Ambit Biosciences and Astellas Enter Strategic Partnership to Research, Develop and Commercialize FLT3 Kinase Inhibitors in Multiple Indications

Ambit to Receive a US$40 Million Upfront Cash Payment; Up to US$350 Million in Pre-Commercialization Milestones, Double-Digit Royalties with Option to Co-promote and Share Profits in U.S.

SAN DIEGO and TOKYO, December 18 - Ambit Biosciences Corporation and Astellas Pharma Inc. today announced
that they have entered into a worldwide agreement to jointly develop and
commercialize FLT3 kinase inhibitors in oncology and non-oncology
indications. This partnership includes AC220, Ambit's lead clinical-stage
investigational drug that entered into a Phase 2 clinical trial earlier this
month in relapsed/refractory acute myeloid leukemia (AML), and other
undisclosed FLT3 kinase inhibitors. AC220 is a novel, orally available,
potent and highly selective small molecule that was specifically designed as
a second generation FMS-like tyrosine kinase-3 (FLT3) inhibitor using Ambit's
proprietary drug discovery engine, KINOMEscan(TM).

The companies will collaborate to develop AC220 for AML and other
indications. The parties will also collaborate on a research and development
program for a series of novel FLT3 inhibitors for a variety of oncology and
non-oncology indications. The companies will share equally in the
responsibilities and expenses for the development of AC220 and any additional
products in the U.S. and Europe, while Astellas will have sole responsibility
to fund development in all other territories. Under the terms of the
agreement, Ambit will receive an up-front cash payment of US$40 million and
will be eligible to receive pre-commercialization payments of up to US$350
million.

Astellas will have sole responsibility for funding and implementing the
commercialization of all products, and Ambit will be entitled to
post-approval milestone payments upon the achievement of certain sales
thresholds, as well as tiered double-digit royalties on net sales. In the
U.S., Ambit will also have the option to co-promote AC220 and other products
under a profit sharing arrangement where Astellas and Ambit share equally in
profits and losses generated from U.S. sales.

"We are pleased to have entered into a great partnership with Ambit,"
stated Masafumi Nogimori, president and chief executive officer of Astellas.
"We believe that AC220, as the most selective and advanced FLT3 kinase
inhibitor, has the potential to provide a new treatment option for AML where
high unmet medical needs exist. Astellas is strongly committed to focus on
oncology and this partnership is a significant milestone to establish our
franchise in oncology."

"With their strategic commitment to the development and commercialization
of innovative oncology products, Astellas is an ideal partner for Ambit,"
said Scott Salka, Chief Executive Officer of Ambit Biosciences. "This
collaboration establishes a comprehensive and global leadership position in
the discovery and development of FLT3 kinase inhibitors, and we look forward
to working closely with Astellas to explore the clinical utility of AC220 in
AML and other indications."

About AC220

AC220, Ambit's lead product candidate, is being developed in
collaboration with Astellas Pharma Inc. and is a novel, potent, highly
selective, orally bioavailable second-generation FLT3 inhibitor. AC220 is
currently under evaluation in a Phase 2 clinical trial designed to support
potential registration of AC220 as monotherapy treatment in adult and elderly
patients with relapsed/refractory AML that have the internal tandem
duplication (ITD) mutation in the FLT3 kinase. AML is one of the most common
types of blood cancers in adults, and the FLT3 kinase is mutated and
constitutively activated in 25-40 percent of such patients. FLT3 ITD
mutations predict poor prognosis and decreased response to existing
treatments, including chemotherapy and hematopoietic stem cell transplant.
Ambit leveraged KINOMEscan(TM), the company's proprietary, high-throughput
method for screening small molecule compounds against a large number of human
kinases, to advance AC220 from initial chemistry to clinical candidate
selection for IND-enabling studies in only 18 months.

About Acute Myeloid Leukemia (AML)

Acute myeloid leukemia is a form of blood cancer. According to the
American Cancer Society, approximately 13,000 new cases of AML will be
diagnosed in the United States in 2008. The median age of a patient with AML
is about 67 years. Standard treatment for patients 60 years or older with AML
includes systemic combination chemotherapy. The median survival for patients
receiving induction chemotherapy, which is associated with high mortality, is
6-11 months, with shorter survival for patients over the age of 60 years. The
five-year survival rate for AML 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 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
employs a novel and proprietary kinase profiling technology, KINOMEscan(TM),
to screen compounds against 442 human kinases.

Ambit's lead compound, AC220, is in clinical development for the
treatment of AML and other indications. Ambit has initiated a Phase 2 pivotal
trial in patients with relapsed or refractory AML and plans to commence
several other clinical studies with AC220 in 2010. Ambit's clinical pipeline
also includes AC480, an oral pan-HER inhibitor that was in-licensed from BMS.
Ambit is conducting Phase 2 studies with AC480 in patients with solid tumor
cancers. Additionally, Ambit has an advancing pool of preclinical candidates
targeting BRAF (in collaboration with Cephalon), JAK2, Aurora, and CSF1R.
Through its KINOMEscan Division, Ambit markets its technology as a profiling
service. For more information, visit www.ambitbio.com.

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 15,000 employees worldwide. The organization is committed to
becoming a global category leader in urology, immunology & infectious
diseases, neuroscience, DM complications & metabolic diseases and oncology.
For more information on Astellas Pharma Inc., please visit our website at
www.astellas.com/en.

    Contact:
    --------
    Ambit Biosciences:                       Astellas Pharma:
    ------------------                       ----------------
    M. Scott Salka                           Corporate Communications
    +1-858-334-2101                          +81-(0)3-3244-3201
                                             www.astellas.com/en/
    Christopher Morl (business development)
    +1-858-334-2134

    Scott Lerman (media)
    The Ruth Group
    +1-646-536-7013
    slerman@theruthgroup.com

    Sara Pellegrino (investors)
    The Ruth Group
    +1-646-536-7002
    spellegrino@theruthgroup.com

For Ambit Biosciences: M. Scott Salka, +1-858-334-2101; Christopher Morl (business development), +1-858-334-2134; Scott Lerman (media), The Ruth Group, +1-646-536-7013, slerman at theruthgroup.com; Sara Pellegrino (investors), The Ruth Group, +1-646-536-7002, spellegrino at theruthgroup.com; For Astellas Pharma: Corporate Communications, +81(0)3-3244-3201