DOI:
10.1039/C4RA15414B
(Paper)
RSC Adv., 2015,
5, 19620-19623
Design, synthesis and biological evaluation of C(6)-indole celastrol derivatives as potential antitumor agents†
Received
28th November 2014
, Accepted 11th February 2015
First published on 12th February 2015
Abstract
Celastrol is a natural triterpenoid which possesses diverse pharmacological activities including potent antitumor activity. Previously, we reported a C(6)-modified celastrol derivative NST001A which possesses cytotoxic activity against Colon 205 cells with a 60 nM IC50 value. To further explore the structure activity relationships, a new class of C(6)-indole substituted celastrol derivatives were designed and synthesized. Biological evaluation of these compounds includes their cytotoxic activity against human hepatocellular carcinoma Bel7402 and human glioblastoma cell line H4. Among all these semisynthetic analogues, compound 4f and 4h displayed excellent in vitro antiproliferative activities against Bel7402 cancer cells (IC50 = 0.02 μM and 0.01 μM, respectively).
Introduction
Triterpenoids are members of a larger family of structurally related compounds known as cyclosqualenoids that are widely distributed in nature.1 Celastrol (Fig. 1) is a quinone methide triterpenoid extracted from the root bark of the traditional Chinese medicine “Thunder of God Vine” (Tripterygium wilfordii Hook F.). It was reported that celastrol has diverse biological activities, such as antitumor,2–4 antioxidant,5–7 anti-inflammatory,5,6,8–10 antifungal,11,12 immunomodulatory,13 protective and antiangiogenic effects.13–15 Celastrol has promising clinical potential as both a therapeutic and chemopreventive agent for various cancers, including gliomas, hepatocellular carcinomas, prostate cancer, renal cell carcinoma, head and neck carcinoma, non-small cell lung carcinoma.16,17 The antitumor activity of celastrol may result from targeting multiple signaling pathways such as NF-Kb,18 Notch,19 HSP90,20 Cdc37, p23, IKKβ, proteasome, Bcr-Abl, and p-Akt,3 which suggests its potential use as an antitumor agent and has attracted many oncologists into the exploration of novel celastrol derivatives.
 |
| Fig. 1 Structure of celastrol and NST001A. | |
Some celastrol analogues like pristimerin and dihydrocelastrol are reported to be inducers of the heat shock response and cytoprotective agents of lethal stress in Hela cells and SH-SY5Y neuronal cells.21 Current structure modifications of celastrol mainly focus on its 20-carboxylic acid group. Klaić22 and Sun23 disclosed that amidation and esterification of the carboxylic acid group of celastrol may influence its bioactivity and the amide linkage is more appropriated for tethering celastrol to a bioprobe. It was found that the carboxyl of celastrol was not required for its apoptotic activity, and the quinone methide moiety is not responsible for its chemical chaperone activity, but is crucial for its cytotoxic activity in cancer cells. In the studies conducted by Westerheide,21 dihydrocelastrol is active as a heat shock promoter. However, it is inactive in the cellular assays.24
Previously, we disclosed a series of C(6)-sulfonated celastrol derivatives as potent antitumor agents.25 Compared with the parent compound celastrol, NST001A displayed good cytotoxicity against Colon 205 cell line and can potently inhibit the growth of Colo 205 xenografts in nude mice with a better safety profile. This study indicated that the C(6)-position of celastrol was suitable for modification.
Indole has a wide range of biological activities and many antitumor agents such as sunitinib,26 SU5416,27 SU6668,28 vinblastine,29 vincristine30 (Fig. 2) share the indole moiety.31 In order to further explore the SAR of C6 position, we designed and synthesized a new series of C(6)-indole substituted celastrol analogues. Biological evaluation of these compounds includes their antiproliferative activity against human hepatocellular carcinoma Bel7402 and human glioblastoma cell line H4.
 |
| Fig. 2 Chemical structures of indole derivatives. | |
Results and discussion
Chemistry
The general route used for the synthesis of the celastrol derivatives is depicted in Scheme 1. Novel C(6)-indole substituted celastrol analogues 3a–3j were synthesized by Friedel–Crafts reaction in good yields (43–88%).32,33 Subsequently, the phenolic hydroxyl group along with the carboxylic acid group were methylated in the presence of potassium carbonate and methyl iodide, and afforded celastrol analogues 4a–4g and 4i. Compound 4h was the only N-substituted indole derivative under the same condition.
 |
| Scheme 1 Synthesis of C(6)-indole analogues of celastrol. Reagents and conditions: (a) scandium(III) triflate, DCM and (b) MeI, K2CO3, DMF. | |
Biological assay
In vitro cell growth inhibition. All synthesized celastrol analogues, along with celastrol as positive control were tested for their cytotoxic activity against human hepatocellular carcinoma Bel7402 and human glioblastoma cell line H4.As shown in Table 1, celastrol exhibited potent anti-tumor effect with IC50 values in the low micromolar range. Compared with the positive control compound, C(6)-indole substituted celastrol 3a showed poor cytotoxicity. When methyl group was introduced to 7-position of indole, the antiproliferative activity against Bel7402 was increased (3j vs. 3a). The 6-substituted indole analogues showed better cytotoxicity than 5-substituted ones (3f vs. 3b, 3g vs. 3d and 3i vs. 3e). In this series, compound 3f and 3i displayed the best in vitro anti-cancer activity (IC50 = 4.52–8.85 μM), which is comparable with celastrol.
