Charles River Laboratories International Inc., a Wilmington drug research company, said Tuesday that it has agreed to buy a 75 percent ownership stake in Vital River, a Chinese provider of pharmaceutical research models and related services. The purchase price is about $27 million.

Charles River said the acquisition would help it expand in the fast-growing Chinese market for drug discovery and development. The transaction, which is expected to be completed in the first quarter of 2013, should add more than 1 percent to Charles River sales next year while boosting earnings per share, the company said.