Pertaining to cisplatin-resistant cell line of A2780 DDP, CholesPt(IV)-Liposome mediated notable potency, possessing an IC
50 of 12.6 µmol/L even exceeding the potency of cisplatin (IC
50 18.6 µmol/L) (
Fig. 2c). Close observation of the IC
50 values of cisplatin in A2780 and A2780 DDP identified the drug resistance characters of A2780 DDP substantially curtailed the drug potency of cisplatin, whose IC
50 was subjected to pronounced rise from 2.7 µmol/L in A2780 to 18.6 µmol/L in A2780 DDP (
Fig. 2c). The reduced drug potency of cisplatin may be attributable to the characteristic impeded drug internalization implicated in the drug resist cells. Of note, cisplatin is elucidated to translocate to the cell interior either through passive diffusion or active transportation with aids of cell membrane protein (copper transporter 1:Ctr1)
[38]. Molecular insight identified the reduced expression of Ctr1 protein in the platinum-resistant cells, which hampers the efficiency of the Ctr1-dependent cisplatin internalization. In accordance, quantification of intracellular platinum affirmed remarkably reduced internalization of cisplatin [also cisPt(IV)] at drug-resistant cell line. Particular noteworthy was reduced intracellular cisplatin at 6 h post incubation as compared to that at 2 h (
Figs. 3a–c), implying the critical impact of platinum-resistant cells, not only feasible of impeding internalization but also capable of prompting efflux of molecular platinum drugs. On the contrary, nanoparticles have been documented to steer an alternative endocytosis pathway rather than Ctr1-mediated transportation. Hence, the cell internalization of CholesPt(IV)-Liposome is envisioned to be unperturbed by the reduced expression of Ctr1 in the drug resistant cells, thereby capable of overcoming the impeded cellular uptake presented by drug resistant cells. Subsequent investigations approved CholesPt(IV)-Liposome capable of overcoming drug resistance, as evidenced by comparable drug potency of CholesPt(IV)-Liposome to A2780 and A2780 DDP, approximate 9.7 µmol/L for A2780 and 11.4 µmol/L for A2780 DDP, respectively (
Figs. 2c and
d). In accordance to comparable IC
50 value of CholesPt(IV)-Liposome in A2780 and A2780 DDP, CholesPt(IV)-Liposome mediated similar level of cell uptake efficiency, which is in stark contrast to aforementioned cisplatin whose cell uptake efficiency was substantially lowered. These results suggest CholesPt(IV)-Liposome adopted an alternative pathway for cell internalization. CLSM observation for intracellular distribution of internalized CholesPt(IV)-Liposome captured colocalization of CholesPt(IV)-Liposome and endosome, confirmed the proposed CholesPt(IV)-Liposome internalized cell through endocytosis (Fig. S7 in Supporting information). Still, marked reduced cell uptake of CholesPt(IV)-Liposome at 4 ℃ as compared to 37 ℃ (
Fig. 3d), again affirmed CholesPt(IV)-Liposome internalized cells through an energy-dependent endocytosis pathway. Moreover, as opposed to aforementioned distinctive efflux character of A2780 DDP for cisplatin, the intracellular platinum was observed to progressive increase (
Fig. 3b), suggesting utilization of liposome by means of integration of lipophilic cholesPt(IV) prodrug in liposome could address the impeded cell internalization drawbacks of the molecular drugs encountered in drug resistant cells.