* FTSEurofirst 300 ends down 0.4 percent
* Actelion leads pharma slide on trial news
* Unilever a top gainer after $3.7 bln Alberto Culver buy
By Simon Jessop and Harpreet Bhal
LONDON, Sept 27 (Reuters) - European stocks ended a thinly traded session in the red and just off lows on Monday, tracking U.S. weakness ahead of fresh macro data, and led by biotech Actelion after a key drug failed a late-stage trial.
The pan-European FTSEurofirst 300 index of top shares closed down 0.4 percent at 1,073.55 points, partially retracing Friday's gains, albeit in low volumes that amounted to just 67 percent of the 90-day average.
"We've seen a bit of a pause for breath today and it seems like the market is running out of a bit of steam," said Joshua Raymond, markets strategist at City Index.
"There is a big focus on economic data this week and if we do see one or two of these underperform then it could convince people to take profit off the table and we could see markets pull back."
Leading the charge to the downside was Swiss-based Actelion, which fell 7.9 percent after its experimental haemorrhage drug clazosentan failed a key Phase III study and now looks unlikely to make it to market.
Across in Britain, AstraZeneca added to the sector woes and ended down 1.6 percent after experimental prostate cancer pill zibotentan also failed a late-stage clinical trial.
Both stocks ended up lagging the European healthcare index, which fell 0.6 percent.
Looking ahead, UK GDP figures on Tuesday and U.S. GDP numbers on Thursday are likely to provide the biggest macro push this week, with consumer sentiment data from both sides of the Atlantic and euro zone inflation data also due.
"Sentiment for stock markets still remains positive overall - the FTSE 100 is still up by around 6 percent for September," said Anthony Grech, head of research at London-based IG Index.
While weak data from Tuesday's announcements could prompt a further sell-off in London, investors had been "happy to buy into any weakness for the past couple of months now, so it looks like shares still have some gains yet to come," he added.
UNILEVER
On the upside, consumer goods giant Unilever's latest billion-dollar deal dominated corporate newsflow.
Food and beverage stocks led the sectoral gainers and ended up 0.9 percent on the back of Unilever's planned buy of Alberto Culver for $3.7 billion in cash.
Other deal news to boost the sector included talk China's Bright Food Group is mulling a $3.2 billion deal for Britain's United Biscuits - in what would be the biggest foreign deal in the sector by a Chinese company.
Pernod Ricard helped support the sector gains with a 1.9 percent rise after French newsletter La Lettre de L'Expansion said the French drinks group had had an excellent summer, with sales up 7 percent against 2008.
Across Europe, the FTSE 100 index ended down 0.5 percent, Germany's DAX was 0.1 percent lower and France's CAC 40 closed down 0.4 percent. (Editing by David Cowell)