Tailoring drug release from long-acting contraceptive levonorgestrel intrauterine systems

The wide array of polydimethylsiloxane (PDMS) variants available on the market, coupled with the intricate combination of additives in silicone polymers, and the incomplete understanding of drug release behavior make formulation development of levonorgestrel intrauterine systems (LNG-IUSs) formidable. Accordingly, the objectives of this work were to investigate the impact of excipients on formulation attributes and in vitro performance of LNG-IUSs, elucidate drug release mechanisms, and thereby improve product understanding. LNG-IUSs with a wide range of additives and fillers were prepared, and in vitro drug release testing was conducted for up to 12 months. Incorporating various additives and/or fillers (silica, silicone resins, silicone oil, PEG, etc.) altered the crystallization kinetics of the crosslinked polymer, the viscosity, and the microstructure. In addition, drug-excipient interactions can occur. Interestingly, additives which increased matrix hydrophobicity and hindered PDMS crystallization facilitated dissolution and permeation of the lipophilic LNG. The influence of additives and lubricants on the mechanical properties of LNG-IUSs were also evaluated. PDMS chemical substitution and molecular weight were deemed to be most critical polymer attributes to the in vitro performance of LNG-IUSs. Drugs with varying physicochemical characteristics were used to prepare IUSs, modeling of the release kinetics was performed, and correlations between release properties and the various physicochemical attributes of the model drugs were established. Strong correlations between first order release rate constants and both drug solubility and Log P underpin the partition and diffusion-based release mechanisms in LNG-IUSs. This is the first comprehensive report to provide a mechanistic understanding of material-property-performance relationships for IUSs. This work offers an evidence-based approach to rational excipient selection and tailoring of drug release to achieve target daily release rates in vivo. The novel insights gained through this research could be helpful for supporting development of brand and generic IUS products as well as their regulatory assessment.