Plexxikon and Roche Enter Partnership to Develop Targeted Cancer Therapeutic Medicine PLX4032

Plexxikon Inc. and Roche today announced they have entered into an
agreement to develop and commercialize PLX4032, Plexxikon's
investigational targeted cancer therapy which selectively inhibits
B-Raf(V600E), a mutated form of the BRAF kinase gene. The BRAF(V600E)
gene has been associated with increased tumor aggressiveness and
decreased survival in many types of cancers and is a common
cancer-causing kinase gene. The BRAF(V600E) gene is found in
approximately 70% of malignant melanomas and a large number of
colorectal and thyroid tumors. PLX4032 may offer a new treatment
modality for the estimated 100,000 cancer patients in the United
States who carry the BRAF(V600E) gene. Plexxikon recently filed an
Investigational New Drug (IND) application for PLX4032 and plans to
initiate a Phase 1 clinical trial by the end of this year.

Separately, Roche Molecular Diagnostics, a business unit of Roche
Diagnostics, and Plexxikon announced they will collaborate on
development of an in vitro assay to screen for the presence of the
BRAF(V600E) mutation in biological samples taken from patient tumors.
An assay that correlates the presence of this mutation with clinical
outcome may aid clinical development of PLX4032.

"As one of the leading pharmaceutical companies in oncology,
together with their commitment to personalized medicine, Roche makes
an ideal partner for the development of this unique compound," said K.
Peter Hirth, Ph.D., chief executive officer of Plexxikon Inc. "We
believe PLX4032 could be a first-in-class oral cancer therapeutic
which selectively targets an oncogenic protein found only in diseased
tissue. Along with our Phase 2 diabetes product and preclinical stage
portfolio in multiple therapeutic areas, PLX4032's entry into
development further validates Plexxikon's discovery platform for novel
drug candidates."

"There is a growing body of evidence demonstrating that agents
such as PLX4032, which selectively inhibit activated kinases, are
increasingly useful in treating cancer and improving patient
outcomes," said Peter Hug, Roche's Global Head of Pharma Partnering.
"We are very excited to partner with Plexxikon for the development of
PLX4032 and other B-Raf(V600E) targeted compounds. This could be a
further example of the potential of personalized medicine. With our
combined expertise in diagnostic and therapeutic development as well
as commercialization, we are confident in our capability to develop
this potential drug to make a difference to patients' lives."

Plexxikon Roche Collaboration

Under the terms of the agreement, Roche will pay Plexxikon $40
million as an upfront payment and a further $6 million in guaranteed
research funding over the next two years. In addition, Plexxikon could
potentially receive up to approximately $660 million over the term of
the collaboration based on the successful completion of a series of
development and commercial milestones for multiple indications and/or
multiple compounds, as well as royalties on potential product sales.

Also under the collaboration, Roche and Plexxikon will jointly
develop PLX4032 and follow on compounds targeting other BRAF kinase
mutations. Plexxikon has filed an IND application for PLX4032, and
will conduct a Phase 1 dose escalation study in patients with cancer,
including melanomas. Roche will have a worldwide, exclusive license to
develop and commercialize PLX4032, in addition to other anticancer
compounds resulting from the partnership. Plexxikon retains the right
to co-promote any product in the collaboration in the United States.

About PLX4032

PLX4032 is a selective inhibitor of the B-Raf(V600E) kinase found
in approximately 70% of malignant melanomas, a large percentage of
colorectal and thyroid cancers and many other tumor types. In
preclinical studies in both melanoma and colorectal cancer models,
PLX4032 reduces tumor size and slows the progression of the tumors
even after the completion of treatment, without evidence of side
effects. Highly selective against B-Raf(V600E) compared to a panel of
over 70 other kinases, PLX4032 is designed to work specifically on
cancer cells, leaving healthy cells untouched in contrast to
chemotherapeutic agents.

