The benefits of photoacoustics for the monitoring of drug stability and penetration through tissue-mimicking membranes

The paper demonstrates the potential of photoacoustics for the identification of the mechanisms underlying drug transport through tissue-mimicking systems. Photoacoustic experiments were performed for a model transdermal delivery system, consisting of drug dithranol (in pharmaceutical form) and dodecanol-collodion (DDC) membrane. The spectroscopic data revealed a single-path photodegradation of dithranol in Vaseline (dithranol → danthrone, characterized by the 1st order decay rate of (7.85 ± 0.31)·10−4 s−1), and a multipath degradation in the DDC system, involving danthrone and the unknown compound (characterized by the absorption band at ~400 nm) as the final products. The desorption experiments performed enabled the identification of the unknown compound as the photodegradation product of dithranol molecules bound to the membrane matrix. The result led to the incorporation of the adsorption effects and heterogeneous structure of the membrane into the hydrodynamical model of mass transport. The model was tested against the photoacoustic depth-profiling data for dithranol permeation through DDC. The analysis allowed the dispersion and advection coefficients to be determined (D′ = (2.05 ± 0.03)⋅10−9 cm2 s−1 and va′ = (−5.55 ± 0.05)⋅10−7 cm s−1, respectively). Moreover, it was found, that the dithranol photodegradation rate in the non-steady state system is slower compared to the steady-state case; the effect was attributed to different permeation rates of dithranol and mobile Vaseline particles through the membrane.