* Pfizer weighs on Dow, S&P 500
* Mallinckrodt to buy Questcor for $5.6 bln
* MannKind stumbles; FDA extends drug review date
* Dow down 0.17 pct, S&P down 0.08 pct, Nasdaq up 0.09 pct (Updates to open, adds quote)
By Chuck Mikolajczak
NEW YORK, April 7 (Reuters) - U.S. stocks were little changed on Monday, as recent weakness in so-called momentum names appeared to abate while a decline in shares of Pfizer weighed on the market.
Declines in momentum shares - fast-growing companies which have outperformed the market recently - have left investors anxious about how much further they may fall.
That weakness continued early on Monday, with names such as TripAdvisor Inc down 1.6 percent to $84.30. However, other names such as Facebook Inc, up 1 percent to $57.33 and Gilead Sciences Inc, up 0.8 percent at $72.76, were able to reverse initial declines.
"There is a difference between that sense you get when it is picking up speed and becoming a one-way juggernaut either way, to the downside or the upside, and we are not seeing that," said Gordon Charlop, a managing director at Rosenblatt Securities in New York.
"We'll stick it out and see where they finally decide to step in but there is nobody here who is sensing there isn't going to be a point where that happens."
Earnings season gets under way this week, with results from financials JPMorgan Chase & Co and Wells Fargo & Co , as well as retailer Bed, Bath & Beyond.
Pfizer Inc, down 2.5 percent to $31.37, also added pressure to the Dow and S&P 500. The company's experimental breast cancer drug in a clinical trial nearly doubled the amount of time patients lived without their disease getting worse, but overall survival was not yet shown to be statistically significant, researchers said.
S&P 500 companies' first-quarter earnings are projected to have increased just 1.2 percent from a year ago, Thomson Reuters data showed. The forecast is down sharply from the start of the year, when growth was estimated at 6.5 percent.
A lackluster first-quarter earnings season impacted by harsh winter weather could spark a pullback, according to some analysts, with investors looking for optimism for the second quarter.
The Dow Jones industrial average was down 27.36 points, or 0.17 percent, at 16,385.35. The Standard & Poor's 500 Index was down 1.53 points, or 0.08 percent, at 1,863.56. The Nasdaq Composite Index was up 3.72 points, or 0.09 percent, at 4,131.44.
Specialty pharmaceuticals company Mallinckrodt Plc agreed to buy drugmaker Questcor Pharmaceuticals Inc for about $5.6 billion to gain access to its multiple sclerosis drug, Acthar Gel. Questcor shares surged 20.9 percent to $82.04 while Mallinckrodt shed 1.2 percent to $61.77.
Energen Corp said it would sell its natural gas utility Alabama Gas Corp (Alagasco) to Laclede Group Inc for $1.28 billion in cash to focus on oil and gas exploration and production. Energen gained 1.9 percent to $83.08 and Laclede lost 2.3 percent to $45.17.
MannKind Corp slumped 8.9 percent to $6.26. The company said the U.S. Food and Drug Administration extended the review date of its inhaled insulin treatment by three months. (Editing by Nick Zieminski and Bernadette Baum)