Drug release testing of long-acting intrauterine systems

Abstract

Performance evaluation of polydimethylsiloxane (PDMS) based long-acting (e.g. 3–5 years) levonorgestrel (LNG) intrauterine systems (IUSs), such as Mirena®, is challenging due to their complex formulation, locally-acting feature, and extremely long duration of drug release. To achieve such long-term release, a large amount of drug (up to 52 mg in Mirena®) must be incorporated as a drug reservoir in the IUS. Consequently, dose dumping or unanticipated changes in the LNG-IUS in vivo release characteristics may give rise to adverse product safety and efficacy. Therefore, it is crucial to understand, and have appropriate control over, the physicochemical properties and in vitro release characteristics of these products. This requires an understanding of the LNG-IUSs drug release mechanism and the development of a sensitive yet robust in vitro release testing method. There have been no previous reports on in vitro drug release and the release mechanism from LNG-IUSs. This is probably a consequence of the extremely slow drug release rate of LNG-IUSs under real-time in-use conditions (e.g., 3–5 years) and therefore it is impractical to obtain complete release profiles (e.g. there is only 60% release in 5 years for Mirena®). Therefore, the development of appropriate accelerated in vitro release methods is imperative. Following preparation of LNG-IUSs, similar to Mirena®, real-time release was tested in (0.9% w/v NaCl) media in a water shaker bath at 37 °C for over 2 years. Addition of surfactant (sodium dodecyl sulfate (SDS)), elevation of temperature, addition of organic solvents (ethanol (EtOH), isopropanol (IPA), tert-butanol (TBA) and tetrahydrofuran (THF)) and a combination thereof were utilized as release media to accelerate drug release for LNG-IUSs. Complete drug release was achieved in 32 and 672 days in THF and TBA hydro-organic media, respectively. The release profile in THF was considered too fast as it may result in change of release mechanism, whereas the release profile in TBA was deemed suitable following model fitting. Model fitting was performed to understand the release characteristics as well as the release mechanisms. The release rate in the hydro-alcoholic media was linearly proportional to the swelling ratio of the PDMS in the corresponding organic solvents. Zero-order, first-order and two-phase models were utilized to fit the release profiles obtained under the different release conditions. The data analysis was comparable using the parameters from different models given the high R2 values. However, the two-phase model was better in terms of the release mechanism of the LNG-IUSs considering the full drug release profile. The present study will facilitate the process of granting of biowaivers through an in vitro approach, thus reducing the necessity for clinical studies. In addition, it will help reduce the regulatory burden without sacrificing product quality of LNG-IUS products.