Thermodynamic and Structural Study of Budesonide-Exogenous Lung Surfactant System

Abstract

The clinical benefits of using exogenous pulmonary surfactant (EPS) as a carrier of budesonide (BUD), a non-halogenated corticosteroid with a broad anti-inflammatory effect, have been established. Using various experimental techniques (differential scanning calorimetry DSC, small- and wide- angle X-ray scattering SAXS/WAXS, small- angle neutron scattering SANS, fluorescence spectroscopy, dynamic light scattering DLS, and zeta potential), we investigated the effect of BUD on the thermodynamics and structure of the clinically used EPS, Curosurf^®. We show that BUD facilitates the Curosurf^® phase transition from the gel to the fluid state, resulting in a decrease in the temperature of the main phase transition (Tm) and enthalpy (ΔH). The morphology of the Curosurf^® dispersion is maintained for BUD < 10 wt% of the Curosurf^® mass; BUD slightly increases the repeat distance d of the fluid lamellar phase in multilamellar vesicles (MLVs) resulting from the thickening of the lipid bilayer. The bilayer thickening (~0.23 nm) was derived from SANS data. The presence of ~2 mmol/L of Ca²⁺ maintains the effect and structure of the MLVs. The changes in the lateral pressure of the Curosurf^® bilayer revealed that the intercalated BUD between the acyl chains of the surfactant's lipid molecules resides deeper in the hydrophobic region when its content exceeds ~6 wt%. Our studies support the concept of a combined therapy utilising budesonide-enriched Curosurf^®.

Keywords: SANS; SAXS/WAXS; budesonide; differential scanning calorimetry; lateral pressure; lung surfactant.

Figure 1

(A) DSC thermograms of the Curosurf^® and BUD/Curosurf^® mixture. The vertical line indicates the Tm of the Curosurf^®. (B) Temperature Tm of the gel-to-fluid phase transition as a function of wt% BUD. The dashed line indicates the Tm of Curosurf^®. The error bars represent the standard deviation of duplicate runs of the same sample.

Figure 2

(A) SAXS and WAXS scattered intensity (in relative units) of Curosurf^® and BUD/Curosurf^® mixtures without (blue) and with 2 mmol/L of Ca²⁺ (red) at 50 °C. (B) Repeat distance (d) of Curosurf^® as a function of wt% BUD at 40 °C (empty blue symbols) and 50 °C (full blue symbols) in the absence or presence of Ca²⁺ (50 °C, full red symbols) in a hydration medium. The error bars are within the size of symbols. Dashed lines indicate the respective d of Curosurf^®.

Figure 3

(A) SANS curves of unilamellar vesicles of Curosurf^® (blue empty circles) and BUD/Curosurf^® with different content of BUD (in wt%) at 40 °C. Curves are shifted along the y-axis for clarity. Inset: Kratky–Porod plot representation of scattering curves (ln I(q).q² vs. q²). (B) Effect of BUD on the thickness of the lipid bilayer d_g of Curosurf^®. The dashed line displays the value of d_g of Curosurf^®. The error bars were derived from the standard deviation of the slope of the Kratky–Porod plot.

Figure 4

(A) Normalised fluorescence emission spectrum of Pyr4PC and Pyr10PC in Curosurf^® bilayers at 37 °C with designated vibronic bands of pyrene monomer (I–V). The spectra are normalised to the intensity of the first monomer maxima, $I_{monomer}^{norm}\left(376\right)=100$. (B) Lateral pressure detected by Pyr4PC (blue) and Pyr10PC (red) fluorescence probes as a function of wt% BUD at 37 °C. The dashed line corresponds to the η_PyrnPC n = 4 and 10 of Curosurf^® without BUD. The error bars represent the standard deviation of the fluorescence intensities measured as a function of time.