- In Alzheimer's disease research, the lion's share of time and resources has gone toward a protein called amyloid, which many have believed to be the root cause of patients' declining brain function. But there's another protein, tau, that has drawn interest as well. Investigating tau-targeting drugs may soon be easier too, as the Food and Drug Administration on Thursday approved an imaging agent that shows how much tau is in a patient's brain.
- While amyloid imaging drugs are already on the market, Thursday's approval was the first one for tau. The FDA cleared Tauvid, developed by Eli Lilly-owned Avid Radiopharmaceuticals, as a way to estimate the size and spread of tau protein clusters, sometimes referred to as tau tangles. The agency specified that Tauvid is to be used in positron emission tomography brain imaging for patients being evaluated for Alzheimer's, and not other conditions such as chronic traumatic encephalopathy.
- Tauvid is a radioactive drug that binds to the parts of the brain where there are tau clusters. Its approval is based on two clinical trials which found a high likelihood that "evaluators" could correctly identify tau pathology from a Tauvid scan. Before, the only way to get an in-depth look at these tangles was through an autopsy.
There are many in the Alzheimer's research community who believe targeting amyloid is the key to treating the underlying disease. And yet, pretty much every amyloid-targeting drug that advanced to late-stage human testing has failed. Cambridge, Massachusetts-based Biogen is the only company to tease out positive results, and even those have been met with rather intense scrutiny.
Setbacks to the amyloid hypothesis have put pressure on scientists and drugmakers to expand their focus.
Tau has become the second largest area of Alzheimer's drug research. Like amyloid, the thinking goes that when tau proteins misfold, they glob together and form aggregates which impair brain cell signaling.
According to the federal database clinicaltrials.gov, there are at least 137 open studies of various tau-based treatments. A 2019 report by the Cleveland Clinic counted 17 drugs intended to modify the course of Alzheimer's disease that specifically target tau, compared to a total then of 38 therapies aimed at amyloid.
While Alzheimer's patients can be symptomatically identified, the disease can still only be definitively diagnosed through a pathological evaluation of the brain after a patient dies. Having the tools to measure the amyloid plaques or tau tangles in patients' brains, therefore, can be important for care and for understanding the effects of drug interventions.
"The fight against AD requires precise and reliable assessments of the two key pathologies of the disease because clinical assessments alone are limited in their ability to accurately diagnose patients," Mark Mintun, vice president of Lilly's pain and neurodegeneration research and development, said in a May 29 statement.
Though both the FDA and Lilly touted the approval of Tauvid, the company said the drug will have limited availability for now. Lilly said availability will later expand "in response to commercial demand and payor reimbursement."
Lilly acquired Avid almost a decade ago for $300 million upfront. Avid's lead program at the time was a drug called florbetapir F 18 (18F-AV-45), which would go on to gain approval in 2012 as a radioactive diagnostic for beta-amyloid plaques.