Development of a Clinically Relevant Dissolution Method for Metaxalone Immediate Release Formulations Based on an IVIVC Model

Purpose The aim of the present work was to classify metaxalone according to the Biopharmaceutics Classification System (BCS), to develop a clinically relevant dissolution method that can be used to predict the oral absorption of metaxalone and to establish an in vitro-in vivo correlation (IVIVC). Methods Solubility of the drug was studied in different pH media and permeability studies were performed using a Caco-2 cell model. The in vitro dissolution and in vivo disposition of metaxalone from 3 different immediate release (IR) tablet formulations were investigated using USP 2 apparatus and a single dose, four-way, crossover bioequivalence study in healthy humans along with an oral solution of the drug, respectively. An IVIVC was established by using a direct, differential based method. Results Metaxalone has been confirmed as a Class II drug according to BCS. Bioavailability studies performed in humans demonstrated that dissolution was the rate limiting step for bioavailability of the drug and one of the test products had significantly improved bioavailability compared to the marketed product Skelaxin®. An IVIVC model was developed that demonstrated an acceptable internal predictability. Conclusion The IVIVC demonstrated that formulation factors play a significant role in dissolution and absorption of metaxalone. A pH 4.5 dissolution medium containing 0.5% NaCl with 0.2% SLS (USP apparatus 2 at 50 rpm) is clinically relevant to predict bioavailability of the drug and is superior to the USP method in terms of the Quality by Design (QbD) concept.