Effect of ipratropium bromide on the interfacial organization of model lung surfactant membranes

The interactions of inhaled drugs with lung surfactant are important for understanding their behavior at the first biological barrier of the respiratory tract. In this work, we investigated the effect of the anticholinergic bronchodilator, ipratropium bromide (IPB), on model lung surfactant membranes composed of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and their 8:2 molar mixture. Langmuir monolayer studies showed that ipratropium bromide has little influence on zwitterionic DPPC, while it interacts strongly with negatively charged DPPG, leading to concentration-dependent expansion of the monolayer, stabilization of the liquid-expanded phase, and a shift of the phase diagram toward higher surface pressures. For the mixed DPPC:DPPG 8:2 model, the effect of the drug was moderate and mainly visible at low surface pressures, where it fluidized the layer and delayed the development of condensed domains. Brewster angle microscopy confirmed these observations by revealing altered domain morphology for DPPC, extended phase coexistence and circular domains for DPPG, and reduced condensation of the mixed model in the presence of ipratropium at lower surface pressures. Compression-expansion experiments mimicking the breathing cycle demonstrated that IPB decreases the reversibility of the mixed monolayer and promotes aggregate formation. GIXD and XRR measurements showed that it does not alter the in-plane or vertical structure of DPPC, but significantly affects DPPG by inducing structural features characteristic of lower-pressure phases and slightly increasing electron density in the headgroup region. Molecular dynamics simulations confirmed preferential localization of ipratropium bromide near DPPG headgroups, driven mainly by electrostatic interactions.