A high-throughput micro-scale workflow to quantify molecularly dissolved drug concentrations under solubilizing conditions

Physiological and artificial solubilizing agents usually enhance apparent solubility of poorly soluble drugs, and in many cases also oral drug exposure. However, exposure may decrease in cases where micellization reduces the molecularly dissolved drug fraction, overriding the solubility advantage. While this information is critical to accurately anticipate the effect of drug micellization on oral absorption, the experimental determination of molecularly dissolved drug concentrations is complex and time consuming. The present study aimed at developing a micro-scale in vitro workflow (comparative micro-scale mass transfer assay, CMMTA) to quantify molecularly dissolved (unbound) drug concentrations in the presence of solubilizing agents. A linear correlation was determined between the cumulative passive permeation of the model drug riluzole (RLZ) and its concentration in the donor buffer solution on a 96-well microtiter sandwich plate (PermeaPlain™). Next, the drug permeation from micellar drug solutions (in fasted and fed state simulated intestinal fluids, FaSSIF and FeSSIF) was measured and the concentration of unbound drug was derived from the aforementioned correlation. The results were validated against established methods to measure free (unbound) drug concentrations, namely equilibrium dialysis and microdialysis. The concentrations of molecularly dissolved RLZ were correctly captured on one single microtiter plate. Both, the standard curve and samples at different solubilizing conditions can be determined simultaneously within a few hours using small quantities of drug substance. Hence, the proposed CMMTA workflow represents a promising screening tool for early-stage drug development.