Maker of microspheres for local delivery of radiation suffers clinical trial setback

Sirtex Medical's SIR-Spheres (green) being delivered into the bloodstream via microcatheter.

Sirtex Medical's clinical trial of its microspheres for targeted delivery of radioactive particles did not live up to expectations. Investors dumped the Australian company's stock in a hurry beginning March 13. It began the following week at $15, or a stunning 60% lower than before the bad news was delivered.

The results of the trial show that adding the company's resin SIR-Spheres to first-line chemotherapy for metastatic colon cancer does not improve progression-free survival, or the amount of time following treatment until the disease worsens or the patient dies of any cause.

"The treatment right now is relegated more to a salvage setting," Sydney-based healthcare analyst Derek Jellinek told Bloomberg. Using the therapy as a first-line treatment alongside chemotherapy would quintuple its market potential, he added.

The SIR-Spheres are used to transport the radioactive element yttrium-90, which delivers beta radiation over a short distance (2.5 millimeters) at the site of liver tumors.

The SIR-Spheres are inserted into the liver's hepatic artery using a microcatheter that travels through the leg's arteries. The spheres reach the liver tumor through the tumor's arterial blood supply. They then become implanted in the tumor because, at a median diameter of 32 microns, they are too large to pass through small blood vessels within the tumor, Sirtex says in a video on its website.

Then, the radiation begins. More than 90% of the radiation is delivered within the first 11 days. More than 10 million microspheres are deployed in each dose, and the targeted delivery minimizes damage to healthy liver tissue, Sirtex says.

Sirtex shares recovered slightly after the initial onslaught of selling. That's because the trial met a secondary endpoint related to efficacy against live tumors. That's significant because up to 90% of metastatic colon cancer patients die of liver failure, the company says. But the share price is still barely half of what it was before the bad news broke.

Full results of the trial of more than 500 people with metastatic colon cancer that had spread to the liver will be revealed at the American Society of Clinical Oncology meeting to be held May 29 to June 2.

Sirtex has annual revenues of close to $200 million thanks to the SIR-Sphere. Revenues would expand greatly if the therapy wasn't limited to use as a last resort.

The Sydney Morning Herald reports that there is speculation that some doctors in the U.S. are using the SIR-spheres as a first-line therapy. The latest clinical trial results could put an end to that practice.

- read the release (PDF)
- here's Bloomberg's take
- here's The Sydney Morning Herald's take

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