It comes just three months after Eli Lilly announced
that it was ending the clinical trial of its Alzheimer's drug solanezumab, after patients taking it demonstrated no signs of improvement compared to those taking a placebo.
"While we are disappointed that a benefit was not observed in this study, our work continues [to study the impact of] verubecestat in people with less advanced disease," said Dr Roger Perlmutter, president of Merck Research Laboratories.
Experts say the failure of the drug on people with mild-to-moderate Alzheimer's was a substantial setback.
"There were high hopes that [the trial] might succeed," Professor John Breitner, Canada research chair in prevention of dementia and professor of geriatric psychiatry and preventive medicine at McGill University told CNN.
"There is a lot of concern and debate in the professional community at this point. It has been very disappointing."
Around 47 million people currently live with dementia worldwide, with Alzheimer's estimated to account for 60 to 80% of dementia cases. This figure is expected to double every 20 years to reach 131 million people in 2050, according to Alzheimer's Disease International.
Years of testing, trials
It has now been over a decade since a new drug for alleviating the symptoms of Alzheimer's was made available. There is currently no treatment that has been proven to slow the impact of the disease, which causes a gradual decline in memory and thinking skills and eventually leaves sufferers unable to care for themselves.
"We really don't know the causal pathway that causes the loss of connection in the brain that results in the loss of cognitive abilities [in Alzheimer's patients]," said Breitner. "Despite 25 years of trying to figure out what is going on and what is the appropriate target for treatment, we still don't know."
There have been many successful studies that have "cured" or reduced memory loss in transgenic lab animals such as rats, which have been bred with human genes as a model for Alzheimer's. But this success has so far never been replicated in humans.
Beta amyloid theory
Verubecestat belongs to a group of experimental Alzheimer's drugs called BACE inhibitors, which work to control an enzyme involved in the formation of abnormal protein clusters in the brain known as amyloid plaques.
While the ultimate cause of cell death and tissue loss in the brain of Alzheimer's patients has never been proven, amyloid plaque build-up between nerve cells in the brain has been long suspected as one of the main causes of the disease.
However, the failure of verubecestat adds to growing concerns that this underlying assumption, known as beta amyloid theory, is wrong.
There are several pharma companies currently trialling BACE inhibitors in the hope of finding a cure, including AstraZeneca and its research partner Eli Lilly, Novartis in collaboration with Amgen, as well as Janssen and Biogen.
"Alzheimer's is a terrible disease and we desperately need treatments. There would be a huge market for a successful drug," said Brietner. "Until now companies have been willing to take the risk [by investing large sums of money in clinical trials]. But at the moment a lot of things are being reassessed."
Continuing the fight against Alzheimer's
"This isn't the end for beta amyloid theory, just yet," said Professor Bryce Vissel, the Roth Fellow and director of the Centre of Neuroscience and Regenerative Medicine at University of Technology Sydney.
"There's a trial of another important drug for Alzheimer's, aducanumab
, being run by Biogen -- another pharmaceutical company -- which has shown interesting and encouraging signs."
"We must continue to pursue this disease," said Vissel. "There's good reason to have hope and optimism that with the pursuit of excellent science and some rethinking about [Alzheimer's], a treatment will become available for this devastating disease in our lifetimes."
The results of Merck's trial of verubecestat on patients with early stage Alzheimer's are expected in February 2019.
Merck's shares fell by nearly 2% following Tuesday's announcement.