The proposed bill aims to cap out-of-pocket drug costs for recipients of Medicare - the U.S. health insurance for those 65 and older - at $2,000 a year, and also provide free vaccines for seniors. Merck Chief Executive Rob Davis said the proposed bill could have a "highly chilling" effect on innovation, but he does not see it hurting sales of Keytruda and Gardasil in the near-term, if it is passed. With Merck expected to lose patent protection for Keytruda in 2028, the company could also consider a subcutaneous or other versions of the drug, which is currently given by intravenous infusion form. "I think there is a path to think about that innovation," said Merck research chief Dean Li. Keytruda has come to the attention of U.S. lawmakers over tax issues. On Wednesday, U.S. Senator Ron Wyden, a Democrat, sent a letter to Merck suggesting the company had avoided billions of dollars of U.S. taxes owed from Keytruda sales in recent years by booking all the profits from the treatment outside of the United States.