GlaxoSmithKline (NYSE: GSK) extended its hostile $2.6 billion takeover offer for Human Genome Sciences Inc. (Nasdaq: HGSI) a second time to allow investors to consider any competing bids the U.S. biotechnology company may receive.

The $13-a-share bid now will expire at 5 p.m New York time July 20, the London-based drugmaker said in a statement today. The bid was to have ended today at 5 p.m. today. Shares of Rockville, Maryland-based Human Genome have closed above the offer price since Glaxo proposed the takeover in May, signaling stockholders expect a higher bid.

The extension means investors will be able to learn the outcome of Human Genome’s effort to seek other bidders before deciding whether to tender their shares to Glaxo. Human Genome’s board has rejected Glaxo’s proposal as inadequate and said set a deadline of July 16 for competing bids.

“The three months that will have passed since the start of this process is more than adequate time for the HGS board to bring it to completion,” Glaxo said today. After July 20, “GSK will consider all available options regarding its offer but can make no assurance that the tender offer will be further extended.”

About 0.2 percent of Human Genome’s shares had been tendered as of late yesterday, Glaxo, which operates its headquarters in Research Triangle Park, N.C., said today.

Human Genome today reiterated its rejection of Glaxo’s offer and said its sale process “continues to be active and fully under way.”

‘Fair Process’

“We are committed to completing our exploration of strategic alternatives as expeditiously as possible and in a manner that ensures that HGS stockholders have the benefit of a complete and fair process,” Human Genome said in a statement. Glaxo has refused to participate in Human Genome’s process of seeking bids.

Human Genome rose 1.5 percent to $13.26 at 9:40 a.m. New York time in Nasdaq Stock Market trading. Glaxo was unchanged at 1,446 pence in London.

By threatening not to extend the offer again, Glaxo is “trying to force the hand of any Human Genome shareholders dithering on the sidelines,” said Nicholas Turner, an analyst at Mirabaud Securities Ltd. in London.

Glaxo, the U.K.’s biggest drugmaker, wants to acquire Human Genome to gain control of Benlysta, a treatment for lupus, as well as albiglutide for diabetes and darapladib for heart disease. The drugmakers cooperate on all three medicines. Glaxo made the hostile offer in May after Human Genome’s board rejected an unsolicited bid in April for the same amount.

Sales Potential

The offer undervalues the sales potential of Benlysta, said Jose Aymami, a fund manager at Merchbanc in Barcelona who holds Human Genome shares.

“This is a product that will have little competition,” Aymami said. “A biologic product with multibillion-dollar sales potential has often carried a multiple of several times sales.”

Aymami cites the current bidding process for Amylin Pharmaceuticals Inc., which has a market value of more than $4.5 billion, compared with analyst estimates of $1.5 billion in potential revenue from its diabetes treatment Bydureon.

Glaxo Chief Executive Officer Andrew Witty has said the offer is “full and fair,” priced at 81 percent more than the stock’s closing price on April 18, the day before Glaxo’s interest became public.

Human Genome said on May 17 it was in talks with “major” pharmaceutical and biotechnology companies about a potential transaction and adopted a so-called “poison pill” shareholder rights plan to deter the hostile takeover.

Bankers from Goldman, Sachs & Co. and Credit Suisse Group AG are advising Human Genome, while Glaxo has hired Lazard Ltd. and Morgan Stanley.