GlaxoSmithKline Plc (GSK) will begin a hostile $2.6 billion tender offer this week for Human Genome Sciences Inc. (HGSI) after the U.S. company, its partner on the Benlysta drug for lupus, rebuffed a takeover approach.

Glaxo will offer $13 a share in cash, or 81 percent above Human Genome’s closing price on April 18, the London-based drugmaker said in a statement today. Rockville, Maryland-based Human Genome announced April 19 that Glaxo had offered to buy the company for that price. Human Genome said the offer was too low, and the company said it would begin a review of strategic alternatives.

“GSK values the long relationship it has with HGS and has clearly stated its preference to complete a transaction on a friendly basis in a timely fashion,” Glaxo said in today’s statement. “GSK remains willing to meet and review its offer with HGS at any time.”

The bid became public last month after Human Genome shares slid 76 percent from their 2011 peak. Sales growth for Benlysta, Human Genome’s first drug on the market, has disappointed investors, and the company may not become profitable for years.

Glaxo won’t participate in Human Genome’s review in part because the U.K. company wants Human Genome shareholders to know that it’s committed to proceeding with the offer, Glaxo said.

The four weeks that have passed since Glaxo made its offer on April 11, together with the 20 business days that the tender offer must remain open, gives Human Genome enough time to complete its review of alternatives, Glaxo said.

Human Genome hired Goldman, Sachs & Co. and Credit Suisse Securities (USA) LLC as financial advisers and Skadden Arps Slate Meagher & Flom LLP and DLA Piper LLP as legal counsel, the company said last month.

Lazard Ltd. (LAZ) and Morgan Stanley (MS) are acting as financial advisers to Glaxo, and Cleary Gottlieb Steen & Hamilton and Wachtell, Lipton, Rosen & Katz are providing legal advice, Glaxo said in April.