Diterpenoids from Andrographis paniculata as natural chemosensitizers

Abstract

Extracts of the medicinal plant Andrographis paniculata Nees (Acanthaceae) are described in literature as showing anticancer properties in leukaemic cell lines [1,2]. The aim of this study was to isolate the main constituents of a commercially available phytotherapeutic preparation of A. paniculata and to determine their chemosensitizing potential using the Nicoletti assay [3]. Chromatographic separation steps resulted in the isolation of the diterpenes andrographolide (1), 14-deoxy-11,12-didehydroandrographolide (2) and the diterpene glucoside neoandrographolide (3). Whereas the individual effects of suboptimal concentrations of the chemotherapeutic etoposide (500 nM) and 20.8µM of 3 showed only weak effects in S-Jurkat cells (15% and 8% apoptotic cells [AC], respectively), their combination strongly induced cell death (64% AC). In contrast, 1 and 2 showed no increase of AC. In order to specify the chemosensitizing effect, we tested the compounds also in X-linked inhibitor of apoptosis proteins (XIAP)-overexpressing Jurkat cells. XIAP overexpression protects Jurkat cells from etoposide-induced apoptosis. Although the combination of etoposide and 2 showed no synergism in S-Jurkat cells, an increased percentage of AC was observed in XIAP-overexpressing cells. For 3 (20.8µM, 3% AC), the chemosensitizing effect could be confirmed (37% AC). We enzymatically cleaved the glucose-moiety of 3 obtaining the diterpene andrograpanin (4). When used in combination with etoposide, a distinct loss of activity was observed, which indicates a major impact of the sugar-moiety on the bioactivity. As expected from a XIAP inhibitor, we found that 3 potentiates the caspase-3 like activity. In conclusion, this study enriches the pharmacological profile of the medicinal plant A. paniculata and elucidates compound 3 as potent, naturally derived small-molecule chemosensitizer in a leukaemic cell line.