NantKwest Announces Updated Clinical Results for Nanatinostat (VRx-3996), a Novel HDAC Inhibitor Being Developed in Partnership with Viracta Therapeutics

Phase Ib/II Clinical Trial Data Reported at 2019 Annual Meeting of
the American Society of Clinical Oncology Shows High Overall Objective
Response Rate (ORR) of 58% and Complete Response Rate (CR) of 33% in
Relapsed/Refractory Lymphoma Patients

CULVER CITY, Calif.–(BUSINESS WIRE)–NantKwest Inc. (Nasdaq:NK)
announced today that its strategic partner Viracta Therapeutics
presented updated clinical data on its HDAC inhibitor, nanatinostat
(VRx-3996) at the Annual Meeting of the American Society of Clinical
Oncology (ASCO) in Chicago, IL, from May 31^st – June 4^th,
2019.

In April 2017, NantKwest announced that it was the lead investor in
Viracta’s Series B financing round. Concurrent with the financing,
NantKwest secured an exclusive license with commercialization rights to
nanatinostat for use in combination with natural killer (NK) cell
therapies, including NantKwest’s NK cell platforms.

Nanatinostat is a Class 1 histone deacetylase (HDAC) inhibitor currently
in phase Ib/II clinical trials (NCT03397706). In preclinical studies,
nanatinostat has been shown to reactivate silenced transgenes in tumor
cells thereby turning them into preferential targets for NK cell
killing, while also serving to broadly stimulate a patient’s immune
system, offering the potential for improved clinical responses in cancer
patients.

The activity of HDAC inhibitors are believed to be based on the
upregulation of natural killer group 2D (NKG2D) ligand expression on
cancer cells, which serve as “eat-me” signals for NK cells and can drive
NK proliferation, activation and cancer cell killing.

NantKwest is preparing to initiate clinical trials that include
nanatinostat in combination with its haNK and t-haNK cell therapy
platforms, which we believe will work synergistically to enhance the
efficacy of the company’s NK cell therapies and further distinguish us
in the market.

Interim results presented at the 2019 annual ASCO meeting from the phase
Ib portion of the ongoing phase Ib/II clinical trial of nanatinostat was
in combination with the antiviral valganciclovir for the treatment of
relapsed/refractory Epstein Barr Virus (EBV)-associated lymphomas. Three
doses of the combination were evaluated with responses seen at all dose
levels. Both drugs are taken orally and can be administered in an
out-patient setting.

EBV-associated cancers are known to be an extremely difficult cancer to
treat. Early clinical data with the combination of nanatinostat and
anti-viral therapy in EBV-associated lymphoma from the phase Ib/II study
has provided encouraging efficacy signals. Currently, there are no
approved treatments for EBV-associated lymphomas that specifically
target the virus.

The combination therapy produced an objective response rate (ORR) of
58%, a complete response rate (CR) of 33% and a disease stabilization
rate (DSR) of 75%. Based on these encouraging results, Viracta
anticipates advancement of the clinical trial to the phase II stage that
will evaluate intermittent dosing of nanatinostat (4 days on and 3 days
off) in combination with daily valganciclovir.

Patrick Soon-Shiong, M.D., Chairman and CEO of NantKwest commented,
“EBV-associated lymphomas represent a heterogenous group of cancers that
are often aggressive and poorly responsive to available therapy. In this
phase Ib stage, we are pleased to see encouraging objective responses
including a 33% complete response rate in relapsed/resistant EBV
positive cancer patients. We look forward to our continued partnership
with Viracta to transition to phase II clinical trials, while also
moving nanatinostat forward in combination with NantKwest’s haNK and
t-haNK cell therapies.”

Ivor Royston, M.D., Viracta’s CEO added, “Our ASCO presentation and
clinical trial update highlights the potential of our proprietary ‘Kick
& Kill’ therapeutic approach to treat a wide range of EBV positive
cancer patients, with responses seen across all doses in both T cell and
B cell lymphomas in our ongoing phase Ib/II study. “This data enable us
to move forward with the phase II portion of our study with a
well-tolerated dose combination of nanatinostat with valganciclovir,
which we expect to initiate in the third quarter of 2019.”

About Nanatinostat

Nanatinostat (VRx-3996) is a histone deacetylase (HDAC) inhibitor that
is being investigated in a range of clinical indications. Nanatinostat
is selective for Class 1 HDACs, including isoforms targeted in Viracta’s
“Kick & Kill” therapeutic approach. Viracta is investigating
nanatinostat in a phase Ib/II clinical study [NCT03397706] in
combination with an antiviral valganciclovir for the treatment of
EBV-associated cancers. Both drugs are taken orally and can be given on
an out-patient basis. Recently, the nanatinostat plus valganciclovir
combination therapy received Orphan Drug Designation (ODD) from the U.S.
Food & Drug Administration (FDA) for three sub-types of EBV-associated
cancers: post-transplant lymphoproliferative disorder (PTLD),
plasmablastic lymphoma, and angioimmunoblastic T cell lymphoma.

