Amgen Inc. said Monday its fourth quarter profit rose 15 percent despite flat sales as the company reduced its expenses.

After the close of market, the Thousand Oaks-based biotech reported net income of $961 million (91 cents per share), compared with $835 million (76 cents) a year ago. Last year the company had additional costs from a restructuring.

Revenue for Amgen, the world’s largest biotech by revenue, was essentially flat at $3.75 billion. Sales rose for its anti-infection drug Neulasta and arthritis treatment Enbrel, but fell for its anemia drugs Aranesp and Epogen, which have been hurt by safety concerns.

Excluding one-time costs, Amgen says it earned $1.12 billion ($1.06) in the quarter, up from $1.09 billion ($1) a year ago. Analysts surveyed by Thomson Reuters on average were expecting a profit of $1.07 per share.

For full year 2008, earnings rose 6 percent to $4.55 per share, as revenue increased 2 percent to $15 billion. For this year, Amgen forecast adjusted earnings of $4.55 to $4.75 per share, in line with the Wall Street consensus. But executives expect revenue of between $14.8 billion to $15.2 billion, below the $15.4 billion analyst consensus.

One reason the company is expected to see only a modest rise in earnings this year is the anticipated cost of getting its new osteoporosis drug denosumab approved and launched, which is expected to happen later in the year.

Prior to the announcement, Amgen shares closed up $1.22, or 2 percent, to $54.41 in trading on the Nasdaq.