jt0082, that is not my understanding. Based on my reading, the 10b5-1, if properly constructed and administered, provides a defense against the presumption that trades were made with MNPI in possession.
Terminations are permitted, but the burden is then on the individual to demonstrate that the termination wasn't made as a result of obtaining MNPI.
In this case, let's say BO discussions with Pfizer commenced on 12/15/2018, and the 10b5-1 plans were put in place after AHA in November of 2018 (which would make sense). If the deal got announced on Monday, and there was a set plan to sell x shares obtained from y option exercises at z price or better within r timeframe, and that plan got cancelled after 12/15/2018 -- and the SEC looked into it, the defense against the presumption would almost certainly fail.
So in a sense you are right -- the plans can be terminated -- but not pursuant to obtaining MNPI that is still pertinent during the planned trading period. Not without personal legal risk.
Modifications, suspension and termination actions are technically all permitted but may weaken the good faith defense. A change is permissible if the participant does not have MNPI at such time.
You'd really be playing with fire if you terminated or amended a 10b5-1 trading plan when AMRN is in it's current position as a BO target - below are two cases where insiders got busted for amending their plans: