Cancer remains a major public health problem worldwide . The three most commonly diagnosed types of cancer among women in 2012 were that of the breast, lung and bronchus, and colorectum, accounting for about half of the estimated cancer cases in women . However, current treatment options for breast cancer are still limited mainly to surgical resection, chemotherapy, and radiotherapy, which are highly aggressive and/or nonspecific and often accompanied with undesirable and potentially serious side effects because anticancer drugs also exert excessive toxicity to healthy tissues and cells [2, 3]. Nanomedicine, especially drug formulation by polymeric nanoparticles, has shown a great deal of promise to provide solutions to such problems in cancer treatment [4, 5]. In recent years, a lot of attention has been paid to the biodegradable polymeric nanoparticles for their passive and active drug targeting to the desired sites after various routes of administration [6, 7]. In addition, the nanoparticles used as drug carriers possess other advantages including a stable structure, high entrapment efficiency, high cellular uptake, more desirable biodistribution, and more reasonable pharmacokinetics as well as preferentially accumulate at the tumor site through the enhanced permeability and retention effect [8, 9]. Polymeric nanoparticles were also found to reduce or overcome drug resistance of tumor cells .
Biodegradable polymers have great application potential in biomedical fields including drug delivery and tissue engineering. Among them, the polyester family including poly(d,l-lactide-co-glycolide) (PLGA), polylactide (PLA), and polyglycolide (PGA) is most extensively investigated due to its good biocompatibility and biodegradability [9, 11]. Despite the well-established importance, this kind of polymers still has limitations in particular applications. It is well known that the autocatalytic effect and the acidic degradation products of these polyesters cause unfavorable effects. In addition, the degradation rate of polyesters such as PLA and PLGA is too slow due to their hydrophobic nature to meet the therapeutic needs [12, 13]. It was also reported that PLA- and PLGA-based nanoparticles can be rapidly cleared in the liver and captured by the reticuloendothelial system (RES) when they are administrated into the blood circulation [14, 15]. These drawbacks could be overcome by the introduction of d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) into the hydrophobic PLA backbone . TPGS, a water-soluble derivative of the natural form of d-α-tocopherol, is formed by esterification of vitamin E succinate with poly(ethylene glycol) (PEG) 1000. It was found that TPGS could improve the aqueous solubility of drugs including taxanes, antibiotics, cyclosporines, and steroids. In addition, TPGS could serve as an excellent molecular biomaterial for overcoming multidrug resistance and as an inhibitor of P-glycoprotein to increase the cytotoxicity and oral bioavailability of antitumor agents .
Though PLGA-based nanoparticles and PLA-TPGS-based nanoparticles have been extensively studied as delivery vehicles of drugs, most of them were focused on making use of linear polymers. In recent years, branched polymers, such as hyper-branched polymers, star-shaped polymers, and dendrimers, have obtained great attention due to their useful mechanical and rheological properties [9, 18, 19]. A star-shaped block polymer is a branched polymer molecule in which a single branch point (core) gives rise to multiple linear chains or arms . In comparison with linear polymers at the same molar mass, nanocarriers based on a star-shaped polymer molecular structure showed a smaller hydrodynamic radius, lower solution viscosity, higher drug content, and higher drug entrapment efficiency [21, 22]. Therefore, in this research, novel delivery systems of star-shaped block copolymers based on PLA and TPGS with unique architectures were developed, which would provide valuable insights for fabricating ideal and useful drug carriers for nanomedicine applications [23, 24]. Cholic acid (CA) is one of the two major bile acids produced by the liver where it is synthesized from cholesterol. It is composed of a steroid unit with one carboxyl group and three hydroxyl groups. CA was chosen as the polyhydroxy initiator due to its biological origin, which may obtain better biocompatibility for polymers incorporated with the CA moiety . Moreover, it was reported that CA-functionalized star-shaped polymers could exhibit faster hydrolytic degradation rates in comparison with linear homopolymers such as PLA and poly(ϵ-caprolactone) (PCL). The existence of the CA moiety in biomaterials could also significantly increase both cell adherence and proliferation .
In this research, the star-shaped block copolymer CA-PLA-TPGS with three branch arms was used for developing a superior nanocarrier of anticancer agents with satisfactory drug content and entrapment efficiency for breast cancer treatment. The star-shaped CA-PLA-TPGS nanoparticles containing paclitaxel (PTX) as a model drug were characterized, and the anticancer effect of nanoparticles was evaluated both in vitro and in vivo.