The news sent the US biotech's shares plummeting nearly 27 per cent from $75.88 to $55.51 in after hours trading. Although this must be tempered against the fact that the shares only reached these highs when Biogen initially announced in October that it was looking for a suitor - at that point the shares were worth around $65 and were as low as $45 back in March. The rumours first started to fly when billionaire corporate raider Carl Icahn bought a 1 per cent share in Biogen and by October had increased that stake to around 3 per cent. Icahn has a reputation for buying into companies he believes are undervalued and then pressurising management into major changes or to sell up completely. Ironically, by then confirming it did want to sell up, Biogen shares rocketed so high, it seems to have put potential buyers off. Pharma companies are increasingly looking to the biotech industry either for acquisitions or simply to ape their seemingly more successful business model. This has led to a slurry of rumours of big-name biotechs being touted as merger targets for the pharma powerhouses. However, it now seems Biogen won't be one of them, for the time being at least.