UPDATE 2-Onyx Pharma calls myeloma data 'impressive'

* Overall response rate 24 percent

* Onyx plans FDA application by year-end

* Shares rise 18 percent (Adds analyst comment, share price)

By Deena Beasley

LOS ANGELES, July 26 (Reuters) - Nearly a quarter of patients with multiple myeloma who had stopped being helped by current medicines responded to Onyx Pharmaceuticals Inc's experimental cancer treatment carfilzomib, according to data from a clinical trial.

Onyx (ONXX.O), whose shares jumped 18 percent, said it planned to use the study results, released on Monday, as the basis for seeking U.S. approval with the Food and Drug Administration before the end of the year.

"We think these are very impressive data," Onyx Chief Executive Officer Tony Coles told Reuters.

He said Onyx would seek an accelerated six-month review by the FDA based on the unmet need of the intravenous drug's target population, as well as its efficacy and tolerability.

"I find it very hard to believe that FDA wouldn't approve this product," said David Moskowitz, an analyst who follows Onyx for Madison Williams.

He said the data showed "a material benefit in very sick patients who have no other alternatives."

"In addition, the safety data is so strong for this product that we believe there is going to be significant differentiation over Velcade," added Moskowitz, referring to Takeda Pharmaceutical Co's (4502.T) widely used multiple myeloma treatment.

Christopher Raymond, an analyst for Robert W Baird, agreed that a 24 percent response rate for carfilzomib was better than expected. But he expressed concern that as a single arm study it did not provide a comparison to another treatment.

"We lack prospective here in terms of approvability," Raymond said.

"Carfilzomib is the same mechanism as Velcade so I think the question that is probably going to come up when FDA reviews this is could you have gotten the same results simply by retreating with Velcade? There is no data that says definitively yes or no," Raymond said, adding that the FDA might prefer to wait for data from any number of broader ongoing carfilzomib studies.

The mid-stage trial involved 266 severely ill multiple myeloma patients whose cancer had worsened after an average of five prior treatment regimens.

The study found that 24 percent of the patients responded to carfilzomib for a median duration of 7.4 months.

Coles said Onyx had expected a response rate for such heavily pretreated patients in the mid-teens with a response duration of four to six months.

A study by the International Myeloma Working Group found that myeloma patients as sick as those enrolled in the Onyx study can expect to respond to therapy 11 percent of the time.

The company did not release full details of the trial, including side effects, but said the drug was well-tolerated and there were no new or unexpected toxicities.

Coles said about 50,000 Americans have multiple myeloma -- a cancer that develops in cells of the bone marrow -- and about 20,000 new cases are diagnosed each year. Around 20 percent of patients are considered "relapsed and refractory," meaning that their cancer no longer responds to standard drugs, including Takeda's Velcade.

Onyx is also studying carfilzomib, a proteasome inhibitor, in patients with earlier-stage myeloma.

Onyx acquired carfilzomib developer Proteolix Inc last year for an upfront cash payment of $276 million and potential milestone payments of up to $535 million.

Moskowitz believes that Onyx itself is now a strong candidate to be acquired by a much larger drugmaker.

"A major innovative product in a market where all patients eventually fail therapy could be very attractive," he said.

Shares of Onyx were up $3.87, or 18 percent, at $25.37 on Nasdaq. (Additional reporting by Bill Berkrot in New York; Editing by Valerie Lee, Lisa Von Ahn and Gunna Dickson)