2.7.1. Drug Release

A Franz diffusion cell (Logan Instruments Ltd., Somerset, NJ, USA) having an exposed surface area of 0.79 cm<sup>2</sup> , was used to carry out in vitro drug release. In short, a semipermeable membrane (MWCO 12–14 kDa) was held between the donor and receptor compartment. Clarithromycin-loaded SLN dispersion (CL1–CL9, 2.5 mg/mL of drug) or control (drug solution in 10% *v*/*v* span 80) was added in the upper compartment. The acceptor compartment (20 mL) contained simulated tear fluid (pH 7.4) with 1% Tween 80 (to maintain sink conditions) [29] and was stirred at 50 rpm. The temperature of the entire assembly was controlled at 37 ± 0.5 ◦C by means of a thermostatic water bath. At various time intervals, aliquots of samples (1 mL) were drawn and substituted with the equivalent amount of buffer held at the same temperature. A control experiment was performed at the same experimental conditions using a similar strength of clarithromycin solution. The samples were later diluted with mobile phase and quantified for clarithromycin. The data collected were analyzed to calculate regression coefficient (r<sup>2</sup> ) and interpret various mathematical models [30].
