<i>tert</i>-Butoxycarbonyl-Modification Driven Disturbance of Molecular Ordering Enables High-Efficiency Dual Drugs Co-Assembly for Synergistic Tumor Inhibition

The development of carrier-free drug delivery systems (CDDS) for tailored drug combinations posed a significant challenge, particularly in achieving efficient co-assembly while maintaining therapeutic efficacy. Herein, we proposed a co-assembly strategy based on molecular engineering. Paclitaxel (PTX) and 7-ethyl-10-hydroxycamptothecin (SN38) were chemically modified with <i>tert</i>-butoxycarbonyl (BOC) groups. The successful incorporation of the BOC groups was confirmed by proton nuclear magnetic resonance and mass spectrometry analyses. Further characterization using polarized light microscopy and X-ray diffraction revealed that this modification significantly reduced the crystallinity of both drugs, while simultaneously disrupting their original ordered stacking structure. Molecular dynamics simulations indicated that BOC modification increased molecular spacing, reduced stacking density, and expanded molecular volume, resulting in a looser molecular packing arrangement. This structural alteration enabled the modified drug molecules to efficiently coassemble with α-tocopherol succinate (α-TOS) into spherical nanoparticles at a nearly predefined mass ratio. The resulting nanoparticles exhibited a high drug loading capacity of 52.66% and remained stable at 4 °C for over 50 days. Notably, these nanoparticles displayed controllable release characteristics at pH 5.0. Both <i>in vitro</i> and <i>in vivo</i> studies demonstrated the BOC-modified drugs retained their bioactivity. When co-assembled with α-TOS, the nanoparticles exhibited a significant synergistic antitumor effect and suppressed tumor metastasis through downregulation of matrix metalloproteinase-9 (MMP-9) expression. This study provided a solid theoretical foundation and innovative approach for the development of CDDS, utilizing molecular-scale regulation for drug co-assembly.