Karyopharm Announces Ongoing Phase 3 SIENDO Study of XPOVIO® (Selinexor) in Patients with Endometrial Cancer Passes Planned Interim Futility Analysis: Data and Safety Monitoring Board Recommends the Study Should Proceed as Planned Without Any Modification

-- Company Expects Top-Line Study Results in the Second Half of 2021 --

NEWTON, MA, USA I November 25, 2020 I Karyopharm Therapeutics Inc. (Nasdaq: KPTI), a commercial-stage pharmaceutical company pioneering novel cancer therapies, today announced that following a pre-specified interim futility analysis for the ongoing Phase 3 SIENDO study, the Data and Safety Monitoring Board (DSMB) has recommended that the study should continue, as previously planned, without the need for adding additional patients to the trial or amending the study protocol.

"We are very excited that the SIENDO study has passed its planned futility analysis and that the study will continue as planned," said Sharon Shacham, PhD, MBA, President and Chief Scientific Officer of Karyopharm. "We believe this is an encouraging development for the study, and more importantly, for patients and families in need of novel treatment options for advanced or recurrent endometrial cancer. There are currently no approved therapies in the maintenance setting for patients with advanced endometrial cancer, making the future trial results from the SIENDO study, particularly important. We expect to report top-line data from this study in the second half of 2021."

The SIENDO study is an ongoing multicenter, blinded, placebo-controlled, randomized Phase 3 study evaluating the efficacy and safety for front-line maintenance therapy with XPOVIO in patients with advanced or recurrent endometrial cancer. Participants with primary stage IV or recurrent disease who had a partial or complete response after a single line of at least 12 weeks of standard taxane-platinum combination chemotherapy are randomized in a 2:1 manner to receive either maintenance therapy of 80mg of XPOVIO taken once per week or placebo, until disease progression. The study is expected to enroll approximately 248 patients. The primary endpoint in the study is progression free survival (PFS) with the goal of the study demonstrating a hazard ratio of 0.6. As of the date of the planned futility analysis, approximately 109 patients had been enrolled in the trial.

About XPOVIO® (selinexor)

XPOVIO is a first-in-class, oral Selective Inhibitor of Nuclear Export (SINE) compound. XPOVIO functions by selectively binding to and inhibiting the nuclear export protein exportin 1 (XPO1, also called CRM1). XPOVIO blocks the nuclear export of tumor suppressor, growth regulatory and anti-inflammatory proteins, leading to accumulation of these proteins in the nucleus and enhancing their anti-cancer activity in the cell. The forced nuclear retention of these proteins can counteract a multitude of the oncogenic pathways that, unchecked, allow cancer cells with severe DNA damage to continue to grow and divide in an unrestrained fashion. Karyopharm received accelerated U.S. Food and Drug Administration (FDA) approval of XPOVIO in July 2019 in combination with dexamethasone for the treatment of adult patients with relapsed or refractory multiple myeloma (RRMM) who have received at least four prior therapies and whose disease is refractory to at least two proteasome inhibitors, at least two immunomodulatory agents, and an anti-CD38 monoclonal antibody. Karyopharm has also submitted a Marketing Authorization Application (MAA) to the European Medicines Agency (EMA) with a request for conditional approval of selinexor in this same RRMM indication. Karyopharm's supplemental New Drug Application (sNDA) requesting an expansion of its current indication to include the treatment for patients with multiple myeloma after at least one prior line of therapy has been accepted for filing by the FDA. In June 2020, Karyopharm received accelerated FDA approval of XPOVIO for its second indication in adult patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL), not otherwise specified, including DLBCL arising from follicular lymphoma, after at least 2 lines of systemic therapy. Selinexor is also being evaluated in several other mid-and later-phase clinical trials across multiple cancer indications, including as a potential backbone therapy in combination with approved myeloma therapies (STOMP), in liposarcoma (SEAL) and in endometrial cancer (SIENDO), among others. Additional Phase 1, Phase 2 and Phase 3 studies are ongoing or currently planned, including multiple studies in combination with approved therapies in a variety of tumor types to further inform Karyopharm's clinical development priorities for selinexor. Additional clinical trial information for selinexor is available at www.clinicaltrials.gov.

Please see XPOVIO Full Prescribing Information available at www.XPOVIO.com.

About Karyopharm Therapeutics

Karyopharm Therapeutics Inc. (Nasdaq: KPTI) is a commercial-stage pharmaceutical company pioneering novel cancer therapies and dedicated to the discovery, development, and commercialization of novel first-in-class drugs directed against nuclear export and related targets for the treatment of cancer and other major diseases. Karyopharm's Selective Inhibitor of Nuclear Export (SINE) compounds function by binding with and inhibiting the nuclear export protein XPO1 (or CRM1). Karyopharm's lead compound, XPOVIO® (selinexor), received accelerated approval from the U.S. Food and Drug Administration (FDA) in July 2019 in combination with dexamethasone as a treatment for patients with heavily pretreated multiple myeloma. In June 2020, XPOVIO was approved by the FDA as a treatment for patients with relapsed or refractory diffuse large B-cell lymphoma. A Marketing Authorization Application for selinexor for patients with heavily pretreated multiple myeloma is also currently under review by the European Medicines Agency. In addition to single-agent and combination activity against a variety of human cancers, SINE compounds have also shown biological activity in models of neurodegeneration, inflammation, autoimmune disease, certain viruses and wound-healing. Karyopharm has several investigational programs in clinical or preclinical development. For more information, please visit www.karyopharm.com.

SOURCE: Karyopharm Therapeutics

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