As the summer grinds to a halt, the rumour mill is speeding up and the latest story is that GlaxoSmithKline is planning a takeover bid for partner Human Genome Sciences, which is developing Benlysta, an eagerly-anticipated experimental lupus drug.

Shares at HGS shot up for a sixth consecutive day, and up over 11% yesterday, amid speculation that GSK may make a $30 per share offer. The excitement centres around Benlysta (belimumab), formerly known as LymphoStat-B, which is seen by analysts as representing a breakthrough in the difficult-to-treat disease.

Belimumab is the first in a new class of drugs called BLyS-specific inhibitors and GSK and HGS have noted that no new drug for lupus has been approved by regulatory authorities in more than 50 years. Last month they unveiled data from the BLISS-52 865-patient year-long Phase III study which showed that “a clinically and statistically significant improvement” was shown in patient response rate for belimumab plus standard of care, versus placebo plus standard of care.

Rumours of an imminent bid are based on the fact that the second of two Phase III trials, BLISS-76, is due to be reported by the end of the year. If GSK makes a bid now, it could well negotiate a very reasonable price as a $30 per share bid would value HGS at over $4 billion.

JMP Securities analyst Liisa Bayko told the Wall Street Journal that "I have no reason to believe [GSK] are more serious now than they have ever been," She has an $18 per share price target on HGS and added that the results from the BLISS-76 study could well change her forecast. "I feel pretty good about that being positive and the stock could go up a lot after that," she said.