Have We Been Barking Up the Wrong Tree?

More of my blog posts deal with dementia than with any other subject and the news about Alzheimer’s disease over the years has been largely dispiriting, so who would have thought that I would leap at the opportunity to write about a new diagnostic test. But with so much of the medical literature relentlessly focused on COVID-19, it’s reassuring to realize that research on other subjects is continuing. The new study does not report a treatment, let alone a cure for Alzheimer’s disease. Furthermore, the prospect of screening healthy individuals to determine their future risk of developing progressive cognitive impairment is ethically fraught. Nonetheless, in the current climate, this report is good news.

It’s good news, and not just because it indicates that not all medical scientists have retooled as corona virus researchers, though it does that. It’s good news, and not just because it means it will be possible to target intervention studies to high risk individuals will permit studies to be carried out on smaller numbers of people and over a shorter period of time, though it means that. It’s good news because it shines a bright light on a long-neglected character in the Alzheimer’s story, the tau protein. 

Back in 1906, when Alois Alzheimer peered into his microscope at tissue from the brain of a patient who had died of the disorder of cognition that would one day bear his name, he identified two unusual substances that he described as plaques and tangles. The plaques, which were located between neurons, would ultimately be found to be composed of a protein known as amyloid. The neurofibrillary tangles, which were located inside the nerve cell bodies, would eventually be identified as a protein called tau. These two substances have been recognized as the hallmarks of Alzheimer’s disease for over a century.

For years, the roles of amyloid and tau were hotly debated. Some researchers felt that amyloid was the result of Alzheimer’s; others were confident it was the cause. Some scientists were more interested in studying amyloid; others directed their efforts towards tau. But over the course of the last 25 years, amyloid has gained the upper hand. Study after study has sought to improve cognition in Alzheimer’s disease by ridding the brain of amyloid-laden plaques—and each time, the approach failed. 

A great deal of excitement was engendered by immunotherapy back in 2001: the idea was to stimulate the body to create antibodies against amyloid with what was essentially a vaccine—but the study had to be stopped because a subset of patients developed meningitis. Then there was enthusiasm about the use of monoclonal antibodies. Several such antibodies have made it to phase 3 trials in which their efficacy was compared to placebo. In 2014, two studies of Bapineuzumab showed no benefit. In 2018, Solznezumab was tried for individuals with mild Alzheimer’s and it was unsuccessful. In the same year, additional negative results were reported for Verubecestat in people with mild to moderate Alzheimer’s. 

All these negative studies don’t exonerate amyloid. Maybe the trials are initiated too late in the course of these disease’s development. Maybe the dose is too low. But with anti-amyloid strategies repeatedly striking out, I can’t help but wonder, as have others who know much more about the science than I do, that we’re looking at the wrong target.

Which is why the new study that focuses on tau is exciting. The authors found that their tau antibody test was able to diagnose Alzheimer’s disease as well as or better than more invasive existing tests—when they used the test in patients all of whom had some kind of neurodegenerative disease. That is, the test did well in answering the question: is this person being tested more likely to have Alzheimer’s or, say, Parkinson’s? That’s a very different question from: is this person normal or does he have Alzheimer’s? Not only was the population in which the test was studied composed exclusively of patients with some neurologic condition, not only did the population include a much larger proportion of people with Alzheimer’s than would be found in the general population, but the subjects were far from ethnically or racially diverse. So, it’s a long way from the article in JAMA to a widely useful diagnostic test.

Despite the test’s preliminary nature, it is a compelling piece of evidence that tau should get more attention. Two weeks before the on-line publication of the JAMA study, a small Swiss pharmaceutical company, AC Immune, announced that together with Johnson & Johnson, it was launching a trial of a vaccine designed to stimulate the body to produce antibodies against tau—leading its stock price to soar by 18.9 percent. Just a few weeks earlier, the giant Swiss pharmaceutical company, Roche, announced it, too, was investing in the development of an anti-tau vaccine. 

It’s too early to say whether the attack on tau will fizzle, much like the previous attacks on amyloid. But maybe, just maybe, it will be a rousing success.

