Results
IDX enhances the
cytotoxicity of cisplatin against NPC cell lines
The human parental NPC cell lines
C17, C666 and EBV-positive/negative NPC cells (NPC43+ve/ NPC43-ve) used
in this study were all positive for ENOX2 expression (Supplementary
Figure 1A and B). We first determine the drug concentrations that caused
inhibition of 50% cell viability (IC50). IDX inhibited
proliferation of NPC43-ve, NPC43+ve, C17 and C666 cells over 120 hours
with IC50 values ranging between 2-4 µM (Figure 1A);
however, NPC43-ve showed higher resistance to IDX treatment. Based on
this initial data, doses ranging from 1-4 µM were selected and used to
further assess the antitumor effects on cell migration and apoptosis.
Cancer cells treated with IDX showed significantly reduced migration
compared with untreated cells, in a dose-dependent manner (Figure 1B).
Likewise, IDX dose-dependently increased late-stage apoptotic cells
compared with the untreated control. (Figure 1C and D). The cis
demonstrated diverse cytotoxicity with IC50 values
ranging from 0.2-1.2 µM in the four tested cell lines (Figure 1E). In
contrast to IDX, NPC43-ve cells demonstrated pronounced sensitivity,
suggesting that cis might be more effective in killing EBV-negative
cells. A combination cytotoxicity study evaluated the interaction
between IDX (1, 2 and 4 µM) and cis (0.6, 1.2 and 2.6 µM). A combination
of IDX and cis enhanced the inhibition of cell proliferation (Figure
1F), with minimal impact observed when cells were co-treated with lowest
dose of cis (0.6 µM). Similarly, Figure 1G demonstrated that cell growth
was not significantly impacted in cells treated with either cis at 0.6
µM or IDX at 1 µM, while cis at 4 µM exhibited far greater inhibition on
cell growth. We therefore selected a combination treatment of cis 1.2 µM
with IDX at 2 or 4 µM resulting in ~10-fold and 20-fold
reductions in cell growth when compared to IDX and cis treatments alone,
respectively (Figure 1H). The combination treatment also resulted in a
reduction in NPC43-ve and NPC43+ve cell migration by 6-fold
(Supplementary Figure 1C).