A Chemical Biology Toolbox Targeting the Intracellular Binding Site of CCR9: Fluorescent Ligands, New Drug Leads and PROTACs

A conserved intracellular allosteric binding site (IABS) has recently been identified at several G protein‐coupled receptors (GPCRs). Starting from vercirnon, an intracellular C−C chemokine receptor type 9 (CCR9) antagonist and previous phase III clinical candidate for the treatment of Crohn's disease, we developed a chemical biology toolbox targeting the IABS of CCR9. We first synthesized a fluorescent ligand enabling equilibrium and kinetic binding studies via NanoBRET as well as fluorescence microscopy. Applying this molecular tool in a membrane‐based setup and in living cells, we discovered a 4‐aminopyrimidine analogue as a new intracellular CCR9 antagonist with improved affinity. To chemically induce CCR9 degradation, we then developed the first PROTAC targeting the IABS of GPCRs. In a proof‐of‐principle study, we succeeded in showing that our CCR9‐PROTAC is able to reduce CCR9 levels, thereby offering an unprecedented approach to modulate GPCR activity.

Bivalent ligands promote endosomal trafficking of the dopamine D3 receptor-neurotensin receptor 1 heterodimer

Bivalent ligands are composed of two pharmacophores connected by a spacer of variable size. These ligands are able to simultaneously recognize two binding sites, for example in a G protein-coupled receptor heterodimer, resulting in enhanced binding affinity. Taking advantage of previously described heterobivalent dopamine-neurotensin receptor ligands, we demonstrate specific interactions between dopamine D3 (D₃R) and neurotensin receptor 1 (NTSR1), two receptors with expression in overlapping brain areas that are associated with neuropsychiatric diseases and addiction. Bivalent ligand binding to D₃R-NTSR1 dimers results in picomolar binding affinity and high selectivity compared to the binding to monomeric receptors. Specificity of the ligands for the D₃R-NTSR1 receptor pair over D₂R-NTSR1 dimers can be achieved by a careful choice of the linker length. Bivalent ligands enhance and stabilize the receptor-receptor interaction leading to NTSR1-controlled internalization of D₃R into endosomes via recruitment of β-arrestin, highlighting a potential mechanism for dimer-specific receptor trafficking and signalling.

Homobivalent Dopamine D2 Receptor Ligands Modulate the Dynamic Equilibrium of D2 Monomers and Homo- and Heterodimers

Dopamine D₂ receptors (D₂Rs) are major targets in the treatment of psychiatric and neurodegenerative diseases. As with many other G protein-coupled receptors (GPCRs), D₂Rs interact within the cellular membrane, leading to a transient receptor homo- or heterodimerization. These interactions are known to alter ligand binding, signaling, and receptor trafficking. Bivalent ligands are ideally suited to target GPCR dimers and are composed of two pharmacophores connected by a spacer element. If properly designed, bivalent ligands are able to engange the two orthosteric binding sites of a GPCR dimer simultaneously. Taking advantage of previously developed ligands for heterodimers of D₂R and the neurotensin receptor 1 (NTSR1), we synthesized homobivalent ligands targeting D₂R. Employing bioluminescence resonance energy transfer, we found that the bivalent ligands 3b and 4b comprising a 92-atom spacer are able to foster D₂R-homodimerization while simultaneously reducing interactions of D₂R with NTSR1. Both receptors are coexpressed in the central nervous system and involved in important physiological processes. The newly developed bivalent ligands are excellent tools to further understand the pharmacological consequences of D₂R homo- and heterodimerization. Not limited to the dopaminergic system, modifying class A GPCRs' dynamic equilibrium between monomers, homomers, and heteromers with bivalent ligands may represent a novel pharmacological concept paving the way toward innovative drugs.

Validation of a new technique for measurement of intracranial pressure with a scintillation counter

Intracranial pressure sensors and subdural and subgaleal sensing tambours were used to measure the pressure difference between the intracranial and subgaleal spaces in two monkeys. The pressure differential was transmitted to fluid bathing a piston, to which an isotope source (145Pm) was attached. The radiation signal emanating through a fixed collimator was detected transcutaneously by a sodium iodide crystal contained within a photomultiplier tube connected to a scintillation counter. After in vitro testing of linearity, in vivo infusion studies were performed. Linearity between intracisternal pressure and radioactivity (r = 0.99; p less than 0.001) was established in the two experimental animals for an interval of 5 months and 1 year, respectively. Autopsy findings confirmed that the sensing tambours became encapsulated with a pseudomembrane that did not attenuate the pressure signal. The results of this investigation suggest that this method for measurement of intracranial pressure without transcutaneous connections may be suitable for long term monitoring of intracranial pressure.