Carina Zhou, Sanghyuk Ko, Tiffany Gu, & Dr. Edward Njoo
Natural products and their analogs have long served as inspiration for the exploration and development of small molecules with therapeutic significance. One such compound is andrographolide, a labdane diterpenoid extracted from the plant Andrographis paniculata, which has been extensively studied as an anti-cancer therapeutic. It is known to function putatively through covalent inhibition of Nf-kB, a transcription factor that modulates tumor survival and metastasis. Remarkably, functionalization of the C-19 hydroxyl and C-17 alkene putatively alters the primary mode of action from inhibition of Nf-kB to the modulation of the Wnt/𝜷-catenin signaling pathway. To interrogate the structure-activity relationship of these two positions, we synthesized a library of andrographolide analogs by protecting the C-19 hydroxyl with large, hydrophobic silyl and trityl ethers and epoxidizing C-17. We also optimized the facio-selective installation of the epoxide on the C-8/17 exo-olefin. MTT assays revealed that all of our C-19 analogs in HCT-116 colorectal cancer cells showed greater anti-proliferation compared to that of andrographolide but had relatively similar IC50 values in MDA-MB-231 metastatic breast cancer cells. Additionally, previous studies show that the installation of a TBDPS protecting group at C-19 and epoxidation of C-17 led to modulation of the Wnt/𝜷-catenin signaling pathway through GSK3𝜷 inhibition. To further investigate the mechanism of action of these analogs, we performed western blots to assess the relative levels of active 𝜷-catenin compared to phosphorylated 𝜷-catenin. Finally, we describe progress towards the first chemical synthesis of an andrographolide A-ring oxetane analog, which is a co-isolated naturally-occurring metabolite of Andrographis paniculata, whose biological activity and synthetic preparation have not been previously described.