Human Genome Sciences said Thursday that its board of directors again rejected the $2.6 billion takeover offer from GlaxoSmithKline.

The two companies have partnerships linking three of HGS's most-promising drugs, which might deter other potential bidders.

Still, HGS says the $13 per share offer is insufficient and urged shareholders not to sell shares to GSK.

GSK is based in London, but has a big Center City presence and facilities in suburban Philadelphia.

GSK had sought to takeover HGS in a friendly way, but after the first rejection, announced that it was going directly to shareholders.

"Our board of directors has concluded unanimously that the GSK offer is inadequate and does not reflect the value inherent in Human Genome Sciences," HGS President and Chief Executive Officer H. Thomas Watkins said in a statement.

HGS said GSK was trying to take advantage of HGS's lower stock price in recent months and that talks have begun with other pharmaceutical companies.

"As part of this process, the company's management, with the assistance of the company's financial advisors, has engaged in discussions with a number of other parties, including major pharmaceutical and biotechnology companies, regarding a potential transaction, has entered into confidentiality agreements with certain parties and is providing those parties an opportunity to engage in a due diligence review of confidential information," the HGS statement said, without specifying which companies.

Thus far Thursday, GSK has not commented.

The HGS stock opened at $14.25 on Thursday. It's 52-week high was $28.38.

The bid for Human Genome Sciences is just the largest of a few recent moves by GSK to enlarge the empire.

The other day GSK said it completed acquisition of 10 million shares of Theravance common stock.

Earlier in the week, GSK said it spent about $98 million for the shares it didn't previously own of the privately-held company Cellzome. Cellzome does work in the proteomics technology area, meaning it researches and scans proteins to define their structures.