(M0930-03-16) Optimizing Storage Conditions to Maintain Chemical Stability and Aerodynamic Performance of Tigecycline Inhalation Powder

Purpose: Tigecycline (TGC) is a broad-spectrum glycylcycline antibiotic with activity against nontuberculous mycobacteria (NTM) [1,2]. However, TGC exhibits chemical instability in aqueous solution, leading to technical challenges in delivery. Furthermore, severe adverse reactions have been noted in response to intravenous delivery of large TGC doses, resulting in poor patient compliance [1-3]. Spray dried microparticle formulations of TGC for use in dry powder inhalers (DPIs) represent an alternative strategy to both localize drug delivery to the lungs for pulmonary NTM infections to reduce systemic effects and mitigate drug stability challenges due to the solid nature of the formulation. Herein, we evaluate the stability of tigecycline inhalation powder in bulk and as prepared for DPI use for four months at three temperatures (22 °C, 4 °C, and -20 °C).

Methods: TGC inhalation powder was prepared via spray drying using a Buchi B-290 benchtop spray dryer as previously reported [4]. The spray dried powder was collected and stored as a bulk powder or in #3 hydroxypropylmethylcellulose (HPMC) capsules shielded from light at 22 °C, 4 °C, or -20 °C with desiccant. Initially and at specified time intervals, the powder was analyzed for physicochemical and aerodynamic stability as a function of storage temperature. Particle morphology, crystallinity, moisture content, and glass transition temperature were analyzed using scanning electron microscopy (SEM), x-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), respectively. TGC chemical stability was monitored using high performance liquid chromatography (HPLC). TGC powders were reconstituted immediately before analysis to minimize solution-based drug degradation. Lastly, the aerodynamic performance was assessed via release of 10 mg powder in a prepackaged #3 HPMC capsule from an RS-01 low resistance inhaler into a Next Generation Impactor (NGI) at 60 L/min for 4 s. The aerodynamic particle size distribution was described through the mass median aerodynamic diameter (MMAD), geometric standard deviation (GSD), and fine particle fraction (FPF; ≤4.46 µm with respect to the emitted dose).

Results: A method was developed using HPLC to assay tigecycline content, with non-degraded tigecycline separated from tigecycline degradation products. The tigecycline content in the bulk powder ranged from 62-75 wt% over 16 weeks regardless of storage temperature, which is in line with the theoretical tigecycline content of 70 wt%. No degradation products were observed in the chromatograms. The bulk powders were further analyzed for crystallinity and glass transition temperature. The powders remained amorphous after 15 weeks at the three storage temperatures. There was minimal influence on storage temperature on the glass transition temperature after 12 weeks. Previous literature indicates that amorphous powders retain physical stability when stored at least 40-50 °C below the glass transition temperature [5]. With glass transition temperatures of 85-95 °C, room temperature storage should be feasible without compromising stability of the powders. Once monthly, the aerodynamic performance of the prepackaged capsules stored at the three temperatures was evaluated. After 16 weeks, it was found that capsule storage at 22 °C significantly increased the MMAD and decreased the FPF (Figure 1). Statistical differences were also found with the capsules stored at 4 °C albeit with a smaller percentage difference from the initial. No differences were observed when the capsules were stored at -20 °C. Storage of the powder in bulk at 22 °C with capsule preparation immediately before analysis yielded an aerodynamically stable powder over 16 weeks (Figure 1), with no differences compared to the initial measurement. This demonstrates that the powder is stable at room temperature over 4 months but is not stable in the capsules as they were prepared. The moisture content after 16-18 weeks of storage was compared between powders stored in bulk and those stored in capsules (Figure 2). The storage in capsules led to greater water content in the powder than storage in bulk at the same temperatures. No significant differences were found in water content across the three temperatures. While increased moisture could explain a decline in aerodynamic performance, it does not explain why variable performance is seen as a result of storage temperature. The morphology of the powder was then examined to compare the powder stored in capsules to that stored in bulk after 18 weeks. From these images (Figure 3), it is evident that morphological changes are only prominent when the powder was stored in capsules at room temperature.

Conclusion: The storage of tigecycline inhalation powder as a bulk powder at 22 °C was found to have physicochemical and aerodynamic stability over 4 months. However, storage in #3 HPMC capsules for use in RS-01 inhalers at 22 °C led to morphological changes and poor aerodynamic performance. While capsules stored at -20 °C demonstrated aerodynamic stability over 16 weeks and represent a path forward, we are currently investigating solutions to mitigate physical and aerodynamic changes to the powders when stored at room temperature to overcome limitations associated with cold-chain storage.

References: [1] Kwon YS, Levin A, Kasperbauer SH, Huitt GA, Daley CL: Efficacy and safety of tigecycline for Mycobacterium abscessus disease. Respiratory Medicine 2019, 158: 89-91.
[2] Ferro BE, Srivastava S, Deshpande D, Pasipanodya JG, van Soolingen D, Mouton JW, van Ingen J, Gumbo T: Tigecycline is highly efficacious against Mycobacterium abscessus pulmonary disease. Antimicrobial Agents and Chemotherapy 2016, 60: 2895-2900.
[3] Jitkova Y, Gronda M, Hurren R, Wang X, Goard CA, Jhas B, Schimmer AD: A novel formulation of tigecycline has enhanced stability and sustained antibacterial and antikeukemic activity. PLoS One 2014, 9: e95281.
[4] Maloney SE, Alshiraihi IM, Singh A, Stewart IE, Mariner Gonzalez J, Gonzalez-Juarrero M, Meibohm B, Hickey AJ: Spray dried tigecycline dry powder aerosols for the treatment of nontuberculous mycobacterial pulmonary infections. Tuberculosis 2023, 139: 102306.
[5] Chang RYK, Chen L, Chen D, Chan HK: Overcoming challenges for development of amorphous powders for inhalation. Expert Opinion on Drug Delivery 2020, 17: 1583-1595.

Acknowledgements: The authors wish to thank Todd Ennis for acquiring the XRPD data. This work was funded by the National Institutes of Health (R01AI155922). Microscopy was performed at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), which is supported by the National Science Foundation, Grant ECCS-1542015.


Figure 1: Aerodynamic performance in terms of mass median aerodynamic diameter (MMAD, left) and fine particle fraction (FPF, right) of TGC dry powder after 16 weeks when stored in a capsule (striped) or in bulk (solid). Statistical analysis performed using a One-Way ANOVA with Dunnett’s multiple comparisons test. *p < 0.05, ***p < 0.005 relative to the initial measurement.


Figure 2: Moisture content of TGC dry powder after 16-18 weeks of storage as a bulk powder or in #3 HPMC capsules. Statistical analysis performed using a One-Way ANOVA with Tukey’s multiple comparisons test. ***p < 0.005.


Figure 3: Scanning electron micrography images of tigecycline inhalation powders after 18 weeks in bulk or in capsules at three storage temperatures.