Structural characterization of cationic liposomes loaded with sugar-based carboranes

Abstract

In this article we report the physicochemical characterization of cationic liposomes loaded with orthocarborane and two of its sugar-containing derivatives. Carboranes are efficient boron delivery agents in boron neutron capture therapy, an anti-cancer treatment based on neutron absorption by 10B nuclei. Cationic liposomes were prepared using the positively charged DOTAP and the zwitterionic DOPE, as a helper lipid. These liposomes are currently used in gene therapy for their ability in targeting the cell nucleus; therefore they can be considered appropriate vectors for boron neutron capture therapy, in the quest of reducing the high boron amount that is necessary for successful cancer treatment. Boron uptake was determined by an original in situ method, based on neutron absorption. The structural properties of the loaded liposomes were studied in detail by the combined use of small angle x-ray scattering and small angle neutron scattering. These techniques established the global shape and size of liposomes and their bilayer composition. The results were discussed in term of molecular properties of the hosted drugs. Differences found in the insertion modality were correlated with the preparation procedure or with the specific shape and lipophilic-hydrophilic balance of each carborane.

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SCHEME 1

Structure of the carborane cage (C₂B₁₀H₁₂) in the ball-and-stick representation. Hydrogen atoms have been omitted for clarity.

SCHEME 2

Sketch of a liposome global structure and length scattering density profile for x-ray scattering (left side) and neutron scattering (right side).

FIGURE 1

Neutron absorption coefficient versus theoretical (i.e., starting) content of glucosyl-carborane (a) and orthocarborane (b) in loaded liposomes.

FIGURE 2

SAXS profile for DOTAP/DOPE pure liposome. Total lipid concentration: 4.2 10⁻¹ mol/dm³. The best-fit parameters are reported in Table 1.

FIGURE 3

SANS profile for DOTAP/DOPE pure liposome. Total lipid concentration: 4.2 10⁻¹ mol/dm³. The best-fit parameters are reported in Table 1.

FIGURE 4

SANS spectra of plain and OCB-loaded liposomes. Total lipid concentration was 4.2 10⁻¹ mol/dm³ for all samples. The q²I(q) versus q-representation is able to evidence the different behavior of the curves in the low q-region.

FIGURE 5

SAXS profiles for plain and carborane-loaded liposomes. The fitting of the pure liposome curve is also shown for comparison. Lipid content was 4.2 10⁻¹ mol/dm³ for all samples.

FIGURE 6

SAXS curves for liposomes loaded with different amounts of glucosyl-carborane and corresponding best fits.

FIGURE 7

Experimental SAXS diagrams of liposomes containing LCOB, added before extrusion, at molar fractions ranging from 0 (pure liposomes) to 0.5.

FIGURE 8

SAXS spectra, with corresponding simulations, of co-vesicled and non-co-vesicled X_LCOB = 0.25 samples.