The present invention is directed to a method of quantifying intracellular metabolite effluxes in permeabilized cancer cells for selecting cancer patients responsive for a cancer treatment, the method comprising the steps of: a) providing a sample of cancer cells taken from a patient; b) permeabilizing said cancer cells; c) incubating said permeabilized cancer cells in a reaction medium for a period of time allowing biological activity of intracellular organelles and accumulation of metabolites produced by said activity into the reaction medium in the presence of a substrate or substrates relating to a metabolite efflux or effluxes of interest, wherein said substrates used are at least glutamine and pyruvate; d) determining the quantity of metabolites relating to said metabolite efflux or effluxes of interest accumulated in the reaction medium during step c); and e) comparing the amounts of metabolites determined in step d) to equal measurements performed on control samples of the same tissue type and assessing the aggressiveness of the cancer cells or the treatment response of the cancer cells to a drug affecting a metabolic pathway or pathways relating to said metabolite efflux or effluxes of interest.