Prostate cancer is the most common cancer and the second leading cause of cancer mortality in men. An estimated 317,000 cases of prostate cancer were diagnosed, and 41,000 were expected to be fatal in the U.S. in 1996. Many cytotoxic chemotherapeutic agents have proven ineffective against metastatic prostate cancer, with only estramustine, suramin, and mitoxanthrone showing low levels of efficacy. During the course of searching for anti-cancer agents from traditional herbal medicine, we isolated alpha-Boswellic acid acetate from Olibanum (Boswellia carterii Birdw), a folk medicine used in the treatment of inflammatory diseases in China and the Middle East without overt toxicity. Alpha-Boswellic acid acetate shows excellent inhibitory activity against the DNA topoisomerase I enzyme: It is 3 times more potent than the standard, camptothecin, in the topoisomerase I relaxation assay. Because the levels of topoisomerase I are significantly higher in human cancerous prostate tissues than in normal tissues, and appear to remain constantly high throughout cell growth, compounds targeted at this enzyme could have the advantages of selectivity and specificity. The effectiveness of alpha-Boswellic acid acetate (as NSC624807) against cancer were evaluated several times by NCI using an in vitro assay system consisting of 57 human tumor cell lines. The compound was effective against several prostate cancer cell lines (androgen-independent PC-3: GI50=3.77x10-7M; DU-145: GI50=3.97x10-7 M) and cell strains derived from fresh surgical specimens of prostate tumor (GI50=3.9x10-7M). In view of its favorable toxicity profile (LD50 greater than 2.0 g/kg) and encouraging preliminary clinical efficacy in brain tumor, we propose to study the structural requirements of this unique natural product, which has a pentacyclic ring system, for topoisomerase I inhibition, and to evaluate its synthetic analogs for improved physical and biological activity.
The specific aims are (1) to isolate a quantity of alpha-Boswell acid and its acetate from Boswellic carterii Birdw by high speed countercurrent chromatography, (2) to establish structure-activity relationship by chemical modifications of the alpha-Boswellic acid molecule, and (3) to evaluate the synthetic analogs in vitro in the topoisomerase I assay and against three prostate tumor cell lines: androgen-independent PC-3 and DU-145, and androgen-dependent LNCaP. These biochemical studies are critical for assessing (1) what structural features are essential for activity, (2) whether these alpha-Boswellic acid acetate analogs act as topoisomerase I inhibitors, and (3) whether we have any new alpha-Boswellic acid acetate analogs with enhanced activity against prostate cancer cells. We believe that these studies are important in providing a better understanding of alpha-Boswellic acid acetate and in identifying the most promising topo-I inhibitors for in vivo experiments and for further development as non-hormonal chemotherapy for prostate cancer.
