Amphiphilic block co-polyesters bearing pendant cyclic ketal groups as nanocarriers for controlled release of camptothecin

Abstract

Amphiphilic block co-polymers consisting of hydrophilic poly(ethylene glycol) and hydrophobic polyester bearing pendent cyclic ketals were synthesized by ring-opening co-polymerization of ε-caprolactone (CL) and 1,4,8-trioxaspiro-[4,6]-9-undecanone (TSU) using α-hydroxyl, ω-methoxy, poly(ethylene glycol) as the initiator and stannous octoate as the catalyst. Compositional analyses indicate that TSU was randomly distributed in the hydrophobic blocks. When the TSU content in the co-polymers increased, the polymer crystallinity decreased progressively and the glass transition temperature increased accordingly. The hydrophobic, anticancer drug, camptothecin (CPT), was successfully encapsulated in the block copolymer nanoparticles. The CPT encapsulation efficiency and release kinetics were strongly dependent on the co-polymer composition and crystallinity. CPT release from nanoparticles constructed from co-polymers containing 0, 39 and 100 mol% TSU in the hydrophobic block followed the same trend, with an initial burst of approx. 40% within one day followed by a moderate and slow release lasting up to 7 days. At a TSU content of 14 mol%, CPT was released in a continuous and controlled fashion with a reduced initial burst and a 73% cumulative release by day 7. The in vitro cytoxicity assay showed that the blank nanoparticles were not toxic to the cultured bone metastatic prostate cancer cells (C4-2B). Compared to the free drug, the encapsulated CPT was more effective in inducing apoptotic responses in C4-2B cells. Modulating the physical characteristics of the amphiphilic co-polymers via co-polymerization offers a facile method for controlling the bioavailability of anticancer drugs, ultimately increasing effectiveness and minimizing toxicity.

Keywords: AMPHIPHILIC BLOCK CO-POLYMER; CAMPTOTHECIN; CONTROLLED RELEASE; CRYSTALLINITY; CYCLIC KETAL; NANOPARTICLES.

Figure 1

¹H NMR spectra of ECT copolymers with varying TSU content. Polymer composition and molecular weight were calculated by comparing integrals of characteristic peaks of the PEG blocks at 3.64 ppm and the typical protons for the monomers (3.95 ppm for TSU and 4.06 ppm for ε-CL). The molecular weight of PEG served as the reference.

Figure 2

¹³C NMR spectra (the carbonyl region) of ECT copolymers with different compositions showing a random distribution of CL and TSU units in the hydrophobic block.

Figure 3

SEC chromatograph of a typical copolymer (ECT2). THF was used as the mobile phase. M_n = 44,900 g/mol; PDI = 1.22.

Figure 4

DSC thermographs (second scan) of ECT copolymers showing the evolution of the glass transition temperature (top) and the melting temperature/melting enthalpy (bottom) as a function of copolymer composition. Heating rate: 10 °C/min.

Figure 5

WAXD patterns of ECT copolymers with different compositions. With an increase in TSU content, the characteristic diffraction peaks for PCL decrease in intensity. Only a diffuse and amorphous halo was detected for ECT5 and ECT10.

Figure 6

DLS histograms of blank nanoparticles (top) and CPT-loaded nanoparticles (bottom).

Figure 7

CryoSEM image of blank ECT2 nanoparticles assembled from ECT2. The nanoparticles are spherical in shape and exhibit a relative narrow size distribution and smooth surfaces.

Figure 8

WAXD diagrams of free CPT (bottom), blank nanoparticles (middle) and CPT-loaded nanoparticles assembled from ECT2 (top).

Figure 9

In vitro release profiles of CPT from ECT nanoparticles in PBS (pH 7.4) at ambient temperature.

Figure 10

Cytotoxicity of blank ECT2 nanoparticles and CPT-loaded nanoparticles to the cultured C4-2B cells. Representative live/dead staining of C4-2B cells cultured in the presence of filtered DI H₂O (5%, v/v, A) and blank ECT2 nanoparticles (95μg/mL, B) for 24 h. Live (green) and dead (red) cell nuclei were stained with SYTO 13 and propidium iodide, respectively. C: Apoptosis assay of vehicle controls, including filtered DI H₂O (5%, v/v), DMSO (0.1%, v/v) and blank ECT2 nanoparticles (95 μg/mL).

Figure 11

Dosage-dependent apoptosis response of C4-2B cells (A) and PC-3 cells (B) to free CPT and CPT-loaded, ECT2 nanoparticles.

Scheme 1

Synthesis of TSU containing block copolymers via ring opening copolymerization of ε-CL and TSU using mPEG as the initiator and Sn(Oct)₂ as the catalyst.