ABCG2 and breast cancer: Indeed, a higher expression of this efflux pump reduces the intracellular concentrations of topotecan, declining its efficacy against its target topoisomerase I. Moreover, our previous article by Di Desidero et al. established that the combination of topotecan plus pazopanib caused a decrease of ABCG2 expression in breast cancer cells with an increase of intracellular levels of topotecan lactone, the active form of topotecan (12).