Potent Heterocyclic Ligands for Human Complement C3a Receptor

Abstract

The G-protein coupled receptor (C3aR) for human inflammatory protein complement C3a is an important component of immune, inflammatory, and metabolic diseases. A flexible compound (N2-[(2,2-diphenylethoxy)acetyl]-l-arginine, 4), known as a weak C3aR antagonist (IC50 μM), was transformed here into potent agonists (EC50 nM) of human macrophages (Ca(2+) release in HMDM) by incorporating aromatic heterocycles. Antagonists were also identified. A linear correlation between binding affinity for C3aR and calculated hydrogen-bond interaction energy of the heteroatom indicated that its hydrogen-bonding capacity influenced ligand affinity and function mediated by C3aR. Hydrogen-bond accepting heterocycles (e.g., imidazole) conferred the highest affinity and agonist potency (e.g., 21, EC50 24 nM, Ca(2+), HMDM) with comparable efficacy and immunostimulatory activity as that of C3a in activating human macrophages (Ca(2+), IL1β, TNFα, CCL3). These potent and selective modulators of C3aR, inactivated by a C3aR antagonist, are stable C3a surrogates for interrogating roles for C3aR in physiology and disease.

Proteinases, Their Extracellular Targets, and Inflammatory Signaling

“…Not surprisingly, the downstream signaling initiated by the complement system-generated peptides via their GPCR targets has much in common with inflammation-related signaling by the PARs (see below) and kinin-stimulated receptors. The recent availability of C3AR1-targeted agonist ligands will enable a more extensive interrogation of this area of interest (Reid et al, 2014).…”

Downsizing Proteins Without Losing Potency or Function

“…For example, a hydrogen-bond accepting nitrogen conferred agonist activity, with much greater potency for imidazoles and oxazoles ( Figure 3) than for oxadiazoles, furans and other heterocycles. Interestingly, there was a linear correlation [9,10] between the C3aR-binding affinity (measured by competition with 125 I-C3a) and the calculated hydrogen-bond acceptor interaction energy (kcal mol -1 ) between water and heteroatom of heterocycles compared with the water dimer (determined using ab initio methods MP2/6-311++G(3d,3p) and corrected for the basis set superimposition error within Gaussian 09). This enabled us to tune agonist potency by rational variation of the heterocyclic component incorporated into the dipeptide mimetics, coupled with changes to other substituents.…”

“…Here we summarize the principle and effectiveness of this idea, which starts with a functionally important amino acid in C3a and rationally grows it into functional surrogates for C3a. We compare activity profiles for the resulting peptidomimetics versus human C3a, all compounds binding to a specific G protein coupled receptor (C3aR) expressed on the plasma membrane surface of human immune and other cell types [9,10].…”

Chemical Approaches to Modulating Complement-Mediated Diseases

“…and has a completely different pharmacological profile from C3a. 22 201 Compound 40 was found to be a weak antagonist in some animal models of inflammatory diseases, [202][203][204] however it has also been reported to be a C3aR agonist in vitro in some cell types, such as transfected RBL and CHO cells, and may also bind to other receptors. 199, 205 48…”