UPDATE 4-Trial size may limit use of Seattle Genetics drug
* Trial size, design limit conclusions -- FDA staff
* FDA staff see significant amounts of missing safety data
* Label for Hodgkin's narrower than expected -- analysts
* Seattle Genetics shares fall 3.6 pct (Rewrites first paragraph; adds analyst comments)
WASHINGTON, July 12 (Reuters) - U.S. drug reviewers might limit the use of a Seattle Genetics Inc (SGEN.O) experimental blood cancer drug due to the narrow scope of its clinical trials, sending the company's shares down 3.6 percent.
In documents released on Tuesday the Food and Drug Administration asked an advisory panel to consider the drug, under the proposed trade name Adcetris, for use in previously treated patients with Hodgkin's lymphoma and anaplastic large cell lymphoma (ALCL). About 9,000 Americans a year are diagnosed with Hodgkin's lymphoma and 3,000 with ALCL.
The reviewers suggested the labeling for Hodgkin's should focus on a smaller patient group than expected, limiting potential sales. According to the FDA documents, the drug should be considered for patients who had already been given a stem cell transplant, rather than on all treated patients as suggested by the company.
The FDA advisory panel votes on the drug on Thursday.
Wall Street analysts noted the FDA reviewers had not raised any unusual concerns about the drug's safety or effectiveness.
Cory Kasimov, a JPMorgan analyst, sees an 85 percent chance the drug will be approved for both diseases.
"That said, we still believe that expansion in earlier stages of disease with larger patient markets is critical to the stock's future valuation," he said in a research note.
Howard Liang, an analyst at Leerink Swann, sees U.S. sales at over $400 million for both types of cancer in 2015.
Seattle Genetics shares closed 74 cents down at $19.80 on the Nasdaq. They hit an all-time high of $21.41 on June 28 after European health regulators accepted an application to market the drug for both Hodgkin's and ALCL in Europe.
"The stock is down probably because there are some hints the FDA may not allow as broad a label as the company is seeking," Liang said.
LIMITED DATA FROM TRIALS
In preliminary trials, Adcetris, given in a 30-minute infusion every three weeks, demonstrated complete response rates and reduced tumor size in more than 90 percent of patients -- 94 percent for Hodgkin's and 97 percent for ALCL, the company said.
The FDA said major side effects for both included nerve damage, infections, reactions from the infusion and myelosuppression, a common side effect from chemotherapy that causes decreased blood cell production.
One patient also dropped out of the Hodgkin's study because of Stevens Johnson Syndrome, a life-threatening skin condition.
However, the mid-stage trials only included 58 patients for ALCL and 102 patients for Hodgkin's, and did not compare Adcetris to another drug. The FDA said these factors "limit the benefit-risk analysis."
The FDA reviewers also noted that no second trial confirmed the clinical benefit of Adcetris in patients with ALCL, as required under priority review, and asked the advisory panel to weigh whether the drug should be given an accelerated approval without such confirmation.
Seattle Genetics Chief Executive Clay Siegall told Reuters in May that annual U.S. sales of Adcetris for Hodgkin's and ALCL would likely amount to less than $1 billion.
But if the drug wins approval as a front-line therapy, or first treatment option, the annual U.S. sales could be "considerably over a billion dollars," Siegall said.
He added that he had a "high level of confidence" Adcetris would win U.S. approval by Aug. 30 -- the early decision date set by the FDA. [ID:nN25149337]
The FDA granted priority review status for the drug, known chemically as brentuximab vedotin, meaning the agency believes the medicine is a potentially significant advance over existing therapies.
Brentuximab vedotin links a tumor-targeting antibody to a cancer-killing chemotherapy drug with the goal of limiting side effects. It is designed to home in on an antigen, or foreign substance, in Hodgkin's lymphoma, several types of T-cell lymphoma and other hematologic malignancies.
Seattle holds the full U.S. and Canadian rights to Adcetris, while Japanese drugmaker Takeda Pharmaceutical Co Ltd (4502.T) has the rights for the rest of the world, with milestone payments and royalties going to Seattle.
On June 27, Seattle said it would receive a $5 million milestone payment under that collaboration agreement after the European Medicines Agency decision.
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