PRESIDIO PHARMACEUTICALS, INC. SUCCESSFULLY COMPLETES PHASE 1A CLINICAL TRIAL OF PPI-461, A POTENT, NEW HEPATITIS C VIRUS (HCV) INHIBITOR

SAN FRANCISCO, Calif. – July 14, 2010 – Presidio Pharmaceuticals, Inc. announced
today the successful completion of a Phase 1a clinical trial of PPI-461, its lead
HCV NS5A inhibitor, in healthy subjects. This first-in-human trial evaluated four
single doses of PPI-461, followed by a five-day, once-a-day treatment regimen
with PPI-461 at the highest tested dose.

The randomized, double-blind, placebo-controlled trial was completed with 40
healthy subjects in the United Kingdom. The trial results indicated that PPI-461
was well-tolerated in all dose regimens. There were no serious adverse events,
and all subjects completed the trial successfully. Among PPI-461 recipients
there were only transient adverse events, none of which were attributed to study
drug by the investigator, and there were no significant changes in safety-related
clinical laboratory assessments.

Pharmacokinetic (PK) analyses showed substantial blood levels of PPI-461 were
rapidly and consistently achieved and were dose proportional. PPI-461 plasma
concentrations were orders of magnitude above those shown to suppress viral
replication in vitro and were maintained at predicted effective concentrations for
more than 24 hours. In the five-day, multi-dose regimen, steady-state PK was
readily achieved, supportive of once-daily dosing.

“These first clinical data of PPI-461 indicate excellent tolerance in healthy
subjects with all tested dosing regimens. The PK profile is very encouraging,
suggesting that plasma concentrations of PPI-461 can be obtained with a
relatively low dose, once-daily (QD) regimen that may provide a continuous
antiviral effect,” said Nathaniel Brown, M.D., Chief Medical Officer.

Presidio Pharmaceuticals, Inc. plans to initiate a Phase 1b proof-of-concept trial
in patients with HCV in Q4 2010 and expects to have early results regarding the
clinical efficacy of PPI-461 near year-end.

ABOUT PPI-461

PPI-461 is a novel, proprietary NS5A inhibitor against hepatitis C virus (HCV),
exhibiting highly potent and selective activity against all genotypes in HCV
replicon assays.

Inhibitors of the HCV NS5A protein represent an exciting, highly potent class that
is distinct from other classes of HCV antivirals that target the viral protease or
replicase. With poor response and tolerance issues associated with the current
standard of care treatment—pegylated-IFN and ribavirin—there is clear need for
more potent and better tolerated inhibitors that can be orally administered in
future combination therapies.

ABOUT PRESIDIO

Presidio Pharmaceuticals, Inc. is a San Francisco-based specialty
pharmaceutical company dedicated to the discovery and development of smallmolecule
antiviral therapeutics for novel and validated targets. For more
information, please visit our website at: www.presidiopharma.com.

Contact: Omar K. Haffar, PhD
415-655-7561
omar@presidiopharma.com