Sophiris slides on prostate cancer data but vows to press on

Micrograph showing the most common prostate cancer
The phase 3 protocol will be submitted to regulators in Europe and the U.S. (Michael Bonert CC BY-SA 3.0)

Sophiris Bio is adamant that it will advance its lead drug topsalysin into phase 3 testing, after a second dose showed no additional benefit in a prostate cancer trial.

The company insists that the midstage data presented after a single administration of the drug is enough to warrant a late-stage program. Shares in the company plunged 40% on the back of the new findings, however, suggesting investors were hoping re-treatment would add to its efficacy.

Topsalysin (PRX302) is being developed as a treatment for localized (stage I or II) prostate cancer that is only activated in the presence of prostate specific antigen (PSA), associated with prostate cancer and benign prostatic hyperplasia (BPH).

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It is administered by injection directly into the prostate tumor, and in the presence of PSA binds to cancer cells and form a pore across their membrane, causing them to rupture. It’s designed to kill tumors without affecting healthy prostate tissue.

In the earlier readout from the phase 2b trial, 10 out of 37 patients treated with drug had a clinical response six months after a single administration of topsalysin, with six of them seeing the total destruction of the tumor. Now, it says that “additional benefit was not observed on targeted biopsy six months after re-treatment” in patients who showed no response to the initial dose.

Piper Jaffray analyst Christopher Raymond says the six-month data with the drug remains “compelling” and he thinks the stock could recover on future catalysts such as agreement on a phase 3 trial protocol.

The latest share crash comes after a tricky few months for Sophiris and the topsalysin program that have caused its shares to yo-yo. The biotech halted patient dosing in June after a patient death, but restarted trials a couple of months later after an investigation found that it was unrelated to the drug or administration procedure.

The lead investigator in the trial, Professor Mark Emberton M.D. of University College London (UCL) in the U.K., said: “We remain encouraged by both the safety and biopsy data from the first administration of topsalysin and are working with Sophiris to design a protocol for a potential phase 3 registration study using a single administration of topsalysin.”

Sophiris’ CEO Randall Woods says the company is in the final stages of finalizing the design for the pivotal trial and will be submitting it to regulators, first in the E.U. and then in the U.S., for feedback.

The latest readout has some additional results that Sophiris reckons back up its optimism for a positive phase 3 outcome. Follow-up biopsy tests reveal that 15 of the 37 patients (41%) had a partial response that was still “clinically significant”, and less than a third (32%) had no response to the drug. The treatment was well-tolerated by patients throughout the study, according to the biotech.

"The results from the second administration in no way impacts our excitement about topsalysin as a potential targeted focal therapy in localized prostate cancer,” says Emberton. Around two thirds of newly-diagnosed prostate cancer patients have stage I or II disease.

Topsalysin also met its endpoints in a 2015 phase 3 study in 479 BPH patients, improving symptoms over 12 months.