This is an interesting and important paper which serves as a cautionary reminder regarding the potential activity of allosteric modulators to have "functionally selective" effects on GPCR signaling (Urban et al., 2007; Allen and Roth, 2011). Thus, it has been known for decades (see Urban et al., 2007, for review) that GPCR "agonists" and "antagonists" can activate distinct signaling pathways and that a drug can appear to be an "agonist" for one pathway and have "antagonist" activity at another. As we have recently suggested, such activity could have potentially therapeutic implications for schizophrenia and related disorders (Allen et al., 2011).

Although it has been recognized for some time that allosteric modulators of GPCR activity may also have functionally selective actions (Sheffler and Conn, 2008) in vitro, it was unknown if these...  Read more

This is an interesting and important paper which serves as a cautionary reminder regarding the potential activity of allosteric modulators to have "functionally selective" effects on GPCR signaling (Urban et al., 2007; Allen and Roth, 2011). Thus, it has been known for decades (see Urban et al., 2007, for review) that GPCR "agonists" and "antagonists" can activate distinct signaling pathways and that a drug can appear to be an "agonist" for one pathway and have "antagonist" activity at another. As we have recently suggested, such activity could have potentially therapeutic implications for schizophrenia and related disorders (Allen et al., 2011).

Although it has been recognized for some time that allosteric modulators of GPCR activity may also have functionally selective actions (Sheffler and Conn, 2008) in vitro, it was unknown if these effects of signaling bias were therapeutically relevant.

What is important here is that Digby et al. rigorously demonstrate signaling bias in vitro and in situ, and then follow these findings up by showing that the signaling "fingerprint" is relevant to the apparent electrophysiological effects of the drugs in vivo.

Although the drugs lacked apparent antipsychotic drug-like actions, they clearly had different effects on synaptic physiology.

Taken together with many other papers of this sort which have appeared over the past several years, the findings reinforce the principle that relying on a single cellular readout for demonstrating the "agonist," "antagonist," or "allosteric" actions of a small molecule could ultimately be problematic for therapeutics.

References:

Allen JA, Roth BL. Strategies to discover unexpected targets for drugs active at G protein-coupled receptors. Annu Rev Pharmacol Toxicol. 2011 Feb 10;51:117-44. Abstract

Allen JA, Yost JM, Setola V, Chen X, Sassano MF, Chen M, Peterson S, Yadav PN, Huang XP, Feng B, Jensen NH, Che X, Bai X, Frye SV, Wetsel WC, Caron MG, Javitch JA, Roth BL, Jin J. Discovery of β-arrestin-biased dopamine D2 ligands for probing signal transduction pathways essential for antipsychotic efficacy. Proc Natl Acad Sci U S A. 2011 Nov 8;108(45):18488-93. Abstract

Sheffler DJ, Conn PJ. Allosteric potentiators of metabotropic glutamate receptor subtype 1a differentially modulate independent signaling pathways in baby hamster kidney cells. Neuropharmacology. 2008 Sep;55(4):419-27. Abstract

Urban JD, Clarke WP, von Zastrow M, Nichols DE, Kobilka B, Weinstein H, Javitch JA, Roth BL, Christopoulos A, Sexton PM, Miller KJ, Spedding M, Mailman RB. Functional selectivity and classical concepts of quantitative pharmacology. J Pharmacol Exp Ther. 2007 Jan;320(1):1-13. Abstract

This is a very useful paper, but it is worth pointing out that there is really no evidence for "ligand bias" or "functional selectivity" of these compounds. The VU analogs are partial agonists but show no bias toward one signal pathway or another. The low ERK and α-arrestin signaling are due to the lower degree of receptor reserve for those two pathways and the requirement for more efficacious agonists to signal. The correlation of the in-vitro studies with the different in-vivo readouts does provide very important guidance for pharmacologists about what properties are needed for M1 agonists to produce different physiological effects. It doesn't, however, give any real information about "biased agonists."

View all comments by Rick Neubig

The discovery of M1 allosteric agonists caused a great deal of excitement because they act on the M1 receptor fairly exclusively. The new research by Digby et al. suggests that this newer class of compounds can influence very specific molecular signaling pathways initiated by M1 activation, and that these pathways mediate specific aspects of cognition. These findings could have profound effects on the development of novel therapeutic agents for cognitive impairments seen in schizophrenia and other severe neuropsychiatric disorders.

View all comments by Anantha Shekhar
View all comments by Amanda Bolbecker

Regarding the comment by Rick Neubig: while it could be argued that the paper does not provide "gold standard" pharmacological data for bona fide functional selectivity/stimulus bias, in fairness to the authors I provide their conclusions directly from the paper:

"Thus, the differential effects of these M1 agonists on CNS responses may reflect a combination of partial agonist activity that is impacted by differences in receptor reserve and by an inherent stimulus bias at M1 so that these compounds are not capable of fully activating some responses, even in systems in which the receptor is highly expressed…. Thus, different levels of M1 expression are likely to contribute to the differential responses to VU0357017 and VU0364572 observed in these studies. However, it was interesting to find that VU0357017 never achieved full efficacy in activation of ERK1/2 phosphorylation, even in cell lines with strong induction of M1 expression to levels that induced high receptor reserve in the calcium mobilization assay. Also, VU0357017 did not induce robust β-arrestin responses in...  Read more

Regarding the comment by Rick Neubig: while it could be argued that the paper does not provide "gold standard" pharmacological data for bona fide functional selectivity/stimulus bias, in fairness to the authors I provide their conclusions directly from the paper:

"Thus, the differential effects of these M1 agonists on CNS responses may reflect a combination of partial agonist activity that is impacted by differences in receptor reserve and by an inherent stimulus bias at M1 so that these compounds are not capable of fully activating some responses, even in systems in which the receptor is highly expressed…. Thus, different levels of M1 expression are likely to contribute to the differential responses to VU0357017 and VU0364572 observed in these studies. However, it was interesting to find that VU0357017 never achieved full efficacy in activation of ERK1/2 phosphorylation, even in cell lines with strong induction of M1 expression to levels that induced high receptor reserve in the calcium mobilization assay. Also, VU0357017 did not induce robust β-arrestin responses in the original cell line or in the TREx hM1 cells. Thus, the differential effects of these M1 agonists on CNS responses may reflect a combination of partial agonist activity that is impacted by differences in receptor reserve and by an inherent stimulus bias at M1 so that these compounds are not capable of fully activating some responses, even in systems in which the receptor is highly expressed."