Phase III 50% succeed. 25% for first-in-class agents, which it sounds like this one is. [2]
So if this phage passes its current phase I trials, it would then have a 5% chance of eventually getting to market. The current probability of success would be even lower because it hasn't yet completed phase I.
Also the mechanism by which it works (eliminating certain protein aggregates in the brain) might not actually be related to the cause of Alzheimer's. Many other drugs targeting these aggregates have failed.
It could be that it's like trying to cure the flu by taking tylenol - sure you get a fever when you get the flu, but it's not the root cause.
But would it ultimately not matter if you aren't cured? To use your analogy, tylenol takes away some of the symptoms, if this takes away the symptoms of Alzheimer/Parkinson's from an outside observer's perspective you're "cured." Even if in reality you aren't and have to take this for the rest of your life.
The analogy is flawed. These proteins might be a cause of the symptoms, or an effect of the disease, but we don't know that they're related. A better analogy might be, that this could be as effective as trying to eliminate a pigeon infestation by sweeping up their feathers and scrubbing out the crap.
Yes, except other drugs designed to clear the plaques have been tested and they've all shown no reduction in symptoms. So my flu analogy was not great :)
But it's absolutely possible that the prior drugs were all tested wrong or didn't quite do what we thought they did. That is not sarcasm, sadly.
People with Alzheimer's would love it too, because the status quo is that your memory deteriorates and then you die, and there's basically nothing you can do about it.
It could be that it's like trying to cure the flu by taking tylenol - sure you get a fever when you get the flu, but it's not the root cause.
But I hope it works!