The cytotoxic effect of ⁶⁴Cu/NOTA-terpyridine platinum conjugate, as a novel chemoradiotherapy agent

Colorectal cancer is one of the most prevalent cancers worldwide that displays both intrinsic and acquired resistance to platinum-based chemotherapeutic agents (Pt-CAs). To overcome such resistance, new classes of Pt-CAs have been proposed, including terpyridine (TP) compounds that targets the G-quadruplex tertiary structure of DNA. Additionally, recent studies indicate a maximum chemoradiation benefit, when radiation is administered with Pt-CAs at their highest concentrations in cancer cell DNA. Accordingly, we synthesized a novel chemoradiotheranostic agent by conjugating a TP moiety with ⁶⁴Cu (⁶⁴Cu-NOTA-TP). The in-vitro cytotoxic effects, cellular uptake, internalization and efflux of ⁶⁴Cu-NOTA-TP was measured for a colorectal cancer (HCT116) and normal fibroblast (GM05757) cells. Radiolabelling NOTA-TP with ⁶⁴Cu resulted in 17530-, 40083- and 66000-fold enhancements in its cytotoxicity against HCT116 cells (EC₅₀=0.017±0.004, 0.012±0.006 and 0.005±0.0002µM) as compared to ^coldCu-NOTA-terpyridine (EC₅₀ = 298 ± 2, 481 ± 25 and 330 ± 51µM) at 24, 48 and 72h post-administration, respectively. More importantly, the cytotoxicity of the ⁶⁴Cu-conjugate toward the HCT116 cells was about 3.8-fold higher than that of GM05757 cells at 24 and 72h. This result was consistent with a 2-3-fold higher internalization of ⁶⁴Cu-conjugate in HCT116 cells relative to GM05757 cells at similar times. The internalized activity of the ⁶⁴Cu-conjugate steadily increased from 0.04 ± 0.02% to 18.7±2.8% over 24h incubation time. Moreover, efflux kinetics of the ⁶⁴Cu-conjugate showed that more than 40% of internalized activity was retained by cancer cells over a 24h. In conclusion, this work presents a novel chemoradiotherapeutic agent with considerable potential for targeted cancer treatment combined with radioisotope imaging.