About Roche

Headquartered in Basel, Switzerland, Roche is one of the world's
leading research-focused healthcare groups in the fields of
pharmaceuticals and diagnostics. As a supplier of innovative products
and services for the early detection, prevention, diagnosis and
treatment of disease, the Group contributes on a broad range of fronts
to improving people's health and quality of life. Roche is a world
leader in diagnostics, a leading supplier of medicines for cancer and
transplantation and a market leader in virology. In 2005 sales by the
Pharmaceuticals Division totaled 27.3 billion Swiss francs, and the
Diagnostics Division posted sales of 8.2 billion Swiss francs. Roche
employs roughly 70,000 people in 150 countries and has R&D agreements
and strategic alliances with numerous partners, including majority
ownership interests in Genentech and Chugai. Additional information
about the Roche Group is available on the Internet (www.roche.com).

Plexxikon Profile

Plexxikon is a leader in the discovery and development of novel
small molecule pharmaceuticals to treat human disease. Plexxikon's
proprietary Scaffold-Based Drug Discovery(TM) platform is being
applied to build therapeutic franchises in metabolic and
cardiovascular disease, inflammation and oncology. This discovery
process integrates a number of state-of-the-art technologies,
including structural screening as one key component that provides a
significant competitive advantage over other drug discovery
approaches. To date, the company has discovered a portfolio of
clinical and preclinical stage compounds in each of these franchises.

Currently, the company's most advanced program is a PPAR
pan-agonist for the treatment of diabetes and related cardiovascular
complications, with the lead candidate now in Phase 2 clinical testing
in collaboration with Wyeth Pharmaceuticals. Other discovery programs
include a dual kinase inhibitor for the treatment of rheumatoid
arthritis and other inflammatory diseases and a renin inhibitor for
hypertension in collaboration with Servier. Plexxikon is seeking
pharmaceutical and biotechnology partners for select collaboration
opportunities. For more information, please visit www.plexxikon.com.

Forward-Looking Statements

This press release contains forward-looking statements. Such
forward-looking statements, include, among others, those relating to
the successful development, approval and launch of a product from
PLX4032; the potential receipt by Plexxikon of substantial payments by
successfully fulfilling certain development and commercial milestones
for multiple indications and/or multiple compounds and successful
launch of a product in the U.S. and other countries; that PLX4032 will
be successfully developed and approved as a product which may be sold
to the public; that Plexxikon will receive royalties from sales of
this product, if successfully launched and that this product, if
developed, could represent a significant step forward in treating
patients. These statements are based on current expectations of future
events. Forward-looking statements are not guarantees of performance.
If underlying assumptions prove inaccurate or unknown risks or
uncertainties materialize, actual results could vary materially from
expectations and projections.

These forward looking statements are subject to numerous risks and
uncertainties. These risks and uncertainties include but are not
limited to, general industry conditions and competition; obtaining
U.S. and other countries regulatory approvals; health care changes in
the U.S. and other countries; unexpected outcomes; product efficacy or
safety concerns; product manufacturing issues; successful marketing of
the product if developed; superior products being brought to market;
loss of key employees; government reimbursement issues; economic
conditions; technological advances and patents attained by
competitors; manufacturing and supply disruptions; challenges inherent
in new product development, including obtaining regulatory approvals;
challenges to patents by competitors or allegations that the product
infringes the patents of third parties; U.S. and other countries
health care reforms; governmental laws and regulations; product
liability claims or litigation risks; governmental investigations; and
trends toward health care cost containment. These risks and
uncertainties also include the risks that clinical trials may not
proceed as planned due to technical, scientific, or patient enrollment
issues, or disagreements with regulatory authorities over trial design
or other matters; that the scale and scope of future clinical and
nonclinical studies may change and will be determined in significant
part by data collected in ongoing and future trials; that further
clinical studies may not reflect the results obtained in early
clinical and nonclinical studies; that ongoing nonclinical studies,
including toxicology studies, will yield currently unanticipated
negative outcomes that could adversely affect planned clinical trials;
that results from the clinical trials will be insufficient to support
additional phase programs without additional trials and consequent
delay in the timetable for potential approval; and that any potential
product may not achieve sales sufficient to earn the royalties
referenced above. The foregoing list sets forth many, but not all, of
the factors that could impact upon the ability to achieve results
described in any forward-looking statements. It is not possible to
predict or identify all such factors and should not consider this list
to be a complete statement of all potential risks and uncertainties.
Neither company assumes any obligation to update any forward-looking
statements as a result of new information or future events or
developments.