Micelles based on biodegradable poly(L-glutamic acid)-b-polylactide with paramagnetic Gd ions chelated to the shell layer as a potential nanoscale MRI-visible delivery system

Abstract

There is much interest in the development of a nanoscale drug delivery system with MRI visibility to optimize the delivery efficiency and therapeutic efficacy under image guidance. Here we report on the successful fabrication of nanoscale micelles based on biodegradable poly( L-glutamic acid)- b-polylactide (PG- b-PLA) block copolymer with paramagnetic Gd3+ ions chelated to their shell. PG- b-PLA was synthesized by sequential polymerization reactions: anionic polymerization of L-lactide followed by ring-opening polymerization of benzyl glutamate N-carboxylic anhydride. The metal chelator p-aminobenzyldiethylenetriaminepenta(acetic acid) (DTPA) was readily conjugated to the side chain carboxylic acids of poly( L-glutamic acid). The resulting copolymer formed spherical micelles in aqueous solution with an average diameter of 230 nm at pH 7.4. The size of PG(DTPA)- b-PLA micelles decreased with increasing pH value. DTPA-Gd chelated to the shell layer of the micelles exhibited significantly higher spin-lattice relaxivity (r1) than a small-molecular-weight MRI contrast agent, indicating that water molecules could readily access the Gd ions in the micelles. Because of the presence of multiple carboxylic acid functional groups in the shell layer, polymeric micelles based on biodegradable PG(DTPA-Gd)- b-PLA may be a suitable platform for the development of MRI-visible, targeted nanoscale drug delivery systems.

Fig. 1

Synthesis of PG(DTPA)-b-PLA (A) and schematic model of the micellar structure with DTPA-Gd chelated to the shell layer (B). a: Naphthalene/K, L-lactide; b: TFA; c: Benzyl glutamate NCA; d: 1 M trifluoromethane sulfonic acid (TFMSA)/thioanisole/trifluoroacetic acid; e: DIC, pyridine, DMAP, p-aminobenzyldiethylenetriaminepenta(acetic acid tert butyl ester); and f: TFA.

Fig. 2

¹H NMR spectra of PG(DTPA)-b-PLA in DMSO-d₆ (A) and its corresponding micelles in aqueous D₂O solution (2 mg/mL) (B). The NMR signals from the PLA segment (circles) disappeared in the micelles.

Fig. 3

Micelle size characterized by dynamic light scattering and TEM. (A): The mean diameters of PG(DTPA)-b-PLA micelles at pH 8.5, pH 7.4, and pH 5.5 were 184.9 nm, 229.6 nm, and 256.1 nm, respectively. (B): TEM of Gd-loading PG(DTPA)-b-PLA micelle in pH 7.4 PBS. Scale bar is 100 nm.

Fig. 4

(A): Excitation spectra of pyrene as a function of copolymer concentration in water. For reasons of clarity, spectra at copolymer concentrations of 0.0005 and 1.0 mg/mL were omitted from the plot. (B): Plot of excitation intensity ratio at I₃₃₄/I₃₃₁ against logarithm of copolymer concentration. The experiment was performed with emission wavelengths of 393 nm. The CMC measured was 12 mg/L.

Fig. 5

Results of the T1 relaxivity (r₁) measurements of Gd-micelle and DTPA-Gd measured at 4.7 T and room temperature.

Fig. 6

Cell viability of micelles formed from PG(DTPA)-b-PLA and Gd-loading PG(DTPA-Gd)-b-PLA evaluated by MTT. The NIH/3T3 cell line was used. PG(DTPA)-b-PLA: grey box; PG(DTPA-Gd)-b-PLA: white box.