br The methyl aspartate receptor NMDAr hypofunction

The -methyl--aspartate receptor (NMDAr) hypofunction hypothesis of schizophrenia correlates disease symptomology with glutamatergic neurotransmission dysfunction., , , Therefore, inhibition of glycine transporter 1 (GlyT1) to elevate synaptic levels of glycine and potentiate NMDAr signaling is a promising therapy for this disease. Current treatments for schizophrenia are associated with unwanted side effects and do not provide sufficient relief across all symptom domains., , , Additionally, FDA-approved GlyT1 inhibitors for schizophrenia remain elusive, with Hoffman-La Roche clinical candidate bitopertin (RG1678) being one of the most advanced to date. Having reached Phase III, the Ritonavir was ultimately withdrawn from development after failing to achieve statistical significance in the improvement of negative symptoms compared to control. As such, treatments for the positive, negative, and cognitive symptoms of schizophrenia remain a significant unmet medical need. We previously reported a series of azepane sulfonamides as GlyT1 inhibitors. Identified through a virtual screening campaign and exemplified by , these compounds were characterized by notable glycine uptake efficiency (GlyT1 IC for : 37 ± 6 nM), low aqueous solubility, and favorable brain-to-plasma ratios (1.6–2.1). As previously noted, is structurally similar to Sanfoi’s withdrawn clinical candidate SSR504734, () and the more potent related N-Me analog . We speculated that by capitalizing on our learnings from development of the former we might increase the potency of through an increase in piperidine lipophilicity. However, rather than evoke a ring expansion to form an azepane, we proposed that conversion to an isoquinuclidine, such as in , might also provide the desired outcome. While quinuclidine isomers of similar to have been reported, also had the potential advantage of having a single chiral center. We were optimistic that this might increase synthetic tractability and facilitate exploration of our hypothesis. As shown in , a comparison of and in a glycine uptake inhibition assay revealed slightly more than a ten-fold decrease in potency. While this did not confirm our hypothesis, we were gratified that such a change still afforded a compound with appreciable in vitro activity. Even more interesting, enantiomer demonstrate activity comparable to . This was surprising, because the chirality of the benzylic position of ( enantiomer) is opposite that of the same position in (,). Suspecting an error in stereochemical assignment, we validated the chirality of both and by vibrational circular dichroism (VCD) and X-ray crystallography (discussed below). A molecular overlay (recreated in MOE) reinforced the concept that similar potencies between and were not unreasonable as there is good overlap of benzamide, aryl, and heterocyclic regions (). We were interested in modifying to be a probe suitable for evaluation in a murine MK801 locomotor activity (LMA) assay. While the potency and solubility (typically high for the compounds described herein) of were acceptable, both lipophilicity, as measured by log, and stability on incubation with human liver microsomes were higher than desired. Reasoning that reductions to the former might also attenuate the latter, we sought to reduce log (<4) in a variety of ways. This included a) heteroaromatic and substituent changes to the benzamide aromatic ring, b) conversion of the unsubstituted phenyl moiety into various heterocycles, and c) ring contraction of the isoquinuclidine itself. In the course of elucidating benzamide SAR to attenuate lipophilicity, simple heterocyclic replacements (Not shown: 1-methyl-imidazol-4-yl, thiazol-4-yl; GlyT1 uptake IC > 20 μM) of the aryl ring were insufficient to retain potency. For benzamides themselves, potency was found to directly depend on the nature and number of ortho substituents. Specifically, linear free energy relationship analysis (LFER) identified both the ortho substituent parameter (E; inverse correlation) and hydrophobic nature (π Hammett constant; positive correlation) of the benzamide -substituents to be good predictors of glycine uptake inhibition. Where such information was not available, a torsion angle analysis could be used to roughly bin compounds according to potency (). In other words, hydrophobic -substituents that perturbed the planarity of amide-aryl conjugation generally were more active at inhibiting glycine uptake.