The
dissolution of inhaled drug particles in the lungs is a challenge
to model using biorelevant methods in terms of (i) collecting a respirable
emitted aerosol fraction and dose, (ii) presenting this to a small
volume of medium that is representative of lung lining fluid, and
(iii) measuring the low concentrations of drug released. We report
developments in methodology for each of these steps and utilize mechanistic in silico modeling to evaluate the in vitro dissolution profiles in the context of plasma concentration–time
profiles. The PreciseInhale aerosol delivery system was used to deliver
Flixotide aerosol particles to DissolvIt apparatus
for measurement of dissolution. Different media were used in the DissolvIt chamber to investigate their effect on dissolution profiles,
these were (i) 1.5% poly(ethylene oxide) with 0.4% l-alphaphosphatidyl
choline, (ii) Survanta, and (iii) a synthetic simulated lung lining
fluid (SLF) based on human lung fluid composition. For fluticasone
proprionate (FP) quantification, solid phase extraction was used for
sample preparation with LC–MS/MS analysis to provide an assay
that was fit for purpose with a limit of quantification for FP of
312 pg/mL. FP concentration–time profiles in the flow-past
perfusate were similar irrespective of the medium used in the DissolvIt chamber (∼0.04–0.07%/min), but these were
significantly lower than transfer of drug from air-to-perfusate in
isolated perfused lungs (0.12%/min). This difference was attributed
to the DissolvIt system representing slower dissolution
in the central region of the lungs (which feature nonsink conditions)
compared to the peripheral regions that are represented in the isolated
lung preparation. Pharmacokinetic parameters (Cmax, Tmax, and AUC0-∞) were estimated from the profiles for dissolution in the different
lung fluid simulants and were predicted by the simulation within 2-fold
of the values reported for inhaled FP (1000 μg dose) administered
via Flixotide Evohaler 250 μg strength inhaler in man. In conclusion,
we report methods for performing biorelevant dissolution studies for
orally inhaled products and illustrate how they can provide inputs
parameters for physiologically based pharmacokinetic (PBPK) modeling
of inhaled medicines.
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