A technical word that encompasses agonists, antagonists etc. is effector, defined in the Wikipedia entry as follows:
In biochemistry, an effector molecule is usually a small molecule that selectively binds to a protein and regulates its biological activity.
However ‘effector’ does tend to be used more in regulation of enzyme activity than signal transduction. I suggest this is preferable to the term ‘activity’, suggested by @DeNovo as it is the targets of agonists etc. that exhibit the biological activity that one may be considering, rather than the agonists themselves. Such effectors modulate the activity of other molecules, so one could even consider using the term ‘modulator’, described in a Wikipedia entry — for the particular case of allostery — as follows:
In biochemistry and pharmacology, an allosteric modulator (allo- from the Greek meaning "other") is a substance which indirectly influences (modulates) the effects of a primary ligand that directly activates or deactivates the function of a target protein.
However I would definitely not use a noun of this sort to convey the idea of your sentence.
I find that more complex and clumsy then necessary. Far better, in my opinion, to express yourself in simpler terms and more directly.
“Addition of a methyl group to the phenyl ring changed it from being
an agonist to an antagonist.”
“Addition of a methyl group to the phenyl ring changed its action from that
of an agonist to that of an antagonist.”
However ‘effector’ might be useful as a portmanteau description of agonists etc. in circumstances when you are listing things.