Laura López

Biased Agonism at G Protein‐Coupled Receptors: The Promise and the Challenges—A Medicinal Chemistry Perspective

Historically, determination of G protein‐coupled receptor (GPCR) ligand efficacy has often been restricted to identifying the ligand as an agonist or antagonist at a given signaling pathway. This classification was deemed sufficient to predict compound efficacy at all signaling endpoints, including the therapeutically relevant one(s). However, it is now apparent that ligands acting at the same GPCR can stabilize multiple, distinct, receptor conformations linked to different functional outcomes. This phenomenon, known as biased agonism, stimulus bias, or functional selectivity offers the opportunity to separate on‐target therapeutic effects from side effects through the design of drugs that show pathway selectivity. However, the medicinal chemist faces numerous challenges to develop biased ligands, including the detection and quantification of biased agonism. This review summarizes the current state of the field of research into biased agonism at GPCRs, with a particular focus on efforts to relate biased agonism to ligand structure.

A new mechanism of allostery in a G protein–coupled receptor dimer

SB269652 (1) is the first drug-like allosteric modulator of the dopamine D2 receptor (D2R), but contains structural features associated with orthosteric D2R antagonists. Using a functional complementation system to control the identity of individual protomers within a dimeric D2R complex, we converted the pharmacology of the interaction between SB269652 and dopamine from allosteric to competitive by impairing ligand binding to one of the protomers, indicating that the allostery requires D2R dimers. Additional experiments identified a “bitopic” pose for SB269652 extending from the orthosteric site into a secondary pocket at the extracellular end of the transmembrane (TM) domain, involving TM2 and TM7. Engagement of this secondary pocket was a requirement for the allosteric pharmacology of SB269652. This suggests a novel mechanism whereby a bitopic ligand binds in an extended pose on one G protein-coupled receptor protomer to allosterically modulate the binding of a ligand to the orthosteric site of a second protomer.

Multi-Receptor Binding Profile of Clozapine and Olanzapine: A Structural Study Based on the New β2 Adrenergic Receptor Template

Schizophrenia is a devastating mental disorder that has a large impact on the quality of life for those who are afflicted and is very costly for families and society.[1] Although the etiology of schizophrenia is still unknown and no cure has yet been found, it is treatable, and pharmacological therapy often produces satisfactory results. Among the various antipsychotic drugs in use, clozapine is widely recognized as one ofthe most clinically effective agents, even if it elicits significant side effects such as metabolic disorders and agranulocytosis. Clozapine and the closely related compound olanzapine are good examples ofdrug s with a complex multi-receptor profile ;[2] they have affinities toward serotonin, dopamine, a adrenergic, muscarinic, and histamine receptors, among others.

Molecular determinants of allosteric modulation at the M1 muscarinic acetylcholine receptor.

Background: BQCA is a selective allosteric modulator of the M1 mAChR. Results: Residues that govern BQCA activity were identified using mutagenesis and molecular modeling. Conclusion: BQCA likely occupies a pocket overlapping prototypical mAChR modulators and gains selectivity through cooperativity with orthosteric ligands. Significance: Understanding the structural basis of BQCA function can provide insight into the design of more tailored allosteric ligands. Benzylquinolone carboxylic acid (BQCA) is an unprecedented example of a selective positive allosteric modulator of acetylcholine at the M1 muscarinic acetylcholine receptor (mAChR). To probe the structural basis underlying its selectivity, we utilized site-directed mutagenesis, analytical modeling, and molecular dynamics to delineate regions of the M1 mAChR that govern modulator binding and transmission of cooperativity. We identified Tyr-852.64 in transmembrane domain 2 (TMII), Tyr-179 and Phe-182 in the second extracellular loop (ECL2), and Glu-3977.32 and Trp-4007.35 in TMVII as residues that contribute to the BQCA binding pocket at the M1 mAChR, as well as to the transmission of cooperativity with the orthosteric agonist carbachol. As such, the BQCA binding pocket partially overlaps with the previously described “common” allosteric site in the extracellular vestibule of the M1 mAChR, suggesting that its high subtype selectivity derives from either additional contacts outside this region or through a subtype-specific cooperativity mechanism. Mutation of amino acid residues that form the orthosteric binding pocket caused a loss of carbachol response that could be rescued by BQCA. Two of these residues (Leu-1023.29 and Asp-1053.32) were also identified as indirect contributors to the binding affinity of the modulator. This new insight into the structural basis of binding and function of BQCA can guide the design of new allosteric ligands with tailored pharmacological properties.

