Background: This article describes the challenges in the discovery and optimization of
mGlu2/4 heterodimer positive allosteric modulators (PAMs).
Methods: Initial forays based on VU0155041, a PAM of both the mGlu4 homodimer and the mGlu2/4
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 mGlu2/4 heterodimer (EC50 = 3.4 μM), but was peripherally
restricted (rat Kp = 0.03). Optimization of this hit led to PAMs with improved potency (EC50s <800
nM) and improved CNS penetration (rat Kp >2, an ~100-fold increase).
Result: However, these new amide analogs of febuxostat proved to be either GIRK1/2 and GIRK1/4
activators (primary carboxamide congeners) or mGlu2 PAMs (secondary and tertiary amides) and not
selective mGlu2/4 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.