GlaxoSmithKline Plc. may pay more than $500 million for rights to an experimental drug that could help tobacco smokers permanently kick their addiction to nicotine.

Glaxo will initially pay Nabi Biopharmaceuticals $40 million for the drug, called NicVAX, while further payments will be conditional on development targets. Nabi will also receive a "double-digit" percentage of any potential sales of the treatment, which is in the second of three phases of clinical tests.

NicVAX blocks nicotine from reaching receptors in the brain by triggering the immune system to produce antibodies that bind to nicotine, making the molecules too large to cross the blood-brain barrier.

"This smoking cessation vaccine technology could be a novel solution to help the millions of smokers who want to stop smoking and remain abstinent," Jean Stephenne, president of Glaxo's biologicals unit, said.