# Flavonoids as Promising Multitarget Agents in Alzheimer's Disease Therapy
## 2 Activities of Flavonoids from Natural Sources
### Inhibition of NFTs Formation  
#### 2.2.1 Inhibition of GSK-3\(\beta\)  
Flavanones  
Naringenin (**6**) exhibits GSK-3\(\beta\) inhibition with an IC\({}_{50}\) of 45.71 uM [52]. Hesperetin, the (25)-3\({}^{\prime}\),5,7-trihydroxy-4\({}^{\prime}\)-methoxyflavanone (**84**) (Figure 9), presents GSK-3\(\beta\) inhibition characterized by an IC\({}_{50}\) of 26.92 uM [66]. However, the corresponding glycosylated flavonones naringin, the (25)-4\({}^{\prime}\),5-dihydroxy-7-[\(\alpha\)-L-rhammopyranosyl-(1\(\rightarrow\)2)-\(\beta\)-D-glucopyranasyloxy]flavan-4-one (**85**) (Figure 9), and hesperidin (**20**), existing also in several citrus fruits, have no effect on the inhibition of GSK-3\(\beta\) (IC\({}_{50}\) > 300 uM) [66]. These results suggest that side group substitutions on the flavonoid scaffold affect the GSK-3\(\beta\) activity in a significant manner as the flavonoids with larger side groups exhibit lower inhibitory activity. The presence of bulkier groups leads to decreased interactions between enzymes and inhibitors [52; 66].  
Figure 8: Structure of flavonoids that inhibit \(\beta\)-secretase.  
Flavones  
Flavone (**86**) (Figure 9), the simplest compound of this class, with no hydroxy groups, presents no inhibitory activity of GSK-3\(\beta\) (IC\({}_{50}>\) 100 \(\upmu\)M) [66]. Very weak inhibitors of the enzyme are also nobiletin (**39**) (IC\({}_{50}=\) 52.48 \(\upmu\)M) and tangeritin (**38**) (IC\({}_{50}>\) 100 \(\upmu\)M) [66], which have multiple methoxy groups as substituents. These results show how the inhibitory activity of GSK-3\(\beta\) is influenced by the number of free hydroxy substituted groups on the flavonoids (hydroxy groups on A and B rings tend to form hydrogen bonds with the active site of the enzyme) [52; 66].  
The GSK-3\(\beta\) inhibitory activity of apigenin (**29**) and luteolin (**34**) is characterized by IC\({}_{50}\) values of 1.91 and 1.51 \(\upmu\)M, respectively, suggesting the importance of the hydroxy groups at C4\({}^{\prime}\), C5 and C7 [66].  
C-glycosIfavones at C6, such as isoorientin (**47**) and isovitexin (**44**) are weak inhibitors of GSK-3\(\beta\), having IC\({}_{50}\) values of 185.00 and 195.00 \(\upmu\)M, respectively [80]. Orientin (**46**), the C-glycosIfavone at C8, is inactive against the enzyme (IC\({}_{50}>\) 5000 \(\upmu\)M). These results show that side bulky group substitutions on the flavonoid scaffold affect the GSK-3\(\beta\) activity in a significant manner, decreasing the interactions between the enzyme and inhibitors [80].  
Flavonols  
Quercetin (**1**) presents high GSK-3\(\beta\) inhibition (IC\({}_{50}=\) 2.00 \(\upmu\)M) [52] and morin (**77**), structurally related to **1**, is also very active against GSK-3 [53] showing how important is the presence of the two hydroxy groups on the ring B. Kaempfer (**76**), with only one hydroxy group on ring B at C4\({}^{\prime}\), exhibits an IC\({}_{50}\) of 3.47 \(\upmu\)M [66], suggesting that one of the two hydroxy groups on ring B should be on C4\({}^{\prime}\).  
Also, it can be concluded that flavones, such as luteolin (**34**) and apigenin (**29**), are more potent compared to flavan-4-ones such as heesperetin (**84**) and naringenin (**6**) [52; 66].