Eli Lilly said on Monday it will buy Loxo Oncology Inc for $8 billion, an expensive bet on a pipeline of cancer drugs that target rare genetic mutations and the biggest acquisition in the drugmaker's 143-year history. The price represents a 68 percent premium to Loxo's Friday share price close, which some Wall Street analysts said was high for a company with only one drug on the market that it shares with a partner.