Magenta Therapeutics Presents Updated Phase 2 Clinical Data on MGTA-456 Cell Therapy at American Academy of Neurology Annual Meeting

— Six-month follow-up on patients with cerebral adrenoleukodystrophy
(cALD) shows stable neurological function scores and early and
persistent resolution of brain inflammation on MRI —

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 that the Company presented
Phase 2 clinical data on its cell therapy, MGTA-456, at the annual
meeting of the American Academy of Neurology in Philadelphia,
Pennsylvania.

MGTA-456 is a cell therapy designed to provide a high dose of
hematopoietic stem cells that are well-matched to the patient. The
Company plans to enroll 12 patients in the ongoing Phase 2 study in
inherited metabolic disorders, which include cALD, Hurler syndrome,
metachromatic leukodystrophy and globoid cell leukodystrophy. The
primary endpoint of the study is neutrophil engraftment after
transplantation. The study is also collecting both short- and long-term
disease-specific outcomes. Data from the first five evaluable patients
treated in this study were highlighted in a poster presented by Ashish
Gupta, MBBS, M.P.H., Assistant Professor, Department of Pediatrics and
Division of Blood and Marrow Transplantation, University of Minnesota.

In a separate oral presentation today, Kevin Goncalves, Ph.D., Magenta
Therapeutics, will highlight preclinical data demonstrating that the
high stem cell dose in MGTA-456 accelerates and improves engraftment of
human microglia in the brains of transplanted mice.

“We are very pleased to see signs of durable disease benefit in patients
with cALD,” said John Davis, M.D., M.P.H., Chief Medical Officer,
Magenta Therapeutics. “cALD is a rapidly progressive disease, and
patients whose disease progresses quickly typically have poor long-term
outcomes. The stable neurological function score and persistent decrease
in brain inflammation in these two patients suggest that we have halted
the inflammatory process associated with the disease which may provide
long-term benefits. We look forward to providing an additional update
from the study before the end of the year.”

Patients with Inherited Metabolic Disorders (IMDs) transplanted with
MGTA-456, a CD34+ Expanded Cell Therapy Product, Show Rapid Engraftment
in Preliminary Phase 2 Trial Results

Key results in patients with cALD:

• Both patients had stable neurological function scores, which remained
  unchanged between baseline and six months post-transplant, suggesting
  progress of the disease has been arrested.
• The Loes score, a method for quantifying the severity of brain
  abnormalities and atrophy found on MRI, also remained stable in both
  patients after six months.
• Both patients showed resolution of gadolinium enhancement on MRI, an
  indicator of brain inflammation, by one month post-transplant, and the
  resolution persisted at six months.
  • Durable resolution of gadolinium enhancement is correlated with
    long-term disease benefit in patients with cALD.

Key results in patients with Hurler Syndrome:

• As previously reported, all three patients with Hurler syndrome
  achieved normal levels of blood leukocyte IDUA enzyme, the enzyme that
  is deficient in untreated patients with Hurler syndrome, by Day 42
  post-transplant. This suggests that transplant with MGTA-456 is
  affecting the disease process in these patients.
  • Normalization of blood leukocyte IDUA enzyme after transplant has
    been significantly associated with improvement in disease.
  • Patients showed a marked decline in urine total glycosaminoglycan
    (GAG), the toxic metabolites implicated in disease, after
    transplant.
  • Both of these findings are correlated with improved long-term
    disease outcomes.

Overall results:

• All five patients met the primary endpoint of neutrophil engraftment
• MGTA-456 was well tolerated

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
mpai@magentatx.com