Two additional hydrogen bonds are formed
Two additional hydrogen-bonds are formed by the -substituted methoxy-group with Ser and Asp. Ser exerts the role of a gate keeper in CK1: Entry of the ATP in the binding-site causes a conformational change into the closed form by Ser, which locks the binding-site and prevents ATP-diffusion out of the binding-site. This in turn stabilizes CK1 in the active conformation. The ability of 1 to form a hydrogen-bond with Ser locks the ATP-binding-site permanently. The position of Ser is fixed in the closed form of the pocket and thereby prevents the replacement of IC261 by ATP. The interaction of a -substituent with Ser was investigated by the introduction of both sterically demanding and functional groups (, )., , These groups should prevent the conformational change into the closed form of the ATP-binding-site. The FDA-approved kinase inhibitor Sunitinib () was included into the investigation for the same rational. 11 did not show any effect on Aβ levels, but an unexpected inverse modulation of the γ-secretase was observed for , which decreased the Aβ levels and increased Aβ levels (). The inverse modulation by as well as indicates the mechanism to be different from total inhibition of Aβ generation. The moderate inhibition of Aβ secretion (Aβ=26%, Aβ=53%, Aβ=26%) by was not observed in the purified γ-secretase assay. This indicates an indirect, potentially upstream mode of action. As the IC261 substitution pattern (2,4,6-trimethoxybenzene) was found to be important for the activity, it was systematically varied into 2,3,4- and 3,4,5-trimethoxybenzene (, ). These two derivatives also exert a significant atomoxetine hcl in total Aβ. Although the o-positions of (Aβ=54%, Aβ=72%, Aβ=65%) are substituted with hydrogens, it is equipotent to (Aβ=55%, Aβ=77%, Aβ=77%). A distinct rise in activity was observed for (Aβ=27%, Aβ=66%, Aβ=47%) with a methoxy-group in -, - and -position. The most important interaction was anticipated to be the indole-nitrogen forming two hydrogen-bonds with Asp and Leu. This crucial interaction was challenged by a methyl-substitution of the nitrogen, expecting a 90% loss of activity. Surprisingly, a rise in Aβ-inhibitory activity of compared to IC261 was observed. The three Aβ-species were significantly decreased to 35% for Aβ and to 65% for Aβ and Aβ each. Particularly this observation was the first major evidence against the involvement of CK1 in our biological assay. IC261 and the three most potent derivatives , and were tested for inhibitory activity on CK1δ and 42 other kinases to evaluate the influence of CK1 on γ-secretase (). Surprisingly only IC261 shows a significant inhibition of CK1δ (81% inhibitory activity, , ). As soon as the substitution pattern differs from 2,4,6-trimethoxybenzene, no significant CK1δ inhibition (>80%) was observed. This is shown, for example, in the case of (2,3,4-trimethoxybenzene), which bears a methoxy-group in the m- instead of the second -position. It exerts an inhibition of a mere 1% under the same conditions (). In addition, no significant inhibitory activity on CK1δ was found for and . Compounds and are inactive on CK1δ yet display significant inhibition (98% and 89%) of the tyrosine kinase Flt3, which plays an important role in leukemia. However, Flt3 is not known to exert effects on APP metabolism. Only the N-methylated indolinone lacked significant inhibition (inhibitory activity <20%) for all kinases tested, which confirms the relevance of N-methylation of indolinones in kinase inhibition. This N–H bond is essential for the inhibition of most kinases by indolinones. The inhibitory activity of , , , on CK1δ cannot be correlated to the inhibition of γ-secretase activity which is shown in for the Aβ level reduction at a concentration of 10μM. These data agree with three possible explanations: (1) IC261 does not directly affect γ-secretase activity but a degradation product is responsible for γ-secretase inhibition, (2) IC261 is coincidently a weak γ-secretase inhibitor, which independently targets CK1δ or CK1ε activity. (3) Another, yet unidentified kinase is responsible for the cellular activity. To investigate a direct effect on γ-secretase activity, IC261 was tested in a cell-free assay using lipid-reconstituted purified γ-secretase and purified APP C100-His as substrate for Aβ generation, that is, under conditions, where no additional metabolism is present ().