About EBV-Associated Cancers

Approximately 95% of the world’s adult population is infected with
Epstein-Barr Virus (EBV). Infections are commonly asymptomatic or
associated with mononucleosis. Following infection, the virus remains
latent in a small subset of lymphatic cells for the duration of the
patients’ life. Cells containing latent virus are increasingly
susceptible to malignant transformation. Patients who are
immunocompromised are at an increased risk of developing EBV lymphomas.
In addition, EBV is also associated with a variety of solid tumors,
including nasopharyngeal carcinoma and gastric cancer.

About NantKwest

NantKwest, a member of the NantWorks ecosystem of companies, is an
innovative clinical-stage immunotherapy company focused on harnessing
the power of the innate immune system by using the natural killer cell
to treat cancer and virally induced infectious diseases.

NantKwest is uniquely positioned to implement precision cancer medicine,
with the potential to change the current paradigm of cancer care.
Natural Killer (NK) cells are a safeguard in the human body designed to
recognize and detect cells under stress due to cancer or viral
infection. NantKwest’s “off-the-shelf” activated NK cell platform is
designed to destroy cancer and virally infected cells from the body. The
safety of our NK cells as well as their activity against a broad range
of cancers have been tested in phase I clinical trials in Canada and
Europe as well as in multiple phase I and II clinical trials in the
United States. In addition to being a universal cell-based therapy that
does not require individualized patient sourcing or matching, our NK
cell products have been largely administered in the outpatient setting
as an “off-the-shelf” living drug.

With the capacity to grow active killer cells as a cancer therapy, our
NK cells have been designed to induce cell death against cancers and
virally infected cells by several mechanisms, including: (i) innate
killing, whereby all of our NK platforms recognize the stress proteins
typically found on cancer cells, which, upon binding, release toxic
granules to immediately kill their targets; (ii) antibody-mediated
killing with our haNK® platform, which are NK cells engineered to
express antibody receptors that can bind to therapeutic antibody
products, thereby enhancing the cancer cell killing effect of that
antibody; and (iii) Chimeric Antigen Receptor directed killing using the
taNK® platform, which includes NK cells engineered to incorporate
chimeric antigen receptors (CARs) to target tumor-specific antigens
found on the surface of cancer cells. All three modes of killing
(innate, antibody-mediated, and CAR directed killing) are employed by
our t-haNK™ platform, which is an innovative combination of our aNK,
haNK® and taNK® platforms in a single product.

Our haNK®, and t-haNK™ platforms have been designed to address certain
limitations of CAR T-cell therapy including the capability to infuse
cell therapy in an outpatient setting which allows for potential
reduction of risk for serious cytokine storms and protracted serious
adverse events. In phase I and II clinical trials in patients with late
stage cancer, our NK cells have been administered as an investigational
outpatient infusion safely with greater than 500 infusions to date at a
dose of 2 billion cells per infusion.

By leveraging an integrated and extensive genomics and transcriptomics
discovery and development engine, together with a pipeline of multiple,
clinical-stage, immuno-oncology programs, we believe NantKwest is
uniquely positioned to be the premier immunotherapy company and
transform medicine by delivering living drugs in a bag and bringing
novel NK cell-based therapies to routine clinical care.

NK-92, aNK, haNK, taNK, and t-haNK are trademarks of NantKwest, Inc.

Forward-Looking Statements

This press release contains forward-looking statements within the
meaning of the Private Securities Litigation Reform Act of 1995.
Forward-looking statements include statements concerning or implying
that NantKwest will be successful in improving the treatment of cancer.
Risks and uncertainties related to this endeavor include, but are not
limited to, obtaining FDA approval of NantKwest’s NK cells as well as
other therapeutics as part of the NANT Cancer Vaccine platform as a
cancer treatment.

Forward-looking statements are based on management’s current
expectations and are subject to various risks and uncertainties that
could cause actual results to differ materially and adversely from those
expressed or implied by such forward-looking statements. Accordingly,
these forward-looking statements do not constitute guarantees of future
performance, and you are cautioned not to place undue reliance on these
forward-looking statements.

These and other risks regarding NantKwest’s business are described in
detail in its Securities and Exchange Commission filings, including in
NantKwest’s Quarterly Report on Form 10-Q for the quarter ended March
31, 2019. These forward-looking statements speak only as of the date
hereof, and we disclaim any obligation to update these statements except
as may be required by law.

About Viracta Therapeutics, Inc.

Viracta is a clinical-stage drug development company focused on
advancing novel epigenetic therapeutics derived from its proprietary
“Kick & Kill” therapeutic approach to benefit patients with
viral-associated cancers and other serious diseases. Viracta has entered
into partnerships with Shenzhen Salubris Pharmaceutical Co., Ltd. to
bring treatments for EBV-associated cancers to China, and with
NantKwest, Inc. to utilize nanatinostat in combination with their
clinical-stage Natural Killer (NK) cell immunotherapy. Viracta plans to
enter into additional geographic and combination therapy partnerships.

For additional information please visit www.nantkwest.com
or www.viracta.com.

Contacts

Media Contact:
Jen Hodson
562-397-3639
[email protected]