Thermoresponsive 2-hydroxy-3-isopropoxypropyl hydroxyethyl cellulose with tunable LCST for drug delivery

Abstract

Thermoresponsive polymer 2-hydroxy-3-isopropoxypropyl hydroxyethyl celluloses (HIPECs) were successfully synthesized, characterized, and applied for thermoresponsive drug delivery. The lower critical solution temperature (LCST) of HIPEC could be easily tuned from 21.1 to 56.1 °C as the molar substitution (MS) increased from 1.21 to 2.88. Dynamic light scattering and transmission electron microscopy experiments revealed that HIPEC can self-assemble into nano-sized aggregates, and their size could be changed by variation in temperature. Additionally, the critical aggregation concentration (CAC) ranged from 0.101 to 0.805 g L^-1 by changing MS of HIPEC. In vitro drug delivery studies indicated that the amphotericin B (AmpB) release rate was much faster at temperatures above LCST; approximately 95% of the drug was released from aggregates in 40 h. MTT assays were conducted to evaluate the cytotoxicity of HIPEC, and the observation of the Hoechst 33342 living cell stain using confocal laser scanning microscopy confirmed the high cell viability as HIPECs were used.

Scheme 1. Synthesis pathway of thermoresponsive HIPECs.

Fig. 1. (a) Absorbance changes for HIPEC-1∼5 aqueous solutions (5 g L⁻¹) with a heating rate of 1 °C min. (b) Effect of MS on LCST. (c) Reversible changes of optical absorbance against temperature fluctuation for HIPEC-3 aqueous solutions (5 g L⁻¹) (inset: photographs of HIPEC-3 aqueous solution at 25 °C and 40 °C). (d) Absorbance variations of HIPEC-3 aqueous solutions (5 g L⁻¹) with different NaCl concentrations during heating. (e) Effect of NaCl concentration on LCST.

Fig. 2. (a) Excitation spectra of pyrene (6 × 10⁻⁷ mol L⁻¹) in water in the presence of increasing concentrations of HIPEC-3 (λ_em = 490 nm). Measurements were performed at 10 °C. (b) Plot of I₃₃₈/I₃₃₄ in the excitation spectra versus the concentration of HIPEC-3. (c) Effect of MS on CAC.

Fig. 3. (a) Plots of the average hydrodynamic diameter of HIPECs with different MS versus temperature. TEM images (b) of HIPEC-3 aggregates observed at 25 °C and (c) large aggregates formed after heating the solution to 38 °C.

Fig. 4. (a) Drug release profiles of AmpB-loaded HIPEC-3 aggregates at 35 °C, 36.5 °C and 38 °C in water. (b) Cytotoxicity of HIPEC-3 on normal hepatocytes (HL-7702) at temperatures below and above the LCST. (c, d) Confocal fluorescence images of HL-7702 cells incubated by PI and Hoechst 33342 at the same time. (e) Merged image of (c) and (d), and the corresponding bright-field image. (Blue color: Hoechst 33342 staining occurring in the cell nucleus, λ_ex = 405 nm, λ_em = 421–501 nm; red color: PI background fluorescence occurring outside the cells, λ_ex = 559 nm, λ_em = 577–657 nm).