Auxiliary proteins are the predominant determinants of differential efficacy of clinical candidates acting as AMPA receptor positive allosteric modulators

Positive allosteric modulators (PAMs) of AMPA receptors boost cognitive performance in preclinical and clinical studies. Their therapeutic window is narrow, however, and clinical application will likely only occur if greater discrimination in activity is achieved. Toward that end, we compared the modulatory activity of two PAMs recently considered as clinical candidates, LY451395 (mibampator) and PF-04958242/BIIB104, on recombinant and native AMPA receptors (AMPARs). We found that the principle molecular determinant that shaped modulatory activity of both PAMs on deactivation (recombinant) and decay (synaptic) of AMPARs was the auxiliary protein incorporated into the receptor complexes. AMPARs containing the stargazin/g2 transmembrane AMPAR regulatory protein (TARP) were slowed to a .10-fold degree by both PAMs as compared with those incorporating g8 TARP. Neither subunit composition nor flip/flop splice variation had substantive effect. Similarly, stargazin/g2-containing mossy fiber EPSCs in cerebellar granule neurons were slowed to a ∼5-fold greater degree than EPSCs in hippocampal CA1 pyramidal cell neurons, which express the g8 TARP. LY451395 exhibited greater efficacy than BIIB104 at both synapses. These studies provide insight into the receptor constituents that determine efficacy of sulfonamide PAMs. We conclude that compounds that discriminate between AMPARs complexed with distinct TARPs, and particularly those with lower stargazin/g2 efficacy such as BIIB104, could act as viable procognitive therapeutics.