The US Food and Drug Administration (FDA) has granted accelerated approval to a new cancer drug called selinexor. This pill, which also goes by the market name Xpovio, has been found to be successful in treating multiple myeloma in patients that hadn't had success with other treatments. But at this very early stage the drug comes with some major caveats.

Selinexor works on a completely different mechanism to other cancer drugs. Known as a selective inhibitor of nuclear export, the drug enters a cell and targets the mechanisms that move molecules into and out of the cell. The idea is that cancer cells need to get rid of a certain protein called XPO1 that suppresses tumor growth, so blocking this process makes them easier to destroy.

The drug has recently made it through Phase II trials, which showed that it helped 25 percent of patients diagnosed with multiple myeloma, a pervasive form of blood cancer. In those tests, the patients received 80 mg of selinexor and 20 mg of dexamethasone in pill form, twice a week. These patients had already received several other treatments for the cancer, with no success. The key to its effectiveness seems to be the fact that it works using a different mechanism to the other treatments.

After considering the data from that trial, the FDA approved selinexor for further tests in patients with multiple myeloma, but on the condition that they had already tried four or more existing treatments. That's because the new drug is actually quite risky – it can also affect healthy cells, interfering with their ability to transport proteins in and out. That led to complications such as reduced platelets and red or white blood cells.

"The concerns about side effects are real," says Dan Vogl, the professor who led the first part of the Phase II trial. "It's not an easy drug to take as a patient or to give as a physician. But this (FDA) approval is properly placed. This is a drug we can give patients later in treatment when all other options have failed."

Selinexor is currently going through further studies, including a Phase 3 trial that's testing a lower dose of the drug and combining it with another regular therapy. In others, it's being tested as a treatment for lymphoma, leukemia and sarcoma.