My goal is that any reader can take something away from this Alzheimer's post, regardless of their experience in medicine or science. It's Friday Links-driven, but with far more of my own commentary/explanation than normal.
First up: an article about a clinical trial for Alzheimer's treatment that appears to have failed. While I don't research Alzheimer's, I'm going to argue in this blog post that 1) single "magic bullet" treatments are unlikely to ever work for a chronic complex disease that develops over many decades like Alzheimer's (once the disease has actually manifested symptomatically), but 2) single "magic bullet" prevention methods might work for specific patient sub-populations. In this case, different prevention methods would work for different sub-populations. Meanwhile 3) Broad, non-specific treatments that target multiple biological processes are better for stabilizing Alzheimer's after it's been diagnosed (discussed in the second link). Note that this is NOT the same as combination therapy.
Bapineuzamab is an antibody that recognizes and binds to beta-amyloid, one of the molecules involved in the pathogenesis of Alzheimer's (note that I don't say it's the cause or even a significant cause). For my non-biomedical readers, this is a common treatment strategy nowadays. Say protein X causing disease Y is floating around in the space between cells in your body. Specific antibodies can be made to bind specifically to protein X which physically blocks protein X from damaging other things ("neutralization"). Furthermore immune cells are then able to recognize the antibody bound to protein X, and clear protein X from the body. This strategy is used in treatment of diseases like rheumatoid arthritis and lymphoma, and it works wonders.
But Bapineuzamab failed to have any effect on Alzheimer's progression in this Pfizer trial. Why? I'm going to use my extraordinary powers of hindsight (dig deep- you probably have this ability too!) and say that it probably has far more to do with the fact that beta-amyloid is just a tiny piece of the puzzle for Alzheimer's than any problem with the drug. Bapineuzamab probably recognizes beta-amyloid just fine, and it might even clear beta-amyloid from the body. But I doubt that clearance of beta-amyloid would have any effect on Alzheimer's. Why? Because it's too late.
The trial looked at treatment of early-to-moderate Alzheimer's, but Alzheimer's develops over decades. For a while, it's just Mild Cognitive Impairment (MCI), but lots of people get MCI and never progress to Alzheimer's. So figuring out a way to predict Alzheimer's progression through things like blood tests and brain imaging is all the rage now (see third link). And that's why the trial focused on Alzheimer's rather than pre-Alzheimer's. But in the patients that do progress, what's going on?
The length of time that it takes for Alzheimer's to develop means that, even if beta-amyloid were to be the ultimate cause, then beta-amyloid can trigger numerous other biological processes that are themselves damaging to the brain. By the time someone has Alzheimer's symptoms, these "secondary processes" are already robust so more damage is occurring independent of beta-amyloid, and a lot of neurons are already malfunctioning or dead. This fundamentally alters the biology of the brain so that treatments are unlikely to reverse anything, and multiple causes of degeneration make it unlikely that single treatment would slow the progression of the disease. I won't review the ginormous body of literature implicating all sorts of things in Alzheimer's pathogenesis, but I'll take a couple as an example.
One way to abstract Alzheimer's (see picture below) is that everything causes everything else. It's a complicated feedback loop (or feedback web) where a bunch of biological processes all cause and worsen each other over a period of many years. For example (highlighting a tiny portion of the feedback web), extracellular amyloid or intracellular tau (hallmarks of protein misfolding) acting on one subset of neurons might interfere with normal breakdown of neurotransmitters, as well as directly causing neurons to fire inappropriately. Too much excitation of nearby neurons results in excitotoxicity (killing those cells) or in inappropriate activation. The brain might remodel in reaction, forming new synapses that result in electrical feedback loops that reinforce each other. The resultant clinical and subclinical seizures might interfere with brain function long after the damage is done. As neurotransmitters start to diffuse inappropriately due to synaptic dysfunction, they start affecting other cells indiscriminately, and damage may occur to brain's extensive blood system. This allows in immune cells that further alter the blood vessels to essentially break down the blood-brain barrier. This lets in various molecules that again might cause excitotoxicity or protein misfolding, and maybe it even lets in bacteria. So damage leads to biological response that causes more damage, leading to further responses etc.
All of these processes cause each other, and all of them cause disease. Targeting a single initiating factor (which varies from patient to patient) might work for prevention, but not for treatment.
