Multidrug resistance (MDR) has been considered as a huge challenge to the effective chemotherapy.Therefore,it is necessary to develop new strategies to effectively overcome MDR.Here,based on the previous research of N-(2-hydroxypropyl)methacrylamide (HPMA) polymer-drug conjugates,we designed an effective system that combined drug-efflux circumvention and mitochondria targeting of anticancer drug doxorubicin (Dox).Briefly,Dox was modified with mitochondrial membrane penetrating peptide (MPP) and then attached to (HPMA) copolymers (P-M-Dox).Our study showed that macromolecular HPMA copolymers successfully bypassed drug efflux pumps and escorted Dox into resistant MCF-7/ADR cells via endocytic pathway.Subsequently,the mitochondria accumulation of drugs was significantly enhanced with 11.6-fold increase by MPP modification.The excellent mitochondria targeting then resulted in significant enhancement of reactive oxygen species (ROS) as well as reduction of adenosine triphosphate (ATP)production,which could further inhibit drug efflux and resistant cancer cell growth.By reversing Dox resistance,P-M-Dox achieved much better suppression in the growth of 3D MCF-7/ADR tumor spheroids compared with free Dox.Hence,our study provides a promising approach to treat drug-resistant cancer through simultaneous drug efflux circumvention and direct mitochondria delivery.