(Reuters) - Two remaining late-stage clinical trials testing an experimental Alzheimer’s drug from Denmark’s Lundbeck have failed, scuppering hopes for the medicine and underscoring the difficulty of developing such treatments.

Lundbeck’s idalopirdine is a so-called 5-HT6 antagonist and is similar to another pill, called intepirdine, being developed by U.S. biotech firm Axovant Sciences.

Unlike some higher profile Alzheimer’s drugs from companies such as Eli Lilly and Merck & Co, idalopirdine was aimed at treating symptoms of the brain disorder, rather than halting progression of the underlying disease.

The failure of Lundbeck’s two latest Phase III trials is not a huge surprise, since the company and its Japanese partner Otsuka had already announced disappointing results from another late-stage trial in September.

Taken together, Lundbeck said on Wednesday, the trials “do not demonstrate efficacy to support a regulatory submission”.

The setback casts further doubt on the 5-HT6 antagonist approach to fighting Alzheimer’s, following discontinuation of another 5-HT6 antagonist last year from Pfizer that had reached mid-stage clinical testing.

Shares in Lundbeck fell more than 4 percent after the company gave its update on idalopirdine alongside fourth-quarter financial results.

Drugs working to block 5-HT6 are designed to promote the release of acetylcholine, a neurotransmitter needed for normal cognition. The idea is to use them alongside the existing drug donepezil to help patients with mild to moderate Alzheimer’s.

Cholinesterase inhibitors, like donepezil, are currently among the few medical options for treating Alzheimer’s patients.

Looking ahead, drugmakers hope to use new types of medicines that target the build-up of certain proteins in the brain that are thought to impair cognition.

The failure of one such closely watched experimental drug from Eli Lilly in November dealt a blow to this concept but experts believe subtly different therapies could still work.