Pharmacological characterisation of allosteric modulators at human mGlu₅

The metabotropic glutamate receptor 5 (mGlu₅) is a Class C G protein-coupled receptor, ubiquitously expressed throughout the central nervous system (CNS). With major roles in cognition, learning and memory, mGlu₅ dysfunction is linked with numerous neurodegenerative and neuropsychiatric disorders, representing a viable therapeutic target. Allosteric modulators bind topographically distinct sites from glutamate and other orthosteric agonists and enhance (positive allosteric modulators, PAMs), inhibit (negative allosteric modulators, NAMs) or do not effect (neutral allosteric ligands, NALs) mGlu₅ function. While mGlu₅ modulators have efficacy in in vivo rodent models of CNS disorders, none have been approved for human use. We hypothesise preclinical optimisation using non-human pharmacological data may contribute to translational failures, as functional studies are predominantly performed using rat mGlu₅ and non-human brain neuronal cultures. Here we are the first to systematically assess and quantify the impact of eleven chemically and pharmacologically diverse mGlu₅ PAMs, NAMs and NALs on human mGlu₅ activity using radioligand binding, intracellular calcium (iCa²⁺) mobilisation and inositol monophosphate (IP₁) accumulation assays. By comparing to published and newly generated data for rat mGlu₅ we show that while modulator pharmacology is relatively consistent across species, ligand dependent species differences in allosteric modulator affinity, cooperativity and probe dependence are evident. Additionally, we report PAM-dependent effects on orthosteric agonist kinetic profiles at human mGlu_5. Together, these data highlight the importance of systematic evaluation of mGlu₅ allosteric ligand activity at human mGlu₅ to improve drug design and overcome potential barriers to translatability to clinical settings.