The Food and Drug Administration said Monday that the response rate seen in a study of a proposed ChemGenex Pharmaceuticals Ltd. (CXSPY, CXS.AU) drug to treat certain type of patients with chronic myeloid leukemia was "low."

The drug, omacetaxine, faces a review by an outside panel of medical experts on Wednesday who serve on the FDA's oncologic drugs advisory committee. ChemGenex has proposed selling the drug under the brand name of Omapro.

Specifically, the company is seeking approval for Omapro to treat patients with a mutation known as T315I that can develop in some patients after being treated with Gleevec. Gleevec, by Novartis AG (NVS, NOVN.VX) is currently the mainstay of treatment for chronic myeloid leukemia or CML.

However, in a briefing document posted to the FDA's Web site Monday ahead of the panel meeting, the agency said there is no commercially available method to test patients to see if they have the T315I mutation. The agency said 35% of patients in the study didn't have their mutation status confirmed when they entered the study.

The agency also said researchers had planned to enroll 100 patients in one study but the company only submitted effectiveness data on 66 patients.

The FDA said it would ask its panel for advice "on the adequacy of the sample size and the clinical significance of the observed responses and their duration for the intended patient population."

In an interview before the FDA posted the review of Omapro, Greg Collier, ChemGenex's chief executive, explained the mutatation also prevents other drugs approved to treat CML, such as Tasigna, also from Novartis and Sprycel from Bristol-Myers Squibb Co. (BMY). Tasigna and Sprycel are approved for use in patients who don't respond to treatment with Gleevec.

Omapro is designed to work through a different mechanism than Gleevec to keep cancer cells from proliferating by stopping the production of certain so-called short-lived oncoproteins that are present in leukemia cells.

One study involving Omapro involved 81 patients with CML who were all at different stages of the disease. The response rate to the drug ranged from 35% to 86% of patients depending on the phase of the disease, with better responses seen among less sick patients.

Chronic myeloid leukemia is one of four types of leukemia, a type of blood cancer in which abnormal white cells are produced, limiting the body's ability to fight infection. About 5,000 new cases are diagnosed each year in the U.S. according to the National Cancer Institute.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com
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