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Harpoon Therapeutics Treats First Patient with HPN424 in Phase 1 Clinical Trial of Metastatic Castration-Resistant Prostate Cancer (mCRPC) Patients

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HPN424, First-in-Class TriTAC™ T Cell Redirection Therapy, Targets
Prostate-Specific Membrane Antigen (PSMA)

Clinical Data Expected in 2019

Harpoon Therapeutics, an immuno-oncology company pioneering a new class
of T cell engaging therapeutics, announced today that the first patient
has been treated with HPN424 in a Phase 1 clinical study of metastatic
castration-resistant prostate cancer (mCRPC) patients. HPN424 is the
first of multiple compounds in development that are based on the
company's TriTAC™ (Tri-specific T
cell Activating Construct)
platform and designed to penetrate solid tumors, have extended serum
half-life, and recruit patients' own T cells to destroy malignant tumor
cells.

The Phase 1 trial is a multicenter, multinational, open-label, ascending
dose study designed to evaluate the safety, tolerability and
pharmacokinetics of HPN424 in approximately 40 patients with metastatic
prostate cancer.

"We are pleased to initiate Harpoon's first clinical trial with HPN424
in patients with advanced prostate cancer, an important milestone that
marks our transition to a clinical-stage company," said Jerry McMahon,
PhD, President and Chief Executive Officer of Harpoon Therapeutics.
"HPN424 is the first TriTAC compound to enter clinical testing and
represents a novel class of T cell therapeutics aiming to achieve
superior efficacy in penetrating and killing solid tumors. Data is
expected in 2019, and should provide the safety assessment,
pharmacokinetics and pharmacodynamics to determine the optimal dose and
regimen for our additional planned trials in prostate cancer, the third
leading cause of cancer deaths for men in the U.S."

In the first phase of the study, HPN424 will be administered once weekly
to patients via intravenous (IV) infusion with dose escalation until a
therapeutic dose level has been reached. Following dose
escalation, Harpoon will further evaluate the safety and efficacy of
HPN424 in additional cohorts of patients at the recommended Phase 2 dose
established in the first phase of the study. For more information about
the trial, please visit clinicaltrials.gov,
identifier number NCT03577028.

"Immunology represents a new frontier for the treatment of prostate
cancer," said Charles G. Drake, MD, PhD, Director of Genitourinary
Oncology at New York Presbyterian/Columbia University Cancer Center,
Co-director of Columbia's Cancer Immunotherapy Program and Scientific
Advisory Board member at Harpoon. "Each year in the U.S., approximately
26,000 men die from prostate cancer following treatment with current
available therapies. HPN424, an innovative biologic designed to activate
T cells in the tumor, is highly potent in killing prostate cancer cells
in preclinical models and could be a promising option for these men with
advanced disease."

"HPN424 is designed as an ‘off-the-shelf' T cell therapy which can
overcome loss of human leukocyte antigen (HLA) expression – a frequent
mechanism by which cancer cells escape T cell recognition," said
Che-Leung Law, PhD, Vice President of Translational Medicine.
"Pharmacological insights gleaned from the study should help inform
development of both HPN424 and our pipeline of additional products -
targeting mesothelin (MSLN), B-cell maturation antigen (BCMA) and DLL3 -
in the near future."

About HPN424

HPN424 is a 50-kD single polypeptide that contains three binding domains
— for human PSMA, human serum albumin and human CD3. In preclinical
studies, HPN424 demonstrated single-digit picomolar potency for
PSMA-dependent T cell killing in a panel of human prostate cancer cell
lines, which translated to in vivo efficacy with efficacious
doses in the low μg/kg range. HPN424 was well tolerated in a multi-dose
safety study in non-human primates and showed a serum half-life of about
80 hours supporting once-weekly administration. AACR_2018_Poster_HPN424

About Harpoon Therapeutics

Harpoon Therapeutics is a clinical-stage immuno-oncology company founded
by Patrick Baeuerle, PhD, a pioneer in the development of T cell
engaging therapies, and MPM Capital. The company is focused on the
discovery and development of novel T cell engaging biologics for the
treatment of cancer. Harpoon Therapeutics created a novel T cell
redirection platform called TriTAC (Tri-specific T cell Activating
Construct) to unleash the targeted cell-killing properties of a
patient's own immune system through T cell activation. This approach has
been optimized to overcome the limitations of existing immunotherapies
to penetrate tissues and extend serum exposure for superior efficacy in
solid tumors. Harpoon is developing a robust pipeline of TriTAC
candidates. Harpoon has initiated a Phase 1 clinical trial for lead
candidate HPN424, a prostate-specific membrane antigen (PSMA)-targeting
TriTAC, and expects to file an investigational new drug application for
HPN536, a mesothelin-targeting TriTAC soon. For more information, please
visit www.harpoontx.com.

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