GlaxoSmithKline could walk away from its $2.60 billion bid to buy Human Genome Sciences if the latter's board does not drop a 'poison pill' adopted last week to thwart the UK drug giant's hostile approach.

A fortnight ago, GSK decided to appeal directly to shareholders with its $13-a-share offer for its long-time partner and co-developer of the lupus drug Benlysta (belimumab). HGS responded by adopting a shareholder rights plan which gives other investors the right to buy more stock at a discount if one shareholder buys 15% or more.

In unveiling the poison pill last week, HGS said the move allows it "to fully engage in its strategic review process and...protect the long-term interests of the company’s stockholders". Following GSK's original approach, the US biotech hired Goldman Sachs and Credit Suisse to explore alternatives, including a potential sale of the company.

GSK was invited to get involved in the process but insists its participation is unnecessary as its bid does not require due diligence. Now it has responded by amended the conditions of the tender offer to address the shareholder rights plan.

Specifically, GSK has added a condition to its offer "requiring HGS to redeem the pill or, alternatively, GSK being satisfied in its reasonable judgment that the pill has been invalidated or is otherwise inapplicable" to its proposed acquisition. The tender offer and withdrawal rights are still scheduled to expire on June 7.