A spate of failed Alzheimer’s drug trials has led the US Food and Drug Administration (FDA) to reassess its requirements for a successful clinical trial. The decision comes as part of a wider effort to treat patients with the neurodegenerative disease at an earlier stage.

In a series of guidelines released on Thursday the FDA outlined new goals for trials which are more appropriate for early-stage patients, including those who have not yet showed signs of memory loss or functional impairment. Such early-stage or asymptomatic patients may be identified through sensitive cognitive screening, imaging tests or biomarkers, with an increased focus on improving patients’ thinking ability and memory.

According to the Centers for Disease Control and Prevention, Alzheimer’s is the sixth-leading cause of death in the US. There are currently no approved treatments that slow the progression of the disease, while the number of failed trials stands at almost 200. The failures come from some of the largest drug developers, including Pfizer, Eli Lilly and Merck.

The most recent failure was announced on Tuesday, with Merck’s halting of a late-stage trial of verubecestat in early-stage patients. Its decision followed findings that the drug was unlikely to work. Similarly, on Wednesday Biogen said that it needed 500 additional patients for its study of aducanumab, which is in its final stages of testing. Investors have taken it as a sign that the trial, the last late-stage study into an Alzheimer treatment by a major drug maker, may fail.

As the biological progression of Alzheimer’s is not fully understood, researchers face the challenge of attempting to tackle a disease which has no discernable time frame or target. The FDA’s decision to adapt its criteria for successful treatment reflects the perceived necessity to widen the scope of clinical trials.

While current guidelines require a drug to cause cognitive and functional improvements to be deemed successful, under the new goals the success of clinical trials could be judged on more subtle biological signals.

“Biomarkers may actually be able to detect who’s on that Alzheimer’s path,” Alzheimer’s Association chief science officer Maria Carrillo said. “If we can … slow down overt symptoms of cognitive problems, why would we wait years for functional evidence, when we could detect it earlier and stop it at the cognitive problem?”

The FDA’s draft guidance “gives the field more confidence in being bold about the trials that we design and then carry out,” Carrillo said. “It’s going to be really important.”

There is some trepidation around the new proposals as they would involve patients and doctors participating in trials without approved measuring symptoms such as memory loss and impaired functionality. However, treating patients in the earliest stages of the disease could save valuable time and money, as well as preventing serious damage in patients.

The guidelines are one of five proposals the FDA released on Thursday, all with the intention of developing treatments for neurological diseases.

“The brain, in many respects, is the last organ system where many aspects of our understanding of the underlying biology of disease remain uncertain,” FDA commissioner Scott Gottlieb said. “Symptoms and progression of neurological diseases can also vary significantly across patients, and even within patients, and across organ systems.”