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Synthesis and Cytotoxicity Assessment of Novel 7-O- and 14-O-Derivatives of Glaucocalyxin A.

BACKGROUND
Rabdosia japonica has been historically used in China as a popular folk medicine for the treatment of cancer, hepatitis, and gastricism. Glaucocalyxin A (GLA), an ent-kaurene diterpene isolated from Rabdosia japonica, is one of the main active ingredients showing potent inhibitory effects against several types of tumor cells. To the best of our knowledge, studies regarding the structural modification and Structure-Activity Relations (SAR) of this compound have not yet been reported.


OBJECTIVE
The aim of this study is to discover more potent derivatives of GLA and investigate their SAR and cytotoxicity mechanisms.


METHODS
Novel 7-O- and 14-O-derivatives of GLA were synthesized by condensation of acids or acyl chloride. The anti-tumor activities of these derivatives against various human cancer cell lines were evaluated in vitro by MTT assays. Apoptosis assays of compound 17 (7,14-diacylation product) were performed on A549 and HL-60 cells by ﬂow cytometry and TUNNEL. The acute toxicity of this compound was tested on mice, at the dose of 300mg per kg body weight.


RESULTS
Seventeen novel 7-O- and 14-O-derivatives of GLA (1-17) were synthesized. These compounds show potent cytotoxicity against the tested cancer cell lines, and almost all of them were found to be more cytotoxic than GLA and oridonin. Of the synthesized derivatives, compound 17 presents the greatest cytotoxicity, with IC50 values of 0.26μM and 1.10μM in HL-60 and CCRF-CEM cells, respectively. Furthermore, this compound induces weak apoptosis of A549 cells but has great potential in stimulating the apoptosis of HL-60 cells. Acute toxicity assays indicate that compound 17 is relatively safe.


CONCLUSION
The results reported herein indicate that the synthesized GLA derivatives exhibit greater cytotoxicity against leukemia cells than against other types of tumors. In particular, the 7,14-diacylation product of GLA is an effective anti-tumor agent. However, the cytotoxicity mechanism of this product in A549 cells is expected to be different than that in other tumor cell lines. Further research is needed to confirm this hypothesis.