Perspective: Implications of Ligand-Receptor Binding Kinetics for Therapeutic Targeting of G Protein-Coupled Receptors

Abstract

The concept of ligand-receptor binding kinetics has been broadly applied in drug development pipelines focusing on G protein-coupled receptors (GPCRs). The ligand residence time (RT) for a receptor describes how long a ligand-receptor complex exists, and is defined as the reciprocal of the dissociation rate constant (k _off). RT has turned out to be a valuable parameter for GPCR researchers focusing on drug development as a good predictor of in vivo efficacy. The positive correlation between RT and in vivo efficacy has been established for several drugs targeting class A GPCRs (e.g., the neurokinin-1 receptor (NK₁R), the β₂ adrenergic receptor (β₂AR), and the muscarinic 3 receptor (M₃R)) and for drugs targeting class B1 (e.g., the glucagon-like peptide 1 receptor (GLP-1R)). Recently, the association rate constant (k _on) has gained similar attention as another parameter affecting in vivo efficacy. In the current perspective, we address the importance of studying ligand-receptor binding kinetics for therapeutic targeting of GPCRs, with an emphasis on how binding kinetics can be altered by subtle molecular changes in the ligands and/or the receptors and how such changes affect treatment outcome. Moreover, we speculate on the impact of binding kinetic parameters for functional selectivity and sustained receptor signaling from endosomal compartments; phenomena that have gained increasing interest in attempts to improve therapeutic targeting of GPCRs.

Figure 1

Receptor residues in the binding pocket of A_2AR that lead to altered RT of ZM241385 upon mutation to alanine (Protein Data Bank (PDB) accession number: 4EIY). (A) Side view of the hydrophobic pocket in the receptor, formed by E169^ECL2, T256^6.58, and H264^ECL3, which is also further stabilized by a water molecule. Mutagenesis of residues that are located in this hydrophobic pocket decreases the RT of ZM241385. (B) Top view of A_2AR; above Y271^7.36 (essential for ZM241385’s binding and decreasing RT when mutating to alanine (∼10-fold)),^, there exists another hydrophobic pocket that is formed by I66^2.64, S67^2.65, and L267^7.32. Upon mutagenesis of these residues, the RT of ZM241385 is increased.

Figure 2

Receptor residues within the CCR5 binding pocket that are crucial for maraviroc’s long RT (PDB accession number: 4MBS). Top view (A) and side view zoom (B) of maraviroc’s binding pocket. E283^7.39 is crucial for the transition from the flexible RL state to the long-lasting R*L state. Other receptor residues, such as W86^2.60 and Y108^3.32, have been shown to be detrimental in shortening the RT when mutated to alanine.

Figure 3

Orthosteric binding pocket and exosite of β₂AR (PDB accession numbers: 6MXT and 4LDO). Superimposed salmeterol−β₂AR and epinephrine−β₂AR structures highlight the additional binding interactions of salmeterol with β₂AR contributing to the longer RT.

Figure 4

E354^6.53bQ increases the RT of GIP(1–42) on GIPR. Both E354^6.53b (A) and E354^6.53bQ (B) interact with the N-terminal nitrogen (Y1) of GIP(1–42). E354^6.53b forms a salt bridge, while the loss of anionic properties by glutamine at this position in (E354^6.53bQ) still allows for a hydrogen bond in the same place.

Figure 5

Graphical overview of how variations in ligand and receptor can alter ligand–receptor RT. Receptor changes affecting ligand–receptor binding kinetics can lead to a shorter RT (B) or to a longer RT (C) for a certain ligand as compared to the wild type (WT) receptor (A). Besides post-translational modifications (PTM) of the same ligand, different endogenous ligands and analogs of endogenous ligands) can also lead to a shorter RT (B) or to a longer RT (C) as compared to an endogenous ligand (A).