Merck & Co , which plans to buy rival drugmaker Schering-Plough Corp later this year, reported lower-than-expected sales and earnings that it attributed to the global economic slowdown.

The company, whose shares fell almost 3 percent in pre-market trading, said on Tuesday it earned $1.46 billion, or 67 cents per share. That compared with $3.33 billion, or $1.52 per share, in the year-earlier first quarter, when it received a big payment from marketing partner AstraZeneca .

Excluding special items, Merck earned 74 cents per share. Analysts on average expected 78 cents per share, according to Reuters Estimates.

Global sales fell 8 percent to $5.39 billion, well below the Reuters Estimates forecast of $5.76 billion.

Our first-quarter results in part reflect the impact of the difficult global economy on patients, providers and payors, but we remain on track to meet our full-year earnings guidance, company Chief Executive Richard Clark said in a release.

Clark said the company's planned acquisition of Schering-Plough would help assure sustainable growth in coming years.

Schering-Plough earlier on Tuesday reported higher-than expected first-quarter earnings as it took a hatchet to its research and administrative spending.

Merck shares were trading at $24.51 in pre-market activity, from their closing price of $25.22 on Monday. Schering-Plough shares edged slightly lower.

(Reporting by Ransdell Pierson; Editing by Derek Caney)