In vitro dissolution testing of respirable size anti-tubercular drug particles using a small volume dissolution apparatus.

A dissolution apparatus that uses a small volume of stationary medium (25 μL) has been developed for in vitro dissolution testing of respirable drug particles and used to evaluate the dissolution of two anti-tubercular drugs, moxifloxacin and ethionamide. Solubilities of moxifloxacin and ethionamide in phosphate buffered saline (PBS, pH 7.4) were 17.68 ± 0.85 mg mL-1 and 0.46 ± 0.02 mg mL-1 whereas in the presence of lung surfactant (0.4% w/v Curosurf® in PBS) solubilities were 20.76 ± 0.35 mg mL-1 and 0.56 ± 0.03 mg mL-1, respectively. A fine particle dose (∼50 µg) of aerodynamically separated moxifloxacin or ethionamide particles (<6.4 µm) was collected onto a glass coverslip using a modified Twin Stage Impinger. The dissolution behaviour of the fine particle dose was evaluated at various perfusate flow rates (0.2, 0.4 and 0.8 mL min-1 of PBS), mucus simulant concentrations (1.0, 1.5 and 2.0% w/v polyethylene oxide in PBS), and in the presence of lung surfactant. The dissolution behaviour of the respirable size particles was observed under an optical microscope and the dissolved drug that diffused into the perfusate was quantified by HPLC. The moxifloxacin particles disappeared quickly and showed faster permeation (<30 min) compared to the ethionamide particles at all the dissolution conditions evaluated. This study demonstrated the differences in the dissolution rates of moxifloxacin and ethionamide particles and may be useful to estimate the residence time of the inhaled dry powder particles in the lungs.