International Journal of Pharmaceutics 437 (2012) 70–79 Contents lists available at SciVerse ScienceDirect International Journal of Pharmaceutics jo ur n al homep age: www.elsevier.com/locate/ijpharm Pharmaceutical nanotechnology Folate-decorated thermoresponsive micelles based on star-shaped amphiphilic block copolymers for efficient intracellular release of anticancer drugs Seyed Jamal Tabatabaei Rezaei a , Mohammad Reza Nabid a,∗ , Hassan Niknejad b , Ali Akbar Entezami c a Department of Chemistry, Shahid Beheshti University, G.C., 1983963113, Tehran, Iran b Nanomedicine and Tissue Engineering Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran c Lab of Polymer, Faculty of Chemistry, University of Tabriz, Tabriz, Iran a r t i c l e i n f o Article history: Received 11 June 2012 Received in revised form 21 July 2012 Accepted 25 July 2012 Available online 4 August 2012 Keywords: Thermoresponsive Star block copolymers Tumor-targeting Intracellular release a b s t r a c t In this study, a new type of folate-decorated thermoresponsive micelles based on the star-shaped amphiphilic block copolymer 4s[poly(-caprolactone)-b-2s(poly(N-isopropylacrylamide- co-acrylamide)-b ′ -methoxy poly(ethylene glycol)/poly(ethylene glycol)-folate)] (i.e., 4s[PCL-b-2s(P(NIPAAm-co-AAm)-b ′ -MPEG/PEG-FA)] (PCIAE-FA)), were developed for the tumor- targeted delivery and temperature-induced controlled release of hydrophobic anticancer drugs. These amphiphilic star copolymers are capable of self-assembling into spherical micelles in aqueous solution with an average diameter of 91 nm. The lower critical solution temperature (LCST) of micelles was around 39.7 ◦ C. The anticancer drug, paclitaxel (PTX), was encapsulated into the micelles. In vitro release studies demonstrated that the drug-loaded delivery system (PTX–PCIAE-FA) is relatively stable at physiologic conditions but susceptible to temperatures above LCST which would trigger the release of encapsulated drugs. The cytotoxicity studies showed that the PTX transported by these micelles was higher than that by the commercial PTX formulation Tarvexol ® . The efficacy of this thermoresponsive drug delivery system was also evaluated at temperatures above the LCST; the results demonstrated that the cellular uptake and the cytotoxicity of PTX-loaded micelles increase prominently. These results indicate that these thermoresponsive micelles may offer a very promising carrier to improve the delivery efficiency and cancer specificity of hydrophobic chemotherapeutic drugs. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent years, considerable interests have been attracted by the biomedical materials, especially by the polymeric micelles self- assembled from amphiphilic block copolymers due to their unique characteristics such as core-shell structure, mesoscopic size range, and prolonged blood circulation (Jabr-Milane et al., 2008; Kwon et al., 1997; Nakayama et al., 2006; Schmaljohann, 2006; Wang et al., 2006; Zhou et al., 2010). The hydrophobic inner core serves as a container for encapsulation of hydrophobic drugs and the outer shell composed of hydrophilic flexible polymers plays a key role in the stabilization of drug-carried micelle and helps these nanopar- ticles to escape from the reticuloendothelial system (RES) uptake after intravenous administration (Gref et al., 2000, 1994). It is known that the ideal nano-based drug delivery system (DDS) should be stable during systemic circulation in the blood- stream with lowest leakage of the loaded therapeutic agents and able to specifically accumulate in the required organ or tissue. Upon accumulation in the target sites, and after being taken up by ∗ Corresponding author. Tel.: +98 21 29902800; fax: +98 21 22431586. E-mail address: m-nabid@sbu.ac.ir (M.R. Nabid). cancer cells, the carrier should release the therapeutic agents quickly in response to the local environment (Cabral and Kataoka, 2010; Hubbell, 2003; Savi ´ c et al., 2003). For the accumulation of carrier system in the targeted site, some targeting moieties such as folate, biotin, peptide, and saccharide are covalently linked to the particle surface, which can recognize and bind to specific recep- tors that are unique to or positively expressed on cancer cells (Li et al., 2011a; Liu et al., 2010; Tai et al., 2010; Xiong and Lavasanifar, 2011). Also, to overcome the intracellular releasing, many novel simulative-responsive micellar systems were introduced recently including the temperature, pH, ionic strength, specific enzymes, and so on (Lee et al., 2005, 2007; Nishiyama et al., 2005; Sengupta et al., 2005). In particular, temperature-responsive mechanisms have been widely investigated, as sensitivity is relatively easy to introduce and effective stimulus in many in vitro and in vivo applications. Poly(N-isopropylacrylamide) is a well-known ther- mosensitive polymer and exhibits a reversible thermoresponsive phase transition from a hydrophilic to a hydrophobic structure at around 32 ◦ C, termed as lower critical solution temperature (LCST) (Boyer et al., 2009; Mougin et al., 2011; Omer et al., 2011; Xu et al., 2007). LCST is well-adjustable by copolymerization of NIPAAm with hydrophilic/hydrophobic vinyl monomers, allowing the con- trol over drug release. Based on the concept of LCST, several block 0378-5173/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijpharm.2012.07.069