Alexion Pharmaceuticals added to S&P 500 index

Standard & Poor's Indices said Monday that it is adding the stock of drug developer Alexion Pharmaceuticals Inc. to its S&P 500 index.

At the close of trading Thursday, Alexion will replace Motorola Mobility Holdings Inc. in the index, S&P said. Google Inc.'s $12.5 billion deal to buy Motorola Mobility is expected to close before Wednesday.

Being added to the S&P 500 could boost Alexion's shares because managers of investment funds that track the S&P 500 will have to buy them in order to mimic the action of the broad-based index.

Alexion, which is based in Cheshire, Conn., has one approved product, Soliris. The drug treats a rare blood disorder called paroxysmal nocturnal hemoglobinuria, or PNH, which causes red blood cells to break down, leading to anemia.

Last month, the company reported first-quarter net income of $45.4 million on revenue of $244.7 million.

Its shares rose $2.99, or 3.4 percent, to $91.75 in extended trading Monday. They closed the regular session up $4.50, or 5.3 percent, at $88.76.