UPDATE 3-Roche, AC Immune drop Alzheimer drug trials after setback

* To halt two late-stage clinical trials of crenezumab

* Roche committed to ongoing clinical studies in Alzheimer's

* Roche was working on crenezumab with AC Immune

* AC Immune shares plunge 37 percent before the bell (Adds AC Immune share movement, analyst comments)

ZURICH, Jan 30 (Reuters) - Roche Holding and partner AC Immune SA will halt two late-stage clinical trials of their crenezumab drug for early Alzheimer's disease, the latest high-profile setback to finding a treatment for the memory robbing disease.

The announcement from Roche and its Swiss-based partner AC Immune came after an interim analysis indicated it was unlikely to hit its primary goal, the Swiss drugmaker said on Wednesday. Nasdaq-listed shares of AC Immune tanked almost 50 percent in response to the news.

Alzheimer's disease is a compelling target for drugmakers as the affected population increases, but treating it has proven elusive, with drugmakers such as Eli Lilly, AstraZeneca Plc and Johnson and Johnson discontinuing trials testing their experimental drugs.

These failures have undermined conviction in the so-called beta amyloid treatment hypothesis, in which a protein forms plaques in the brain and is believed to play a pivotal role in the disorder.

Roche said an Alzheimer's Prevention Initiative (API) study of crenezumab in familial Alzheimer's disease in Colombia would continue and that it remained committed to ongoing clinical studies in Alzheimer's disease, including separate late-stage trials with gantenerumab and a mid-stage anti-tau trial that seek to tackle the debilitating brain-wasting sickness.

AC Immune Chief Executive Officer Andrea Pfeifer said the company was "extremely disappointed" by the news, adding that she was still hopeful the study in Colombia - focusing on cognitively healthy individuals with a genetic mutation who are at risk of developing Alzheimer's disease - would turn out more positively.

Roche shares were largely unchanged, while AC Immune shares fell 47.3 percent to $5.66 and were the top percentage loser across U.S. exchanges among those trading before the bell.

Shares of companies who are still developing Alzheimer's treatments, including Biogen Inc, were down in early trading.

"This latest failure is yet another setback for the anti-Abeta hypothesis and could also impact others working in this space," Brokerage Jefferies analysts wrote in a client note.

Experimental Alzheimer's drugs have had a dismal track record, with more than 100 failures.

However, regulators moved last year to announce plans to encourage testing of medicines for early stage Alzheimer's and help pharmaceutical companies win approval for drugs to treat the disease.

James Pickett, head of research at the UK-based Alzheimer's Society charity, said it was "hugely disappointing" to hear that another drug trial for Alzheimer's had failed, but added; "We're far from losing hope".

"Increasingly, research shows us that while amyloid plays an important part in Alzheimer's, it's not the only issue," he said in an emailed statement. (Additional reporting by Michael Shields in Zurich, Kate Kelland in London and Manas Mishra in Bengaluru; editing by Maria Sheahan and Jason Neely)