The use of spray freeze drying technology to produce nucleic acid formulation for pulmonary delivery

Abstract

Nucleic acid therapeutics have potential for the treatment of various lung diseases including respiratory infections, asthma and chronic obstructive pulmonary disease. Dry powder formulation offers several advantages over liquid aerosol in formulating macromolecules such as better stability and compatibility. Spray freeze drying (SFD) is a drying process that can be used to produce inhaled nucleic acid powder. It involves the atomization of liquid into fine droplets which are instantaneously frozen in a cryogenic liquid, followed by the freeze drying of samples which are sublimed at low temperature and pressure to form porous particles. In this study, SFD technology was investigated to produce inhaled formulation of nucleic acids, with herring sperm DNA (hsDNA) used as a model nucleic acid and mannitol used as a bulking excipient. Different SFD formulations were prepared by varying the solute concentration (1 – 7.5% w/v) and hsDNA concentration (0.25 – 2% w/w) in the feed solution. The particle size distribution and the morphology of the spray freeze dried powders were examined by laser diffraction and scanning electron microscope respectively. The aerodynamic properties of the powder were assessed by next generation impactor (NGI). The fine particle fraction (FPF), which is defined as the fraction of powder exhibited aerodynamic diameter below 5  m, was calculated to determine the optimal formulation for inhalation. The morphology study revealed that particles produced by SFD were overall spherical in shape. At low solute concentration (below 5% w/v), the powders were highly porous and fragmentation could be observed. When the solute concentration was kept at 5% w/v, varying hsDNA concentration did not have a major effect on the particle size distribution, with the median diameter (measured by laser diffraction) around 10 to 15  m. The NGI study showed that the SFD formulations had a FPF between 20 to 30%. As the hsDNA concentration increased from 0.25 to 2% w/w, the FPF increased, and the formulation containing 2% hsDNA w/w exhibited a significantly higher FPF than the others. Overall, this study demonstrated that inhalable nucleic acids dry powder formulations can be successfully produced by SFD. Further studies will be carried out to investigate whether the increase of nucleic acids content could enhance the FPF, and formulations containing different types of nucleic acids such as small interfering RNA (siRNA) and microRNA will be explored.