy that uses the same mechanism. Consistent with the E478 data
presented to date, we believe the results for ASGCT show that AHR
antagonism is a promising method for expanding gene-modified stem cells
while maintaining the editing or transduction rates, which will be
important for patients receiving these therapies.”

Title: A Novel Aryl Hydrocarbon Receptor Antagonist Expands Adult
Human Hematopoietic Stem Cells from Mobilized Peripheral Blood and Bone
Marrow and Increases the Dose of CRISPR/Cas9 Gene-Edited
NSG-Repopulating Cells, Abstract #979
Presenter: Megan
Hoban, Ph.D., Magenta Therapeutics
Presentation Date and Time:
Thursday, May 2, 2019; 10:45 a.m. ET
Session Title: Engineered
Cell Therapies
Room: Heights Courtyard 2

Forward-Looking Statement

This press release contains forward-looking statements and information
within the meaning of The Private Securities Litigation Reform Act of
1995 and other federal securities laws. The use of words such as “may,”
“will,” “could”, “should,” “expects,” “intends,” “plans,” “anticipates,”
“believes,” “estimates,” “predicts,” “projects,” “seeks,” “endeavor,”
“potential,” “continue” or the negative of such words or other similar
expressions can be used to identify forward-looking statements.

The express or implied forward-looking statements included in this press
release are only predictions and are subject to a number of risks,
uncertainties and assumptions, including, without limitation:
uncertainties inherent in clinical studies and in the availability and
timing of data from ongoing clinical studies; whether interim results
from a clinical trial will be predictive of the final results of the
trial; whether results from preclinical studies or earlier clinical
studies will be predictive of the results of future trials; the expected
timing of submissions for regulatory approval or review by governmental
authorities, including review under accelerated approval processes;
orphan drug designation eligibility; regulatory approvals to conduct
trials or to market products; whether Magenta’s cash resources will be
sufficient to fund Magenta’s foreseeable and unforeseeable operating
expenses and capital expenditure requirements; and other risks set forth
under the caption “Risk Factors” in Magenta’s Registration Statement on
Form S-1, as updated by Magenta’s most recent Annual Report on Form 10-K
and its other filings with the Securities and Exchange Commission. In
light of these risks, uncertainties and assumptions, the forward-looking
events and circumstances discussed in this press release may not occur
and actual results could differ materially and adversely from those
anticipated or implied in the forward-looking statements. You should not
rely upon forward-looking statements as predictions of future events.
Although Magenta believes that the expectations reflected in the
forward-looking statements are reasonable, it cannot guarantee that the
future results, levels of activity, performance or events and
circumstances reflected in the forward-looking statements will be
achieved or occur.

Moreover, except as required by law, neither Magenta nor any other
person assumes responsibility for the accuracy and completeness of the
forward-looking statements included in this press release. Any
forward-looking statement included in this press release speaks only as
of the date on which it was made. We undertake no obligation to publicly
update or revise any forward-looking statement, whether as a result of
new information, future events or otherwise, except as required by law.

Contacts

Magenta Therapeutics:
Manisha Pai, Vice President,
Communications & Investor Relations
617-510-9193
[email protected]