Molecular mechanisms of bitopic ligand engagement with the M1 muscarinic acetylcholine receptor.

Background: Bitopic ligands bind concomitantly to orthosteric and allosteric receptor sites. Results: Residues affecting binding and biased signaling of the selective agonists TBPB and 77-LH-28-1 were identified at the M1 muscarinic receptor. Conclusion: Novel bitopic ligand binding poses and mechanisms of receptor activation were identified. Significance: Understanding the basis of bitopic ligand mechanisms can enable the design of selective ligands. TBPB and 77-LH-28-1 are selective agonists of the M1 muscarinic acetylcholine receptor (mAChR) that may gain their selectivity through a bitopic mechanism, interacting concomitantly with the orthosteric site and part of an allosteric site. The current study combined site-directed mutagenesis, analytical pharmacology,and molecular modeling to gain further insights into the structural basis underlying binding and signaling by these agonists. Mutations within the orthosteric binding site caused similar reductions in affinity and signaling efficacy for both selective and prototypical orthosteric ligands. In contrast, the mutation of residues within transmembrane helix (TM) 2 and the second extracellular loop (ECL2) discriminated between the different classes of ligand. In particular, ECL2 appears to be involved in the selective binding of bitopic ligands and in coordinating biased agonism between intracellular calcium mobilization and ERK1/2 phosphorylation. Molecular modeling of the interaction between TBPB and the M1 mAChR revealed a binding pose predicted to extend from the orthosteric site up toward a putative allosteric site bordered by TM2, TM3, and TM7, thus consistent with a bitopic mode of binding. Overall, these findings provide valuable structural and mechanistic insights into bitopic ligand actions and receptor activation and support a role for ECL2 in dictating the active states that can be adopted by a G protein-coupled receptor. This may enable greater selective ligand design and development for mAChRs and facilitate improved identification of bitopic ligands.

Discovery of novel inhibitors of amyloid β-peptide 1-42 aggregation.

Alzheimers disease, characterized by deposits of amyloid β-peptide (Aβ), is the most common neurodegenerative disease, but it still lacks a specific treatment. We have discovered five chemically unrelated inhibitors of the in vitro aggregation of the Aβ17-40 peptide by screening two commercial chemical libraries. Four of them (1-4) exhibit relatively low MCCs toward HeLa cells (17-184 μM). The usefulness of compounds 1-4 to inhibit the in vivo aggregation of Aβ1-42 has been demonstrated using two fungi models, Saccharomyces cerevisiae and Podospora anserina, previously transformed to express Aβ1-42. Estimated IC(50)s are around 1-2 μM. Interestingly, addition of any of the four compounds to sonicated preformed P. anserina aggregates completely inhibited the appearance of SDS-resistant oligomers. This combination of HTP in vitro screening with validation in fungi models provides an efficient way to identify novel inhibitory compounds of Aβ1-42 aggregation for subsequent testing in animal models.

Synthesis, binding affinity and SAR of new benzolactam derivatives as dopamine D3 receptor ligands

A series of new benzolactam derivatives was synthesized and the derivatives were evaluated for their affinities at the dopamine D(1), D(2), and D(3) receptors. Some of these compounds showed high D(2) and/or D(3) affinity and selectivity over the D(1) receptor. The SAR study of these compounds revealed structural characteristics that decisively influenced their D(2) and D(3) affinities. Structural models of the complexes between some of the most representative compounds of this series and the D(2) and D(3) receptors were obtained with the aim of rationalizing the observed experimental results. Moreover, selected compounds showed moderate binding affinity on 5-HT(2A) which could contribute to reducing the occurrence of extrapyramidal side effects as potential antipsychotics.

