Karyopharm is seeking approval of selinexor in combination with low-dose dexamethasone for the treatment of patients with relapsed/refractory multiple myeloma who have received at least three prior therapies and whose disease is refractory to at least one proteasome inhibitor, at least one immunomodulatory imide drug, and an anti-CD38 monoclonal antibody.
This disease is referred to as “triple-class refractory” multiple myeloma. At this stage, patients have exhausted every effective treatment option and are faced with the choice of supportive care or recycling previously successful drugs. Their median overall survival time is 3-5 months.
Karyopharm submitted its the New Drug Application using the Accelerated Approval pathway, arguing that the drug meets an unmet medical need and can be approved on surrogate endpoints – in this case, overall response rate.
The modified intent-to-treat analysis comprised 122 patients. The overall response rate was 25.4% with a median response duration of 4.4 months. Two patients had a complete response; six had a very good partial response; and 23 had a partial response.
Some committee members, however, said it would be impossible to tease out how much of the response could be due to the co-administration of 20 mg dexamethasone with each dose. In a phase 1 dose-ranging study of selinexor as monotherapy, it produced only one partial response in 56 patients. And, FDA pointed out, historical studies have shown response rates of 10%-27% for high-dose dexamethasone.
However, those in favor of the drug pointed out that the STORM patients were steroid-refractory, and that a 25% response rate would be unlikely on low-dose dexamethasone alone. This is proof of the company’s claim that the steroid works synergistically with selinexor, they said.
These members also pointed out that even a few years ago, there simply were no patients like the STORM cohort. Only recently have these patients lived long enough to develop resistance against all therapeutic lines, so it’s unrealistic to use historical data to judge what a reasonable response rate looks like in this situation.
Committee members also choked on STORM’s adverse event (AE) profile. All patients experienced at least one treatment-emergent AE, and 60% had at least one serious AE. Most (88.6%) required a dose modification due to an AE, and 28.5% discontinued due to one. The most common AEs were thrombocytopenia, anemia, nausea, fatigue, and decreased appetite. The company said these were “typically reversible and manageable with dose reductions.”
Additionally, there were 23 deaths in the trial. About half (13) were due to disease progression, but the remainder were due to a fatal treatment-emergent AE. Two of these (one pneumonia and one sepsis) were directly due to selinexor, the company said.
Despite the committee’s concerns, 16 of the 17 speakers described positive experiences with selinexor. They universally acknowledged that “it’s a hard drug to take,” and that side effects need to be managed proactively. But they also said, universally, that the drug has brought them additional months of good-quality life, decreased lengthy hospital stays, enabled them to participate in important family events, and even travel. Some also expressed the hope that selinexor would be a bridge drug, decreasing their disease burden enough that they could qualify for other clinical trials of new investigational drugs.
Only Stephanie Fox-Rawlings, PhD, of the National Center for Health Research, urged a delay. “Even if these adverse events are manageable, they harm patients’ quality of life,” she said. “This may be acceptable to some, but if the drug can’t provide a meaningful benefit then they are not worth it and in this clinical trial there was no improvement noted in quality of life. This drug has serious risks and we don’t know if it works.”
Dr. Fox said she was “very glad” that Karyopharm has completed recruitment for its phase 3 randomized study, dubbed BOSTON. BOSTON will assign active patients to once-weekly 100 mg selinexor plus weight-dosed bortezomib, plus twice-weekly 20 mg dexamethasone. The comparator group will receive weight-based bortezomib twice a week and 20 mg dexamethasone four times a week. Patients who progress can cross over to the active arm. The company hopes for even better results, saying that the proteasome inhibitor has also shown a synergistic effect with selinexor. Results are expected in 2020.
“The BOSTON study doesn’t solve anything,” retorted committee member David Harrington, PhD, emeritus professor of biostatistics at the Dana-Farber Cancer Institute. “It’s a different clinical profile, different dosing, a different combination of agents, and it doesn’t isolate the single-arm activity of selinexor.”
