Astellas and Ambrx Initiate Collaboration for Discovery and Development of Next-Generation Antibody Drug Conjugates for

  Astellas and Ambrx Initiate Collaboration for Discovery and Development of
            Next-Generation Antibody Drug Conjugates for Oncology

PR Newswire

TOKYO and SAN DIEGO, April 4, 2013

TOKYO and SAN DIEGO, April 4, 2013 /PRNewswire/ --Astellas Pharma Inc.
(Tokyo: 4503, "Astellas") today entered into a collaboration with Ambrx Inc.
("Ambrx") for the discovery and development of novel antibody drug conjugates

ADCs allow for the targeted delivery of drugs to the target tissue. Ambrx
creates optimized ADCs using its site-specific conjugation technology along
with proprietary linkers and payloads. In the preclinical setting, Ambrx ADCs
have demonstrated high potency and a wider therapeutic index than ADCs created
using conventional non-specific conjugation.

Ambrx will receive an upfront payment of $15 million from Astellas, as well as
up to $285 million in potential near and long-term research, development,
regulatory and sales-based milestones for an undisclosed number of targets for
ADCs in oncology. A portion of these milestones, as well as royalties on any
net sales, will be contingent on eventual successful commercialization of
products developed as a result of this partnership. Astellas will receive
worldwide rights to develop and commercialize ADCs for oncology. Additional
terms of the collaboration are not disclosed at this time.

"Agensys, Inc., an affiliate of Astellas which specializes in therapeutic
antibody research and development in cancer, has significant experience with
ADCs as oncology therapeutics and is looking to further expand its
capabilities in this area. Ambrx offers a novel approach to allow creation of
site-specific and highly stable conjugations that have the potential to
further optimize drug delivery to tumor cells," said David Stover, Ph.D.,
Senior Vice President and Agensys Site Head.

Added Lawson Macartney, chief executive officer of Ambrx, "We recognize
Astellas as a leader in the development of innovative therapeutics for
oncology and are proud to initiate this collaboration. We look forward to
developing these therapeutics while also advancing our broad pipeline of
partnered and wholly owned therapeutic candidates with best-in-class

About Ambrx

Ambrx Inc. is a clinical stage biopharmaceutical company using an expanded
genetic code to create best-in-class biotherapeutics, including antibody drug
conjugates and proteins with improved pharmacologic properties. The company is
developing ARX201, a long-acting growth hormone that has successfully
completed Phase 2b clinical trials. Ambrx has collaborations to discover and
develop products incorporating Ambrx technology with Bristol-Myers Squibb, Eli
Lilly and Company and several undisclosed companies. Ambrx is advancing a
robust portfolio of product candidates that are optimized for efficacy, safety
and ease of use in multiple therapeutic areas. For additional information,

About Astellas

Astellas Pharma Inc., located in Tokyo, Japan, is a pharmaceutical company
dedicated to improving the health of people around the world through the
provision of innovative and reliable pharmaceuticals. Astellas has
approximately 17,000 employees worldwide. The organization is committed to
becoming a global category leader in Urology, Immunology (including
Transplantation) and Infectious Diseases, Oncology, Neuroscience and DM
Complications and Kidney Diseases. For more information on Astellas Pharma
Inc., please visit the company website at 

Ambrx     Ian Stone 
          David Schull
          (212) 845-4271
Astellas  Corporate Communications
          TEL: +81-3-3244-3201

SOURCE Ambrx Inc.

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