UPDATE 2-U.S. FDA staff questions small ChemGenex study

* FDA cites small study, low response rate

* Agency questions lack of available test for mutation

* ChemGenex: Risks, benefits of Omapro acceptable (Adds details from FDA documents, company comment)

WASHINGTON, Feb 8 (Reuters) - A small study of ChemGenex Pharmaceuticals Ltd's CXS.AX experimental drug for leukemia patients with a certain genetic mutation showed a low response rate, U.S. health regulatory staff said in documents released on Monday.

The Australian drugmaker submitted data on how well the injectable drug, called Omapro, worked in just 66 patients, although plans for the study had called for 100 enrollees, U.S. Food and Drug Administration staff said.

FDA officials released the documents ahead of Wednesday's public meeting of outside advisers to discuss whether to recommend approval of the drug.

Omapro, or omacetaxine mepesuccinate, is a secondary treatment aimed at chronic myeloid leukemia patients who do not improve with Novartis AG's (NOVN.VX) (NVS.N) Gleevec. CML is a type of cancer that originates in bone marrow, the spongy tissue that makes blood cells.

While Bristol-Myers Squibb Co's (BMY.N) Sprycel and Novartis's Tasigna also aim to treat patients after unsuccessful Gleevec treatment, Omapro targets those who test positive for a specific genetic mutation known as Bcr-Abl T315I.

However, there is no commercially available method of detecting this mutation, the FDA wrote.

The FDA said it would seek advice from its advisory panel on the Omapro study, including the sample size, "the clinical significance of the observed responses and their duration in the intended patient population," and the drug's overall benefits and risks.

While the FDA usually follows its panelists' advice, it is not bound by their recommendation.

In a separate document, ChemGenex said its drug showed acceptable risks and benefits with manageable side effects. Reactions to the drug can include platelet and bone marrow disorders as well as anemia, according to the FDA.

Omapro is "the only therapeutic treatment alternative for a patient population with a poor prognosis and no proven treatment options," the company wrote. Patients using the drug would self-inject it twice a day for seven days each month, according to ChemGenex.

In an interview on Friday, ChemGenex Chief Executive Officer Greg Collier said testing for the genetic mutation was available in some hospital labs. "Without a drug approved," he added, "there's not a huge demand to get this test done as regularly as it should be."

While the target patient population is small, it is hard to pinpoint an exact market for the drug because the Bcr-Abl T315I mutation is not routinely tested, he told Reuters.

About 30,000 people in the United States have chronic myeloid leukemia, according to Collier. Two-thirds undergo Gleevec, and roughly 40 percent of them develop a resistance and need other options. About half of those with resistance have some sort of mutation, with 20 percent having the one Omapro targets, he said.

ChemGenex also has plans to develop Omapro in Europe, the Middle East and parts of Africa as part of a deal with Hospira Inc (HSP.N). (Reporting by Susan Heavey; Editing by Gerald E. McCormick and Lisa Von Ahn)