Abstract

Allosteric modulation of mGlu₁ represents a viable therapeutic target for treating numerous CNS disorders. While multiple chemically distinct mGlu₁ positive (PAMs) and negative (NAMs) allosteric modulators have been identified, drug discovery paradigms have not included rigorous pharmacological analysis. In the present study, we hypothesised existing mGlu₁ allosteric modulators possess unappreciated probe dependent or biased pharmacology. Using HEK293A cells stably expressing human mGlu­_1, we screened mGlu₁ PAMs and NAMs from divergent chemical scaffolds for modulation of different mGlu₁ orthosteric agonists in intracellular calcium (iCa²⁺) mobilisation and inositol monophosphate (IP₁) accumulation assays. Operational models of agonism and allosterism were used to derive estimates for important pharmacological parameters such as affinity, efficacy and cooperativity. Modulation of glutamate and quisqualate-mediated iCa²⁺ mobilisation revealed probe dependence at the level of affinity and cooperativity for both mGlu₁ PAMs and NAMs. We also identified the previously described mGlu₅ selective NAM PF-06462894 as an mGlu₁ NAM with a different pharmacological profile to other NAMs. Differential profiles were also observed when comparing ligand pharmacology between iCa²⁺ mobilisation and IP₁ accumulation. The PAMs Ro67-4853 and CPPHA displayed apparent negative cooperativity for modulation of quisqualate affinity, and the NAMs CPCCOEt and PF-06462894 had a marked reduction in cooperativity with quisqualate in IP₁ accumulation and upon extended incubation in iCa²⁺ mobilisation assays. These data highlight the importance of rigorous assessment of mGlu₁ modulator pharmacology to inform future drug discovery programs for mGlu₁ allosteric modulators.

Significance Statement mGlu₁ positive and negative allosteric modulators have therapeutic potential in multiple CNS disorders. We show that chemically distinct modulators display differential pharmacology with different orthosteric ligands and across divergent signalling pathways at human mGlu₁. Such complexities in allosteric ligand pharmacology should be considered in future mGlu₁ allosteric drug discovery programmes.