* Futures up: Dow 45 pts, S&P 4.25 pts, Nasdaq 8.5 pts
* Gilead falls as Express Scripts chooses AbbVie treatment
* Biocryst gets FDA OK for intravenous flu drug
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, as oil prices continued to show signs of stabilizing.
* 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. However, Brent pulled back to last trade down 14 cents to $61.24 while U.S. crude was last off 0.4 percent at $56.91.
* 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 as 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 lost 6.6 percent to $101.30 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 jumped 10.2 percent to $12.30 in light premarket trade after the U.S. Food and Drug Administration approved its intravenous flu drug.
* Rising energy shares lifted European equities, while Greece's bourse advanced after its prime minister offered Sunday to bring pro-European independents into the government.
* Asian shares took their cues from Wall Street's strong finish last week and moved higher, also aided by the recovery in oil prices.
Futures snapshot at 7:35 a.m.:
* S&P 500 e-minis were up 4.25 points, or 0.21 percent, with 80,495 contracts changing hands.
* Nasdaq 100 e-minis were up 8.5 points, or 0.2 percent, in volume of 14,883 contracts.
* Dow e-minis were up 45 points, or 0.25 percent, with 12,172 contracts changing hands. (Editing by Bernadette Baum)