Both stocks popped and sold off.
MEIP has been under $2 since March 2015, there are plenty of people who bought under $2 and if they have been holding for some time, they may want out.
Because there are many people looking to exit, it's easy for the stock to fall.
Once weaker hands drop out, the stock will likely continue higher.
This is why it is never worth holding stocks through events.
It's always best to wait and then act.
Now that the partnership is known and the unknown costs are off MEI, the stock should drift higher over time.
So, yeah, you might be disappointed in today's actions. This is probably going to pop higher once Helsinn's cost of investment (per share) is known.
Remember, sales are still a way off.