Drugmaker Retrophin Inc RTRX.PK said it offered to buy the rest of Transcept Pharmaceuticals Inc TSPT.O shares it does not already own for $4 per share, valuing Transcept at about $250 million.
Retrophin's offer represents an 11 percent premium to Transcept's closing share price of $3.59 on the Nasdaq on Wednesday.
Shares in Transcept, which has a market capitalization of about $62 million, rose about 9 percent to $3.90 in after-market trade, below the offer price, suggesting that investors did not expect a higher offer.
New York-based Retrophin, which is developing treatments for diseases affecting children, said last week it sent a letter to the Transcept board offering to buy the company for $3.50 per share.
Point Richmond, California-based Transcept rejected the offer and, instead, adopted a poison pill to prevent stockholders from acquiring over 4.99 percent of its shares.
Transcept has struggled to lift sales of its lead drug, Intermezzo, since it hit the market in April 2012. The company's revenue, mainly made up of Intermezzo sales, fell to $200,000 in the quarter ended June from $500,000 a year earlier.
Intermezzo is used to treat people who have difficulty returning to sleep after waking up in the middle of the night.
(Reporting by Zeba Siddiqui in Bangalore; Editing by Sriraj Kalluvila)