Article Cite This: Mol. Pharmaceutics 2018, 15, 1361−1370
Enhancing the Spreading Behavior on Pulmonary Mucus Mimicking Subphase via Catanionic Surfactant Solutions: Toward Effective Drug Delivery through the Lungs Gokce Alp† and Nihal Aydogan*,‡ †
Department of Chemical Engineering and ‡Department of Chemical Engineering, Hacettepe University, Beytepe 06800, Ankara, Turkey S Supporting Information *
ABSTRACT: Effective and efficient spreading of drug formulations on the pulmonary mucosal layer is key to successful delivery of therapeutics through the lungs. The pulmonary mucus layer, which covers the airway surface, acts as a barrier against therapeutic agents, especially in the case of chronic lung diseases due to increased thickness and viscosity of the mucus. Therefore, spreading of the drug formulations on the airways gets harder. Although spreading experiments have been conducted with different types of formulations on mucus-mimicking subphases, a highly effective formulation is yet to be discovered. Adding surfactant to such formulations decreases the surface tension and triggers the Marangoni forces to enhance the spreading behavior. In this study, catanionic (cationic + anionic) surfactant mixtures composed of dodecyltrimethylammonium bromide (DTAB) and dioctyl sulfosuccinate sodium salt (AOT) mixed at various mole ratios are prepared and their spreading behavior on both mucin and cystic fibrosis (CF) mucus models is investigated for the first time in the literature. Synergistic interaction is obtained between the components of the DTAB/AOT mixtures, and this interaction has enhanced the spreading of the formulation drop on both the mucin and CF mucus models when compared with the spreading performances of selected conventional surfactants. It is proposed that the catanionic surfactant mixtures, especially when mixed at the molar ratios of 8/2 and 7/3 (DTAB/AOT), improve the spreading even on the cystic fibrosis sputum model. As it is vital to transport a sufficient amount of drug to the targeted region for the treatment of diseases, this study presents an important application of the fundamentals of colloidal science to pharmaceutical nanotechnology. KEYWORDS: catanionic surfactants, Marangoni flow, spreading, mucus, pulmonary drug delivery
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INTRODUCTION The successful delivery of drug formulations to targeted areas within the body is as important as their development. The human lungswith their inherent properties such as large surface area and low catabolic enzymatic activityoffer a favorable route for drug delivery applications.1−3 The treatment of lung diseases via the lungs is of particular interest to researchers; however, the thick and highly viscous mucus which covers pulmonary surfaces presents a major challenge for drug delivery via this route. In the case of several lung diseases, the composition of this layer changes and acts as an even stronger barrier against drug delivery, making it even harder for formulations to spread uniformly on the airways.4−6 Aerosolized surfactant formulations are in use for the treatment of chronical lung diseases. Unfortunately, the aerosol droplets cannot spread as effectively and homogeneously on the mucus layer as desired. As a result, the treatment of several mortal lung diseases cannot be sufficiently achieved and there is still no truly effective treatment for mortal lung diseases such as cystic fibrosis.7,8 The primary reason behind this problem is that the © 2018 American Chemical Society
formulations used in such treatments are usually dissolved in saline solutions and therefore possess high surface tension.9 As such, methods for maintaining a homogeneous and uniform spread of droplets after their deposition on the surfaces of the airway are critical and must be investigated in order to increase the effects of such means of treatment. Spreading of a thin film on a viscous surface such as mucus depends on the surface tension difference between the phases and the surface tension gradient between the center and the edge of the spreading film in the radial direction. Solutions, which do not include surfactant, possess constant surface tension. Therefore, spreading of these kinds of solutions is driven by the capillary imbalance and it proceeds until the surface tensions at the three-phase contact line are balanced.10 In this case, the spreading rate can be defined by Tanner’s Law; R(t) = Ω3/10(γt/η)1/10, where Ω is the drop volume, γ is the Received: January 25, 2018 Accepted: February 13, 2018 Published: February 13, 2018 1361
