* Breast cancer study fails to prove added Kadcyla benefit
* Ending Phase III trial with Alzheimer's drug gantenerumab
* Roche shares fall 5.4 percent, most in five years
* MorphoSys down 9.3 pct, ImmunoGen off 27 pct in premarket (Adds latest shares, more reaction, Breakingviews link)
By Silke Koltrowitz and Ben Hirschler
ZURICH/LONDON, Dec 19 (Reuters) - Roche suffered a double blow on Friday as two clinical studies of drugs for breast cancer and Alzheimer's disease failed, sparking the biggest percentage fall in its stock in five years.
The Swiss drugmaker said it was ending a late-stage study of experimental Alzheimer's drug gantenerumab after it failed to prove effective, underlining the difficulty of treating the memory-robbing disease and removing "blue sky" upside for the shares.
Commercially, however, the failure of Roche's new drug Kadcyla to demonstrate an added benefit in breast cancer was more damaging as it hurts the company's hopes of broadening sales in an disease area it has long dominated.
Fabian Wenner, an analyst at Kepler Cheuvreux, said consensus forecasts for annual Kadcyla sales of 2.5 billion Swiss francs ($2.55 billion) in 2020 would now "have to come down significantly", while Bernstein's Tim Anderson said earnings per share forecasts for the 2017-20 period would fall by around 5 percent.
Roche's late-stage study in women with previously untreated advanced HER2-positive breast cancer showed three of its approved treatments -- Herceptin, Perjeta and Kadcyla -- helped patients live longer without their disease worsening.
But the two treatment arms containing Kadcyla did not prove superior, undermining expectations that Kadcyla could become the new standard of care for treating many women.
Shares in Roche, which in recent years has boasted one of the most productive research pipelines in the industry, fell 5.4 percent by 1240 GMT on the double setback.
Germany's MorphoSys, Roche's partner on the Alzheimer's project, declined 9.3 percent, while U.S. firm ImmunoGen, which contributes technology to Kadcyla, lost 27 percent in pre-market Nasdaq trade.
NOT GIVING UP
The setback in Alzheimer's is disappointing for the wider field, since many experts had hoped the final-stage Phase III study testing the drug in patients with so-called prodromal disease, or pre-dementia, would produce a better outcome.
Researchers argue it makes more sense to give drugs early, before patients' brains are wrecked by the condition.
Alzheimer's, the most common form of dementia, already afflicts 44 million people worldwide and this figure is set to triple by 2050, according to campaign group Alzheimer's Disease International.
Unlike heart disease and cancer, which have seen major strides in drug development, no new therapies have been approved to treat Alzheimer's in a decade.
Roche suffered a setback with another experimental Alzheimer's drug earlier this year, but the company insisted it was not giving up.
"This is the first Phase III trial to evaluate a potential disease-modifying medicine in this early prodromal stage of Alzheimer's disease. We remain committed to investigating new medicines for this devastating illness," said Roche Chief Medical Officer Sandra Horning.
Gantenerumab continues to be studied in another Phase III trial involving patients with later stage disease and Roche has two other experimental Alzheimer's drugs in Phase II tests, crenezumab and RG1577. ($1 = 0.9811 Swiss francs) (Editing by Keith Weir and David Evans)