利用位向選擇保護策略合成Dioscin類似物作為有潛力之抗癌藥物

Abstract

皂素為天然存在之配醣體，具有相當多元化之結構以及生物活性，其結構上主要分成三萜類和類固醇類兩大類。類固醇類皂素通常具有紅血球細胞溶血作用及抑制腫瘤生長的活性，其活性需要醣基部份之結構，但是其醣基在分子層面的角色仍未明。皂素由於結構上的複雜和純物質取得不易，發展一個實際可行的方法進一步針對醣基部分的修飾將可對皂素結構與活性的關係進一步了解，將有機會發現新的治療物質。 Dioscin (diosgeninyl 3b-O-(2,4-di-O-a-L-rhamnopyranosyl)-b-D-glucoside)屬於固醇類之spirostan皂素，為最常見於薯蕷科植物塊莖的一種皂素，具有抗癌活性。為了探討結構與活性的關係，我們設計和用保護基策略合成一系列的衍生物，以葡萄糖為dioscin的結構核心，並利用位向選擇保護策略得到3,6-di-O-benzyl-4-O-levulinoyl-2-O-((2-nitro)phenylacetyl)-D-glucopyranosyl trichloroacetimidate (141)，與6a-O-acteyl-chlorogenin (145)醣化後獲得化合物 164，之後藉由選擇性去保護、與不同的醣基提供者醣化和全面的去保護得到dioscin分子資料庫。 在生物活性探討方面，我們利用細胞存活率測定其對前列腺癌細胞PC-3造成之影響，結果顯示給予10 uM劑量，所有化合物對於PC-3細胞株皆無生長抑制作用，而2'-OH接上L-fucosyl (97)或L-rhamnosyl (104)為此一系列衍生物中具有最佳之活性，但其GI50皆大於30 uM，整體而言，此dioscin衍生物對於前列腺癌細胞並無抗癌活性，未來將會針對其他癌細胞株進行測試。

Saponin, a naturally occurring glycoside with diversities of structures and bioactivities, can be structurally divided into 2 major types-triterpenoidal and steroidal glycosides. The common features of steroidal spirostan saponins are hemolytic and cytotoxic where the sugar units are often required for function in vivo, but their roles at the molecular level are not well-understood. Due to the complexity of structures and limited availability of homogeneous saponins, development of practical methods for modification of the carbohydrate domain of natural products may lead to a better understanding of the structure activity relationships (SAR) of saponins and have better chance to identify new therapeutics. Dioscin (diosgeninyl 3b-O-(2,4-di-O-a-L-rhamnopyranosyl-b-D-glucoside)), one of the most common steroidal spirostane saponins found in rhizome of Dioscoreaceae, exhibits potent antitumor activities. Thus, in order to study the SAR of dioscin, a series of analogues were designed and synthesized by the protection strategy. Glucose, as a common core structure of dioscin, was chosen and orthogonal protected to 3,6-di-O-benzyl-4-O-levulinoyl-2-O-(2-nitrophenylacetyl)-D-glucopyranosyl trichloroacetimidate 141. Compound 141 was then subjected to glycosylate with 6b-O-acteyl-chlorogenin 145 to give compound 164 which was selectively deprotected, glycosylated with various glycosyl donors, and global deprotected to obtain a library of dioscin analogues. The effect of these analogues on human prostate carcinoma (PC-3) was studied via SRB assay, and the results showed that the compound had no inhibition of growth at 10 uM level. Only 97 and 104 with 2'-O- L-fucosyl and L-rhamnosyl, respectively, displayed weak cytotoxicities at a dose of 30 uM (still with GI50 ＞30 uM). Current study demonstrated that these dioscin analogues were not cytotoxic toward PC-3. Further studies of these derivatives for other cancer cell lines will be performed.