Our own search towards aryl imidazole derived GSMs led
Our own search towards aryl-imidazole derived GSMs led to the discovery of the benzimidazole derivative (). It is one of the most potent GSMs to date, suffering however from sub-optimal drug-like properties. More recently, we have described the design and synthesis of bicyclic triazolo-derivatives, such as triazolopiperidine (), as potent in vitro and in vivo GSMs with an improved drug-like profile. These compounds resulted from a conformational restriction of benzyltriazoles , which in turn derived from the introduction of a methylene spacer in between the central triazole ring (C) and the aromatic Dipraglurant D, in this case the 4-F-phenyl group. While this work was in progress, an alternative approach to improve the potency of , while maintaining its physicochemical properties (MW<400, log<4), was investigated. Considering the influence of torsion between the C and D rings on potency,, other spacers in that would disrupt the planarity between these rings were also evaluated (). Starting from derivative , small substituents were added on the benzylic position with the intent of pointing the D ring towards an active conformation, for instance by hindering free rotation, as in (). This modification had no effect on potency. Replacement of the linker with a carbonyl function, as in ketone or amide , led to a significant decrease in activity. An extended sp-carbon linker on the other hand, such as the -alkene in , led to a 10-fold increase in potency. A saturated 2-atom spacer, such as in aniline and benzylamine was probably too flexible and did not manage to bring the IC under 100nM. Changing the C linker in to a heteroatom (–) led to the discovery of the highly potent aniline , significantly more active than its phenol- and sulfide-analogs ( vs and ). This result would indicate that the H-bond donor capability of may contribute to the increase in potency (vide infra). Furthermore, the trifluoromethyl substituent on the 3-position of the aromatic D ring brought a 13-fold increase in potency versus the non-substituted analog . Restriction of the free rotation of , by cyclizing the aniline into the aromatic ring (), did not improve the IC. Swapping the position of the aromatic and the methyl substituents on the triazole ring in (compound , ) also had a detrimental effect on potency. Aniline (MW=428, log=4.9) was selected for further evaluation. When was tested orally in mice at 30mg/kg it showed no significant effect (−17%) on Aβ peptides levels 4h after dosing, despite high compound brain levels (5.7μM, B/P=0.42). This compound was however highly bound to brain proteins, with the fraction of unbound compound in brain () under the detection limit (<0.05%). It can be hypothesized that the free brain concentration of was not high enough for in vivo activity. Nevertheless, in the benzimidazole series of many derivatives with <0.1% demonstrated robust in vivo activity. Further exploration around this hit was aimed at identifying similarly potent analogs that would also demonstrate in vivo efficacy. It was found that modifications on the aniline ring () resulted in modulation of both in vitro and in vivo activity. Thus, substitution at 3- and 3,5-positions (compounds – in ) with both electron withdrawing and electron donating substituents maintained the in vitro potency of the hit, compound being one of the most potent GSMs in this series. As for , this potency again did not translate in vivo in mice, as Aβ42 levels were not significantly reduced 4h after administration. Compounds and were also highly bound to brain tissue and thus, despite the high compound brain levels, in both cases the free brain concentration () was probably not high enough for in vivo activity (estimated as less than 2nM and 4nM, respectively). In mouse, the free brain concentration of and was at least 20 fold less than the IC in vitro as indicated by the coverage ratio (CR=/mIC, <0.05, see ). Substituents at the 4-position of the aniline ring did not bring an improvement in potency (, not tested in vivo). A substituent present at the 2-position of the aniline (–) turned out to be essential for in vivo potency. Mono 2-substituted anilines, either with electron withdrawing () or lipophilic electron donating groups (), had moderate in vitro activity, but showed good modulation of Aβ peptides levels in vivo. For and , the free brain concentrations were higher than their respective IC in vitro (CR=1.7 and 2.7, respectively). A polar methoxy substituent was weakly active () and it was not tested in vivo. Additional substitution at the 5-position (compounds and ) increased the in vitro potency and maintained the in vivo activity. Compound had high brain and plasma levels and improved B/P ratio (close to 1).