《Table 2–The production yield and particle size distribution (measured by laser diffraction) of spra

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《Spray freeze drying of small nucleic acids as inhaled powder for pulmonary delivery》

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Production yield is an important factor to consider in preparing powder formulation of nucleic acids which are more expensive than the small drug molecules.Table 2 shows the production yield of the spray freeze dried formulations，and all formulations had a satisfactory yield，with at least 83%w/w yield obtained.Most of the formulations had a yield above 90%w/w.The effect of solute concentration on powder properties was first examined.A series of DNA formulations was prepared at various solute concentration from 1%to 7.5%w/v，while the DNA concentration was kept at 1%w/w.As shown in Table 2，the volumetric diameter of the particles was measured by laser diffraction.The median diameter of the particles ranged from 8.7μm to 12.6μm.Formulation formed with 1%w/v solute concentration was generally smaller than the rest of the formulations in the same series.The median diameter of 5-D-1.00（5%w/v solute concentration）was significantly larger than that of 1-D-100（1%w/v solute concentration）（P<0.05） .Fig.2 shows the morphology of the particles examined by SEM.Overall，the particles of different formulations were spherical in shape.As expected，spray freeze drying of liquid with lower solute concentration（1%and 2.5%）produced particles of higher porosity，according to the SEM images，which is a qualitative evaluation.As shown in Fig.2A-D，these particles appeared to be more fragile，as fragments were easily noticed in the images.In addition，the low magnification images，Fig.2B and 2D，showed that the powders of these formulations were highly aggregating.On the other hand，the particles produced with higher solute concentration（5%and 7.5%），as demonstrated in Fig.2E-H，appeared as individual，discrete particles.The particle size data from laser diffraction showed that the higher the solute concentration，the lower the span value，indicating that the particle size distribution was more uniform as solute concentration increased.It was obvious that the D10value of formulation with low solute concentration was much smaller，possibly due to the presence of the small fragmented particles and debris.Judging from the SEM images，the spray freeze dried powders have to be prepared with at least 5%w/v solute concentration to be considered as‘robust’enough as inhaled formulations.