U.S. stock index futures drifted lower 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.

Everybody is going to be eyeing the next week with the start of the new year and a lot of information coming -- ISM numbers, and so on and so forth, said Rob Russell, president of Russell & Co in Dayton, Ohio.

(Today) will give investors moment to pause and reflect on a great last couple of quarters here in the U.S.

S&P 500 futures shed 4.2 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 36 points, and Nasdaq 100 futures dipped 1.75 points.

CVS Caremark Corp agreed to buy Universal American Finanical Co's Medicare prescription drug business for about $1.25 billion.

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

Miner BHP Billiton's acquisition strategy was back in the spotlight as market talk resurfaced it was looking at a $40 billion-plus bid for Anadarko Petroleum Corp , although banking sources said they were unaware of any imminent offer.

Wireless pioneer Craig McCaw, the chairman of Clearwire Corp , has decided to resign, the company said late Thursday. Clearwire, majority-owned by Sprint Nextel Corp , said McCaw's decision was not due to any disagreements with the company on any matters relating to its operations.

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. [nL3E6NV03S]

European shares were on track to post a 7.5 percent gain for 2010, although trading volume dwindled by midday Friday as several markets were closed for the year. <.EU>

Most Asian stock markets ended the year with solid annual gains and further advances were expected in 2011, but worries about the U.S. economy and Europe's festering debt crisis will keep investors on edge well into the new year.

(Reporting by Chuck Mikolajczak; editing by Jeffrey Benkoe)