1,2,3-Triazole-Containing Compounds as Anti–Lung Cancer Agents: Current Developments, Mechanisms of Action, and Structure–Activity Relationship

Lung cancer is the most common malignancy and leads to around one-quarter of all cancer deaths. Great advances have been achieved in the treatment of lung cancer with novel anticancer agents and improved technology. However, morbidity and mortality rates remain extremely high, calling for an urgent need to develop novel anti–lung cancer agents. 1,2,3-Triazole could be readily interact with diverse enzymes and receptors in organisms through weak interaction. 1,2,3-Triazole can not only be acted as a linker to tether different pharmacophores but also serve as a pharmacophore. This review aims to summarize the recent advances in 1,2,3-triazole–containing compounds with anti–lung cancer potential, and their structure–activity relationship (SAR) together with mechanisms of action is also discussed to pave the way for the further rational development of novel anti–lung cancer candidates.


INTRODUCTION
Lung cancer has high morbidity and represents the leading cause of cancer-related deaths (around 20% of all mortalities in cancer) (Nasim et al., 2019;Willis et al., 2019;Bade and Dela Cruz, 2020;Majem et al., 2020;Salehi et al., 2020;. With the development of novel anticancer agents and improvement of technology, lung cancer treatment has achieved great progress in recent decades, but the morbidity and mortality rates remain high and the overall 5-year survival rate is only around 15% (Gray et al., 2019;Coakley and Popat, 2020).
In recent years, a variety of 1,2,3-triazole-containing compounds are designed, synthesized, and evaluated for their anti-lung cancer activity, and some of them have the potential to act on dual-/multi-targets simultaneously (Jain and Piplani, 2019;Farrer and Griffith, 2020;Sahu et al., 2020). This review summarizes the current developments, mechanisms of action, and structure-activity relationships (SARs) of 1,2,3-triazole-containing compounds with anti-lung cancer potential to pave the way for further rational development.
The antiproliferative SAR of 1,2,3-triazole-tethered chalconematrine hybrids 12a-c (IC 50 : 5.01-12.72 μm, MTT assay) against A549 cells reveals that the electron-withdrawing group at the R position could boost up the activity, and the representative compound 12a (IC 50 : 5.01-7.31 μm) not only exhibits the highest activity against the four tested cancer cell lines but also displays low cytotoxicity (IC 50 : 39.21 μm) toward normal NIH/3T3 cells . A mechanistic study reveals that compound 12a could induce the apoptosis of A549 cells concentration-dependently in the A549-xenografted nude mouse model, and it also (10 mg/kg, tail vein injection) shows 85.4% tumor growth inhibition (TGI) after treatment for 16 days without causing obvious toxicities. Accordingly, compound 12a could be considered as a promising candidate for the chemotherapy of lung cancer.

1,2,3-TRIAZOLE-CONTAINING INDOLE DERIVATIVES
Indoles, which could inhibit various enzymes and receptors, such as HDAC, proviral insertion site in moloney murine leukemia virus (Pim), and tubulin in cancer cells, are scaffolds to avail for developing novel anticancer agents (Guo and Diao, 2020;Xu and Xu, 2020). Thus, combination of 1,2,3-triazole with indole represents a promising strategy to develop novel anticancer candidates that are effective against drug-sensitive and drugresistant cancers.
The majority of spirooxindole-derived morpholine-fused 1,2,3-triazoles 16 (IC 50 : 1.87-23.44 μm, MTT assay) are active against A549 cells, and an SAR study implied that the para bromobenzyl group is more favorable than the alkyl group at the N-1 position of oxindole moiety (Senwar et al., 2015). In particular, the activity of compound 16a (IC 50 : 1.87 μm) is comparable to that of doxorubicin (IC 50 : 1.98 μm) but 3.8-fold superior to that of 5-fluorouracil (IC 50 : 7.24 μm). Further investigations demonstrated that compound 16a could arrest the A549 cells at the G2/M phase of the cell cycle and induce apoptosis in A549 cells through collapse of the mitochondrial membrane potential as well as elevation of intracellular ROS levels. Overall, compound 16a has the potential to act as a lead compound for the development of novel anticancer agents against lung adenocarcinoma A549 cancer cells.
An antiproliferative SAR study of 1,2,3-triazole-containing isatin compounds 17 (IC 50 : 0.99-27.33 μm, MTT assay) against NCI-H23 lung cancer cells disclosed that substituents at the C-3 and C-5 positions have great influence on the activity . Introduction of oxime to the C-3 position could increase the activity, and the relative contribution order is methoxime > oxime > ketone. Electron-donating methoxy and halogen atoms at the C-5 position are preferred, whereas movement to the C-7 position causes great loss of activity. In particular, the most active compound 17a (IC 50 : 0.65 μm, MTT assay) is around 2 and 13 times superior to doxorubicin (IC 50 : 1.12 μm) and 5fluorouracil (IC 50 : 8.84 μm), respectively. Some other 1,2,3triazole-containing isatin compounds also hold certain activity against A549 cells, and the representative compounds 18a,b (IC 50 : 14.7 and 9.6 μm, MTT assay) demonstrate the highest activity Fan et al., 2018;. Mechanistic studies indicate that hybrid 18a could induce cell apoptosis, cause cell cycle arrest at the G2/M phase, and lead to collapse of the mitochondrial membrane potential . etoposide and teniposide, have already been approved for cancer therapy, revealing that podophyllotoxin/epipodophyllotoxin derivatives are promising as novel anticancer agents Routh and Nandagopal, 2017).

