Paclitaxel (PTX) is a major chemotherapeutic drug that is effective against a wide variety of cancers, particularly breast, ovarian and lung cancer. For a weakly basic chemotherapeutic drug such as PTX, the development of the acidic tumor microenvironment (Warburg effect) has a remarkable impact on therapeutic resistance. The present approach takes advantage of the acidic tumor microenvironment by incorporating lansoprazole (LAN), a proton pump inhibitor (PPI), with PTX as a potent therapeutic combination that is capable of reversing PTX resistance. To deliver optimal amounts of the drugs to neoplastic cells, a nano drug delivery system was selected. To design the nanoformulation process in a limited framework, typical formulation parameters were optimized and validated by the application of response surface methodology (RSM) using Box-Behnken design (BBD). On the basis of critical quality aspects, the experimental design helped to determine the optimal particle size (243.7 nm), zeta potential (-9.14 mV) and encapsulation efficiencies (88.91% and 80.35% for PTX and LAN respectively). The optimized formulation (PTX-LAN-PLGA-NPs) exhibited sustained release profiles over 384 hours for both the encapsulated drugs. The Korsmeyer-Peppas model was found to be the best fitted model for the release kinetics, where the release mechanism follows Fickian diffusion. In anti-tumor efficacy experiments using Michigan Cancer Foundation-7 (MCF-7) breast cancer cells, the PTX-LAN-PLGA-NPs exhibited a steep decrease in cell viability compared to the pure drugs. Taken together, the results strongly support that incorporation of PTX and LAN in nanoparticles (NPs) is a promising approach for cancer chemotherapy.