(M1230-09-50) The Hidden Potential: Enhancing Doxycycline Dissolution and Aerodynamics with HPMC and Gelatin for Pulmonary Drug Delivery

Purpose: The aim of this research was to enhance the solubility of doxycycline, a poorly soluble antibiotic, for the treatment of pulmonary infections to leverage its antibiotic and anti-inflammatory effects. Given the limited repertoire of excipients approved for the oral inhaled pathway, this study explores and compares the use of two approved polymers to form amorphous solid dispersions (ASD) of doxycycline: hydroxypropyl methylcellulose (HPMC) and gelatin. The objective was to create inhalable microspheres that improve doxycycline's effectiveness by increasing its solubility and ensuring its bioavailability at pulmonary pH levels.

Methods: Doxycycline was combined with either HPMC or gelatin to form ASDs using the spray drying (SD) technique. Feed solutions were prepared by dissolving doxycycline in an acidic solution (pH 2) with each polymer in varying ratios, then spray dried in a Büchi Mini Spray Dryer B-290 (Büchi, Labortechnik AG, Switzerland). The spray drying conditions were adjusted to maximize microspheres' aerodynamic behavior according to a previous work (1). Encapsulation efficiency was determined by dissolving the ASDs in acidified water (pH 2) and assessing the drug content to adjust subsequent calculations. Franz cells dissolution testing was used to determine the release kinetics of doxycycline from the ASDs in phosphate-buffered saline (PBS) at pH 7.4 and 37°C (2). To determine if the ASD was achieved, dynamic scanning calorimetry (DSC) was performed on the formulations with the highest dissolution rates obtained from the dissolution study using a DSC131 (SETARAM Inc., Cranbury, NJ, USA). Aerodynamic properties of the formulations were assessed using a Next Generation Impactor (NGI) (Copley Scientific Limited, United Kingdom). Each formulation was aerosolized using a High-Resistance RS01 Monodose DPI (Berry Global), with an airflow of 63 L/min. A total of 20 mg of powder formulations were loaded into HPMC capsules, size N°3 (Capsugel-Lonza), to perform the aerodynamic characterization.

Results: The spray drying process yielded high encapsulation efficiency for both HPMC and gelatin-based formulations ( >62% EE). Optimal results were observed when microspheres were formulated with 32% polymer ( >96.2% EE), regardless of the polymer used. Franz cell dissolution profiles indicated better performance for the DOX-HPMC formulations when HPMC was present at low ratios (DOX: HPMC 80:20 ~ 90% DOX released at 1 hour) (Figure 1). Conversely, DOX-Gelatin formulations performed better with intermediate amounts of gelatin (DOX: Gelatin 64:36 = 101% DOX at 1 h). Overall, DOX-HPMC and DOX-Gelatin formulations displayed about 30 times higher dissolution rates compared to pure doxycycline. The DSC analysis showed that both formulations exhibited partial amorphization of DOX, retaining its characteristic melting and degradation peaks but shifting both to higher temperatures (Figure 2). Thus, the presence of the polymers proved to be not just useful to achieve the ASD but also served as a thermoprotective component by increasing the degradation temperature from 200 to  220⁰C. Aerodynamic evaluation revealed high fine particle fraction (FPF) and respirable fraction (RF) values, indicating effective pulmonary deposition. The best-performing formulations tested for HPMC and gelatin displayed an FPF of 85.31 ± 10.09 and 66.12 ± 3.79, respectively(Figure 3).

Conclusion: Both HPMC and gelatin spray-dried microspheres significantly enhanced the dissolution rate and aerodynamic properties of DOX due to the partial amorphization of the drug. These DPI formulations are promising powders using FDA-approved polymers that have not been used before for the oral inhalation route, making them suitable for inhalation therapy. These findings suggest that spray drying these polymers to form ASDs can improve the bioavailability and effectiveness of doxycycline for treating pulmonary infections by enabling a fast-dissolving antibiotic. This approach could be used to improve the dissolution of other poorly soluble inhaled drugs, serving as an aerodynamic platform to enable the use of drugs that have not been explored due to the low liquid volume in the lungs for drug dissolution.

References: 1. Behrend-Keim B, Castro-Muñoz A, Monrreal-Ortega L, Ávalos-León B, Campos-Estrada C, Smyth HDC, et al. The forgotten material: Highly dispersible and swellable gelatin-based microspheres for pulmonary drug delivery of cromolyn sodium and ipratropium bromide. Int J Pharm. 2023 Sep 25;644:123331.
2. Salama RO, Traini D, Chan HK, Young PM. Preparation and characterization of controlled release co-spray dried drug–polymer microparticles for inhalation 2: Evaluation of in vitro release profiling methodologies for controlled release respiratory aerosols. Eur J Pharm Biopharm. 2008 Sep 1;70(1):145–52.

Acknowledgements: These group would like to thank Fondecyt-Chile for the grant Fondecyt de Iniciación 11190987 that funded the studies presented, the Center for Research Development and Innovation in Bioactive Products, and the Magister en Bioactividad de Productos Naturales y de Síntesis.

Figure 1: Franz cell dissolution test to evaluate the dissolution profile of the amorphous solid dispersion formulations. Dissolution profiles of A) DOX-HPMC and B) DOX-GA GA microspheres. Percentages in the nomenclature represents the fraction of DOX in the formulation.

Figure 2: Dynamic scanning calorimetry profiles of the Doxycycline (blue) , and its combinations with hydroxypropyl methylcellulose (HPMC) (magenta) and Gelatin (green). The endothermic peaks represent the melting point of DOX and the exothermic peaks its degradation signal. The displacement to the higher temperatures are the proof of partial amorphization.


Figure 3: Aerodynamic particle size distribution of the formulations of the dry powder formulation with the highest dissolution rates.