ISOLATION OF DIOSGENIN FROM Smilax kraussiana MEISN EX. KRAUSS AND SYNTHESIS OF ITS DERIVATIVES AS ANTICANCER AGENTS

Abstract

Plants remain a major source of novel drugs for the treatment of various diseases like cancer. The search for safe anticancer drugs from plants has led to the discovery of camptothecin and taxol. Smilax kraussiana is used in herbal medicine to treat tumors, veneral and skin diseases. Many plants including S. kraussiana are still underexploited despite their ethnomedicinal properties. This study was designed to isolate and characterise the constituents of S. kraussiana, synthesise active derivatives from the most bioactive isolate and evaluate the anticancer activities of the extracts and compounds.

The leaves and stems of S. kraussiana were collected from Onigambari Forest Reserve, Ibadan and authenticated at Forestry Research Institute of Nigeria (Voucher number: FHI 108799), Ibadan, Oyo State. The samples were air-dried, pulverized and successively extracted with hexane, ethyl acetate and methanol. The extracts were subjected to chromatographic techniques to obtain pure isolates. Structural elucidation of the isolated compounds was done using 1D, 2D Nuclear Magnetic Resonance and Mass Spectroscopic methods. Isolated diosgenin was synthetically modified by oxidation, reduction, and condensation reactions. The extracts, isolates and synthesised compounds were evaluated for anticancer activities against four human cancer cell lines; leukaemic (K-562), hepatic (WRL), breast (MCF-7) and colorectal carcinoma (COLO) at 20, 50 and 100 µM concentrations using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and Sorphordamine assays. Tamoxifen was used as positive control for both assays. Data were analysed using descriptive statistics.

The successive extraction of S. kraussiana with hexane, ethyl acetate and methanol yielded 14.0, 20.0 and 11.5 g of the extracts respectively. Chromatographic separation of hexane, ethyl acetate and methanol extracts resulted in four, nine and six compounds respectively. The isolates are mainly triterpenoids, steroids and fatty acids, while the most abundant isolate was diosgenin (3β-hydroxy-5-spirostene) (80.0 mg = 0.5 %). The modification of diosgenin via two synthetic reaction schemes yielded fifteen and twelve compounds respectively. The new analogues obtained from the first scheme included (22β)-25-oxo-27- nor-furost-5-en-3β-acetate, (22β)-25-hydroxy-3β-yl-27-nor-furost-5-en-3β-acetate, (22β)- (Z)-26-(4′-nitrobenzylidene)-3β-yl-furost-5-en-3β-acetate, (22β)-26-(3′,4′,5′-

ii

trimethoxybenzylidene)-3β-yl-furost-5-en-3β-acetate, 3β-acetoxy-furost-5-en-26-aldoxime and 3β-acetoxy-27-nor-furost-5-en-25-ketoxime. The new compounds from the second scheme included (22β,25R)-3β-acetoxy-spirost-5-en-7-one, (22β,25R)-3β-acetoxy-spirost- 5-en-7-ketoxime, (22β,25R)-spirost-3,5-dien-7-one, (22β,25R)-3β-acetoxy-7-(4′- nitrobenzylidene)-spirost-5-en-3β-yl, (22β,25R)-3β-acetoxy-spirost-5-en-3β-yl-7-(ethyl-3′- propanoate)-ketoxime and (22β,25R)-3β-acetoxy-spirost-5-en-3β-yl-7-(ethyl-4′-butyrate)- ketoxime. Nineteen out of the 27 synthesised compounds are reported for the first time. Ethyl acetate and methanol extracts of S. kraussiana exhibited cytotoxic activity against WRL and COLO cell lines with IC50 of 46.1 and 90.0 µM, but showed low inhibition on K-562 and MCF-7 with IC50 of 113.0 and 236.0 µM respectively. Hexane extract exhibited low activity against the four cell lines with IC50 between 130.0 and 310.0 µM. The most bioactive isolate, diosgenin showed cytotoxic activity against the four cell lines by suppressing the viability of cells with IC50 between 12.3 and 38.0 µM, while active synthesised compounds inhibited the growth of the four cell lines with IC50 between 7.5 and 35.5 µM.

The phytochemical constituents of Smilax kraussiana extracts justify their use in herbal medicine. The isolated compounds are reported for the first time from the plant. The synthesised diosgenin derivatives could serve as lead compounds for further investigation as anticancer agents.