Table 1 In vitro cell growth inhibitiona

|
Compounds |
R |
IC50 (μM) H4 |
IC50 (μM) Bel7402 |
Each value was reproduced in three experiments. |
Celastrol |
— |
2.09 |
1.55 |
3a |
H |
56.22 |
>100 |
3b |
5-Me |
67.00 |
55.24 |
3c |
5-OMe |
24.39 |
45.42 |
3d |
5 F |
31.75 |
34.30 |
3e |
5-Br |
69.76 |
18.91 |
3f |
6-Me |
6.66 |
5.52 |
3g |
6-F |
60.90 |
11.88 |
3h |
6-Cl |
7.83 |
6.53 |
3i |
6-Br |
8.85 |
4.52 |
3j |
7-Me |
80.48 |
34.19 |
Overall, another series of celastrol derivatives showed better anticancer activity (Table 2). Compared with the positive control compound celastrol, C(6)-indole substituted analogue 4a showed moderate cytotoxicity. The cytotoxicity was maintained when 5-substituted indole was introduced into the C6 position of celastrol except compound 4b. On the other hand, 6-substituted indole derivatives showed entirely different antiproliferative activity. Compound 4f showed excellent in vitro anticancer activity on Bel7402 cell line while 6-bromo substituted analogue 4i lost its cytotoxicity. Interestingly, the only N-substituted indole derivative 4h displayed the best antiproliferative activity in this series (IC50 = 2.03 μM and 0.01 μM, respectively).
Table 2 In vitro cell growth inhibitiona

|
Compounds |
R |
IC50 (μM) H4 |
IC50 (μM) Bel7402 |
Each value was reproduced in three experiments. |
Celastrol |
— |
2.09 |
1.55 |
4a |
H |
5.87 |
20.45 |
4b |
5-Me |
>100 |
>100 |
4c |
5-OMe |
4.59 |
0.51 |
4d |
5-F |
4.66 |
1.58 |
4e |
5-Br |
5.44 |
1.65 |
4f |
6-Me |
7.88 |
0.02 |
4g |
6-F |
14.56 |
23.91 |
4h |
6-Cl, N-Me |
2.03 |
0.01 |
4i |
6-Br |
>100 |
>100 |
Conclusions
In summary, a new series of semisynthetic C(6)-indole substituted celastrol derivatives were designed and synthesized. Part of these analogues showed similar in vitro anticancer activity to the parent compound celastrol. Notably, compound 4f and 4h showed excellent antiproliferative activity on Bel7402 cancer cell line. To the best of our knowledge, this work is the only example of celastrol derivatives bearing a C–C bond at the C-6 position with potent in vitro anti-cancer activity. The formation of C–C bond at this position results in the inactivation of Michael acceptor, which suggests the Michael acceptor is not necessary for its anti-tumor activity. The present encouraging results give a promising future optimization of the series.
Acknowledgements
This work was supported by the National Science and Technology Major Project (2011ZX11401-026) in China. We acknowledge Dr Li for cell growth inhibition test and Dr Shi for the 1H-NMR, 13C-NMR, IR, [α], m.p. and HRMS test.
Notes and references
- P. Dzubak, M. Hajduch, D. Vydra, A. Hustova, M. Kvasnica, D. Biedermann, L. Markova, M. Urban and J. Sarek, Nat. Prod. Rep., 2006, 23, 394–411 RSC.
- B. Aggarwal, S. Prasad, B. Sung, S. Krishnan and S. Guha, Curr. Colorectal Cancer Rep., 2013, 9, 37–56 CrossRef PubMed.
- R. Kannaiyan, M. K. Shanmugam and G. Sethi, Cancer Lett., 2011, 303, 9–20 CrossRef CAS PubMed.
- H. W. Lee, K. S. Jang and K. H. Chun, BMB Rep., 2014, 47, 697–702 CrossRef PubMed.
- A. C. Allison, R. Cacabelos, V. R. Lombardi, X. A. Alvarez and C. Vigo, Prog. Neuropsychopharmacol Biol. Psychiatry, 2001, 25, 1341–1357 CrossRef CAS PubMed.
- K. Faust, S. Gehrke, Y. Yang, L. Yang, M. F. Beal and B. Lu, BMC Neurosci., 2009, 10, 109 CrossRef PubMed.
- C. Wang, C. Shi, P. Yang, M. Yang, H. Sun and C. Wang, Eur. J. Pharmacol., 2014, 744, 52–58 CrossRef CAS PubMed.
- S. Boridy, G. M. Soliman and D. Maysinger, Nanomedicine, 2012, 7, 1149–1165 CrossRef CAS PubMed.
- A. Salminen, M. Lehtonen, T. Paimela and K. Kaarniranta, Biochem. Biophys. Res. Commun., 2010, 394, 439–442 CrossRef CAS PubMed.
