Cell-based assays:Layout 1 14/1/10 19:58 Page 71
Assays
a certain physiological pathway. That’s still a lot how else would you screen? Unless you’re screen-
of questions there. ing for surrogate agonists, otherwise you’re screen-
So until we can understand that and I’m talking ing against the natural ligand, right?
about GPCRs, we can’t really think about develop-
ing an assay based on that. David Marks: Yes. But one thing is that so far
there’s still not a lot of information about the rele-
Kevin Pfleger: But the point that I would make vance of these heterodimers or dimers, even using
there is at the moment we’re going round and a natural ligand.
round in circles. Because you’re absolutely right.
We don’t know whether these heterodimers are rel- Richard Eglen: Do you think, David, that as the
evant. So we need to actually look into animal technology becomes reduced to practice to screen
models, to see whether these heterodimers are rel- at dimers, that people will do it anyway because
evant. But we can’t do that until we have tool com- what have they got to lose? And they can find com-
pounds that can look at those heterodimers. So at pounds, to Kevin’s point?
the moment the field is in a state where it’s hard to
identify heterodimers specifically because we don’t David Marks: But the problem potentially is that
have the compounds. what Kevin is also saying, if you screen in that con-
text, you may be missing a lot of other things out-
David Marks: But you have agonists, you have nat- side of that context.
ural ligands. So why can’t you use the natural lig-
ands to investigate that? Richard Eglen: Oh, I’m not saying they do this
only, I say they’re doing this in addition.
Kevin Pfleger: The natural ligands for a het-
erodimer? Arthur Christopoulos: I think part of it is... I hope
we’ve come to the tail end of the dimer hysteria
David Marks: The natural ligand for... phase, for want of a better way of putting it.
Where everyone and anyone’s... if you stick two
Martin Valler: ...that encourages the formation of things together, you can find something in a recom-
the... binant cell and call it dimerisation. Now I think
there are guidelines that are starting to emerge that
Kevin Pfleger: Right, if you speak to a lot of peo- are more strict along the lines of criteria and expec-
ple they would suggest that heterodimerisation is tations, of which Kevin is more aware of than I am.
constituative, and the heterodimers form in the ER Including native tissue relevance, physiological,
and traffic up together. pathophysiological, all of that. So the bar has been
set very high now in saying that a dimer in a par-
Arthur Christopoulos: The problem with the natural ticular heterodimer is particularly relevant for a
ligand is that it will behave one way if the monomer given condition. And I think probably what we’re
is there. And if you co-express the receptors, it will going to see is a lot fall by the wayside that don’t
behave a different way. Is that due to the het- meet this criteria, and there are a few that will
erodimer formation, or is that due to the crosstalk emerge. Having said that, it might imply – and
between the two receptors in an intracellular man- again, I’m not a dimer expert – that they are going
ner? One of the difficulties is interpreting that. to end up in the minority. But the targets you find
are probably real. I don’t know.
David Marks: One of the problems is that you
don’t try to investigate that first with a natural lig- David Marks: I think that if, in any particular sys-
and or ligands, because with a heterodimer you tem, or target, if there is information where a het-
may have two natural ligands... if you don’t try to erodimer is relevant, I’m sure we will use it as a
investigate the relevance of that interaction first, way for screening. But until there is demonstrated
and you start screening for compounds, that will relevance to a certain heterodimer, I’m not sure a
work on that heterodimer, then how are you going lot of companies would want to screen in two or
to know how to interpret those new compounds. three different assays. If you have a heterodimer,
that means you have to screen against three differ-
Arthur Christopoulos: I would suggest that most ent assays.
people looking at any dimer situation, they must be
looking at natural ligands for their receptor, or Arthur Christopoulos: I think I mentioned this in
Drug Discovery World Winter 2009/10 71
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