Long-term Storage Stability of Inhalable Phage Powder Formulations: A Four-Year Study

The growing potential of bacteriophage therapy as an alternative treatment for pulmonary infections caused by multidrug-resistant bacteria has been increasingly recognized. This study aimed to evaluate the long-term stability of spray-dried phage powder formulations for pulmonary delivery, focusing on both biological activity and physicochemical properties. Three phages, PEV1, PEV20, and PEV61, were selected for formulation based on their host range against clinical strains of Pseudomonas aeruginosa. Eight spray-dried formulations, developed with varying proportions of lactose as a stabilizer and leucine as a moisture protectant and powder dispersion enhancer, were stored under controlled conditions at 4°C/15% relative humidity (RH) and 20°C/15% RH for four years. Over this period, phage titers declined with reduction ranging from 0.97 log₁₀ in the most stable formulation to 2.49 log₁₀ in the least stable one. Formulations with higher lactose concentrations (70-80%) demonstrated better preservation of biological activity. While the overall particle morphology remained unchanged, some thread-like elongated features protruding from the particle surfaces were observed, particularly in powders stored at 20°C/15% RH. However, there was a decline in the fine particle fraction (FPF) 50 - 60% to 27-44%. These findings showed the potential of spray-dried phage powders as a viable option for long-term storage to retain bioactivity, but the aerosol performance can be compromised.