Nanosecond electric pulses are equally effective in electrochemotherapy with cisplatin as microsecond pulses

Abstract

Background: Nanosecond electric pulses showed promising results in electrochemotherapy, but the underlying mechanisms of action are still unexplored. The aim of this work was to correlate cellular cisplatin amount with cell survival of cells electroporated with nanosecond or standardly used 8 × 100 μs pulses and to investigate the effects of electric pulses on cisplatin structure.

Materials and methods: Chinese hamster ovary CHO and mouse melanoma B16F1 cells were exposed to 1 × 200 ns pulse at 12.6 kV/cm or 25 × 400 ns pulses at 3.9 kV/cm, 10 Hz repetition rate or 8 × 100 μs pulses at 1.1 (CHO) or 0.9 (B16F1) kV/cm, 1 Hz repetition rate at three cisplatin concentrations. Cell survival was determined by the clonogenic assay, cellular platinum was measured by inductively coupled plasma mass spectrometry. Effects on the structure of cisplatin were investigated by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry.

Results: Nanosecond pulses equivalent to 8 × 100 μs pulses were established in vitro based on membrane permeabilization and cell survival. Equivalent nanosecond pulses were equally efficient in decreasing the cell survival and accumulating cisplatin intracellularly as 8 × 100 μs pulses after electrochemotherapy. The number of intracellular cisplatin molecules strongly correlates with cell survival for B16F1 cells, but less for CHO cells, implying the possible involvement of other mechanisms in electrochemotherapy. The high-voltage electric pulses did not alter the structure of cisplatin.

Conclusions: Equivalent nanosecond pulses are equally effective in electrochemotherapy as standardly used 8 × 100 μs pulses.

Keywords: cisplatin; electrochemotherapy; electroporation; nanosecond pulses.

Figure 1

Cell survival of (A) CHO and (B) B16F1 cells at different cisplatin concentrations determined by the clonogenic assay for non-electroporated (non-EP) cells (black circles) and cells electroporated with 25 x 400 ns pulses at 3.9 kV/cm, 10 Hz repetition rate (dark blue squares), 1 × 200 ns pulse at 12.6 kV/cm (light blue diamonds) or 8 × 100 μs pulses at 1.1 (CHO) or 0.9 (B16F1) kV/cm, 1 Hz pulse repetition rate (orange triangles). Bars represent standard deviation, asterisks (*) show statistically significant differences (p < 0.05) to the survival of non-electroporated cells without cisplatin. Survival data were combined from the previous⁸ (for non-electroporated cells and cells electroporated with 25 × 400 ns and 8 × 100 μs pulses) and the present study (for B16F1 cells, additional non-electroporated CHO cells and CHO cells electroporated with 1 × 200 ns pulse).

Figure 2

Pt amount in cell pellets of (A) CHO and (B) B16F1 cells after 25 min incubation at different extracellular cisplatin concentrations in non-electroporated (non-EP) cells (black circles) and cells electroporated with 25 x 400 ns pulses at 3.9 kV/ cm, 10 Hz repetition rate (dark blue squares), 1 × 200 ns pulse at 12.6 kV/cm (light blue diamonds) or 8 × 100 μs pulses at 1.1 (CHO) or 0.9 (B16F1) kV/cm, 1 Hz pulse repetition rate (orange triangles). Bars represent standard deviation, asterisks (*) show statistically significant differences (p < 0.05) to the measured number of cisplatin molecules in non-electroporated cells at the same extracellular cisplatin concentration.

Figure 3

Cell survival as a function of the number of cisplatin molecules per cell for (A) CHO cells and (B) B16F1 cells in non-electroporated (non-EP) cells (black circles) and cells electroporated with 25 x 400 ns pulses at 3.9 kV/cm, 10 Hz repetition rate (dark blue squares), 1 × 200 ns pulse at 12.6 kV/cm (light blue diamonds) or 8 × 100 μs pulses at 1.1 (CHO) or 0.9 (B16F1) kV/cm, 1 Hz pulse repetition rate (orange triangles). Bars represent standard deviation. Survival data were combined from the previous (for non-electroporated CHO cells and CHO cells electroporated with 25 × 400 ns and 8 × 100 μs pulses) and the present study (for B16F1 cells, additional non-electroporated CHO cells and CHO cells electroporated with 1 × 200 ns pulse).

Figure 4

¹H NMR spectra of cisplatin, showing the signals for hydrogens of NH₃ ligands labeled with asterisks (*). Spectra were recorded in a) D₂O, b) D₂O containing 154 mM NaCl, c) 90% H₂O/10% D₂O and d) 90% H₂O/10% D₂O containing 154 mM NaCl treated with 25 × 400 ns pulses (blue), 8 × 100 μs pulses (green) or no pulses (red).

Figure 5

The mechanism of cisplatin uptake into cells is not completely elucidated. In non-electroporated cells, cisplatin enters partially through passive diffusion and facilitated diffusion through ion channels including LRRC8 volume-regulated anion channels (VRAC) and membrane transporters like copper transporter 1 (CTR1) and organic cation transporters (OCTs). In electroporated cells, more cisplatin can enter through the permeabilized cell membrane (pore is a symbolic presentation of increased membrane permeability even though the mechanisms behind electroporation are more complex – refer to34).