Labile

adjective

1. Liable to slip, err, fall, or apostatize.
2. Apt or likely to change.
3. (of a compound or bond) Kinetically unstable; rapidly cleaved (and possibly reformed).

   "Certain drugs can be conjugated to polymer molecules with a linkage that is labile at low pH to effect controlled release in a cellular endosome."

4. (of a verb) Able to change valency without changing its form; especially, able to be used both transitively and intransitively without changing its form.