Bio-Inspired Amphoteric Polymer for Triggered-Release Drug Delivery on Breast Cancer Cells Based on Metal Coordination

Abstract

Nanoscale coordination polymers are promising vehicles for anticancer drug delivery because their surface composition and particle size can be tuned to exploit the enhanced permeability and retention effect, and their reversible interaction with metal cations enables triggered drug release at the tumor site. Here, we develop a novel nanoscale coordination polymer using the diblock copolymer poly(2-methacryloyloxyethyl phosphorylcholine)-block-poly(serinyl acrylate) (PMPC-b-PserA) and demonstrate its use for encapsulation of a hydrophobic drug and triggered drug release to induce breast cancer cell apoptosis in vitro. The zwitterionic PMPC block was inspired by the antifouling structure of cell membranes, and the PserA block was inspired by the amphoteric amino acids of proteins. The polymer was synthesized by reversible addition-fragmentation chain transfer polymerization, and a mixture of the polymer and FeCl₃ self-assembled into nanoparticles via complexation of Fe³⁺ with PserA, with the hydrophilic PMPC block at the particle surface. At a molar ratio of Fe³⁺ to serA of 3:1, the hydrodynamic diameter of the particles was 22.2 nm. Curcumin, a natural water-insoluble polyphenol used to enhance the effects of chemotherapeutics, was encapsulated in the particles as an oil-in-water emulsion, with an encapsulation efficiency of 99.6% and a particle loading capacity of 32%. Triggered release of curcumin was achieved by adding deferoxamine, an FDA-approved Fe³⁺ chelating agent; curcumin release efficiency increased at higher deferoxamine concentrations and lower pH. Triggered release of curcumin induced apoptosis in human triple-negative breast cancer cells; cell viability decreased to 34.3% after 24 h of treatment with the curcumin-loaded nanoparticles and deferoxamine, versus >80% viability without deferoxamine to trigger drug release. The biocompatibility, tunable composition and size, high hydrophobic drug loading, and triggered-release capability of this nanoscale coordination polymer make it well-suited for use in anticancer drug delivery.

Keywords: anticancer drug delivery; diblock zwitterionic copolymer; nanoscale coordination polymer; polymeric colloid; triggered release.

Figure 1.

(A) IR spectra of PMPC-b-PserA and PMPC-b-PserA-Fe³⁺. (B) UV-vis spectra of deionized water solutions of FeCl₃ (blue), PMPC-b-PserA (black), and a mixture of FeCl₃ and PMPC-b-PserA (red).

Figure 2.. Characterization of NCP nanoparticle formation and size.

(A) UV-vis spectra of NCP nanoparticles at Fe³⁺ to serA molar ratios of 3 (a), 6 (b), and 9 (c). Inset: photographs of corresponding colloid solutions. (B) DLS analysis of the polymer and NCP nanoparticles showed that the average hydrodynamic diameter varied from 9.7±1.5 to 111.4±28.3 nm when the Fe³⁺/serA was increased from 0 to 9. (C) TEM image of NCP nanoparticles (Fe³⁺/serA = 3), showing a spherical particle morphology. (D) Size distribution of NCP nanoparticles (Fe³⁺/serA = 3); the particle diameter varied from 16 to 25 nm (average diameter, 20.5±1.9 nm).

Figure 3.. Characterization of NCP nanoparticles with and without DFO.

(A) UV-vis spectra of NCP nanoparticles (Fe³⁺/serA=3) with DFO (red) and without DFO (black). After DFO addition, the broad absorption of NCP nanoparticles between 200 and 450 nm diminished, and a peak at 430 nm appeared. (B) Concentrations of DFO-Fe³⁺ complexes for NCP nanoparticles treated with different amount of DFO at different solution pH values. (C) DLS analysis of the NCP nanoparticles (Fe³⁺/serA = 3) with and without DFO (2.6 µg/µL) at pH 5.5, 6.5, and 7.4.

Figure 4.. Characterization of Cur@NCPs.

(A) UV-vis spectra of curcumin in acetone (orange) and NCP nanoparticles in deionized water before (red) and after (blue) entrapping curcumin. (B) TEM image of Cur@NCPs. (C) Size distribution of Cur@NCPs. (D) Encapsulation efficiency and loading capacity of Cur@NCPs using different volumetric ratios of CHCl₃ to deionized water. Control and Control indicate encapsulation efficiency and loading capacity of a 1 mL PMPC-b-PserA solution (10mg/mL) without Fe³⁺ with addition of 200 µL curcumin in CHCl₃ (4 mg/mL).

Figure 5.. Triggered release of curcumin.

(A) UV-vis absorption spectra of Cur@NCPs with DFO (red) and without DFO (black). (B) Release of curcumin from Cur@NCPs with DFO (red) and without DFO (black) at different solution pH values, at an estimated molar ratio of DFO/Fe³⁺ of 1.

Figure 6.. Induction of breast cancer cell apoptosis in vitro.

(A) Viability of triple-negative breast cancer cells treated polymer (360 µg/mL), FeCl₃ (360 µg/mL), DFO (990 µg/mL), FeCl₃ (360 µg/mL) with DFO (990 µg/mL), NCPs (360 µg/mL), and NCPs (360 µg/mL) with DFO (990 µg/mL). (B) Viability of breast cancer cells treated with Cur@NCPs at different concentrations, at DFO/Fe³⁺ molar ratios of 0, 1, and 1.5. **p<0.01, ***p<0.001, ^#no statistical significance (p>0.05).

Scheme 1.

Schematic of the synthesis of curcumin-loaded nanoscale coordination polymers (Cur@NCPs) and their disassembly and release of curcumin, which induces the apoptosis of breast cancer cells (MDA-MB-231). The diblock copolymer PMPC-b-PserA self-assembles into NCP nanoparticles via complexation of Fe³⁺ and PserA. Cur@NCPs are produced by partitioning of curcumin into the hydrophobic particle core via oil-in-water emulsion. Addition of the chelating agent deferoxamine mesylate (DFO) in an acidic solution causes disassembly of the Cur@NCPs, curcumin release, and cell apoptosis.

Scheme 2.

Synthesis of the diblock copolymer PMPC-b-PserA by RAFT polymerization. (m = 55, n = 25)

Scheme 3.

Self-assembly of PMPC-b-PserA into NCP nanoparticles in water via Fe³⁺ chelation. Addition of Na₂CO₃ induces formation of the Fe-oxyhydroxides core, with hydrophilic PMPC at the particle surface.

Scheme 4.

Mechanism of DFO- and proton-induced disassembly of PMPC-b-PserA NCP nanoparticles.