Armed with this knowledge we looked at ways to
Armed with this knowledge, we looked at ways to reduce the log of through nitrogen incorporation while still maintaining potency via installation of ortho substituents such as chloro, methyl, bromo, and thiomethyl (). In general, pyridine analogs were less active ( vs ; vs ; vs ) than their simple phenyl counterparts, with 3-pyridyl amides evoking the most dramatic potency reductions ( & ). The most active compounds, however, were those containing chloro, methyl, or thiomethyl ortho substituents (–), and, in particular, 4-pyridyl analog reinforced the concept that log could be reduced in select cases with only a slight decrease in uptake efficiency. Despite reductions in lipophilicity, we saw no improvement in predicted human clearance ( vs ; vs ) for pyridine matched pairs, and all compounds contained a low to moderate efflux liability. Improvements in metabolic stability were only observed in a very small number of benzamide modifications that involved introduction of polar ortho substituents such as for and (hu CL <4 μL/min/mg). While very potent, both compounds carried additional liabilities such as concerns around potentially mutagenic anilines () or excessively high efflux (). A similar exercise aimed at reducing lipophilicity and/or exploring areas for scaffold elaboration was conducted through modification of the pendant phenyl ring using racemic material for ease of accessibility. Unfortunately, glycine uptake inhibition decreased in nearly all cases compared to racemic (). Only data for meta substitution ( & vs ) suggested that there was room for a potency increase, and this was accompanied by a commensurate increase in lipophilicity. Furan was also promising as this was the first example of a phenyl replacement that appeared to retain a significant degree of potency while lowering log; disappointingly this had an adverse impact on metabolic stability. Generally, efflux ratios were not investigated for the majority of these samples as these were racemates. It is worth noting that 4-pyridine analog is the only chiral Protein A/G synthesis in and so must be compared to chiral rather than . There was roughly a threefold decrease in potency upon incorporation of a nitrogen at the 4-position of the phenyl ring, no significant change in metabolic liability despite a 1.5 log unit drop in log, and efflux liability persisted (efflux ratio: 4.7). We opted to prepare a small number of additional analogs to finish our exploration of phenyl ring SAR using the more potent scaffold of 2,6-dimethylbenzamide (). Encouraged by the data for , 2-methylfuran was prepared in an attempt to mitigate any potential metabolism associated with the 2-position of the furan ring itself. While potency was retained and lipholicity and human CL decreased, efflux was unchanged. Similar potency and efflux results were obtained with 4-pyridine . The last two compounds of involved scaffold modifications similar to those described by Sanfoi and Taisho Pharmaceuticals respectively. While we synthesized a number of potent sulfones (and a few sulfonamides) such as , all suffered from high predicted human clearance and efflux liabilities. Lastly, pyrazole , despite the promise suggested by bromide , failed to realize any gain through expansion off the meta position of the phenyl ring. What remained elusive within the isoquinuclidines were structural modifications leading to a reduction in both clearance and efflux. We suspected that -dealkylation was at least partly to blame, and, through a combination of changes in the nitrogen substituent and isoquinuclidine itself, we were able to demonstrate this conclusively (). Thus, for des-methyl analog , in vitro microsomal clearance decreased by fourfold (34 vs 121 μL/min/mg) compared to . Removal of a methylene spacer to afford a series of azabicyclo[2.2.1]heptanes (ABHs; –) reduced log by roughly half a log unit and afforded comparable potency ( vs ). As before, removal of the -alkyl substituent within the ABHs () reduced predicted human clearances by roughly fivefold. Similarly, decreasing ring size further to establish a series of azabicyclo[2.1.1]hexanes (ABXs; –) was accompanied by another drop in lipophilicity and potency relative to parent isoquinuclidines, however, this series also had overall lower liabilities from a microsomal stability perspective. Consistent with isoquinuclidine and ABH compounds, des-methyl ABX had the most favorable result in this regard but also the largest drop-off in glycine uptake inhibition.