- K. F. Wong, Y. Yuan and J. M. Luk, Clin. Exp. Pharmacol. Physiol., 2012, 39, 311–320 CrossRef CAS PubMed.
- D. Q. Luo, H. Wang, X. Tian, H. J. Shao and J. K. Liu, Pest Manage. Sci., 2005, 61, 85–90 CrossRef CAS PubMed.
- Y. X. Zhou and Y. L. Huang, Chin. Med. J., 2009, 122, 1666–1673 CAS.
- D. Y. Kim, J. W. Park, D. Jeoung and J. Y. Ro, Eur. J. Pharmacol., 2009, 612, 98–105 CrossRef CAS PubMed.
- H. Ni, W. Zhao, X. Kong, H. Li and J. Ouyang, Acta Haematol., 2014, 131, 102–111 CrossRef CAS PubMed.
- X. Pang, Z. Yi, J. Zhang, B. Lu, B. Sung, W. Qu, B. B. Aggarwal and M. Liu, Cancer Res., 2010, 70, 1951–1959 CrossRef CAS PubMed.
- P. Ge, X. Ji, Y. Ding, X. Wang, S. Fu, F. Meng, X. Jin, F. Ling and Y. Luo, Neurol. Res., 2010, 32, 94–100 CrossRef CAS PubMed.
- R. Kannaiyan, K. A. Manu, L. Chen, F. Li, P. Rajendran, A. Subramaniam, P. Lam, A. P. Kumar and G. Sethi, Apoptosis, 2011, 16, 1028–1041 CrossRef CAS PubMed.
- K. C. Chiang, K. H. Tsui, L. C. Chung, C. N. Yeh, W. T. Chen, P. L. Chang and H. H. Juang, PLoS One, 2014, 9, e93151 Search PubMed.
- X. N. Wang, Q. Wu, X. Yang, L. S. Zhang, Y. P. Wu and C. Lu, Chin. J. Cancer, 2010, 29, 385–390 CrossRef PubMed.
- L. M. Zanphorlin, F. R. Alves and C. H. Ramos, Biochim. Biophys. Acta, 2014, 1840, 3145–3152 CrossRef CAS PubMed.
- S. D. Westerheide, J. D. Bosman, B. N. Mbadugha, T. L. Kawahara, G. Matsumoto, S. Kim, W. Gu, J. P. Devlin, R. B. Silverman and R. I. Morimoto, J. Biol. Chem., 2004, 279, 56053–56060 CrossRef CAS PubMed.
- L. Klaic, R. I. Morimoto and R. B. Silverman, ACS Chem. Biol., 2012, 7, 928–937 CrossRef CAS PubMed.
- H. Sun, L. Xu, P. Yu, J. Jiang, G. Zhang and Y. Wang, Bioorg. Med. Chem. Lett., 2010, 20, 3844–3847 CrossRef CAS PubMed.
- S. Abbas, A. Bhoumik, R. Dahl, S. Vasile, S. Krajewski, N. D. Cosford and Z. A. Ronai, Clin. Cancer Res., 2007, 13, 6769–6778 CrossRef CAS PubMed.
- K. Tang, Q. Huang, J. Zeng, G. Wu, J. Huang, J. Pan and W. Lu, Molecules, 2014, 19, 10177–10188 CrossRef PubMed.
- L. Q. Chow and S. G. Eckhardt, J. Clin. Oncol., 2007, 25, 884–896 CrossRef CAS PubMed.
- T. A. T. Fong, L. K. Shawver, L. Sun, C. Tang, H. App, T. J. Powell, Y. H. Kim, R. Schreck, X. Wang and W. Risau, Cancer Res., 1999, 59, 99–106 CAS.
- A. D. Laird, P. Vajkoczy, L. K. Shawver, A. Thurnher, C. Liang, M. Mohammadi, J. Schlessinger, A. Ullrich, S. R. Hubbard and R. A. Blake, Cancer Res., 2000, 60, 4152–4160 CAS.
- L. H. Einhorn and J. Donohue, Ann. Intern. Med., 1977, 87, 293–298 CrossRef CAS PubMed.
- T. Tsuruo, H. Iida, S. Tsukagoshi and Y. Sakurai, Cancer Res., 1981, 41, 1967–1972 CAS.
- M. E. Welsch, S. A. Snyder and B. R. Stockwell, Curr. Opin. Chem. Biol., 2010, 14, 347–361 CrossRef CAS PubMed.
- D. A. Evans, K. A. Scheidt, K. R. Fandrick, H. W. Lam and J. Wu, J. Am. Chem. Soc., 2003, 125, 10780–10781 CrossRef CAS PubMed.
- G. Desimoni, G. Faita, M. Toscanini and M. Boiocchi, Chem.–Eur. J., 2008, 14, 3630–3636 CrossRef CAS PubMed.
Footnote |
† Electronic supplementary information (ESI) available: Details of experimental procedures, cytotoxicity assay, characterization data, 1H and 13C NMR spectra of new compounds. See DOI: 10.1039/c4ra15414b |
|
This journal is © The Royal Society of Chemistry 2015 |
Click here to see how this site uses Cookies. View our privacy policy here.