The Human Genome Sciences Laboratories and Offices building in Rockville. (Jose Luis Magana/Reuters)

The tender offer for $13 a share was originally set to expire at midnight on June 7, but that deadline became untenable after HGS’s board of directors initiated a poison pill in hopes of derailing the hostile takeover.

The companies have been embroiled in a buyout dispute for nearly two months. Glaxo made an unsolicited bid for the company in April worth $2.6 billion, but that price was rejected because HGS officials said it undervalued the firm’s long-term financial prospects.

HGS urged its shareholders again on Friday to rebuke the tender offer and wait for the board of directors to complete a review process of its strategic alternatives that began in April.

“GSK declined to enter the process and, through its unsolicited tender offer, seeks to circumvent, disrupt and prematurely end the Company’s process to the disadvantage of HGS stockholders,” HGS said in a statement.

Some shareholders have filed a lawsuit claiming the board of directors has not acted in their best interest.

Glaxo and HGS are development partners on Benlysta, a drug that was approved by the Food and Drug Administration last March to treat systemic lupus. The companies have two other drugs in the pipeline, including one to treat cardiovascular disease.