Bicontinuous Cubic Liquid Crystals as Potential Matrices for Non-Invasive Topical Sampling of Low-Molecular-Weight Biomarkers

Abstract

Many skin disorders, including cancer, have inflammatory components. The non-invasive detection of related biomarkers could therefore be highly valuable for both diagnosis and follow up on the effect of treatment. This study targets the extraction of tryptophan (Trp) and its metabolite kynurenine (Kyn), two compounds associated with several inflammatory skin disorders. We furthermore hypothesize that lipid-based bicontinuous cubic liquid crystals could be efficient extraction matrices. They comprise a large interfacial area separating interconnected polar and apolar domains, allowing them to accommodate solutes with various properties. We concluded, using the extensively studied GMO-water system as test-platform, that the hydrophilic Kyn and Trp favored the cubic phase over water and revealed a preference for locating at the lipid-water interface. The interfacial area per unit volume of the matrix, as well as the incorporation of ionic molecules at the lipid-water interface, can be used to optimize the extraction of solutes with specific physicochemical characteristics. We also observed that the cubic phases formed at rather extreme water activities (>0.9) and that wearing them resulted in efficient hydration and increased permeability of the skin. Evidently, bicontinuous cubic liquid crystals constitute a promising and versatile platform for non-invasive extraction of biomarkers through skin, as well as for transdermal drug delivery.

Keywords: DOTAP; X-ray diffraction; bicontinuous cubic liquid crystal; bilayer partitioning; cancer-related biomarkers; glycerol monooleate; humidity scanning (HS) QCM-D; kynurenine; non-invasive extraction; tryptophan; tryptophan-to-kynurenine ratio.

Figure 1

The partial phase diagram of GMO/DOTAP/H₂O. Black circles represent different phases determined with SAXD. Red solid lines are approximate phase boundaries. Lα—lamellar phase, C_G—cubic gyroid (Ia3d) phase, C_D—cubic double diamond (Pn3m) phase, C_P—primitive cubic (Im3m) phase.

Figure 2

Swelling of the lattice parameter normalized by the monolayer thickness as a function of lipid weight fraction in pure water (left) and in 150 mM NaCl solution (right). Triangles represent the Gyroid cubic phase (C_G, space group Ia3d), circles represent the Double Diamond cubic phase (C_D, space group Pn3m), and squares represent the primitive cubic phase (C_P, space group Im3m). Color code for GMO/DOTAP composition: cyan—97.5/2.5 (w/w), yellow—95/5 (w/w), blue—90/10 (w/w), green—85/15 (w/w), and red—80/20 (w/w).

Figure 3

Sorption isotherm of pure GMO (left) and dissipation (right) as a function of relative humidity for GMO (solid line), DOTAP (dotted line), and GMO/DOTAP 90/10 (w/w) (dashed line).

Figure 4

The effect of two-hour in vivo skin application. Diffraction patterns of GMO/DOTAP (90/10 (w/w)) with 60% (w/w) water cubic phase (Im3m) measured before (dark violet) and after (dark red), and fully swollen GMO measured before (dark green) and after (light green).

Figure 5

The partial phase diagram of GMO/DOTAP in 150 mM NaCl solution. Black circles represent different phases determined with SAXD. Red solid lines are approximate phase boundaries. L_α—lamellar phase, C_G—cubic gyroid (Ia3d) phase, C_D—cubic double diamond (Pn3m) phase.

Figure 6

Partitioning of tryptophan (triangles) and kynurenine (squares) into the lipid bilayer of a fully swollen GMO cubic phase (Pn3m). Filled symbols represent partitioning after 1 week, and empty symbols represent partitioning after 2 weeks. * Theoretically determined values using Chemicalize v. 19.7.0, 2019, ChemAxon sofware.

Figure 7

Interfacial area per unit cell (triangles, ×10³ Ų) and per unit volume (circles, m²·cm⁻³). The relationship between K_Q/w and interfacial area with respect to K_bl/w (10 (squares); 3 (triangles); 1.5 (diamonds); 0.1 (crosses) and 0.01 (circles)).