pH-dependent, thermosensitive polymeric nanocarriers for drug delivery to solid tumors

Abstract

Polymeric micelles are promising carriers for anti-cancer agents due to their small size, ease of assembly, and versatility for functionalization. A current challenge in the use of polymeric micelles is the sensitive balance that must be achieved between stability during prolonged blood circulation and release of active drug at the tumor site. Stimuli-responsive materials provide a mechanism for triggered drug release in the acidic tumor and intracellular microenvironments. In this work, we synthesized a series of dual pH- and temperature-responsive block copolymers containing a poly(ε-caprolactone) (PCL) hydrophobic block with a poly(triethylene glycol) block that were copolymerized with an amino acid-functionalized monomer. The block copolymers formed micellar structures in aqueous solutions. An optimized polymer that was functionalized with 6-aminocaproic acid (ACA) possessed pH-sensitive phase transitions at mildly acidic pH and body temperature. Doxorubicin-loaded micelles formed from these polymers were stable at blood pH (~7.4) and showed increased drug release at acidic pH. In addition, these micelles displayed more potent anti-cancer activity than free doxorubicin when tested in a tumor xenograft model in mice.

Figure 1

LCST of pH-dependent and thermo-sensitive polymeric micelles. (A) The LCSTs of P1-βA, P1-ACA, and P2-ACA micelle solutions as a function of pH value (B) Phase transition of P1, P2, P1-ACA, and P2-ACA micelles with 0, 3.4, and 8.4 mol% of amino acid groups at neutral condition as a function of temperature, respectively. (C) Phase transition of P1-ACA micelle solution as a function of temperature at different pH buffer solution.

Figure 2

The particle size distribution of P1-ACA micelles at different temperature in pH 5.3 buffer solution as measured by dynamic light scattering.

Figure 3

Transmission electron micrograph of P1-ACA micelles.

Figure 4

Critical micelle concentration determination of P1-ACA micelles. Plot of the intensity ratio I₃₃₇/I₃₃₄ of pyrene as a function of P1-ACA concentration in aqueous solution.

Figure 5

The cumulative DOX release profiles from (A) P1-ACA and (B) P0 micelles in buffer solutions of pH 5.3 and pH 7.4 at 37°C.

Figure 6

Determination of IC₅₀ of free DOX, DOX-loaded P1-ACA micelles and DOX-loaded P0 micelles to MDA-MB-435 cells. Cell viability was determined by MTS assay and expressed as % viability compared to control untreated cells. By student t-test, IC₅₀ of the three samples are significantly different (p<0.01).

Figure 7

In vivo tumor reduction in xenograft tumor-bearing mice dosed with empty P1-ACA micelles, DOX, and DOX loaded in P0 or P1-ACA.micelles. (A) Growth inhibition of subcutaneous melanoma tumors induced by multiple intravenous injections of 5% glucose, empty P1-ACA micelles, free DOX, DOX-loaded P0 and P1-ACA micelles (8 mg DOX/kg mouse) (b) Normalized (to t = 0) body weights of treated mice. * indicate a statistically significant difference from P1-ACA micelles and untreated mice, or mice treated with empty micelles, DOX, or DOX-loaded P0 micelles, using the one-way ANOVA analysis, p < 0.05.

Scheme 1

Synthesis of P(TEGMA-co-NMAA)-b-PCL.