Ambit Biosciences and Astellas Announce Upcoming Presentation of AC220 Data at the 16th Annual Congress of the European Hematology Association

SAN DIEGO and TOKYO, June 9, 2011 -

Ambit Biosciences Corporation and Astellas Pharma Inc. today announced
that the first presentation of interim data from a global multi-center Phase
2 study evaluating the investigational drug AC220, a potent and selective
FLT3 inhibitor. The data will be presented in an oral session at the upcoming
European Hematology Association (EHA) meeting in London. The abstract (#1019)
can be accessed at the EHA Congress website at
eha.eurocongres.com/16th/. Details of the presentation are as follows:

Title: "A Phase II Open-Label, AC220 Monotherapy Efficacy (ACE) Study in
Patients with Acute Myeloid Leukemia (AML) With FLT3-ITD Activating
Mutations: Interim Results"

Date & Time: Sunday, June 12, 2011, from 9:00am to 9:15am GMT (London)

Session Title: Acute Myeloid Leukemia - Clinical 2

Location: Hall S4, ExCeL London

Presenter: Mark Levis, MD, PhD, Associate Professor, Oncology and
Medicine, Division of Hematologic Malignancies, Johns Hopkins (Baltimore,
Maryland, USA)

About AC220

AC220 is being developed in collaboration between Ambit Biosciences and
Astellas Pharma Inc., and is a novel, potent, highly selective, orally
bioavailable FMS-like tyrosine kinase-3 (FLT3) inhibitor. AC220 is currently
under evaluation in a Phase 2 clinical trial as mono-therapy treatment for
adult and elderly patients with relapsed/refractory AML that have an internal
tandem duplication (ITD) mutation in the FLT3 gene. AML is one of the most
common types of blood cancers in adults, with ITD mutations in the FLT3 gene
occurring in 25-30 percent of AML patients. FLT3 ITD mutations confer poor
prognosis, with early relapse and lower survival following treatment with
existing therapies, including chemotherapy and hematopoietic stem cell
transplant.

About AML

Acute myeloid leukemia is a form of blood cancer. According to the
American Cancer Society, approximately 13,000 adults were newly diagnosed
with AML in 2009 in the United States with approximately 9,000 expected to
die of the disease in that year. AML is generally a disease of older people
and is uncommon before the age of 40. The average age of a patient with AML
is 67 and median survival for these patients is less than six months. The
five-year survival rate for all AML patients is less than 15 percent.
According to a report from Decision Resources, the U.S. AML market is
expected to more than double by 2015.

About the Ambit/Astellas Collaboration

In December 2009, Ambit and Astellas entered into a global strategic
partnership agreement to jointly research, develop and commercialize FLT3
kinase inhibitors in multiple indications, including the lead investigational
compound, AC220. The companies are presently evaluating AC220 in a Phase 2
clinical trial in relapsed and refractory AML patients that have the internal
tandem duplication (ITD) mutation in the FLT3 gene. The companies are also
collaborating on a comprehensive development program to explore the utility
of AC220 in other AML patient subpopulations. Additionally, the companies are
collaborating on a research and development program for additional FLT3
inhibitors for a variety of oncology and non-oncology indications. The
companies share equal responsibility and expenses for the development of
products in the US and Europe, while Astellas has sole responsibility in the
rest of the world. Astellas will be responsible for implementation of
commercialization activities worldwide. Ambit received a $40 million up-front
payment upon entering into the collaboration agreement, and is eligible to
receive up to $350 million in development milestone payments, undisclosed
sales milestones, and tiered, double-digit royalties on global revenues.
Ambit also has an option to co-promote products in the U.S. where Astellas
and Ambit share equally all profits and losses generated from U.S. sales.

About Ambit Biosciences

Ambit Biosciences is a privately-held biopharmaceutical company engaged
in the discovery and development of small molecule kinase inhibitors for the
treatment of cancer, inflammatory disease, and other indications. Ambit's
lead compound, AC220, is a novel, potent, highly selective, orally
bioavailable FMS-like tyrosine kinase-3 (FLT3) inhibitor, and is currently
under clinical investigation in patients with relapsed or refractory AML.
Ambit is developing AC220 in collaboration with Astellas Pharma Inc. as part
of a worldwide agreement to jointly develop and commercialize FLT3 kinase
inhibitors in oncology and non-oncology indications. In addition to AC220,
Ambit's clinical pipeline includes AC480, an oral pan-HER inhibitor, and
AC430, an oral JAK2 inhibitor. Ambit also has a pipeline of preclinical
candidates which includes CEP-32496, a BRAF inhibitor licensed to Cephalon.

About Astellas

Astellas Pharma Inc., located in Tokyo, Japan, is a pharmaceutical
company dedicated to improving the health of people around the world through
provision of innovative and reliable pharmaceuticals. Astellas has
approximately 16,000 employees worldwide. The organization is committed to
becoming a global category leader in Urology, Immunology & Infectious
Diseases, Oncology, Neuroscience, and DM complications & Metabolic Diseases.
For more information on Astellas Pharma Inc., please visit our website at
www.astellas.com/en.

Contacts: Ambit Biosciences: Alan Fuhrman, Chief Financial Officer, +1-858-334-2133; Doug Sherk/Jenifer Kirtland, EVC Group, +1-415-896-68 Media: Janine McCargo, EVC Group, +1-646-528-4034. Astellas Pharma Inc: Corporate Communications, +81-3-3244-3201 .