LONDON (AP) — GlaxoSmithKline says it will persist with its bid to take over U.S. company Human Genome Sciences despite the target’s new ‘poison pill’ defense.

In a statement issued after the market closed on Thursday, GSK said it believes its $13 per share offer represents full value for HGS, its partner in developing new drug treatments.

Rockville, Maryland-based HGS on Thursday announced its defensive move, which will dilute holdings if anyone attempts to acquire 15 percent or more of its stock without board approval.

GSK’s offer closes on June 7.

GSK and Human Genome Sciences split sales of the injectable biotech drug Benlysta, which last year became the first new drug approved for lupus in 50 years. Sales so far have fallen short of expectations, with monthly revenue averaging $11 million, but some analysts believe the drug could grow into a billion-dollar blockbuster.

HGS said Thursday that Glaxo’s offer failed to capture the “significant upside potential” represented by Benlysta and the company’s pipeline of drugs including darapladib, for treatment of cardiovascular disease, and albiglutide, for type 2 diabetes.

GSK said it believes the earliest the first Phase III trial for darapladib can finish is in 2013 and the earliest the second trial can finish is sometime in 2014.

GlaxoSmithKline said it owns “the vast majority of the economics” associated with albiglutide and darapladib.

“HGS is entitled to certain conditional milestones and a mid-single-digit royalty if albiglutide is successfully commercialized. For darapladib, if successfully developed, HGS is entitled to a 10 percent royalty and an option to co-promote and receive up to 20 percent profit share but will also need to make a proportionate contribution to investment for costs of launch and promotion,” GSK said.