Stabilized Reversed Polymeric Micelles as Nanovector for Hydrophilic Compounds

Abstract

Small hydrophilic drugs are widely used for systemic administration, but they suffer from poor absorption and fast clearance. Their nanoencapsulation can improve biodistribution, targeted delivery, and pharmaceutical efficacy. Hydrophilics are effectively encapsulated in compartmented particles, such as liposomes or extracellular vesicles, which are biocompatible but poorly customizable. Polymeric vectors can form compartmental structures, also being functionalizable. Here, we report a system composed of polymeric stabilized reversed micelles for hydrophilic drugs encapsulation. We optimized the preparation procedure, and calculated the critical micellar concentration. Then, we developed a strategy for stabilization that improves micelle stability upon dilution. We tested the drug loading and delivery capabilities with creatine as a drug molecule. Prepared stabilized reversed micelles had a size of around 130 nm and a negative z-potential around -16 mV, making them functional as a drug carrier. The creatine cargo increased micelle size and depended on the loading conditions. The higher amount of loaded creatine was around 60 μg/mg of particles. Delivery tests indicated full release within three days in micelles with the lower cargo, while higher loadings can provide a sustained release for longer times. Obtained results are interesting and encouraging to test the same system with different drug cargoes.

Keywords: amphiphilic copolymers; crosslinking; hydrophilic cargo; reversed micelles.

Figure 1

Chemical scheme of reversed micelles synthesis: (a) modification of the commercial mPEG-b-PLGA –OH terminated copolymer to obtain product 2; (b) modification of product 2 by adding a terminal DTT unit to obtain the product 3; (c) modification of the product 3 by adding two lipoic acid units per copolymer molecule to obtain the product 4; (d) intermolecular crosslinking reaction to obtain the stabilized structure of the reversed micelles, Step I: reduction of a few LA units by DTT, Step II: reduction of LA units by –SH on the copolymer molecules.

Figure 2

Preliminary analysis performed to select the best solvent for preparation of reversed micelles: evaluation of water vesicles in the selected solvent by varying the percent water weight ((a) DMC, (b) CHL); mean photon counts obtained in water vesicles measurements ((c) DMC, (d) CHL); preliminary evaluation of suitable polymer concentrations by measuring the size of vesicles/micelles formed during time ((e) DMC, (f) CHL).

Figure 3

Optimization of the reversed micelles recipe: (a) calculation of the critical micellar concentration from DLS analysis—the plot reports one of the three performed experiments as an example; reversed micelle size, measured by DLS, at 2, 4, 6, 8, and 10 min after the addition of water to the polymer solution with concentrations (b) 2.5, (c) 3.3, and (d) 5.0 mg/mL.

Figure 4

Characterization of stabilized reversed micelles: (a) mean photon count and (b) mean diameter calculated in dilution tests; (c) mean diameter and (d) z-potential evaluated in three repeated syntheses.

Figure 5

Characterization of creatine-loaded stabilized reversed micelles: (a) mean diameter and (b) z-potential of micelles loaded with different starting amounts of creatine; (c) amount of loaded creatine (μg) by varying the starting creatine solution concentrations, normalized to the actual amounts of micelles (mg); (d) percent of released creatine through time (μg) with respect to the actual amounts loaded, in the legend formulations are labeled according to the amount of creatine loaded per mg of RMs. In plots (a–c), the x-axis reports the starting concentration of the creatine solution used during the loading procedure.