br Introduction br In the last few decades despite
In the last few decades, despite significant progress in diagnosis and therapeutic approaches, eﬀective treatments for cancers are limited. This is due to the frequently encountered resistance of cancer Rottlerin to antineoplastic agents [1,2]. Doxorubicin·HCl (Dox) is an anthracycline drug widely used in breast cancer chemotherapy, which final target location is nucleus. Despite its high antitumor activity, the eﬀectiveness of Dox is severely limited by the development of the resistance of breast cancer cells. The resistance of breast cancer cells arises from the over-expression of adenosine triphosphate-binding cassette (ABC) trans-porter proteins, such as P-glycoprotein, which can actively increase the drug eﬄux, leading to inadequate Dox accumulation in the nucleus . It is reported that less than 0.4% Dox enter the cell nucleus after cellular internalization . So far, however, most current Dox loaded nano-carriers are designed to improve serum half-life and cell internalization eﬃcacy, posing an apparent drawback in these systems. Specifically, free Dox released from the nanocarriers into the cytoplasm are re-ex-posed to the eﬄux pump, which has not solved the problem of drug resistance. Therefore, a nuclear-uptake nanodrug delivery system, which delivers Dox directly to the nucleus is expected to address the challenge of drug resistance. There have been several nucleus targeted drug delivery carriers reported , including the cell penetrating peptides (CPPs)  and amino-riched cationic polymers . However,
the hemagglutination and toxicity of CPPs and such cationic polymers, have restricted the further development of direct nuclear delivery platform. To resolve these issues, it is highly desirable to develop a general, safe and eﬀective platform for direct nuclear delivery.
Nucleolin is the most abundant nucleolar phosphoprotein in the nucleus of normal cells, but in metastatic and rapidly dividing breast cells is overexpressed in the cytoplasm and cell membrane [8–10]. It has been reported that the nucleolin in the cytoplasm of breast cancer cells actively and continuously migrated into the nucleus, which en-dowed nucleolin with natural nuclear targeting function [11–13]. Ap-tamers are small single-stranded DNA or RNA oligonucleotide seg-ments, which bind to their targets, such as proteins with high aﬃnity and specificity [14–16]. In contrast to antibodies, aptamers have lim-ited immunogenicity and are unlikely to develop resistance. Aptamer AS1411, a 26-mer DNA aptamer, has been confirmed to selectively bind to nucleolin with a high binding aﬃnity (Kd is in the picomolar to low nanomolar range) [17–20]. The design of targeting drug delivery system based on aptamer AS1411 and nucleolin had been investigated wildly in the field of biomedical [21,22]. In addition, Dox·HCl has been proved could load into the double-stranded regions of aptamer AS1411, forming a physical complex (Ap-Dox) through noncovalent intercala-tion. So, it could be possible to exploit the shuttling properties of nu-cleolin to traﬃc the Ap-Dox to the nuclear, bypassing eﬄux pump.
However, it is considered that when the aptamers are exposed to the
blood, it may be degraded by the serum nucleases . In addition, the small particle size of Ap-Dox complex might be cleared quickly from the bloodstream by the reticuloendothelial system (RES) and renal, leading to a reduced circulation half-life and inadequate accumulation in the tumor site. In view of this, Liposome was regarded as an ideal vehicle to deliver the Ap-Dox complex into the tumor cells because of its high biocompatibility, low toxicity and capability of carrying water-soluble drugs [24–26]. Ap-Dox complex loading in the liposome could prevent the degradation by the serum nucleases and rapid clearance by the RES . Herein, in this study, we formulated a multifunctional liposome (Lip(Ap-Dox)), in which Ap-Dox complex was loaded for reversing drug resistance of MCF-7/Adr cells. The Ap-Dox complex was released after the liposomes were taken up by tumors cells, and then migrated into the nucleus via aptamer AS1411 and nucleolin interaction, thus preventing drug eﬄux and enhancing therapeutic eﬀect.