Pfizer said it will pay $850 million for the rights to an experimental Merck treatment for cancer, dubbed MSB0010718C, that taps the body's anitbodies to help fight tumors. Pfizer and Darmstadt, Germany-based Merck agreed to jointly finance the further development and marketing of the drug, while Merck stands to gain up to an additional $2 billion in bonus payments depending on the treatment's success.