Block copolymer micelles with acid-labile ortho ester side-chains: Synthesis, characterization, and enhanced drug delivery to human glioma cells

Abstract

A new type of block copolymer micelles for pH-triggered delivery of poorly water-soluble anticancer drugs has been synthesized and characterized. The micelles were formed by the self-assembly of an amphiphilic diblock copolymer consisting of a hydrophilic poly(ethylene glycol) (PEG) block and a hydrophobic polymethacrylate block (PEYM) bearing acid-labile ortho ester side-chains. The diblock copolymer was synthesized by atom transfer radical polymerization (ATRP) from a PEG macro-initiator to obtain well-defined polymer chain-length. The PEG-b-PEYM micelles assumed a stable core-shell structure in aqueous buffer at physiological pH with a low critical micelle concentration as determined by proton NMR and pyrene fluorescence spectroscopy. The hydrolysis of the ortho ester side-chain at physiological pH was minimal yet much accelerated at mildly acidic pHs. Doxorubicin (Dox) was successfully loaded into the micelles at pH 7.4 and was released at much higher rate in response to slight acidification to pH 5. Interestingly, the release of Dox at pH 5 followed apparently a biphasic profile, consisting of an initial fast phase of several hours followed by a sustained release period of several days. Dox loaded in the micelles was rapidly taken up by human glioma (T98G) cells in vitro, accumulating in the endolysosome and subsequently in the nucleus in a few hours, in contrast to the very low uptake of free drug at the same dose. The dose-dependent cytotoxicity of the Dox-loaded micelles was determined by the MTT assay and compared with that of the free Dox. While the empty micelles themselves were not toxic, the IC50 values of the Dox-loaded micelles were approximately ten-times (by 24 hours) and three-times (by 48 hours) lower than the free drug. The much enhanced potency in killing the multi-drug-resistant human glioma cells by Dox loaded in the micelles could be attributed to high intracellular drug concentration and the subsequent pH-triggered drug release. These results establish the PEG-b-PEYM block copolymer with acid-labile ortho ester side-chains as a novel and effective pH-responsive nano-carrier for enhancing the delivery of drugs to cancer cells.

pH-Sensitive Micelles for Targeted Drug Delivery Prepared Using a Novel Membrane Contactor Method

“…6 To achieve pH sensitivity, the hydrophobic block of the copolymer can be modified to introduce acidliable bonds which degrade at mildly acidic pH, causing the micelle to collapse, thus releasing the encapsulated drug. The examples of pH sensitive groups are acetal bonds [7][8] and poly(ortho ester) side chains 9 that allow chemical conjugation of drugs to the side chain.…”

The Influence of Drug Loading on Caveolin-1 Mediated Intracellular Internalization of Doxorubicin Nanomicelles in vitro

“…The SMA micelle gives the potential to achieve a wide range of drug loading depending on the characteristics privileges, as this parameter will also precondition a slow or a fast release of the drug from the micelle [19][20][21]. This system differs from most micellar sytem engineered where the drug release rate shows pronounced biphasic characteristics [22][23][24][25]. We generated three SMA micelles with doxorubicin loadings of 4.4, 14.5 or 28.4% (Table 1).…”

pH‐Responsive Drug‐Delivery Systems

“…Of the isolated organs, the renal fluorescence intensity at 24 h after a single injection was higher than other organs ( Figure 3C). [27] Other examples of organic-materials-based pH-responsive drug-delivery systems have also been reported, including poly(b-l-malic acid), [28] poly(ethyleneimine)-poly(ethylene glycol), [29] P(2-(methacryloyloxy)-ethyl phosphorylcholine)-b-P(2-methoxy-2-oxoethyl methacrylate), [30] linear dendritic block copolymers composed of polyamidoamine dendrimer and poly(ethylene glycol) with or without galactose, [31] amphiphilic diblock copolymer that consists of a hydrophilic poly(ethylene glycol) block and a hydrophobic polymethacrylate block with acid-labile ortho ester sidechains, [32] block ionomer complexes of poly(ethylene glycol)-block-poly(4-vinylbenzylphosphonate) and cationic surfactants, [33] poly(ethylene glycol)-b-poly(l-histidine)-b-poly(l-lactic acid)-b-poly(ethylene glycol), [34] carboxymethyl dextran-coated liposomes, [35] chitosan oligosaccharide/ arachidic acid-based nanoparticles, [36] chitosan salts coated with stearic acid, [37] amphiphilic carboxymethyl-hexanoyl chitosan/poly(acrylic acid) hybrid macromolecules, [38] comb-like amphiphilic copolymers with an acetal-functionalized backbone based on poly[(2,4,6-trimethoxybenzylidene-1,1,1-tris(hydroxymethyl) ethane methacrylateco-poly(ethylene glycol) methyl ether methacrylate], [39] PEGylation of aliphatic dendritic polyester by forming acetal linkages, [40] mixed micelles of poly[(d,l-lactide)-co-glycolide)]-poly(ethylene glycol)-folate and poly(b-amino ester)-poly(ethylene glycol)-folate, [41] unimolecular micelles formed by dendritic amphiphilic block copolymers poly(amidoamine)-poly(l-lactide)-b-poly(ethylene glycol) conjugated with anti-CD105 monoclonal antibody (TRC105) and 1,4,7-triazacyclononane-N,N',N-triacetic acid (NOTA, a macrocyclic chelator for 64 Cu), [42] composite microparticle Reprinted from reference [27]. Copyright 2013 Elsevier Ltd.…”