# Carbazole Derivatives as STAT Inhibitors: An Overview
## 3 STATs Inhibitors
### Carbazoles as STAT Inhibitors
Furthermore, mahanine lowered, in a concentration-dependent way, other client proteins such as B-Raf, A-Raf, mutated p53, GSK3b, and PKCb at 24 h in MIAPCa-2 and AsPC-1 up to a dose of 30 \(\upmu\)M. Besides, the authors proved to decrease mahanine concentration (10-20 \(\upmu\)M), demonstrating its efficacy in MIAPCa-2 cells after 24 h of treatment. In fact, mahanine diminished the protein level of Akt and CDK-4 at 15 \(\upmu\)M, while B-Raf, STAT3 and Bcl-XL were conspicuously reduced at 17.5 \(\upmu\)M. As previously mentioned, the molecular chaperone Hsp90 has a crucial function in the survival of cancerous cells in which it is extensively expressed. As a result, this makes it a promising target for chemotherapeutic agents. Indeed, it mediates the folding, assembly, and maturation of many client proteins, among which are HER2, EGFR, PI3K, Akt, B-Raf, STAT3, GSK3b, Cdk4, mutated p53, and steroid receptors that participate in malignant cancer development [105]. In silico studies demonstrated the binding capability of mahanine to Hsp90 via noncovalent weak interactions, in a polar pocket, apart from the ATP-binding cavity. Mahanine constituted two hydrogen bonds with Hsp90, the first with the side chain oxygen of Glu47 through its NH group and the second with the side chain oxygen of Asn51 through its OH group. Moreover, van der Waals interactions with Arg46, Ile43, Gly132, Gln133, Met130, Ser129, Phe138, Ile131, and Gly137 were established. The most advantageous configuration of the ligand with the protein had a binding energy of \(-\)7.6 kcal mol\({}^{-1}\), with a micromolar binding affinity (K\({}_{\text{D}}\) = 3.16 \(\upmu\)M) [100]. In 2013, [PERSON] et al. [32], in their preliminary study, synthesized a series of \(N\)-alkylcarbazole derivatives to evaluate their potential STAT3 inhibitory activity. Their studies, indeed, highlighted that a crucial role in modulating the lipophilic properties of the carbazole was played by the substituent in N9. They analyzed a small series of carbazoles \(N\)-alkylated with C5, C6, and C7 alkyl chains [106]. Additionally, the alkyl chains were functionalized with dimethyl 5-hydroxyisophthalate (for derivatives **13**) or methyl salicate (for derivatives **14**) as substituents (Figure 9). Their findings highlighted the ability of compounds **13a-c**, among others, to inhibit STAT3 phosphorilation and its nuclear translocation in acute monocytic leukemia at 50 \(\upmu\)M with different potencies, amounting to 50%, 90%, and 95%, respectively. A crucial element for this effect seems to be the length of an alkyl linker inserted in these compounds. They obtained these outcomes by performing  
Figure 8: Structure of mahanine (**12**). EMSA and Western blot analysis in THP-1 cells treated with IL-6 (20 \(\upmu\)g/mL) for 15 min. Indeed, IL-6 augmented the STAT3 DNA binding activity as indicated by EMSA/supershift experiments with anti-STAT3 antibody. Furthermore, obtained data suggested a time- and dose-dependent effectiveness for **13a-c**. These compounds, however, could not block IFNy-induced STAT1 nor TNF-\(\alpha\) LPS-induced NF-kB activation. Phosphorylation on specific tyrosine residues and consecutive translocation into the nucleus are crucial to activate STATs. Their studies assessed that compounds **13a-c** lowered, at different levels, IL-6-induced tyrosine705 phosphorylation of cytosolic STAT3 without impacting the whole STAT3 protein. Concerning the structure-activity relationship, the insertion of an \(N\)-alkyl chain added an interaction point to the target protein. Slight changes of its terminal phenyl ring significantly modified the activity profile (compare **13a-c** with **14a-c**). Another important feature to improve the activity is the length of the alkyl linker as observed by comparing **13a** with **13b** and **13c**) [15; 32].