Importantly, many of the damaging processes are variations of normal brain biology. For example, microglia (kind of like the brain's immune system) see damaged neurons and eat them up. If you remove the source of damage, it is perfectly possible they continue eating up neurons instead of letting the neurons recover after the damage. There are medical examples where a disease has manifested for so long that cells that normally act to ameliorate a disease are "locked in" to their action even when it's no longer necessary, and they end up causing damage themselves (for example tertiary hyperparathyroidism). I think of this as part of the more general inflammatory response that occurs whenever there is damage- all sorts of immune cells react to initial damage and can end up causing more damage than the original insult (if you think that makes no evolutionary sense, it actually does. I might expand on that in a future post).
What about prevention? Note that things like excitotoxicity, brain remodeling, and breakdown of the blood-brain barrier can in turn cause beta-amyloid buildup. So there's no reason why beta-amyloid had to be the initial insult- in many (most?) patients beta-amyloid is probably secondary to another biological process. In these cases, drugs targeting beta-amyloid production and degradation probably would not have any preventative effect, because beta-amyloid was not the initial cause. However, there are subsets of patients where beta-amyloid is implicated as a major genetic cause (mutations that affect beta-amyloid production like in Down's syndrome, ApoE4, and presenilin). Prevention using Bapineuzamab is conceivable in those patients, as it would stop the secondary processes from occurring in the first place. However, if we found in another sub-population that inappropriate inflammation due to immune system malfunction (kind of like an autoimmune disease), then prevention would involve anti-inflammatory drugs (Aspirin? IVIg? Prednisone?). Thus, preventative measures for Alzheimer's would be specific to the patient's initial cause(s) of degeneration, which can vary widely depending on the patient.
On the bright side, a very small trial showed stabilization of Alzheimer's with treatment with IVIg (intravenous immunoglobulin). IVIg is simply the mix of the collection of antibodies isolated from multiple human volunteers. There are antibodies against everything- bacteria, toxins, some human proteins, etc. The authors here speculate that there is an antibody targeting amyloid-beta, tau, or some other molecule. While this might be part of the picture, I worry that researchers might go after specific antibodies (which is just like the above Bapineuzamab trial). I challenge the notion that IVIg's broad and non-specific effects are a disadvantage. While one might think it's not 'optimized' for Alzheimer's treatment, the very fact that Alzheimer's involve a complicated web of numerous biological processes means that we need to target them all at the same time. Thus, a non-specific treatment with numerous antibodies doing many different things might in fact be the key to IVIg's efficacy.
For example, IVIg is used in the treatment of autoimmune disease, dampening down immune responses. How it accomplishes that is unclear (and is a bit counter-intuitivee since antibodies MEDIATE the immune response), but it is believed that it both interferes with the specific endogenous antibody that causes disease, as well as flooding the system with so many antibodies that it diverts the immune system from inflammation. Perhaps IVIg is dealing with the inflammatory component of Alzheimer's? Perhaps it interferes with the endogenous cells/antibodies that are damaging the brain? Thus, trying to narrow down the treatment to a single antibody or a few antibodies would eliminate some of the broad effects of IVIg that would be critical for influencing the numerous biological processes. This is different from combination therapy because we're looking for one or two treatments to influence many things (100+) things at once, rather than multiple (3-5) treatments for multiple (3-5) things.
Also note that IVIg only stabilized the disease, it didn't reverse anything. That's because the damage is done- the neurons are dead and the brain has remodeled itself. At this point, Alzheimer's could only be reversed by making new neurons by stem cell therapy. Because many of those dead neuronal circuits likely encoded specific memories and personality traits, we would need to find a way to program those back into the new neurons. Those would be Sci-Fi technologies that haven't even been imagined yet.
More on prevention: you need to be able to predict who is going to get the disease in order for a prevention to fulfill a cost-benefit analysis (since preventative treatments might have their own side effects, and you don't want to expose people who will never get the disease to unnecessary risk). This brain imaging study suggests that this is possible, at least in one inherited subtype of the disease.
http://www.newscientist.com/article/dn22128-alzheimers-villain-cures-multiple-sclerosis-in-mice.html
I'll just leave with you an interesting tidbit- they used beta-amyloid injected into the body cavity of mice to reverse multiple sclerosis (MS). What? Isn't beta-amyloid bad? But this sort of goes with my idea that injecting IVIg "distracts" the immune system from attacking brain cells, just like beta-amyloid might "distract" the immune system from attacking myelin sheaths in MS. I think the lesson here is: We need to think outside of the box and consider counter-intuitive treatments to deal with these complex diseases.