Rescinding the DNR order--for anti-amyloid drugs

The pharmaceutical firm Biogen issued a stunning press release this week: it would seek FDA approval for its anti-Alzheimer’s drug, Aducanumab—the very same drug the company had pronounced a failure just last March, after preliminary analyses indicated the drug was very unlikely to achieve its objectives. Dennis Selkoe, a prominent Alzheimer’s researcher at the Brigham and Women’s Hospital in Boston, wrote in STAT (a health-oriented news website owned by the Boston Globe) that he believed that “aducanumab is the breakthrough we have been waiting for.” But is it?

Aducanumab is by no means the first drug studied that targets beta amyloid, a protein widely believed to play an important role in the development of Alzheimer’s disease. An article in Nature Reviews (Drug Discovery), published shortly after the first analysis of the Aducanumab data, put the drug in perspective: a monoclonal antibody that binds to beta amyloid, it is the fourth such drug to make it to a Stage III clinical trial. And all three of its predecessors, as well as another five anti-amyloid drugs that have a different mechanism of action, were abysmal failures leading Nature Reviews to proclaim "Anti-Amyloid Failures Stack Up as Alzheimer's Antibody Flops." Maybe what this new analysis of the Aducanumab data has done is to reverse the Do-Not-Resuscitate label previously attached to monoclonal antibodies and instead put them on life support. So, what, exactly, does the new data analysis show? 

The details will be released to the FDA when Biogen files with the FDA to approve the drug. Thus far what we know is what Biogen has included in its press release. Biogen conducted two randomized, double blind, controlled studies, giving the monoclonal antibody (presumably by injection, like other monoclonal antibodies) or placebo to a total of 3285 patients and following them, some for 78 weeks. One study, sporting the acronym ENGAGE, produced negative results even upon re-analysis, although a subset of ENGAGE patients who received a high dose of the drug appeared to respond. The other study, named EMERGE, showed a reduction in the rate of clinical decline in treated patients compared to those receiving placebo. In other words, the study patients did not improve nor did they plateau; rather, they got worse more slowly. Unfortunately, the press release doesn’t report the absolute magnitude of the change, opting instead to say that the scores on the Mini-Mental State Examination (MMSE), a commonly used measure of cognition, fell 15 percent less with treatment than with placebo. Similarly, the rating on the ADAS-Cog13, another measure of cognitive function, declined 27 percent less in those on the drug. Finally, and perhaps most encouragingly, the decline in the ability to perform basic daily tasks (as measured by the Alzheimer’s Disease Cooperative Study Activities of Daily Living Inventory) was 40 percent lower in those receiving Aducanumab than in their counterparts who received placebo. 

These are not spectacularly impressive results—the patients continued to worsen and the difference between treated individuals and untreated individuals, at least on measures such as the MMSE, was very small. But what we don’t know is whether the slowed rate of decline will persist over time or whether, as with drugs that are currently on the market such as donepezil (Aricept), there is a one-time effect. We don’t know whether the effect would be greater if the medication were used earlier in the progression of Alzheimer’s disease, perhaps in people with mild cognitive impairment. We don’t know whether the drug will prove to be more effective in those with “pure” Alzheimer’s disease, that is, without concurrent vascular changes. 

What we can predict is that if the drug is approved by the FDA, Biogen, whose stock price soared after the press release, will aggressively market the drug. An estimated 5.8 million Americans have Alzheimer’s disease, according to Alzheimer’s Association statistics released in 2019: since the time from diagnosis to death is 5-7 years, roughly one-fifth of the affected individuals or over a million people likely have early Alzheimer’s and are potential Aducanumab consumers. This could be a dream come true for the pharmaceutical company--a blockbuster specialty drug. (Blockbuster drugs are prescribed to at least one million people a year and specialty drugs are very expensive biologicals such as monoclonal antibodies). And if the sales price for the drug is anything like that of other newly marketed monoclonal antibodies, it may be upwards of $1000 per month (Dupilumab, brand name Dupixent, a new monoclonal antibody used in the treatment of asthma and/or nasal polyps, has a list price of $37,000 per year). 

Biogen stockholders will benefit enormously; how much people with Alzheimer’s disease will benefit is less clear. But on a more positive note, as Dennis Selkoe commented in STAT, for the first time we have evidence that it is possible both to design molecules that target amyloid and to administer them to real people with minimal toxicity and some evidence of benefit. That truly is good news.