Differing activity profiles of the stereoisomers of 2,3,5,6TMP-TQS, a putative silent allosteric modulator of a7 nAChR

Abstract

Many synthetic compounds to which we attribute specific activities are produced as racemic mixtures of stereoisomers, and it may be that all the desired activity comes from a single enantiomer. We have previously shown this to be the case with the a7 nicotinic acetylcholine receptor positive allosteric modulator (PAM) 3a,4,5,9b-Tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[ c]quinoline-8-sulfonamide (TQS) and the a7 ago-PAM 4BP-TQS. Cis-trans-4-(2,3,5,6-tetramethylphenyl)-3a,4,5,9b-tetrahydro- 3H-cyclopenta[c]quinoline-8-sulfonamide (2,3,5,6TMPTQS), previously published as a "silent allosteric modulator" and an antagonist of a7 allosteric activation, shares the same scaffold with three chiral centers as the aforementioned compounds. We isolated the enantiomers of 2,3,5,6TMP-TQS and determined that the (2) isomer was a significantly better antagonist than the (+) isomer of the allosteric activation of both wild-type a7 and the nonorthosterically activatible C190A a7 mutant by the ago-PAM GAT107 (the active isomer of 4BP-TQS). In contrast, (+)2,3,5,6TMPTQS proved to be an a7 PAM. (2)2,3,5,6TMP-TQS was shown to antagonize the allosteric activation of a7 by the structurally unrelated ago-PAM B-973B as well as the allosteric activation of the TQS-sensitive a4b2L159M mutant. In silico docking of 2,3,5,6TMP-TQS in the putative allosteric activation binding site suggested a specific interaction of the (2) enantiomer with a7T106, and allosteric activation of a7T106 mutants was not inhibited by (2)2,3,5,6TMP-TQS, confirming the importance of this interaction and supporting the model of the allosteric binding site. Comparisons and contrasts between 2,3,5,6TMP-TQS isomers and active and inactive enantiomers of other TQS-related compounds identify the orientation of the cyclopentenyl ring to the plane of the core quinoline to be a crucial determinate of PAM activity.