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A drug-release study was conducted to understand the release pattern of a drug from a patch to the surface of the skin. This
study which predicts the availability of the drug on the skin surface for skin permeation. Drug release from each formulation
was carried out in a USP-type V, dissolution apparatus using 20% v/v PEG 400 in normal saline at 37 ± 0.5 °C (12). The cumulative
amount of drug released per cm2 of patch was plotted against time for formulations I and II (see Figure 5). The true absorbance of the drug was measured
by deducting the absorbance of the control sample from the absorbance of the test sample. The drug-release study showed that
drug release sharply increased for 5 h to a mean cumulative amount of 1.5 mg/cm2 from both formulations. The release patterns and rates changed afterward, and the drug released in comparatively slower
rates till the study was continued. At 72 h, drug release was about 2 mg/cm2 of patch for both formulations. The drug present on the patch-matrix surface and near the matrix surface initially released
more drug. However, the duration of the release from the patches might be less because of the increased amount of time required
for drug molecules to reach the patch surface through the entanglement of polymeric network of the patch matrix.
The in vitro skin-permeation study of a drug predicts the drug's in vivo skin-permeation performance. In the present study, in vitro skin permeation was carried out in a modified Keshary–Chein diffusion cell using cadaver skin. The cumulative amounts of
the drug permeated through skin from each cm2 of patch area was about 500 μg in the first 2.5 h in both formulations. Then, the skin permeation of the drug in both formulations
slowly increased with a similar permeation pattern until the 50-h point (see Figure 6). A similar drug-permeation pattern
was maintained in Formulation II until 72 h. The trend varied for Formulation I, where the drug permeated more between 50
and 70 h, compared with the drug in Formulation II. Drug permeation was high during the first few hours. This observation
may result from the faster release of the drug (see Figure 5) from the patch surface and near to the patch surface in the
patch matrix. During the first 10 h, drug permeation was fast, then it gradually slowed. The slowdown could have resulted
from the availability of the drug on the skin surface as depicted in the drug-release data (see Figure 5).
