The drug loading content and encapsulation efficiency of the nanomedicines were determined by analytical reverse-phase high-performance liquid chromatography (RP-HPLC). Briefly, lyophilized SN38-loaded NPs (10 mg) were dissolved in acetonitrile (200 μL) and, subsequently, the NP solutions were added to NaOH (0.5 M, 200 μL) and stirred for 30 min at 37 °C to release the SN38 molecules. The suspension was centrifuged to collect the supernatant, and the SN38 content was quantitatively determined by HPLC. RP-HPLC was performed using a Hitachi Chromaster 5000 system with a YMC-Pack ODS-A column (5 μm, 250 × 4.6 mm) at a flow rate of 1.0 mL/min. UV detection for SN38 was performed at 378 nm. All of the runs used linear gradients of acetonitrile (solvent A) and water (solvent B) containing 0.1% trifluoroacetic acid (TFA). The encapsulation efficiency (EE) and percentages of drug loading (DL) of SN38 in NPs were calculated according to Equations (1) and (2):
where WSN38inNP, Wfeed, and Wtotal represent the amount of SN38 encapsulated in PEG5k-PLA8k NPs, the SN38-equivalent amount fed for NP fabrication and the total amount of nanomedicines, respectively.
where WSN38inNP, Wfeed, and Wtotal represent the amount of SN38 encapsulated in PEG5k-PLA8k NPs, the SN38-equivalent amount fed for NP fabrication and the total amount of nanomedicines, respectively.