Technical Note: Hydrocortisone as a Performance Verification Test Reference Standard for In Vitro Release Testing

Abstract

INTRODUCTION Over the last two decades, in vitro release testing (IVRT) using vertical diffusion cells has become the standard approach for assessing the performance of topical semisolid dosage forms. More recently, studies have indicated that IVRT should be considered as a performance verification test for these types of dosage forms (1). Hauck et al. have further suggested that a 1% hydrocortisone cream be considered as a reference standard for such verification testing, despite its high variability in IVRT studies, because of the consistent release-rate profiles that have been observed and reported since its development more than two decades ago (1–4). Further examination of the potential sources of this high variability is warranted if hydrocortisone is to be established as the reference standard for IVRT verification testing. Membrane soaking has been reported as an essential procedure for studying the in vitro release of hydrocortisone from ointment formulations but unnecessary for creams and lotions (3). Despite being unnecessary for certain topical formulations, a soaking step is often included in IVRT experiments as a way to standardize protocols between laboratories since it is required to evaluate ointments (1). Typically, a solution of isopropyl myristate (IPM), a surfactant reported to be representative of skin lipids (5), and ethoxylated aliphatic amine (ethomeen) is used to soak the membranes for up to one hour before the experiment. Studies have been carried out to evaluate the impact of soaking a polysulfone membrane on the in vitro release rate of hydrocortisone from a 1% hydrocortisone cream. Figure 1 and Table 1 indicate that soaking the membrane for thirty minutes before initiating the experiment leads to a three-fold increase in the release rate of hydrocortisone. These data indicate that although soaking the membrane is not necessary to monitor the release of hydrocortisone, the inclusion of the soaking step enhances release. Further, the effect of adding ethomeen to the soaking solution is minimal, suggesting that this component may not be critical for enhancing the release rate of hydrocortisone. The sensitivity of hydrocortisone release rate to minor variations in temperature has also been assessed. Figure 2 and Table 1 illustrate the importance of maintaining a constant and accurate temperature during the IVRT experiment. A reduction in temperature by just one degree (i.e., 32 °C to 31 °C) leads to a small but significant change in the measured release rate of hydrocortisone as assessed by the Wilcoxon Rank Sum or Mann–Whitney statistical test. Therefore, care should be taken to minimize heat loss during IVRT experiments as they are typically performed at 32 °C. These results warrant additional work to evaluate the sensitivity of hydrocortisone release rate to common experimental parameters within an IVRT study to better understand the reported variability.