Portola to Initiate Phase 2 Study Evaluating the Safety and Effectiveness of Its Factor Xa Inhibitor Antidote PRT4445 in

Portola to Initiate Phase 2 Study Evaluating the Safety and Effectiveness of 
Its Factor Xa Inhibitor Antidote PRT4445 in
Reversing the Anticoagulant Activity of Investigational Factor Xa
Inhibitor Edoxaban 
SOUTH SAN FRANCISCO, CA -- (Marketwired) -- 06/26/13 --  Portola
Pharmaceuticals, Inc. (NASDAQ: PTLA) today announced that it has
entered into a clinical collaboration agreement with Daiichi Sankyo
Company, Limited to conduct a Phase 2 study evaluating the safety and
effectiveness of its investigational Factor Xa inhibitor antidote
PRT4445 at several different doses to reverse the anticoagulant
effects of Daiichi Sankyo's investigational oral, once-daily, direct
Factor Xa inhibitor edoxaban in healthy volunteers. PRT4445 has a
tentatively approved International Nonproprietary Name (INN) of
andexanet alfa. 
Andexanet alfa is a recombinant protein designed to reverse the
anticoagulant activity in patients treated with a Factor Xa inhibitor
who suffer an uncontrolled bleeding episode or undergo emergency
surgery. Edoxaban is an investigational anticoagulant that
specifically and reversibly inhibits Factor Xa, an important factor
in the coagulation system that leads to blood clotting. 
The collaboration will be in effect throughout the Phase 2 study, for
which Daiichi Sankyo will provide development and regulatory
guidance. Portola retains all global development and
commercialization rights for andexanet alfa. Daiichi will make an
undisclosed cash payment to Portola upon initiation of the edoxaban
study. 
Clinical trial results suggest that patients treated with novel oral
anticoagulants may experience uncontrolled bleeding (annually between
1 and 4 percent) and some may require emergency surgery. The lack of
an effective reversal agent, or antidote, for patients on Factor Xa
inhibitors is an unmet need as currently there is no such agent
approved for use against Factor Xa inhibitors. 
About Andexanet Alfa
 Andexanet alfa is a novel recombinant protein
designed to reverse the anticoagulant activity in patients treated
with a Factor Xa inhibitor who suffer an uncontrolled bleeding
episode or need to undergo emergency surgery. Andexanet alfa is
similar to native Factor Xa, but has been modified to restrict its
biological activity to the reversal of the anticoagulant
 effects of
Factor Xa inhibitors. Andexanet alfa acts as a Factor Xa decoy that
binds and sequestors direct Factor Xa inhibitors in the blood. Once
bound to andexanet alfa, the Factor Xa inhibitors are unable to bind
to and inhibit native Factor Xa. The native Factor Xa is then
available to participate in the coagulation process and restore
hemostasis. 
Results from a Phase 1 single ascending dose safety and tolerability
study of andexanet alfa conducted by Portola in 32 healthy volunteers
in the United States showed no apparent safety signals, including no
thrombotic adverse events.  
About Portola Pharmaceuticals, Inc. 
 Portola is a biopharmaceutical
company focused on the development and commercialization of novel
therapeutics in the areas of thrombosis (blood clots), other
hematologic disorders and inflammation for patients who currently
have limited or no approved treatment options. Portola's current
development-stage portfolio consists of three compounds discovered
through its internal research efforts and one discovered by Portola
scientists during their time at a prior company. Portola's two lead
programs address significant unmet medical needs in the area of
thrombosis.  
Portola's lead compound, betrixaban, is an investigational, novel,
oral, once-daily inhibitor of Factor Xa in Phase 3 development for
extended duration prophylaxis (preventive treatment) of a form of
thrombosis known as venous thromboembolism (VTE) in acute medically
ill patients. Currently, there is no anticoagulant approved for
extended duration VTE prophylaxis in this population.  
Portola's second lead development candidate, andexanet alfa, is a
recombinant protein designed to reverse the anticoagulant activity in
patients treated with a Factor Xa inhibitor who suffer an
uncontrolled bleeding episode or need to undergo emergency surgery.
Portola has entered into collaboration agreements with Bristol-Myers
Squibb Company and Pfizer Inc., with Bayer Pharma AG and Janssen
Pharmaceuticals, Inc., and with Daiichi Sankyo to study andexanet
alfa with Eliquis(R) (apixaban), XARELTO(R) (rivaroxaban) and
edoxaban, respectively, in Portola's Phase 2 studies. Portola retains
full, worldwide commercial rights with respect to andexanet alfa.  
Portola's third product candidate, PRT2070, is an orally available
kinase inhibitor being developed for hematologic (blood) cancers and
inflammatory disorders. PRT2070 inhibits spleen tyrosine kinase (Syk)
and janus kinases (JAK), enzymes that regulate important signaling
pathways. Subject to regulatory approval, Portola plans to initiate a
Phase 1/2 clinical study of PRT2070 in 2013 in patients with B-cell
hematologic cancers who have failed or relapsed on existing marketed
therapies or products in development, including patients with
identified mutations. Portola's fourth program, PRT2607 and other
highly selective Syk inhibitors, is partnered with Biogen Idec Inc. 
Forward-looking statement
 Statements contained in this press release
regarding matters that are not historical facts are "forward-looking
statements" within the meaning of the Private Securities Litigation
Reform Act of 1995. Because such statements are subject to risks and
uncertainties, actual results may differ materially from those
expressed or implied by such forward-looking statements. Such
statements include, but are not limited to, statements regarding the
use of betrixaban as a Factor Xa inhibitor, the use of PRT4445, which
has a tentatively approved International Nonproprietary Name (INN) of
andexanet alfa, to reverse the anticoagulant activity in patients
treated with a Factor Xa inhibitor, the use of PRT2070 or PRT2607 as
kinase inhibitors and any of Portola's clinical trials, including
Portola's Phase 3 APEX study for betrixaban, Phase 2 proof-of-concept
studies for PRT4445 and Phase1/2 study for PRT2070. Risks that
contribute to the uncertain nature of the forward-looking statements
include: the accuracy of Portola's estimates regarding its ability to
initiate and/or complete its clinical trials; the success of
Portola's clinical trials and the demonstrated efficacy of Portola's
product candidates thereunder; the accuracy of Portola's estimates
regarding its expenses and capital requirements; regulatory
developments in the United States and foreign countries; Portola's
ability to obtain and maintain intellectual property protection for
its product candidates; and the loss of key scientific or management
personnel. These and other risks and uncertainties are described more
fully in Portola's filings with the Securities and Exchange
Commission, including without limitation its Registration Statement
on Form S-1 that was originally filed with the Securities and
Exchange Commission on April 12, 2013, and the amendments thereto.
All forward-looking statements contained in this press release speak
only as of the date on which they were made. Portola undertakes no
obligation to update such statements to reflect events that occur or
circumstances that exist after the date on which they were made. 
Contacts: 
Mardi Dier
CFO 
Portola Pharmaceuticals 
ir@portola.com  
Paul Laland 
BrewLife 
plaland@brewlife.com
415.946.1071