Terminology summary
Reference ligand for bias: The ligand that is, by definition,
unbiased. The bias of other tested ligands are quantified relative to
this reference.
Reference ligand for Emax: If the reference ligand for bias
does not produce a full receptor response, a separate reference ligand
for Emax can be used to exploit the full window of response. For the
Gs pathway, some studies use forskolin, which activates
adenylate cyclase directly, rather than a reference ligand to determine
the maximal response. Both the reference compound for bias and the
ligand tested for bias will have their Emax values assigned relative to
the reference ligand(s) for Emax for each pathway.
Biased ligands/signaling (using any reference
ligand)
When using any ligand (e.g. a tool compound or clinically relevant
ligand), the statement that a tested ligand is biased can only be made
relative to the reference ligand (which itself can have any bias).
Recommendation 2: Our recommendation here is to report clearly
the identity of the reference compound and to restrict the conclusions
to the comparison made. To facilitate cross-study comparison, it is also
recommended to consider if a reference ligand can be chosen that has
been used repeatedly for the given receptor and pathways before.
Commonly used reference ligands are available in the receptor pages in
the Guide to Pharmacology database (Armstrong et al., 2020).
Caveat: When the reference ligand is not chosen based on its
signaling bias and is not the principal endogenous agonist, the results
cannot be used to determine pathway- or physiology-bias (see Table 1 and
the two next sections).