Karyopharm Therapeutics (KPTI) - Get Karyopharm Therapeutics, Inc. Report announced results from a mid-stage clinical trial Tuesday in which an experimental drug, selinexor, demonstrated a 20% tumor response rate in multiple myeloma patients no longer responsive to four or five currently approved therapies.

Based on Tuesday's results, Karyopharm intends to enroll additional advanced multiple myeloma patients into the study to collect more data. If the 20% response rate to treatment with selinexor holds up, the company intends to petition the U.S. Food and Drug Administration for permission to seek accelerated approval.

Karyopharm expects to have top-line results from the expanded mid-stage study in early 2018.

Shares of Karyopharm are down 5.8% to $10 in Tuesday premarket trading. The stock had already moved 48% higher over the previous five trading days as investors anticipated the selinexor study results.

The quickest development path for a cancer drug in the U.S. often involves testing an experimental drug in patients who have run out of currently approved treatment options.

This is the strategy Karyopharm is using for selinexor, but so much medical progress against multiple myeloma, a cancer affecting the bone marrow, has been made in recent years that "no treatment option" patients have already been exposed to as many as five approved drugs. These include Revlimid and Pomalyst from Celgene (CELG) - Get Celgene Corporation Report , Amgen's (AMGN) - Get Amgen Inc. Report Kyprolis, Takeda's (TKPYY) Velcade, and most recently, Darzalex from Johnson & Johnson (JNJ) - Get Johnson & Johnson (JNJ) Report .

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Karyopharm recruited 78 multiple myeloma patients and treated all of them with selinexor, a pill. The overall response rate was 20.5%, based on an independent review. For patients refractory to four or five previous therapies, the response rate to selinexor remained at 20%.

The company didn't provide details on the duration of response except to say that "several" patients remained on the study without further cancer progression. No details on the safety of selinexor were provided but the company said the drug was well tolerated.

"We are currently unaware of any other therapy, oral or intravenous, reporting such activity in these difficult-to-treat patients who have exhausted all available therapies," said Dr. Keith Stewart of the Mayo Clinic and lead investigator of the selinexor study, in a statement.

The next step for Karyopharm will be to enroll an additional 120 multiple myeloma patients into the study and report response rate results in early 2018. If the results remain consistent, the company will seek permission from the FDA to file for accelerated approval. Karyopharm's argument to the FDA will be, essentially, that selinexor should be approved because multiple myeloma patients refractory to five previous therapies have no other treatment option.

The company also intends to start a randomized phase III study of selinexor in multiple myeloma to confirm the drug's efficacy and safety -- a step FDA will require as a condition for accelerated approval.

Karyopharm's clinical and regulatory strategy could backfire if another competing drug is approved for multiple myeloma before FDA reviews selinexor. In that case, the company's argument that selinexor is a viable treatment of last resort will be moot.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.