A pH-responsive prodrug delivery system of 10-HCPT for controlled release and tumor targeting

We synthesized a pH-responsive conjugate of 10-hydroxycamptothecin-thiosemicarbazide-linear polyethylene glycol 2000 (PEG2000). The conjugate was confirmed by matrix-assisted laser desorption time of flight mass spectrometry, ¹H NMR, and ¹³C NMR. The water solubility of the prodrug was increased by over 3,000 times; much longer body circulation time, higher tumor-targeting ability, and reduced toxicity were observed, compared with commercial 10-HCPT injection. The linker contains a pH-sensitive hydrazone bond, which breaks under low pH conditions in the tumor microenvironment. The conjugates showed good stability in phosphate-buffered saline (pH 7.4) and rat plasma. This amphiphilic conjugate could self-assemble into nanosized micelles of 80-100 nm. Cytotoxicity assay results indicate significantly higher efficacy of the conjugate (IC₅₀ [half maximal inhibitory concentration] =0.117 µM on SW180 cells) than 10-HCPT solution (IC₅₀ =0.241 µM on SW480 cells). Cellular uptake analysis suggested its rapid internalization and nuclear transport. Pharmacokinetic analysis of the conjugates demonstrated that the conjugate circulated for a longer time in the blood circulation system (T_2/1 =10.516±1.158 h) than did 10-HCPT solution (T_2/1 =1.859±1.385 h), and that it also enhanced the targeting and mean residence time (MRT_0-inf =39.873±4.549 h) in the tumor site, compared with 10-HCPT (MRT_0-inf =9.247±1.026 h). Finally, the conjugate demonstrated an increased tumor growth inhibition effect (TIR =82.66%±7.175%) in vivo and lower side effects than 10-HCPT (TIR =63.85%±5.233%). This prodrug holds great promise in improving therapeutic efficacy and overcoming multidrug resistance.