pH-sensitive micelles self-assembled from polymer brush (PAE- g-cholesterol)- b-PEG- b-(PAE- g-cholesterol) for anticancer drug delivery and controlled release

Abstract

A novel amphiphilic pH-sensitive triblock polymer brush (poly(β-amino esters)-g-cholesterol)-b-poly(ethylene glycol)-b-(poly(β-amino esters)-g-cholesterol) ((PAE-g-Chol)-b-PEG-b-(PAE-g-Chol)) was designed and synthesized successfully through a three-step reaction, and their self-assembled polymeric micelles were used as hydrophobic anticancer drug delivery carriers to realize effectively controlled release. The critical micelle concentrations were 6.8 μg/mL, 12.6 μg/mL, 17.4 μg/mL, and 26.6 μg/mL at pH values of 7.4, 6.5, 6.0, and 5.0, respectively. The trend of critical micelle concentrations indicated that the polymer had high stability that could prolong the circulation time in the body. The hydrodynamic diameter and zeta potential of the polymeric micelles were influenced significantly by the pH values. As pH decreased from 7.4 to 5.0, the particle size and zeta potential increased from 205.4 nm to 285.7 nm and from +12.7 mV to +47.0 mV, respectively. The pK_b of the polymer was confirmed to be approximately 6.5 by the acid-base titration method. The results showed that the polymer had sharp pH-sensitivity because of the protonation of the amino groups, resulting in transformation of the PAE segment from hydrophobic to hydrophilic. Doxorubicin-loaded polymeric micelles were prepared with a high loading content (20%) and entrapment efficiency (60%) using the dialysis method. The in vitro results demonstrated that drug release rate and cumulative release were obviously dependent on pH values. Furthermore, the drug release mechanism was also controlled by the pH values. The polymer had barely any cytotoxicity, whereas the doxorubicin-loaded system showed high toxicity for HepG2 cells as free drugs. All the results proved that the pH-sensitive triblock polymer brush and its self-assembled micelle might be a potential delivery carrier for anticancer drugs with sustained release.

Keywords: anticancer; controlled release; drug delivery; micelle; pH-sensitive.

Figure 1

Scheme of drug loading and controlled release from triblock polymer brush (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol) dependent on pH values. Abbreviations: (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol), (poly(β-amino esters)-g-cholesterol)-b-poly(ethylene glycol)-b-(poly(β-amino esters)-g-cholesterol); DOX, dox orubicin; PAE, poly (β-amino esters); PEG, poly(ethylene glycol).

Figure 2

The synthetic route of the polymer (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol). Abbreviation: (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol), (poly(β-amino esters)-g-cholesterol)-b-poly(ethylene glycol)-b-(poly(β-amino esters)-g-cholesterol).

Figure 3

¹H NMR spectrum of polymer brush (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol) in d-CDCl₃. Abbreviation: (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol), (poly(β-amino esters)-g-cholesterol)-b-poly(ethylene glycol)-b-(poly(β-amino esters)-g-cholesterol).

Figure 4

The potentiometric titration of polymer (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol) solutions. Abbreviation: (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol), (poly(β-amino esters)-g-cholesterol)-b-poly(ethylene glycol)-b-(poly(β-amino esters)-g-cholesterol).

Figure 5

Plot of intensity ratios (I₃₃₇/I₃₃₅) as function of logarithm of the polymer brush concentrations in different PBS buffer solutions (pH 7.4, 6.5, 6.0, and 5.0). Abbreviation: PBS, phosphate-buffered saline.

Figure 6

Particle size (A) and zeta potential (B) of polymeric micelles dependent on pH values. Typical TEM images (C) of micelles in different solutions with pH 7.4 (left) and 5.0 (right). Abbreviation: TEM, transmission electron microscopy.

Figure 7

In vitro drug release profiles (A) and mechanism analysis (B) of DOX-loaded (PAE-g-Chol)-b-PEG-b-(PAE-g-Chol) micelles.

Figure 8

Cytotoxicity for HepG2 cells treated with polymer brush for 48 h (A) or free DOX or DOX-PMs for 24 h (B) or 48 h (C) in concentration specified. Abbreviations: DOX, doxorubicin; DOX-PM, doxorubicin-loaded polymeric micelle.