GlaxoSmithKline has extended its offer to acquire Human Genome Sciences without upping its $2.60 billion bid, after hardly any shares have been tendered.

The UK drugs giant has pushed back the date to June 29 for its unsolicited tender offer to acquire all the outstanding common shares of HGS at a price of $13.00 each. It was previously scheduled to expire on June 7 and by then, just over 474,000 shares had been tendered.

Although its offer has been repeatedly rejected by the board at HGS, GSK insists the bid "represents full and fair value and is in the best interests of both companies' shareholders". The firm claims it provides "immediate liquidity at a substantial premium while eliminating further exposure to the significant execution risk inherent in HGS achieving its future growth objectives".

GSK went on to say the offer reflects the value of the two companies' lupus drug Benlysta (belimumab).as well as the late-stage cardiovascular treatment darapladib and albiglutide, currently in Phase III for the treatment of type 2 diabetes, plus HGS's operating and financial assets. It  "continues to believe that now is the appropriate time in the evolution of the GSK/HGS relationship for the companies to combine".

HGS is still not impressed and noted that less than 1% of its outstanding shares have tendered into the GSK offer. The board of directors noted that its "exploration of strategic alternatives", including a potential sale "continues to be active and fully underway.

GSK declined to enter the review and management at the US firm says the latter seeks to "circumvent, disrupt and prematurely end the company’s process to the disadvantage of HGS stockholders". It has been reported previously that GSK will seek the backing from HGS shareholders to replace the entire board with 12 independent directors.