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, CA, June 4, 2019 — 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 31st – June 4th, 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/ll 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.

Media Contact:

Jen Hodson

NANT

562-397-3639

Jen@nant.com