Behavior of Uptake of Moisture by Drugs and Excipients

Recently, we reported that light accelerates the rate of moisture gain by a hygroscopic drug, ethambutol (1). This physical phenomenon was observed when ethambutol and its combinations with other antituberculosis drugs were subjected to stability studies in chambers set at 40
C and 75% relative humidity (RH) in the absence and the presence of light. The study was later extended to packed and unpacked marketed antituberculosis products to determine whether those formulations also exhibited different behavior in the absence and the presence of light. The same phenomenon was found to occur in the case of even unpacked and blisterpacked products (2). In the meantime, a study was also undertaken to determine whether this phenomenon was applicable, in general, to other drug substances and excipients. For the study, 54 pure drug substances and excipients were exposed to accelerated conditions of 40
C and 75% RH in the absence and the presence of light. The results are presented in this article.

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