DOI: 10.1021/acs.molpharmaceut.8b00086 Mol. Pharmaceutics 2018, 15, 1361−1370
Article
Molecular Pharmaceutics surface tension of the liquid−vapor interface, and η is the viscosity of the spreading liquid.11 The addition of surfactant to these formulations decreases the surface tension of the solution. As a result, the surface tension difference between the solution and the subphase on which the formulation will spread is increased. This difference triggers the Marangoni forces to enhance the spreading behavior of the solution on the subphase. The surface tension gradient between the center and the spreading front surface of a deposited drop induces the Marangoni flow to spread the drop outward radially to larger areas, i.e. from low to higher surface tension.9,12 As spreading continues, due to the adsorption of surfactant monomers on the air/liquid interface, bulk surfactant concentration decreases and the spreading process continues until the surface tension gradient is vanished. In this case, the spreading radius (R) of a drop can be expressed with the power law, R(t) ≈ tα in which α is defined as the spreading exponent. It is shown that when Marangoni forces dominate the spreading behavior, the effective spreading power reaches to values higher than 0.1, between 0.25 and 1 in particular.11,13 In the relevant literature, several spreading experiments are described in which aqueous soluble surfactant solutions are used on mucus mimicking subphases and findings suggest that using surfactants for enhancing the spreading offers a good alternative to options for drug delivery via the lungs. However, there is still a lack of clarity about which type of surfactant would be best and further studies are still required to develop an effectively spreading surfactant formulation. Recently Stetten et al. showed that aerosolization of phospholipid dispersions has the ability to decrease the surface tension and thus these dispersions can spread on different subphases by inducing the Marangoni flow. In contrast, direct injection of these dispersions on the subphases did not succeed in terms of spreading and could not provide the same results as the aerosolized solutions.14 Catanionic (cationic + anionic) surfactant mixtures are useful for a wide variety of applications including drug delivery, owing to their advantageous interfacial and bulk properties such as possessing lower CMC and providing a lower surface tension value than those of the individual components.15,16 Due to the synergistic interaction between the anionic and cationic surfactants, they are able to form different types of aggregates, and these structures can be used to encapsulate hydrophilic/ hydrophobic substances.17−19 There are also studies on the spreading of catanionic mixtures in which the spreading was studied on polyethylene films and it was found that the spreading performances of this type of mixtures were comparable with the those of trisiloxanes, i.e. superspreaders.20,21 In this study catanionic surfactant mixtures were utilized for the development of effective spreading formulations on the mucus mimicking subphases. To our knowledge, there have been no other studies until now in which these catanionic mixtures have been used to enhance the spreading of the drug formulations on the pulmonary airway surfaces. In this paper, an investigation into improving the spreading characteristics of aqueous solution drops on both the mucus and cystic fibrosis sputum models by using catanionic surfactant mixtures consisting of DTAB (cationic surfactant) and AOT (anionic surfactant) is presented. As a result of the synergistic interaction between the amphiphiles, the interfacial characteristics of the resulting aggregates had changed the spreading behavior of the drops. As the usage of catanionic mixtures to increase the spreading efficiency of the aqueous formulations
on the mucus mimicking subphases will be, to the best of our knowledge, the first time in the literature, this study holds a critical position for the following studies including drug delivery through pulmonary airways. Obtaining an enhanced spreading behavior of drug formulations is considered to be an essential application, which should be accomplished since it is the first step toward a successful drug delivery. In this manner, this study presents a good example of the adaptation of the fundamentals of colloid science and physical chemistry to pharmaceutical applications on biointerfaces.
