Astellas Enters into Partnership With Optimer on Life-Saving Antibiotic Fidaxomicin

Product Already Filed With European Medicines Agency

STAINES, England, February 7, 2011 - Astellas Pharma Europe Ltd. announced today that it has entered into an
exclusive collaboration and license agreement with Optimer Pharmaceuticals
Inc., San Diego for the development and commercialisation of fidaxomicin, an
investigational antibiotic for Clostridium difficile infection (CDI) in
Europe and certain other countries in the Middle East, Africa and the
Commonwealth of Independent States (CIS).

Fidaxomicin is an orally administered, macrocyclic antibiotic with a new
mechanism and narrow spectrum of action. This novel medicine is currently
being reviewed for market authorisation in both the US and Europe for the
treatment of CDI and reduction in the risk of recurrence when used for
treatment of initial CDI. Optimer filed the marketing authorisation
application (MAA) for fidaxomicin with the European Medicines Agency (EMA) on
2 August 2010, and the EMA accepted it for review as treatment for patients
with Clostridium difficile infection (CDI) and for the recurrences of CDI in
the EU. Additionally, although Astellas will not have marketing rights to
fidaxomicin in the United States, Optimer filed a New Drug Application (NDA)
for the product with the Food and Drug Administration (FDA) late last year,
and the FDA has accepted that application and has granted the company's
request for a six-month Priority Review, assigning a Prescription Drug User
Fee Act goal date of 30 May 2011.

To date, two phase III clinical studies have been completed showing that
fidaxomicin successfully met the primary endpoint of non-inferiority when
comparing the treatment to oral vancomycin in achieving clinical cure -
defined as patients requiring no further CDI therapy two days after
completion of study medication, as determined by the investigator.
Furthermore in both trials, fidaxomicin was statistically superior to
vancomycin in global cure and in reducing recurrences of CDI by up to 47%.
This evidence is of great significance given that the medical community has
yet to find a successful and reliable treatment for recurrent CDI.

"We believe the strengths of Astellas' world-class anti-infective
business capabilities including established relationships with payers and
hospitals in Europe and other markets, combined with Optimer's novel
therapeutic for CDI, represents the most effective way to address a serious,
unmet health need," said Mr. Masao Yoshida, President and CEO of Astellas
Pharma Europe Ltd. "We look forward to bringing fidaxomicin to these markets
to help patients and providers address this serious life threatening
disease."

Under the terms of the agreement, Astellas is granted exclusive rights to
develop and commercialise fidaxomicin in Europe and certain other countries
in the Middle East, Africa and the Commonwealth of Independent States (CIS).
Astellas will pay EUR50 million up front and approximately EUR115 million in
milestone payments on achievement of pre-specified development and sales
milestones.

"We expect the Astellas collaboration will help Optimer realize the full
potential of fidaxomicin and will help position this medication in these
countries as the first line of treatment, both for treating CDI and reducing
recurrences," said Pedro Lichtinger, Optimer's President and CEO. "CDI poses
a significant cost burden on the healthcare system, and we believe, if
approved, fidaxomicin will provide a cost-savings opportunity for hospitals
and payers, especially when used in populations at risk of recurrence such as
the elderly, patients with a prior episode, those taking concomitant
antibiotics, immuno compromised patients or those with renal impairment."

Clostridium difficile, commonly referred to as "C.difficile" or "C.diff"
is a bacteria that causes serious illness- through infecting the inner lining
of the colon and producing toxins that cause inflammation of the colon,
severe diarrhoea and in the most serious cases, death. Patients typically
develop CDI following the use of broad-spectrum antibiotics that disrupt the
normal gastrointestinal (gut) flora, thus allowing Clostridium difficile
bacteria to flourish and produce harmful side effects.

Clostridium difficile in Europe is associated with significant health and
economic burden, and incidence of this serious disease has been increasing
across Europe over the last 10 years. Treatments that provide both a high
success rate and low infection recurrence rate are, therefore, welcomed in
order to combat this illness.

Astellas already has an ongoing commitment to combating
infectious disease through the worldwide launch of its injectable antifungal,
the echinocandin Mycamine(R) (micafungin), and in September 2009, the launch
in the United States of Vibativ(R) (telavancin) an injectable antibacterial
for the treatment of adults with infections caused by gram-positive pathogens
including MRSA. Astellas is currently liaising with the European regulatory
authorities to license Vibativ for the treatment of adults with Nosocomial
Pneumonia and complicated Skin and Soft Tissue Infections (cSSTIs). In
addition, Astellas is continuing to develop its franchise in infectious
disease with the development of isavuconazole, an antifungal for the
treatment of invasive fungal infections, including Aspergillosis. The
compound is currently in Phase III trials. This new partnership is an
important step to further expand our business and to reinforce our franchise
in infectious diseases.

About Astellas Pharma Europe Ltd:

Astellas Pharma Europe Ltd., located in the UK, is a European subsidiary
of Tokyo-based Astellas Pharma Inc. Astellas is a pharmaceutical company
dedicated to improving the health of people around the world through the
provision of innovative and reliable pharmaceuticals. The organisation is
committed to becoming a global company by combining outstanding R&D and
marketing capabilities and continuing to grow in the world pharmaceutical
market. Astellas Pharma Europe Ltd. is responsible for 21 affiliate offices
located across Europe, the Middle East and Africa, an R&D site and three
manufacturing plants. The company employs approximately 3,900 staff across
these regions. For more information about Astellas Pharma Europe, please
visit www.astellas.eu.

About Optimer Pharmaceuticals:

Optimer Pharmaceuticals, Inc. is a biopharmaceutical company
focused on discovering, developing and commercializing hospital specialty
products to treat serious infections and address unmet medical needs. Optimer
has two anti-infective product candidates in development, fidaxomicin and
Pruvel(TM) (prulifloxacin). Fidaxomicin is a narrow spectrum antibiotic being
developed for the treatment of Clostridium difficile infection. The FDA
granted Optimer's request for a six-month Priority Review of fidaxomicin, and
has assigned a Prescription Drug User Fee Act (PDUFA) goal date of May 30,
2011. Optimer has also filed a MAA with the European Medicines Agency (EMA)
for fidaxomicin. Pruvel(TM) is a prodrug in the fluoroquinolone class of
antibiotics being developed as a treatment for infectious diarrhea.
Additional information can be found at www.optimerpharma.com.

    Contacts for inquiries or additional information
    Astellas Pharma Europe Ltd
    Corporate Communications
    Mindy Dooa
    Tel: +44(0)1784-419408
    www.astellas.eu

Contacts for inquiries or additional information: Astellas Pharma Europe Ltd, Corporate Communications, Mindy Dooa, Tel: +44(0)1784-419408