U.S. stocks were set for a lower open on Friday as investors appeared poised to grab profits after a recent rally that has the S&P on track for its best December performance in nearly two decades.

The benchmark index has gained 6.6 percent so far this month, closing Wednesday at its highest level since September 8, 2008, and has risen in 17 of the last 21 sessions. The index is on course for its biggest December gain since 1991, when it rose 11.2 percent.

From its July low the S&P has risen 23 percent, boosted by improving economic data, positive earnings reports and stimulus measures by the U.S. Federal Reserve. Investors will closely watch a host of data next week for any incentives to take profits.

(Investors are) locking in gains, very low volume, anticipation of low volume and some traders simply not wanting to be exposed over the New Year's weekend, said Tim Ghriskey, chief investment officer of Solaris Asset Management in Bedford Hills, New York.

The market has had a big run here, and if there is some negative event over the weekend, it is probably is a bit vulnerable.

S&P 500 futures shed 3.1 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures lost 34 points, and Nasdaq 100 futures dipped 1.5 points.

Volume is expected to remain weak as the U.S. government and many businesses observe the New Year's holiday on Friday.

Drugstore chain CVS Caremark agreed to buy Universal American's Medicare prescription drug business for about $1.25 billion. Universal American jumped 36.9 percent to $20 in premarket trade, while CVS slipped 0.7 percent to $34.76 in light early electronic trading.

U.S.-listed shares of IMAX Corp surged 21.4 percent to $32.60 premarket after a report in Britain's Daily Mail suggested Sony Corp <6758.T> was contemplating a bid of at least $40 per share for the big-screen movie company.

Fertilizer producer K+S AG sued Dow Chemical Co's Rohm & Haas unit, saying it paid the company too much for its Morton Salt unit and is owed a refund.

(Reporting by Chuck Mikolajczak; editing by Jeffrey Benkoe)