By Sten Stovall

LONDON--GlaxoSmithKline PLC (GSK) Friday kept up the pressure on Human Genome Sciences Inc. (HGSI) by extending its offer for the U.S. biotech company to July 20, four days after the target company's deadline for finding alternative suitors.

Britain's biggest drug maker, Human Genome Science's long-time partner, has taken its $2.6 billion bid for the pioneer of gene-based medicine discovery direct to shareholders through a tender offer after HGS spurned the $13-a-share offer, initially made in April, as being too low.

HGS has also adopted a "poison pill" shareholder rights plan in a bid to fend off Glaxo and invited other companies to make a counter offer.

But no counter bidder has emerged.

Glaxo insiders say that is due to the U.K.-based group's partnerships with HGS around key drugs. Glaxo co-markets lupus drug Benlysta with HGS. The biotech also has financial interests in other experimental drugs under development at Glaxo, including darapladib for cardiovascular disease and once-weekly diabetes drug albiglutide.

"Extension of the tender offer to 20 July will provide HGS shareholders the opportunity to evaluate the outcome of the HGS Board's process relative to GSK's offer," Glaxo said Friday.

"Based on circumstances at that time, GSK will consider all available options regarding its offer but can make no assurance that the tender offer will be further extended."

With 198.5 million shares in issue, Glaxo's offer price bid would value HGS at around $2.6 billion. At 1240 GMT, Glaxo shares were down 0.3% at 1,441 pence, giving the U.K. company a market value of about GBP72.02 billion.

-Write to Sten Stovall at Sten.Stovall@DowJones.com

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