InVentiv Health Inc. (NASD: VTIV), a pharmaceutical services and staffing firm, announced an agreement Thursday to be acquired by private equity firm Thomas H. Lee Partners L.P. in a deal valued at $1.1 billion.

InVentiv shareholders would receive $26 for each share. The deal is expected to close in the third quarter pending necessary approvals.

Somerset NJ-based inVentiv also announced first-quarter earnings on Thursday. The company said first-quarter revenue in its clinical division, which includes clinical staffing services, rose 5% to $54.0 million from $51.2 million in the first quarter of 2009.

Total first-quarter revenue at inVentiv rose 4.5% to $269.4 million. The company posted net income of $10.3 million in the first quarter, up 31.7% from the same period last year.

InVentiv's businesses other than staffing include pharmaceutical advertising and marketing, pharmaceutical sales, and patient services.