A Human Genome Sciences shareholder is suing the US biotech in a bid to stop the firm from using its 'poison pill' to foil GlaxoSmithKline's hostile $2.60 billion takeover bid.

The filing was made by plaintiff Duane Howell on behalf of HGS shareholders asking a court in Maryland to issue a temporary restraining order preventing the board of directors from adopting a shareholder rights plan unveiled earlier this month. The poison pill gives other investors the right to buy more stock at a discount if one shareholder buys 15% or more.

In response last week, GSK added a condition to its $13-a-share offer for its long-time partner and co-developer of the lupus drug Benlysta (belimumab), "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.

Mr Howell's lawsuit backs GSK's position as he states in court papers that the poison pill was “not a reasonable response” and one that “will operate to rob the company and its shareholders of the opportunity to obtain a premium change of control transaction”. He went on to ask the court to suspend the pill pending a rapid hearing, and accused HGS management of “acting to entrench themselves…for their own financial interests”.

HGS which has dismissed GSK's bid as too low and  opportunistic, believes the lawsuit is without merit.

GSK melanoma combo goes head-to-head with Roche's Zelboraf

Meantime, GSK has announced the start of a Phase lll programme evaluating the investigational melanoma combination of dabrafenib and trametinib.

Specifically, the drugs giant is running two trials; the first will look at dabrafenib (its BRAF inhibitor) with the MEK inhibitor trametinib to see if the combo is a more effective treatment in patients with BRAF V600E or V600K mutation-positive metastatic cutaneous melanoma than using dabrafenib as a single agent.

The second, arguably more interesting trial, will pit dabrafenib-trametinib against Roche's already-approved BRAF inhibitor Zelboraf (vemurafenib) in a head-to-head trial aimed at finding out which treatment improves the overall survival of patients with metastatic melanoma most.

Rafael Amado, head of oncology R&D for GSK, said that while recent clinical findings with BRAF inhibitors in melanoma "have shown significant improvement over chemotherapy, eventually tumours become resistant to these inhibitors leading to disease relapse". He added that "recent scientific advances have led us to the hypothesis that the combination of BRAF and MEK inhibitors may delay resistance”.