Sophiris Bio said Monday that 10 patients who were administered its experimental prostate cancer drug a second time didn’t see any additional benefit.
The news sent the San Diego, CA-based biotech’s stock plunging 40 percent.
Sophiris (NASDAQ: SPHS) already had encouraging mid-stage clinical trial data testing one dose of its drug, topsalysin, in prostate cancer patients. In June, the company said 10 of 37 patients with localized prostate cancer in its Phase 2b study who received the treatment reported a clinically significant response. Six of the 10 saw ablation, or elimination, of the treated tumors. Another 15 patients showed a partial response.
In a continuation of that study, 11 patients who had responded to topsalysin without experiencing side effects to the first dose, but had not seen complete ablation, received a second dose.
Six months later, biopsies revealed that doubling down on the treatment didn’t help, the company said Monday.
Throughout the study, the treatment was well-tolerated by patients, the company said. One of the patients who received a second dose died the day of the administration of a heart attack, but an investigation determined that the death was unlikely to be related to topsalysin, according to Sophiris.
Topsalysin is a form of the protein proaerolysin that Sophiris has engineered to be activated only by prostate-specific antigen, an enzyme found only on the surface of prostate cells. Following an injection into and around a tumor, the protein’s activation spurs it to kill nearby cells. It’s part of a crop of noninvasive techniques called focal therapies, which are meant to destroy small tumors inside the prostate while sparing most of the gland’s normal tissue.
Notwithstanding the failure of a second dose of topsalysin to provide clinical benefit, Sophiris said it would move forward testing a single dose of the drug in Phase 3.
“Taking into account the observed efficacy and safety profile to date following a single administration, we believe urologists would welcome a treatment like topsalysin for men with clinically significant localized prostate cancer,” Mark Emberton, the trial’s principal investigator, and University College London Faculty of Medical Sciences dean, said in prepared remarks.
This year about 164,690 new cases of prostate cancer are expected to be diagnosed.
Most patients are diagnosed with the localized form of the disease, meaning that it hasn’t spread beyond the prostate.
While many cases of prostate cancer—the second most common form of cancer in men in the U.S.—grow slowly, some patients eventually need radiation or surgery to stem its spread. However, the treatments often come with negative side effects, including incontinence and impotence. Sophiris’s therapy is meant to allow patients to delay or avoid those treatments.
The company said it is finalizing its proposed Phase 3 study design, and plans to submit it first to European regulators, then the FDA. It’s also trying to figure out how to bankroll such a study.
Sophiris is also testing the drug in late-stage studies as a treatment for men with lower urinary tract symptoms caused by an enlarged prostate, also known as benign prostatic hyperplasia.
The company relocated to San Diego from Vancouver, BC, in 2011.