Vaginal gel drug delivery systems: understanding rheological characteristics and performance.

INTRODUCTION: Vaginal gels are used for a wide range of clinical and pharmaceutical applications. Gel performance in vivo, including spreading ability, retention and drug release behavior, is closely related to rheological properties. Hence, a comprehensive rheological characterization of candidate gel formulations is important in screening and designing appropriate vaginal gels to achieve optimal clinical performance. AREAS COVERED: In this review, the basic destructive (flow) and non-destructive (oscillation and creep) techniques, commonly used in the assessment of gels, are introduced. The main rheological properties discussed in this work include viscosity, storage modulus, loss modulus, loss tangent and strain growth under small stress loads. In particular, this paper reviews the rheological methods used in characterizing vaginal gels and discusses the factors that may influence rheological performance. Recent advances in rheological methods, the use of advanced rheological methods and the challenges facing formulation scientists are also reviewed. EXPERT OPINION: The complex and dynamic environment of the vagina requires a comprehensive understanding of the rheological performance of vaginal gels. The establishment of suitable rheological tests to appropriately define such characteristics may facilitate the selection of a gel that avoids leakage. The ideal gel platform must provide adequate coating with minimal leakage. This is extremely difficult to obtain as it requires the formulation of a gel with a suitable viscoelastic balance.