In all cell lines, NP delivery increased the cytotoxicity of the drug combination as compared with free formulation (FF). Both separate and combination NPs were equally effective. Bars, SD. As depicted by the combination index (CI) plots, synergism increased between gemcitabine and cisplatin when delivered by NP formulation (white data points), regardless of whether they were separate or combination NPs, compared with free formulation (black data points). CI>1, antagonism; CI=1, additivity; CI<1, synergism.