Drug loading and encapsulation efficiency

The loading and encapsulation efficiency of paclitaxel in self-made nanoparticles and Abraxane were obtained by determining the amounts of free paclitaxel and total paclitaxel. To obtain the free paclitaxel in these particles, approximately 25mg of paclitaxel nanoparticles was firstly suspended in 10mL of PBS. Then, 0.5mL of the suspensions was drawn out and placed into the upper chamber of a centrifuge tube matched with an ultrafilter (Sartorius Vivaspin 500μL, 30k MWCO PES, Germany). Finally, the ultrafiltrate containing free drug was acquired after centrifugation for 30min at 12,000rpm. To determine the total drug in paclitaxel nanoparticles, approximately 10mg of self-made nanoparticles or Abraxane was added into 10mL of cold acetonitrile, followed by vortex for 5min and centrifugation at 12,000rpm for 5min. Thereafter, the supernatant was used for further analysis [28].

The amount of free paclitaxel in filtrate and the total amount of paclitaxel encapsulated and unencapsulated in nanoparticles were both detected by high-performance liquid chromatography (HPLC). The separation was performed on a reverse-phase silica column (Agela VenusiL MP-C18, 4.6×250mm, 5μm). The mobile phase consisted of acetonitrile and water (47.5: 52.5, V/V) and was pumped at a flow rate of 1.0mL/min. The detection wavelength was 227nm, and the injection volume was 20μL [29].

The encapsulation efficiency and drug loading were obtained according to Eqs (1) and (2), respectively, where W_{total drug} indicates the amount of paclitaxel in the whole nanoparticle suspension, W_{free drug} indicates the amount of paclitaxel unencapsulated in the nanoparticles, and W_{nanoparticles weighed} indicates the amount of paclitaxel nanoparticles used for determination.