US STOCKS-Futures advance after three-day rally; data on tap

* Futures up: Dow 60 pts, S&P 5.75 pts, Nasdaq 2.75 pts

* Gilead falls as Express Scripts chooses AbbVie (Xetra: 4AB.DE - news) treatment

* Biocryst gets FDA OK for intravenous flu drug (Adds premarket actives data)

By Chuck Mikolajczak

NEW YORK, Dec 22 (Reuters) - U.S. stock index futures advanced on Monday, after the S&P 500 notched its best weekly performance in almost two months, ahead of data on the housing market.

* Brent crude hit a high of $62.97 and WTI crude rose to as much as $58.53 amid investor confidence there would be no substantial price loss in the run-up to the new year. Brent last traded down 21 cents at $61.17 while U.S. crude was last off 0.7 percent at $56.74 after Saudi Arabia indicated it could increase its output.

* The benchmark S&P index rose 3.4 percent last week, boosted by a 5 percent jump over three sessions, after the U.S. Federal Reserve said it would take a "patient" approach toward raising interest rates and oil prices appeared to stabilize. The S&P energy index surged nearly 10 percent for the week.

* Investors await existing home sales data, due out at 10:00 a.m. (1500 GMT). Expectations are for a decline of 1 percent.

* Trading volume is expected to be light this week due to the Christmas holiday, which could increase volatility. U.S. equity markets will operate a shortened session on Wednesday and be closed on Thursday.

* Gilead Sciences (NasdaqGS: GILD - news) lost 10.2 percent to $97.35 in premarket trade. Express Scripts, the nation's largest pharmacy benefit manager, has lined up a cheaper price from AbbVie Inc on its newly approved hepatitis C treatment and, in most cases, will no longer cover Gilead's treatments.

* BioCryst Pharmaceuticals (NasdaqGS: BCRX - news) jumped 5.3 percent to $11.75 in premarket trade after the U.S. Food and Drug Administration approved its intravenous flu drug.

* Achillion Pharmaceuticals Inc (NasdaqGS: ACHN - news) said it would test a combination of two of its experimental hepatitis C drugs which showed promise in separate studies. Its shares rose 16.1 percent to $16.50 before the opening bell with over 2.2 million shares changing hands, the most actively traded issue on the Nasdaq.

* Caesars Entertainment Corp said it would merge with affiliate Caesars Acquisition in an all-stock deal. Caesars Entertainment gained 9.7 percent to $14.80 in light premarket trade while Caesars Acquisition rose 11 percent to $10.50.

Futures snapshot at 8:11 a.m. EST (1311 GMT):

* S&P 500 e-minis were up 5.75 points, or 0.28 percent, with 91,013 contracts changing hands.

* Nasdaq 100 e-minis were up 2.75 points, or 0.06 percent, in volume of 17,847 contracts.

* Dow e-minis were up 60 points, or 0.34 percent, with 13,465 contracts changing hands. (Editing by Bernadette Baum)