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EXPERIMENTAL SECTION Materials. Dioctyl sulfosuccinate sodium salt (AOT), dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), perfluorooctanesulfonate (PFOS), Pluronic F127, TritonX 100, and Tween 20 were all obtained from Sigma-Aldrich and used as received. Mucin from porcine type III, bound sialic acid 0.5−1.5% partially purified powder and bovine serum albumin (BSA), lecithin, sodium chloride (NaCl), sodium dihydrogen phosphate (NaH2PO4·H2O), disodium hydrogen phosphate dodecahydrate (NaH2PO4.12H2O), sodium azide (NaN3), calcium chloride (CaCl2), and Tris were also purchased from SigmaAldrich and used without further purification. The water used in all experiments was ultrapure water with a resistivity of 18.3 MΩ·cm (Millipore, USA). Methods. Preparation of the Subphases. The pulmonary mucus model was mimicked by using 5% mucin solution (w/ w), which was prepared in 0.9% NaCl and %0.01 NaN3 solution by magnetic stirring for 12 h at 300 rpm. The cystic fibrosis sputum model was mimicked by following a procedure described in detail elsewhere and by the references therein.22,23 In brief, mucin, lecithin, and BSA were dissolved in the buffer (85 mM Na+, 75 mM Cl−, and 20 mM HEPES, pH 7.4) to achieve final concentrations of 60 mg/mL, 3.2 and 32 mg/mL, respectively. Then the resulting solution was magnetically stirred at 4 °C for 48 h. To mimic in vivo conditions better, also a subphase composed of a natural lung surfactant mixture layer on the mucus model was prepared. Natural pulmonary surfactant mixture was prepared by bronchioalveolar lavage of bovine with 0.15 M NaCl at 4 °C.24 After several procedures that were described elsewhere and by the references therein, a surfactant pellet was obtained.25 The resulting pellet was used to extract the hydrophobic components of lung surfactant via the wellknown Bligh and Dyer method.26 After the extraction, vesicular suspensions were obtained by vortexing the predried films in buffer for 4 h as described.25 All of the subphase models were prepared freshly prior to use, stored at 4 °C, and used within 1 week. Surfactant solutions were prepared above their critical micelle concentrations in phosphate buffer saline (PBS), pH 7.4 and used freshly. Surface Tension and Light Scattering Measurements of Surfactant Solutions and Subphases. The equilibrium surface tension values of the phases play a key role in this process. Surface tension measurements of the subphases, surfactants with different molecular structures (cationic, anionic, and nonionic surfactants), and their catanionic mixtures were implemented by using the Wilhelmy plate method (K9 ET-S, Krüss) at air/aqueous solution interface. Their equilibrium surface tensions at their critical micelle concentrations were 1362
DOI: 10.1021/acs.molpharmaceut.8b00086 Mol. Pharmaceutics 2018, 15, 1361−1370
Article
Molecular Pharmaceutics
Briefly, a standard volume of surfactant solutions (2.5 μL) was dropped via an automatic Hamilton microsyringe driven by a microprocessor-controlled motor system and the drop was formed at the tip of a stainless steel blunt needle. The drop profile was acquired using a CCD camera, right after its contact with the substrate and transferred to a computer with which the contact angle was calculated. The spreading dynamics were analyzed by assuming a power law relation for θC(t) of the form θC(t) ∝ t−x as in the method described by Nonomura et al. given in detail.29 Surfactant solutions were prepared in the same concentrations as those in the spreading experiments performed with particle tracking. Postdeposition Spreading Behavior. Surface tension−time measurements of a previously determined point were performed using a Langmuir−Blodgett (LB) trough (Kibron, Finland) in order to analyze the spreading behavior of the surfactants after the convective spreading phase. In short, mucin solution was placed in a Petri dish as done in the spreading experiments, and the LB probe was placed at a 2 cm distance from the walls of the Petri dish , where the final area of the spreading drops could not reach it. After the surface tension of the mucin reached steady-state, the spreading formulation was applied with a Hamilton syringe to the center of the subphase and the variation at the surface tension was recorded.
determined and are presented in the Supporting Information (Table S1). Prior to each measurement, the equipment was calibrated by using ultra pure water with a resistivity of 18.3 MΩ·cm. Also, the platinum plate was rinsed with water and flame cleaned with a Bunsen burner, before each measurement. The reproducibility of the experimental results was confirmed by repeating all measurements at least three times. The standard errors for the measured surface tensions were determined to be
