GlaxoSmithKline (NYSE: GSK) reported a 13 percent drop in first-quarter net profit compared with a year earlier, when earnings were boosted by the sale of a unit.

The pharmaceutical company said Wednesday that profit after tax was 1.325 billion pounds ($2.2 billion), down from 1.525 billion pounds a year earlier. Revenue was up nearly 1 percent to 6.64 billion pounds. Earnings were 42 cents per share.

Profit in the first quarter of 2011 was boosted by the 1.2 billion-pound sale of the company’s stake in Quest, a U.S. medical lab operator, and its North American interests in Zovirax cold sore cream.

GlaxoSmithKline shares were down 2.4 percent  after the report was released.

“This quarter marked continued progress for the Group as we returned to reported sales growth, delivered additional R&D pipeline output and maintained our focus on returns to shareholders through dividend growth and share repurchases,” GSK Chief Executive Officer Andrew Witty said in a statement.

“Despite continued economic pressure and political instability in many markets and several demanding comparators with Q1 last year, total sales rose 2 percent. This performance reflects the resilience of our business and the investments we have made to increase the breadth and mix of the Group.

“The U.S. Pharmaceuticals and Vaccines business grew 9 percent this quarter. Growth benefited from incremental revenue related to the conclusion of our co-promotion agreement for Vesicare together with growth in Advairand an encouraging performance from new products, particularly in oncology.”

GSK operates its U.S. headquarters in RTP.

Read the full earnings report here.

Watch video of GSK CFO discussing results here.