Merck & Co.’s second-quarter earnings dropped 52 percnet on acquisition and other charges as sales of its key drugs weren’t as strong as in recent quarters.

The drug giant also narrowed its 2010 earnings forecast while trimming the top end of its sales view for the year.

Merck’s results have been skewed by October’s acquisition of Schering-Plough as part of a consolidation wave in the pharmaceutical business. Meanwhile, the company’s results had been bolstered lately by continued strong sales for its biggest drug, asthma and allergy treatment Singulair, as well as its cardiovascular drugs Vytorin and Zetia.

But in the most-recent quarter, sales of Singulair were flat, while Vytorin fell 7.9 percent and Zetia rose 1.4 percent.

Overall, Merck reported a profit of $752.4 million, or 24 cents a share, down from $1.56 billion, or 74 cents a share, a year earlier. There were 48 percent more shares outstanding in the most recent quarter in the wake of the Schering-Plough purchase. Excluding merger and other impacts, earnings rose to 86 cents from 83 cents. Revenue jumped 92 percent, to $11.35 billion on the acquisition.

Analysts polled by Thomson Reuters had most recently forecast earnings of 83 cents on $11.46 billion in revenue.

Shares closed at $35.06 Thursday and were inactive premarket. The stock has risen 17 percent in the last year.