Discovery of natural antiprotozoals from medicinal plants Saussurea costus and Carica papaya

Medicinal plants have been an important source for the discovery of therapeutic agents for infectious diseases. In order to explore their potential an appropriate selection of plant species is important. In our attempt to find hits targeting antiprotozoal diseases, we utilized an extract library setting, and ethnomedicinal information. A library of 1800 plant and fungal extracts was screened for in vitro effects against Trypanosoma brucei rhodesiense STIB 900 strain and Plasmodium falciparum K1 strain. The ethyl acetate extract of Saussurea costus roots, and the methanolic extract of Carica papaya leaves were selected for further studies. HPLC-based activity profiling enabled the localization and identification of the active constituents of these plants extracts. Sensitive hyphenated analytical methods such as HPLC-PDA-ESI-TOF-MS and microprobe NMR were used for structure elucidation of the isolated compounds. X-ray crystallography was used in combination with electronic circular dichroism to determine the absolute configuration of selected compounds.
The ethyl acetate extract of S. costus roots potently inhibited the growth of T. b. rhodesiense in vitro. HPLC-based activity profiling led to the identification of four sesquiterpene lactones. Three structurally related sesquiterpene lactones that originated from different sources were also investigated. All compounds exhibited profound activity against T. b. rhodesiense with IC50 values between 0.8 – 21.9 µM. Cytotoxicity was tested on rat myoblast L-6 cells, where IC50 values of 1.6 to 19.4 µM were observed, and provided selectivity indices (SI) between 0.5 and 6.5. The most active compounds in this study were the germacranolides costunolide, parthenolide, and eupatoriopicrin.
The leaves of the Indonesian ethnomedicinal plant C. papaya are a known antimalarial remedy. So far, the active principles have not been investigated from a phytochemical and pharmacological point of view. HPLC-based activity profiling of the methanolic extract from C. papaya leaves against P. falciparum led to the discovery of five alkaloids and four flavonol glycosides. All compounds exhibited in vitro antimalarial activity against P. falciparum K1 strain, albeit to varying degrees. Three dimeric alkaloids showed potent activity with IC50 values ranging from 0.2 to 1.8 µM, and SI from 24.2 to 107.5. The isolated flavonol glycosides were less active, with IC50 values between 13.2 – 16.8 µM, and selectivity indices of more than 9. Lower activity was observed for the two monomeric alkaloids (IC50 = 77 µM). Carpaine (IC50 of 0.2 µM; SI of 107.5) was the most interesting compound in this study and was, hence, selected for further evaluation of its in vivo pharmacological properties using a 4-day suppressive assay on mice. However, only a reduction of parasitemia by 11.9% was observed. With the aid of X-ray crystallography and ECD calculation, the absolute configuration for carpaine was established as 1S,11R,13S,14S,24R,26S. Carpaine represents a new scaffold for anti-plasmodial drugs. An analysis of carpaine content by means of UPLC-MS/MS was pursued with 28 leaf samples from Indonesia and one from India. The carpaine content varied from 0.02 to 0.31%.