AstraZeneca has thrown in the towel on an experimental antidepressant targeting nicotine receptors after the drug licensed from Targacept failed in two remaining clinical trials, further thinning out its pipeline.

While the news had been widely expected, since two previous Phase III studies also missed their targets, it hit shares in Targacept hard and the stock tumbled.

Britain's second-biggest drugmaker will take an impairment charge of $50 million, representing the rest of the value of TC-5214 following a partial writedown late last year.

TC-5214 was initially deemed a potential multibillion-dollar-a-year seller as the first in a new class of medicines that work by calming neuronal nicotine receptors in the brain.