By Ben Hirschler

LONDON (Reuters) - An experimental treatment for leukaemia being developed by Danish biotech company Genmab (GEN.CO: Quote, Profile, Research) and GlaxoSmithKline (GSK.L: Quote, Profile, Research) has proved effective in a pivotal Phase III trial, the companies said on Thursday.

Genmab Chief Executive Lisa Drakeman told Reuters the result demonstrated ofatumumab's potential to become a multibillion-dollar seller. It could reach the market as early as mid-2009, she said.

The antibody drug met its goal in the study of difficult-to-treat patients with chronic lymphocytic leukaemia (CLL), producing a response rate well in excess of the 25 percent minimum needed.

"We hoped that we would be better than the 25 percent, which was the primary endpoint, and we've just gone way beyond that and it's very, very exciting," Drakeman said in a telephone interview.

An objective response rate of 51 percent was achieved in patients who were "refractory," or failed to respond, to the existing treatments of fludarabine and Campath.

Campath, or alemtuzumab, is an antibody drug for leukaemia from Bayer (BAYG.DE: Quote, Profile, Research) and Genzyme (GENZ.O: Quote, Profile, Research).

A second group of patients refractory to fludarabine and for whom Campath was inappropriate because of their bulky tumours, saw an objective response rate of 44 percent.

The positive results seen from the 138 patients clears the way for the new medicine -- the most important product in Genmab's pipeline -- to be submitted to U.S. and European regulators for approval as early as this year.