Progress in the structural prediction of G protein-coupled receptors: D3 receptor in complex with eticlopride.

Predicting the three‐dimensional structure of ligand–receptor complexes involving G protein‐coupled receptors (GPCRs) is still a challenging task in rational drug design. To evaluate the reliability of the GPCR structural prediction, only a couple of community‐wide assessments have been carried out. Our participation in the last edition, DOCK2010, involved the blind prediction of the dopaminergic D3 receptor in complex with the D2/D3 selective antagonist eticlopride for which the crystal structure has been recently released. Here, we describe a methodology that succeeded to produce a correctly predicted eticlopride‐D3 receptor complex out of three submitted models. Ranking the obtained models in the correct order is the main challenge due to subtle structural differences in the complex that are not sufficiently captured by conventional scoring functions. Importantly, our work reveals that a correct ranking is obtained by including a more sophisticated description of conformational ligand energy on binding. All in all, this case study highlights the current progress in modeling GPCR complexes and underlines that in silico modeling can be a valuable complement in GPCR drug discovery. Proteins 2011;

Synthesis, Binding Affinity, and Molecular Docking Analysis of New Benzofuranone Derivatives as Potential Antipsychotics

The complex etiology of schizophrenia has prompted researchers to develop clozapine-related multitarget strategies to combat its symptoms. Here we describe a series of new 6-aminomethylbenzofuranones in an effort to find new chemical structures with balanced affinities for 5-HT2 and dopamine receptors. Through biological and computational studies of 5-HT2A and D2 receptors, we identified the receptor serine residues S3.36 and S5.46 as the molecular keys to explaining the differences in affinity and selectivity between these new compounds for this group of receptors. Specifically, the ability of these compounds to establish one or two H-bonds with these key residues appears to explain their difference in affinity. In addition, we describe compound 2 (QF1004B) as a tool to elucidate the role of 5-HT2C receptors in mediating antipsychotic effects and metabolic adverse events. The compound 16a (QF1018B) showed moderate to high affinities for D2 and 5-HT2A receptors, and a 5-HT2A/D2 ratio was predictive of an atypical antipsychotic profile.

Mechanistic Insights into Allosteric Structure-Function Relationships at the M1 Muscarinic Acetylcholine Receptor

Background: Selective and potent positive allosteric modulators (PAMs) of the M1 mAChR have been recently described. Results: Use of structural analogues and mutagenic mapping identified the mechanistic basis for increased PAM activity. Conclusion: Combined analytical, structure-function, and modeling approaches uncover allosteric mechanisms at the M1 mAChR. Significance: New chemical space can be explored in the development of tailored M1 mAChR PAMs. Benzylquinolone carboxylic acid (BQCA) is the first highly selective positive allosteric modulator (PAM) for the M1 muscarinic acetylcholine receptor (mAChR), but it possesses low affinity for the allosteric site on the receptor. More recent drug discovery efforts identified 3-((1S,2S)-2-hydroxycyclohexyl)-6-((6-(1-methyl-1H-pyrazol-4-yl)pyridin-3-yl)methyl)benzo[h]quinazolin-4(3H)-one (referred to herein as benzoquinazolinone 12) as a more potent M1 mAChR PAM with a structural ancestry originating from BQCA and related compounds. In the current study, we optimized the synthesis of and fully characterized the pharmacology of benzoquinazolinone 12, finding that its improved potency derived from a 50-fold increase in allosteric site affinity as compared with BQCA, while retaining a similar level of positive cooperativity with acetylcholine. We then utilized site-directed mutagenesis and molecular modeling to validate the allosteric binding pocket we previously described for BQCA as a shared site for benzoquinazolinone 12 and provide a molecular basis for its improved activity at the M1 mAChR. This includes a key role for hydrophobic and polar interactions with residues Tyr-179, in the second extracellular loop (ECL2) and Trp-4007.35 in transmembrane domain (TM) 7. Collectively, this study highlights how the properties of affinity and cooperativity can be differentially modified on a common structural scaffold and identifies molecular features that can be exploited to tailor the development of M1 mAChR-targeting PAMs.