br To develop NP based anti
To develop NP-based anti-cancer formulations, a number of design parameters must be considered. The polymers selected should be bio-compatible, biodegradable, and have low toxicity. Chitosan (CS) is one material which has been widely recognized as suitable for the con-struction of such NPs: it is a natural cationic carbohydrate, with good biocompatibility, low immunogenicity, and the potential to facilitate transport across cell membranes . Importantly, under the action of enzymes in vivo, CS is degraded to the endogenous species water and carbon dioxide . CS can also be used for pH-responsive drug de-livery by virtue of its elevated solubility in slightly acidic environments (such as the tumor microenvironment) . In order to enhance spe-cific recognition of a DDS by tumor tissues (so-called “active tar-geting”), nanocarriers usually require further modification with tumor-homing ligands, such as peptides, antibodies, nucleic acids or small molecules . For instance, folic Latrunculin A (FA) receptors are over-expressed on tumor cells. Hence, grafting FA to NPs can endow them with an active targeting capacity. CS NPs in general have high bio-compatibility, but it is often desirable to add additional surface mod-ifications to e.g. increase circulation times and inhibit an immune re-sponse. However, this typically introduces extraneous materials (e.g. N-isopropylacrylamide, poly(ethylene glycol), gold nanoparticles, meso-porous silica) which can lead to systemic toxicity in healthy tissues. To overcome this issue, a strategy based on the selection of biocompatible copolymers containing hydrophobic segments to provide a lipophilic core for drug storage has been proposed .
Oleanolic acid (OA) is a naturally occurring pentacyclic triterpenoid which can be isolated from a range of plant species, particularly those belonging to the Oleaceae family. It possesses many interesting biolo-gical activities, for example being reported to have antioxidant, anti-inflammatory, and anti-fibrosis effects . Previous studies have found that OA could improve renal function and protect hepatocytes Chemical Engineering Journal 369 (2019) 134–149
against oxidative stress in vitro, with an OA treatment exerting anti-inflammatory effects through down-regulating pro-inflammatory fac-tors (e.g. IL-6, IL-1β, TNF-α) [15,18]. In addition, OA is able to affect different stages of tumorigenesis, suppressing tumor initiation and promotion, and inducing cell apoptosis [19–21].
Tumor cell invasion through the stroma is influenced by signals from the microenvironment, including the extracellular matrix (ECM), cytokines, and stromal cells . In several types of cancer, the fibrotic stroma provides a physical barrier to inhibit the distribution and pe-netration of antitumor drugs . However, the activities of matrix metalloproteinases (MMPs) can be eliminated by OA, suggesting it has potential for ameliorating the fibrosis of tumor stromal cells . All these findings indicate that OA could act as a potent adjunct to cancer chemotherapy .
Multidrug resistance (MDR) is the main cause of failure in the chemotherapy of cancer patients . MDR is mainly related to the overexpression of membrane transporter proteins (P-glycoprotein (P-gp) and the multidrug resistance protein (MRP)), which act to expel anticancer drugs from cancer cells . This profoundly decreases the sensitivity of the tumor to chemotherapy. The action of OA against multidrug resistant tumor cells overexpressing MDR-associated factors has been examined , and it was found that OA could enhance the cytotoxic effects of anticancer agents owing to inhibition of MRP and P-gp, and the consequent reversal of MRP-mediated efflux. This allows OA to impart a chemosensitizing effect .
In this study, we functionalized CS with FA to generate a FA-CS system with active targeting properties. Subsequently, OA was con-jugated to the CS backbone (yielding FA-CS-g-OA) to prolong blood circulation and enhance therapeutic efficacy. In addition, the in-troduction of hydrophobic OA will result in an amphiphilic polymer system which can self-assemble to form spheres with a core-shell structure upon addition to water. These will have a hydrophobic core into which low-solubility drugs such as doxorubicin (DOX) can be loaded. This approach is illustrated in Scheme 1. By bringing together CS, FA, OA and DOX into a simple nanoformulation, we prepare a novel system which can simultaneously i) be taken up selectively by breast cancer cells; ii) give accelerated drug release at the slightly acidic pH of the tumor microenvironment; iii) prevent the occurrence of MDR; and