Shares in Danish biotechnology firm Genmab have surged and slumped over the last couple of days as analysts pondered a possible takeover bid from Biogen-Idec and then went cool on the idea.

The takeover speculation, which stemmed from UK newspapers and suggested that interest from Biogen could prompt partner GlaxoSmithKline into making a move, was quickly dampened by the news that Goldman Sachs has placed 2.5 million shares of Genmab with institutional investors on behalf of the US firm Medarex.

This represents over half of Medarex’ 4.7 million-share stake in the firm and the announcement of the sale led to a decline in Genmab’s share price of over 8%. The US company owns around 11% of the firm, which was spun out of Medarex in 1999, and the latter firm has been reducing its stake gradually over the last year.

There does not seem to be anything sinister about this move from Medarex, which returned all the rights to the antibody HuMax-Inflam to Genmab last September. However eyebrows were raised at the possibility of a takeover and analysts are saying that there is not much chance of one in the near future.

Analysts at ING issued a research note saying that Genmab's lead drug candidate, the cancer and rheumatoid arthritis treatment HuMax-CD20 (ofatumumab), could come up in direct competition against Biogen's Rituxan (rituximab). “We find it immensely unlikely that the anti-trust authorities would allow Biogen to have ownership of the only two drugs in this class,” the analysts wrote, “thus we view Biogen as a very unlikely bidder".

GSK is also being ruled out as a potential buyer given that it already owns the worldwide rights to HuMax-CD20.