Hard to say how high it will go. HD is a limited market, but success there will be considered by many to be a good indication that Dr. Tanzi et al. are onto something big. If AD is positive, then look out! The potential market would be vast - extremely so. I am long with a fair amount (for me) invested, not because I think that PBT2 is a sure thing but because I see such a high upside if it does pan out. Invest only what you can afford to lose, but not investing could also be a costly error.
Ditto. I've invested what I can afford to lose, and the highest probability outcome is that the drug fails and the stock tanks. But the upside if it works counterbalances this - at least in my view - even though that's a low probability outcome. This isn't an investment, it's a gamble, but I like the odds / payoff combination and I'm having fun.