Exploiting Synergistic and Positive Interactions Between Plant Extracts Towards Rational Antimalarial Phytotherapeutic and Evidenced-Based Drug Discovery

Abstract

Ethnopharmacological evidence of the folk use of Nefang, a polyherbal product composed of Mangifera indica (bark and leaf), Psidium guajava, Carica papaya, Cymbopogon citratus, Citrus sinensis, Ocimum gratissimum (leaves), for the treatment of malaria has been reported previously [1]. In vitro antiplasmodial activities of the constituent plant extracts on both 3D7 (CQ-sensitive) and Dd2 (multi-drug resistant) strains of P. falciparum were analyzed using the SYBR Green 1-based fluorescence assay [2, 3], and the interactions between various extract pairs analyzed using a fixed-ratio drug combination approach [4]. Effective concentration 50 (EC50) values were determined by nonlinear regression curve-fitting using GraphPad prism 6.0, and used to calculate the Fractional Inhibitory Concentration 50 (FICA= EC50A in combination/EC50A) and combination indices (CI) (CIA/B= FICA+FICB) for each pair. The derived EC50 values were as follows (3D7/Dd2): MiB-65.33/34.58, MiL-82.56/40.04, Pg-47.02/25.79, Cp-1188/317.5, Cc-723.3/141, Cs-184.4/105.1, Og-778.5/118.9, Nefang-96.96/55.08 (µg/mL), and the best extract pairs gave EC50 ratios (and CI) of: MiB/Pg-6.07/4.53 (CI = 0.351), MiL/Pg-7.18/4.62 (0.358), MiB/Cs-6.34/19.25 (0.366), MiL/Cs-9.63/25.27 (0.482), Pg/Cs-6.22/25.36 (0.483), Cs/Og-21.78/24.64 (0.414) when combined at equipotency ratios, indicating apparent synergism and/or additive interactions between the different extracts[5]. Taken together, this study confirms the antimalarial activities of Nefang and has identified extract pairs with possible positive interactions for exploitation towards a rational, evidence-based and phytotherapeutic drug discovery in malaria. Acknowledgements: Thanks go to Institut Pasteur, Korea for the internship/training course in Biosafety and Drug Discovery.