Eli Lilly said on Monday it w ill 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 repres ents 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 o n the market that it shares with a partner.  Loxo shares surged 66 percent to $232.65, close to the offer price of $235 per share, and prompted stock price rises for a other small developers of targeted cancer therapies, including Array BioPharma, Blueprint Medicines Corp and Clovis Oncology Inc. Lilly shares rose 0.5 percent to $115.28.   The cash deal comes on the heels of Bristol-Myers Squibb Co's agreement last week to buy Celgene Corp for $74 billion in the largest pharmaceutical deal ever, spurring investors' hopes of a new wave of large healthcare acquisitions.  Loxo gained prominence in 2017 - just three years after going public - with impressive clinical trial results showing its drug to be highly effective on cancers driven by a single gene mutation known as TRK fusion, regardless of where in the body the tumors originated.  These patients, with more than 17 different types of advanced cancer but all with the same genetic mutation, had run out of other treatment options. Yet some 80 percent who received Loxo's first drug, a pill called larotrectinib, experienced dramatic, often long-term improvement.