(W1030-06-37) Investigating the Physical Stability of GENE-A Dry Powder for Inhalation Using Solid-State Techniques

Purpose: GENE-A is a small molecule developed as a dry powder inhaler (DPI) formulation. A rigorous particle size control, i.e. D₅₀: 1.6-2.2µm, D₉₀: 3.2-4.2µm, is required for pulmonary delivery of the active pharmaceutical ingredient (API) for optimal aerosol performance. However, substantial API particle size increase was observed under stress stability conditions despite the disappearance of measurable surface amorphous content (AE) generated upon initial jet milling of the API, which is often considered the primary driver for particle growth. Therefore, an extensive study employing diverse solid-state techniques was designed to obtain a mechanistic understanding of particle size changes under select storage conditions. Specifically, factors such as AE and surface energy (SE) of the jet milled API were monitored during storage to understand their impact on particle growth. In addition, inclusion of magnesium stearate (MgSt) at two different concentrations was also explored to determine its feasibility in inhibiting API particle growth under these storage conditions.

Methods: GENE-A was micronized to the desired particle size distribution (PSD) range for pulmonary delivery using a jet-mill, without and with MgSt (added at two different concentrations of 0.7 and 2.1% w/w). MgSt was mixed with GENE-A using an acoustic mixer prior to jet-milling. Micronized neat API and API-MgSt blends were stored at 5°C/0% RH, 40°C/33% RH, and 80°C/11% RH. Powder samples were analyzed periodically (0-6M) using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), laser diffraction (for PSD), differential scanning calorimetry (DSC), and inverse gas chromatography (iGC) for SE determination. AE in micronized samples (generated upon milling) was determined from a correlation curve of the crystallization enthalpies of API samples milled for different time periods (obtained via DSC) with the actual percentage of AE of these milled samples determined directly using solid-state NMR. SEM micrographs of T0 and stability samples were subjected to image processing and energy dispersive X-ray (EDX) chemical mapping to determine surface roughness and distribution of MgSt in the API blends respectively.

Results: Of all the stability samples, only the neat, micronized API showed substantial increase in particle size, i.e. > 28% and 35% increase in D₅₀ and D₉₀ values, respectively, at 80°C/11%RH between T1-6M, despite the disappearance of amorphous content at T=1M (Figure 1). For micronized samples containing MgSt, the D₅₀ and D₉₀ values did not exceed the desired range specified for pulmonary delivery, thereby clearly demonstrating the inhibitory effect of MgSt on particle growth (Figure 1). Interestingly, AE persisted under all storage conditions at 2.1% MgSt samples over 6M as compared to the neat API (AE detected only at 5°C/0%RH and 40°C/33%RH at 1M) and 0.7% MgSt samples (AE detected at 40°C/33%RH and 5°C/0%RH), which indicates that efficiency of MgSt as a protective barrier may be concentration dependent (Figure 1). EDX confirmed that 2.1% MgSt stability samples showed greater homogeneity in MgSt distribution than the 0.7% MgSt mixtures. SEM images of neat, micronized API at 80°C/11% RH showed substantial surface smoothening at T=3M compared to T0 samples (Figure 2). This smoothening was greatly reduced in 0.7% MgSt and negligible in 2.1% MgSt (Figure 2). Image processing corroborated this visual observation, with calculated change in surface roughness being the highest for neat API (54%), followed by 0.7% MgSt (34%), and 2.1% MgSt (17%) (Figure 3). High-resolution SEM revealed merging of small particulates within larger particles, thereby leading to surface smoothening for the neat, micronized API stored at 80°C/11% RH. For this sample, a marked decrease in SE was observed at T=1-6M, which correlated well with increase in particle size. For high temperature and RH stability conditions, SE values decreased over time in samples containing MgSt, despite particle growth inhibition. This may be attributed to the presence of MgSt which is known to reduce the surface energy of powder blends^1,2.

Conclusion: Pronounced particle growth, exceeding the desired PSD range for pulmonary delivery, was observed in GENE-A for only the neat, micronized API samples stored at 80°C/11% RH over 6M, despite absence of AE at T=1M. MgSt was found to be an effective inhibitor of particle growth at all stability conditions, even at a low concentration of 0.7% w/w. Our systematic long term storage stability study shows this excipient to be a promising mitigation strategy for the development of DPI formulations where particle size increase upon storage is a serious risk that can jeopardize drug product performance. Finally, changes in SE correlated well with particle size changes, albeit only for micronized neat API, where an increase in PSD led to a marked decrease in SE. However, no meaningful correlation was obtained between SE and PSD of stability samples containing MgSt, where decrease in SE was observed even upon inhibition of particle growth.

References: 1. Swaminathan, V., Cobb, J., & Saracovan, I. (2006). Measurement of the surface energy of lubricated pharmaceutical powders by inverse gas chromatography. International journal of pharmaceutics, 312(1-2), 158-165.
2. Thool, P., Stancill, C., Bui, M., & Mao, C. (2022). Evaluation of Time Consolidation Effect of Pharmaceutical Powders. Pharmaceutical Research, 39(12), 3345-3357.

Acknowledgements: The authors would like to thank Juan Bian (Analytical Chemistry) for providing the PSD method and Deepak Prasad (Drug Product Manufacturing) for assistance with jet milling. The authors declare no conflict of interest for this study.


Figure 1. PSD values (D50 and D90) of neat, micronized GENE-A, GENE-A+0.7% MgSt (micronized), and GENE-A+2.1% MgSt (micronized) at T0 and during storage (1-6M). n=3, mean ±SD values are plotted. Dotted lines represent the limit of desired D50 and D90 values for DPI formulation


Figure 2. SEM images of micronized GENE-A (with and without MgSt), stored at 80°C/11%RH (T0 and 3M samples)


Figure 3. Change in surface roughness (%) of neat, micronized GENE-A, GENE-A+0.7% MgSt (micronized), and GENE-A+2.1% MgSt (micronized) from T=0-3M. % change values are normalized to the T0 sample of the respective MgSt level (0, 0.7 and 2.1 % w/w)