Merck ( MRK - Free Report ) is expected to face several hurdles over the next few years that could affect its long-term growth trajectory, starting with the anticipated loss of exclusivity of its blockbuster PD-L1 inhibitor, Keytruda, in 2028. The company's biggest revenue driver, Keytruda, is approved for several types of cancer indications and accounts for around 50% of the company's sales.