* 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, whose shares jumped 18 percent, said it
planned to use the study results, released 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 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)