Challenges in the discovery and optimization of mGlu2/4 heterodimer positive allosteric modulators

Background: This article describes the challenges in the discovery and optimization of mGlu(2/4) heterodimer Positive Allosteric Modulators (PAMs). Methods: Initial forays based on VU0155041, a PAM of both the mGlu(4) homodimer and the mGlu(2/4) heterodimer, led to flat, intractable SAR that precluded advancement. Screening of a collection of 1,152 FDA approved drugs led to the discovery that febuxostat, an approved xanthine oxidase inhibitor, was a moderately potent PAM of the mGlu(2/4) heterodimer (EC50 = 3.4 mu M), but was peripherally restricted (rat K-p = 0.03). Optimization of this hit led to PAMs with improved potency (EC(50)s <800 nM) and improved CNS penetration (rat K-p >2, an similar to 100-fold increase). Results: However, these new amide analogs of febuxostat proved to be either GIRK1/2 and GIRK1/4 activators (primary carboxamide congeners) or mGlu(2) PAMs (secondary and tertiary amides) and not selective mGlu(2/4) heterodimer PAMs. Conclusion: These results required the team to develop a new screening cascade paradigm, and exemplified the challenges in developing allosteric ligands for heterodimeric receptors.