Tuning multiple arms for camptothecin and folate conjugations on star-shaped copolymers to enhance glutathione-mediated intracellular drug delivery

Micelle behaviors have been studied intensively for linear block copolymers, and amphiphilic polymers with a non-linear architecture, including star polymers, dendrimers, and hyperbranched polymers are showing advantages in the loading of anticancer drugs. In the current study, multiarmed amphiphilic copolymers with drug conjugates were developed for the construction of micelles to alleviate premature drug release during circulation and to achieve glutathione (GSH)-mediated intracellular drug delivery. Folic acid (FA) was grafted on some of the arms of 4-armed poly(ethylene glycol) (4-arm-PEG) as the targeting groups of micelles, and epsilon-caprolactone (epsilon-CL) was copolymerized with PEG from the rest of the arms, followed by camptothecin (CPT) conjugation through dithiodipropionic acid. The effect of the number of arms used for FA conjugation and the length of PCL blocks on micelle properties, reductive sensitivity, cellular uptake, and antitumor activities were evaluated. The amphiphilic copolymers with star-shaped miktoarms were self-assembled into micelles with an average size of around 100 nm and critical micelle concentrations (CMCs) as low as 0.8 mu g mL(-1). The distribution of hydrophobic PCL segments in more arms resulted in micelles with lower CMCs, higher structural stability, and greater sensitivity of CPT release in the presence of GSH. But, the conjugation of FA on fewer arms led to lower FA densities and less efficient cellular uptake of the micelles. In view of the performance of cellular uptake, CPT loading, and reduction-sensitive release, micelles from 4-armed copolymers with FA conjugation on two arms and PCL copolymerization on another two arms resulted in the most significant antitumor activities. It is suggested that the composition and arm structure of amphiphilic copolymers with star-shaped miktoarms show significant effects on micelle behavior, and optimal antitumor activities can be achieved through judicious selection of multiple arms for conjugating targeting groups and chemotherapeutic drugs.