A method of obtaining one or more fractions from a plant material of Withania somnifera (WS) is disclosed. The method includes subjecting the plant material to hydro-alcoholic extraction in presence of a water-insoluble solvent to obtain at least one extract. The method further includes subjecting the at least one extract obtained from the hydro-alcoholic extraction to at least one of de-pigmentation, de-fatting and detoxification process to obtain the one or more fractions. The one or more fractions thus obtained contain Withaferin A in a concentration greater than concentrations of other pharmacologically active ingredients present in the one or more fractions. The one or more fraction thus obtained and one or more compositions containing the one or more fractions are effective in inhibiting proliferation of mammalian cancerous cells.
