Sepracor shareholders are voicing their concern over the company’s proposed $2.6 billion sale to Dainippon Sumitomo Pharma in a lawsuit filed yesterday in Delaware.

Less than one week after announcing the deal between the Japanese firms, investors are calling the price negotiated—$23 per share in cash—inadequate and unfair. Sepracor, however, insists that the offer price represents a 48 percent premium over the stock’s six-month average price, and a 27.6 percent premium over the closing price of Sepracor’s common stock as of September 1.

Credit Suisse analyst Scott Hirsch stated in a letter to investors that while Sepracor’s sales are close to $400 million a year, its dwindling development pipeline (mostly respiratory and central nervous system treatments) doesn’t make the company appealing to a large number of buyers.

“In our view, if a US firm wanted Sepracor, that likely would’ve happened already, as there have been plenty of lookers over the years,’’ Hirsch stated. “We think Dainippon Sumitomo is more interested in the sales platform and operating leverage than the revenue stream.’’

The two investors filing the suit claim that Sepracor didn’t leave much room for negotiating after the bid was made. They noted the inclusion of a $77.4 million termination fee and a “no solicitation” provision giving Dainaippon first chance to counter other offers. The complaint also states that Sepracor “did not undertake to canvas the market prior to entering into the proposed merger, and thus failed to inform themselves of the inherent fair value of the company.”

Sepracor could not be reached for comment by deadline.