Development and characterization of nanovesicles and colloidosomes for small-molecule delivery

Abstract

Controlled diffusion of bioactive ingredients is of interest to food, cosmetic, and drug industries. To characterize delivery systems and interactions with small molecules, a model hydrophobic molecule, 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl nitroxide (PTMIO), which is an electron paramagnetic resonance (EPR)-active spin probe was used. The objectives of this research were to study (1) the effect of a solid fat barrier to NLC on the diffusion of PTMIO and (2) how the fatty acid packaging of phospholipid structures affects the release of PTMIO from liposomal vesicles (LV). Colloidosomes, microcapsules with a self-assembled coating, were formed to study the effect of a solid barrier on diffusion. Liquid lipid nanoparticles (LLN; refined coconut oil stabilized by lecithin) and solid lipid nanoparticles (SLN; palm oil stabilized by caseinate) were prepared by high pressure homogenization. Dispersions were adjusted above and below the isoelectric point of caseinate (pH 3.5 and 7) to form colloidosomes. Self-assembly of SLN onto the surface of LLN was studied by controlled mixing of the ratios (2:1 to 1:5) (LLN:SLN) of the two dispersions. Particle size and zeta potential were found by dynamic light scattering (DLS) and phase-analysis light scattering (PALS), respectively. The average zeta potential of LLN was -43.61 ± 2.28 mV at both pHs while SLN was 46.71 ± 5.93 mV at pH 3.5 and -42.74 ± 5.58 mV at pH 7. At a volumetric ratio of (1:3) (LLN:SLN), the mixture appeared to have self-assembled, without evidence of bridging flocculation. The mixture was 431.03 ± 7.2 nm in diameter and had a zeta potential of 32.48 ± 2.01 mV at pH 3.5, with a particle size significantly larger than either LLN or SLN (p < 0.05) at pH 3.5, suggesting the particles aggregated. LLN and the mixture were characterized for diffusion by adding PTMIO. PTMIO was reduced by ascorbate and monitored with EPR in-situ. PTMIO was quenched significantly slower in colloidosomes at pH3.5 than at pH 7 (p < 0.05). EPR spectra were simulated by a downhill simplex to separate fractions of PTMIO at the lipid and aqueous phases. Diffusion of PTMIO was limited from colloidosomes at pH 3.5. Saturated (80H or 90H) and unsaturated (PC75 or 90G) phospholipids were used to make nanovesicles to study LV. Because LV can change morphology as molecules are incorporated, a carrier lipid (refined coconut oil), was added at 0, 0.5, or 1.0% to LV before homogenization. LV particle size and zeta potential were characterized by DLS and PALS, respectively. All phospholipids increased in particle size with carrier lipid fraction (p < 0.05). Zeta potential differed by phospholipid (p < 0.05); however, all LV with negative voltages. Differential scanning calorimetry was used to characterize LV. Crystallization of 90H lecithin onset at ca. 53°C and extended from ca. 45°C to 43°C with the addition of carrier lipid. To study diffusion, PTMIO was loaded into LV and studied by the aforementioned EPR method. Carrier lipid fraction did not affect the diffusion (p > 0.05). Reduction of PTMIO in 90H LV was slower than other phospholipids (p > 0.05).