A second investor has sued Express Scripts Holding Co. in Delaware federal court over a planned $67 billion acquisition by Cigna Corp., alleging Express Scripts withheld key information in connection with the deal.

The suit, filed Thursday in Delaware federal court by shareholder Walter Stern, targeted Express Scripts’ public disclosures after the company announced the merger in March. According to the complaint, the St. Louis-based pharmacy benefits management company and its directors had failed to provide adequate details about the firm’s financial projections, as well as potential conflicts of interest by its financial adviser, Lazard Freres & Co.