Developing aqueous lipid formulations with low surface tension behavior at physiological conditions and stability against aggregation
The main goal of the thesis is developing aqueous lipid formulations with good stability and with low surface tension behavior at physiological conditions for exogenous lung surfactant therapy. The thesis contains thermodynamic models for pH prediction of phosphate buffer solutions, experimental results for characterization of the lipid dispersions produced with a method developed at Purdue at physiological conditions, a new dimensionless model for interpreting the stabilization mechanism of the dispersion from a fundamental point of view, and finally new results from new lipid formulations which have improved the lipid dispersion properties. The thermodynamic model shows that pH depends primarily on the concentration ratio R of the two monobasic to dibasic sodium phosphate salts, and secondarily on their concentrations and on the concentrations of the supporting electrolytes such as NaCl or KCl. Various ideal and non-ideal solution thermodynamic models are presented. Model predictions using the non-ideal extended Debye-Hückel (D-H) equation agree with the data up to ca.± 0.1 pH units at 298 K and 310 K. When CaCl2 is added to a standard phosphate buffer saline (PBS), up to 3 mM, a phosphate salt often precipitates, affecting the free Ca2+ ion concentration, the phosphate ion concentrations, and the pH. The effect of the buffer composition and the dispersion preparation protocol on the dynamic surface tension (DST) and vesicle sizes of aqueous dipalmitoylphosphatidylcholine (DPPC) dispersions was studied. Two protocols, with a new method and an old method (Bangham method), were used in preparing the DPPC dispersions. The DPPC dispersions prepared with the new method contained mostly vesicles and were quite stable at 25 or 37 °C. The DPPC dispersions of 1000 ppm at 37 °C with the new method produced, at pulsating area conditions at 20 cycles per minute, low dynamic surface tension minima (DSTM, γmin), lower than 10 mN/m. When a 1000 ppm DPPC dispersion was mixed with a stable solution of 1000 ppm BSA (bovine serum albumin), it became colloidally unstable, aggregating within minutes, implying that heterocoagulation between lipid vesicles and albumin takes place. The heterocoagulated dispersion produced high DSTM because the lipid transport rate to the interface became slower. Moreover, the protein may have been transported to the surface faster and adsorb more than the lipid at the surface. The colloidal dispersion stability at 25° C of aqueous dispersions of sonicated DPPC (dipalmitoylphosphatidylcholine) vesicles was quantitatively evaluated from the Fuchs-Smoluchowski stability ratio W. Data of average particle size vs time were obtained with dynamic light scattering measurements. Zeta potentials results imply that there was some contribution of the double-layer electrostatic forces to the dispersion stability. A new dimensionless model of the DLVO theory for spherical particles was formulated, focusing on the conditions for the existence of a positive interaction potential energy maximum, &PHgr;max, which is linked to W. DLVO calculations of &PHgr;max, and W with error analysis show that the charged DPPC vesicles tested are quite more stable than predicted. DPPC lipid vesicles were modified for reducing aggregation with other vesicles or with the protein with the addition of a small weight fraction of a neutral "PEGylated" lipid, with a covalently bonded PEG (polyethyleneglycol) group. The mixed vesicles were found to be quite more stable than the DPPC vesicles, remaining stable for months, apparently stabilized by steric forces. The colloidal stability at the initial stages of coagulation was evaluated quantitatively from the Fuchs-Smoluchowski stability ratio W. When the modified lipid vesicle dispersion was mixed with the albumin, the vesicles showed no tendency to aggregate with the albumin molecules for days, also probably because of steric repulsion between the PEGylated lipid and the protein. Finally, the mixed lipid dispersions maintained their low DSTM as the DPPC vesicles without the albumin, and also in the presence of albumin. The results have implications on the use of DPPC or DPPC-based lipids in treating alveolar respiratory diseases without albumin inhibition of their surface tension lowering ability.
Franses, Purdue University.
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