Novartis AG said on Monday its experimental leukemia drug works in about half of all patients who develop resistance to Gleevec, the leading drug for the most common type of chronic myeloid leukemia, or CML.

Novartis, which hopes to launch the drug, Tasigna, by the middle of next year, will be battling for market share with Bristol-Myers Squibb Co., whose rival drug Sprycel won approval earlier this year to treat the same group of patients.

Data presented at the annual meeting of the American Society of Hematology showed that 51 percent of patients with CML characterized by a genetic abnormality called the Philadelphia chromosome (designated Ph+ CML) who took Tasigna, achieved a complete return to normal of their white blood cell counts. Normalization of blood cell counts is a measure of disease stabilization.

The Philadelphia chromosome characterizes about 95 percent of cases of CML.

The data, which reflects more mature results from a trial initially presented earlier, showed that in 34 percent of the 279 patients who took part, the Ph chromosome was undetectable after six months or more of treatment with Tasigna.

When the Ph chromosome becomes undetectable, the odds of a patient living longer go up, said David Epstein, president of Novartis’s oncology division, in an interview.

About 100,000 CML patients worldwide currently take Gleevec, which is also made by Novartis, and about 4,500 new patients are diagnosed in the United States each year, Epstein said.

Gleevec, which generates sales of about $2.5 billion a year, is typically the first treatment used for the disease. But Novartis says as many as 13 percent of patients - others say more - become resistant to the drug within five years, becoming candidates for either a higher dose of Gleevec or an alternative therapy.

While Bristol-Myers and Novartis will both be competing for those patients, both companies also have their eye on the much bigger market of patients who have not been treated with other therapies. The two companies are testing their drugs in that population.

Data so far suggests that while Sprycel may be more potent than Tasigna, it may also lead to more side effects.

“Getting the optimal balance between efficacy and tolerability will ultimately determine which drug becomes the first line drug of choice,” Epstein said.

Sprycel interferes with a greater number of proteins than Gleevec, while Tasigna is more narrowly focused than either, concentrating its attack solely on the Bcr-Abl protein, which is considered most directly implicated in causing the disease. Typically, the more specific a drug, the fewer the side effects, Epstein said.

Analysts at JP Morgan forecast 2007 sales of Sprycel of $250 million, rising to $750 million by 2010. However, those sales could go up significantly if doctors begin to use it as a first line therapy, they said.

To date, the analysts said, use of Sprycel has been slower than expected because some oncologists are choosing to increase the dose of Gleevec instead of switching patients to Sprycel.