LONDON - An experimental drug from Micromet reduced levels of cancer cells to undetectable levels in nine out of 12 patients with a severe form of leukemia, interim data from a clinical study showed on Friday.

Blinatumomab is a new kind of antibody drug that primes the body's own killer T-cells to directly attack tumour cells.

The 75 percent response rate represented a boost for the Maryland-based company's first product, which is still several years away from reaching market but has been attracting attention from scientists because of the novel way it works.

The Phase II study is due to enroll 25 adults with acute lymphoblastic leukemia (ALL) who have previously relapsed after standard chemotherapy. Interim results from the first 12 patients were unveiled at a medical congress in London.

ALL is a relatively rare cancer in which malignant immature white blood cells are overproduced and crowd out normal cells in the bone marrow. The average five-year survival rate for adults after first relapse is 7 percent.

Study leader Max Topp of the University of Wuerzburg said blinatumomab, given intravenously, had shown an unprecedented level of efficacy.

"These results are particularly striking relative to the fact that the majority of enrolled patients had characteristics typically associated with a dismal outlook," he said.

All nine patients helped by the drug had a complete molecular response, with no evidence of cancer cells in their bone marrow. Six achieved complete remission (CR) and three had the next-best outcome of CR with partial recovery of blood counts.

Two patients in the study suffered fully reversible central nervous system problems, causing treatment to be interrupted, and one suffered cytokine release syndrome, which was mitigated in subsequent patients by modifying the dose.

Micromet chief executive Christian Itin told Reuters the data presented at the meeting of the European Hematology Association was "very encouraging", as the standard approach of intense chemotherapy achieved CR rates of only 17-45 percent.

ALL afflicts around 11,000 people a year, half of whom have a high medical need, making it a small but potentially lucrative market opportunity.

Itin noted that Clolar, made by Sanofi's Genzyme unit, sold for $93,000 a year as a treatment for paediatric ALL.

Longer-term, blinatumomab is also being tested as a treatment for non-Hodgkin's lymphoma, another blood cancer, suggesting it could eventually generate hundreds of millions of dollars in sales, according to industry analysts.

Micromet owns full rights to the product, after AstraZeneca

handed back North American licensing rights in 2009.

Itin said Micromet planned to market the drug itself in the United States but might seek a partner elsewhere, although it would bide its time before striking any deal.

"It is not something that we are in any rush to do because there are still significant steps ahead of us that should drive significant value into the programme," he said in an interview.