A trial of an experimental drug for Alzheimer’s disease called aducanumab suggests the drug could be a breakthrough. Aducanumab performed well in the trial, which was designed to determine whether aducanumab is safe for human use and what its side effects and correct dosage range are, but the main results of the study have been overshadowed by two side observations the investigators included.

The first observation notes that the drug is proficient at the job it was designed to do, which is to remove deposits of protein in the brain called amyloid plaques that are characteristic of Alzheimer’s dementia. Investigators measured this aspect of its performance via positron emission tomography (PET), seeing “almost complete clearance” of the plaques in those who received the treatment.

The second observation, the one drawing so much attention, is that patients who received aducanumab experienced slower cognitive decline than those in the placebo group. This suggests the drug is clinically efficacious—a property that all previous investigational treatments have lacked.

Observations suggesting the clinical efficacy of aducanumab are exciting, but efficacy isn’t what this trial was designed to test, and in a field governed by the principles of scientific inquiry, drawing conclusions about a variable that wasn’t under direct study is strictly verboten. Those who study and treat Alzheimer’s are especially wary of getting their hopes up this early in the process due to a long string of flameouts by once-promising experimental treatments. Fortunately, the next research phases that aducanumab will undergo are focused almost entirely on clinical efficacy, and two trials are already underway. Results of these trials will be eagerly anticipated going forward.

Image by National Institute on Aging