Pharmacokinetic-modifying effects of glycerol in inhaled medicines

Student thesis: Doctoral ThesisDoctor of Philosophy


The pressurised metered dose inhaler (pMDI) is the most widely used inhalation dosage form for the treatment of respiratory diseases. In some solution-based pMDIs, glycerol is incorporated to modify the aerodynamic particle size distribution of the emitted aerosol droplets. However, evidence is emerging that glycerol may influence drug distribution (pharmacokinetics) after aerosol particles deposit in the lungs. This work investigated the effect of glycerol on (i) the physicochemical characteristics of drug aerosols including their dissolution, (ii) the molecular biophysics of model lung interfaces, (iii) drug uptake by respiratory epithelial cells, and (iv) the pulmonary pharmacokinetics of inhaled beclometasone dipropionate (BDP) in rats. 
Two licensed pMDIs as well as two pharmaceutically-equivalent BDP inhalers formulated with the absence and presence of glycerol, but designed to produce equivalent-sized aerosols were studied. Particles were collected and characterised using a wide range of analytical techniques. Molecular interactions between glycerol and dipalmitoylphosphatidylcholine (DPPC) monolayers and bilayers representing lung interfaces were investigated using biophysical methods and in silico modelling. The effect of glycerol on intracellular uptake of BDP by Calu-3 respiratory epithelial cells was determined using a specially-developed infrared spectroscopy assay. Finally, aerosols were administered intratracheally into rats to determine the effect of glycerol on the concentration-time profiles of BDP in the lungs and blood. 
The licensed BDP pMDIs were strikingly different in fundamental physicochemical characteristics, and this was mirrored by the in-house formulations engineered to exhibit identical aerodynamic particle size distribution. Compared to glycerol-free aerosols, the glycerol-containing particles were different in terms of morphology (less porous), solid state chemistry (different polymorphic content), and dissolution (slower rate). Glycerol was found to preferentially interact with the hydrophilic region of DPPC, thus causing an increase in the size of the phospholipid headgroup solvation shell with an increase in the molecular area of a DPPC monolayer from 52 to 68 Å2 and a reduction in gel phase DPPC bilayer thickness by ~3 Å. These findings were supported by in silico simulations in which the orientation of the headgroup was more parallel to the membrane plane in the presence of glycerol and bridging between adjacent DPPC headgroups provided a mechanism for a membrane stiffening effect with the potential to modify cell permeability to drugs. Glycerol-induced effects on drug transport were confirmed by measuring in situ cell uptake and pharmacokinetic profiles in rats were predicted in silico and experimentally measured in vivo to quantify impacts on inhaled drug delivery. After application of 100 μM BDP to Calu-3 cells, the cumulative BDP concentrations inside the cells at 24 h in the absence and presence of glycerol were 54 and 34 μM, respectively, implying that glycerol restricts the flux of BDP into living Calu-3 cells. Finally, preliminary data indicated that glycerol delays BDP absorption from the airway into the bloodstream as seen by higher concentrations remaining in the rat lungs at 1 h after dosing, albeit non-significant at this time point, as well as lower Cmax and later Tmax in predicted profiles. 
Together, these data provide compelling evidence that glycerol modifies the absorption and distribution of inhaled drugs by physicochemical changes to drug aerosols and biological interactions with lung membranes.
Date of Award1 Oct 2019
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorBen Forbes (Supervisor) & Richard Harvey (Supervisor)

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