By Deena Beasley
CHICAGO (Reuters) - Shares of Exelixis Inc fell 20 percent on Monday after the company reported data over the weekend showing that its experimental cancer drug caused the deaths of six patients in a clinical trial.
Analysts said Exelixis shares were also affected by new trial results showing that a rival drug being developed by Bayer and Algeta helped prostate cancer patients live longer.
The Phase 2 data presented here at a meeting of the American Society of Clinical Oncology showed that treatment with the Exelixis drug, cabozantinib, led to significant tumor shrinkage in several different types of solid tumors, including 24 percent of patients with metastatic ovarian cancer.
Exelixis Chief Executive Michael Morrissey said the safety statistics are consistent with targeted cancer therapies like cabozantinib that block a pathway used by tumor cells to secure blood vessels.
Cowen & Co analyst Eric Schmidt said the rate of treatment-related deaths, 1 percent, was "no different from what we have seen for every other Phase 1 and 2 trial here at ASCO."
Interim results from the midstage trial also showed that cabozantinib shrank bone malignancies in 76 percent of prostate cancer patients.
Of 108 prostate cancer patients so far evaluated by bone scan, 21 had complete resolution, and 61 had partial shrinkage of metastatic bone lesions, which can lead to bone fractures, severe pain and eventual death.
The disease remained stable in 23 other patients, or 21 percent, and worsened in three.
For the overall trial, which involved several different types of solid tumors, six patients died from cabozantinib-related side effects: one breast cancer patient from respiratory compromise, one lung cancer patient from hemorrhage, one ovarian cancer patient from bowel perforation and another from intestinal perforation, one pancreatic patient from gastrointestinal hemorrhage and one unexplained prostate cancer patient death.
"While drug safety is of less concern in cancer indications than in others, the apparent morbidities associated with cabo use will confound interpretation of clinical benefit in a trial designed to show anything less than overall survival," Canaccord analyst George Farmer said in a research note.
Morrissey said the company plans to start a Phase 3 prostate cancer trial in the second half of this year with a combined goal of reducing pain and bone malignancies.
Piper Jaffray analyst Edward Tenthoff also said the reported deaths were affecting shares of Exelixis on Monday.
"The company is exploring lower doses, but the concern is that cabo will not retain the impressive efficacy seen to date," he said in a note to investors.
Morrissey said Exelixis plans to move forward with the current daily 100 mg dose of the drug.
Exelixis expects to have initial results around midyear from a trial of the drug as a treatment for thyroid cancer.
Cabozantinib, also known as XL184, is an oral drug designed to limit blood supply to tumors and block two segments of a pathway used by cancer cells to grow and spread.
The Bayer/Algeta drug, Alpharadin, is designed to treat patients with advanced disease whose cancer has spread to their bones. It works by mimicking many of the behaviors of calcium in the bone.
Shares of Exelixis fell $2.20 to close at $8.69 on the Nasdaq, off an earlier low at $8.03.
(Editing by Matthew Lewis, Maureen Bavdek, Dave Zimmerman and Steve Orlofsky)