1,2,3-TRIAZOLE-CONTAINING PYRIDINE DERIVATIVES
Pyridines are potential inhibitors of CDK, EGFR, PI3K, and RGGT, and some pyridine-based agents, such as masitinib, have already been applied in clinical practice or under clinical trials for the treatment of cancers (Goel et al., 2016;Prachayasittikul et al., 2017). Thus, combination of 1,2,3triazole with pyridine may provide opportunities for the development of novel anticancer agents.
An antiproliferative SAR study of 1,2,3-triazole-containing pyridine derivatives 38 against A549 cells demonstrates that the methyl group on the 1,2,3-triazole motif and the fluoro on the phenyl ring are advantageous to the activity (Xiong et al., 2020). The morpholino group is essential for high activity, while replacement by pyrrolidinyl, thienyl, and alkyl groups leads to great loss of activity. In particular, compounds 38a-c (IC 50 : 3.22-6.43 μm, MTT assay) are more potent than golvatoinib (IC 50 : 8.14 μm), and the most active compound 38b could block cells in the G0/G1 phase. 1,2,3-Triazole-containing isoxazole-thiazole-pyridine hybrids 39 (IC 50 : 0.01-10.22 μm, MTT assay) has promising activity against A549 cells, and an SAR study implies that electron-withdrawing groups, especially the nitro group on the phenyl ring, could greatly increase the activity (Yakantham et al., 2019). The most active hybrid 39b (IC 50 : 0.01 μm) is 308-fold more potent than etoposide (IC 50 : 3.08 μm) against A549 cells, so it could serve as a lead compound for further development of novel anti-lung cancer candidates.
1,2,3-Triazole-containing imidazopyridines 42 (IC 50 : 0.51-47.92 μm, MTT assay) show considerable activity against A549 cells, and an SAR study illustrated that a substitution at the R 2 position, regardless if the substituent is an electron-donating or electron-withdrawing group, could reduce the activity as compared with hydrogen (Sayeed et al., 2018). In particular, compounds 42a,b (IC 50 : 0.51 and 0.63 μm) are more potent than nocodazole (IC 50 : 1.47 μm) against A549 cells. Flow cytometry reveals that these compounds result in A549 cell cycle arrest at the G2/M phase, and further studies indicates that these compounds could inhibit tubulin and induce cell death by apoptosis.
1,2,3-Triazole-containing asiatic acid derivatives 61 (IC 50 : 2.67-39.87 μm, MTT assay) show promising activity against A549, NCI-H460, and NCI-H460/DOX lung cancer cell lines, and an SAR study reveals that the electron-withdrawing group at the ortho position of the phenyl ring is favorable to the activity . The representative compound 61a (IC 50 : 2.67-4.84 μm) is the most active against the three tested lung cancer cell lines, and the RI value is 1.08, suggesting its potential to fight against drug-resistant lung cancer. Moreover, compound 61a (IC 50 : > 50 μm) is nontoxic toward normal HL-7702 cells, and the SI values are > 10.3. Mechanistic studies illustrate that compound 61a is a potential NF-κB inhibitor and could induce apoptosis and suppress cell migration. Accordingly, rational design of 1,2,3-triazole-containing asiatic acid derivatives may offer a new class of NF-κB inhibitors with the ability to suppress cancer cell migration and induce apoptosis.

MTT
assay) show broad-spectrum antiproliferative activity, and compound 62a (IC 50 : 3.53 μm) is comparable to celastrol (IC 50 : 3.02 μm) against A549 cells . An SAR study demonstrates that the 1,2,3-triazole moiety is not essential for the activity and that the removal of this moiety exerts minimal influence on the activity.
1,2,3-Triazole-containing rapamycin derivatives (IC 50 : 12.4-17.9 μm, MTT assay) show higher activity than the parent rapamycin (IC 50 : 18.1 μm) against A549 cells, and compound 73 (Figure 12, IC 50 : 12.8 μm) could induce apoptosis and cell cycle arrest in A549 cells . Moreover, compound 73 inhibits the phosphorylation of mTOR and its downstream key kinases 4EBP1 and p70S6K1 in A549 cells, revealing that this compound could also display effective inhibitory effect on the mTORC1 signaling pathway as rapamycin. Accordingly, compound 73 has the potential to be developed as a new mTOR inhibitor against lung cancers.
1,2,3-Triazole-containing bergenin derivatives 74 (IC 50 : 1.86-23.05 μm, MTT assay) are endowed with considerable activity against A549 cells, and an SAR study demonstrates that the trifluoromethyl group on the phenyl ring is beneficial for the activity . In particular, compound 74a (IC 50 : 1.86 μm, MTT assay), which is comparable to doxorubicin (IC 50 : 1.98 μm), demonstrates potent activity. Cell cycle analysis show that compound 74a could induce G2/M phase arrest and lead to the accumulation of cyclin B1 protein. Cell-based tubulin polymerization assays and docking studies implies that compound 74a disrupts tubulin assembly by occupying the colchicine binding pocket of tubulin.

CONCLUSIONS
Lung cancer, as a cancer with the highest morbidity, is the leading cause of cancer-related deaths and has already posed heavy burden on the world health system, which makes an urgent need to develop novel anti-lung cancer agents. 1,2,3-Triazole could readily interact with diverse enzymes and receptors in organisms through weak interaction and has been considered as a privileged structure in medicinal chemistry. 1,2,3-Triazole-containing agent CAI synergizes with sorafenib to combat NSCLC through the inhibition of NANOG and aggravation of apoptosis, indicating that 1,2,3-triazole-containing derivatives are useful scaffolds to develop novel anti-lung cancer agents.

AUTHOR CONTRIBUTIONS
TL, FG, and GH wrote section 1-6. WL and XS wrote section 7-11. FG and GH collected the whole manuscript. TL and XS corrected the manuscript. All authors read and approved the final manuscript.