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UPDATE 1-Seattle Genetics, Astellas drug rapidly shrinks tumors in bladder cancer study

Tamara Mathias

cancer study@ (Adds expert comment, share movement)

June 3 (Reuters) - An experimental drug developed by Seattle Genetics Inc and Astellas Pharma Inc led to significant, rapid tumor shrinkage in patients with advanced bladder cancer who had been previously treated with immunotherapy and chemotherapy in a midstage trial, according to data presented on Monday.

Seattle Genetics shares rose 4.3% to $67.84.

In the 125-patient study of the drug, enfortumab vedotin, the overall response rate was 44%, meaning tumors were at least 30% smaller for those patients. That included a complete response, or no detectable cancer, for 12% of patients.

Responses were similar even among patients with the worst prognosis, including those whose cancer had spread to the liver, patients who had received three or more previous therapies and those who did not respond to treatment with an immuno-oncology drug, such as Merck & Co's Keytruda, researchers reported.

Current treatment options for those patients are extremely limited.

"These data have the potential to change the treatment course of advanced urothelial (bladder) cancer," Dr. Daniel Petrylak of the Yale Cancer Center, who led the study, said in a statement.

Enfortumab vedotin is a new type of treatment called an antibody-drug conjugate. It combines an antibody that targets a specific protein on the surface of tumor cells with a payload of chemotherapy that penetrates and kills cancer cells.

"Basically, it's a smart bomb," said Petrylak, who presented the data at the American Society of Clinical Oncology (ASCO) meeting in Chicago.

The companies, which share development costs and eventual profits, believe the data to be strong enough to seek accelerated approval with U.S. regulators this year.

"In a heavily treated population of patients, you're seeing significant objective response rates that are durable," said Dr. Robert Dreicer, an oncologist from the University of Virginia Cancer Center and an ASCO expert, said at the meeting. "I would support accelerated approval. I hope the FDA shares that view."

In addition to presenting tumor response data - the primary goal of the trial - researchers said most responses occurred within the first cycle of treatment and reported an overall duration of response of 7.6 months.

While the trial was not designed to demonstrate a statistically significant benefit on overall survival, half the patients were still alive after 11.7 months. Typical median overall survival for metastatic bladder cancer is about 9 months.

The companies are enrolling patients for a confirmatory late-stage trial testing the drug against standard chemotherapy.

"Our priority is to focus on areas with high unmet need and pursue targeted therapies for hard-to-treat cancers where few therapies exist," Andrew Krivoshik, head of oncology for Japan-based Astellas said in a phone interview.

Researchers were particularly encouraged by the significant tumor shrinkage seen in 38% of patients whose bladder cancer had spread to the liver, as they tend to be among the most difficult to treat with a very poor prognosis.

The American Cancer Society estimates 17,670 people will die from bladder cancer in the United States this year. (Reporting by Tamara Mathias in Bengaluru and Julie Steenhuysen in Chicago; editing by Bill Berkrot)