Tumour-treating fields (TTFields): Investigations on the mechanism of action by electromagnetic exposure of cells in telophase/cytokinesis

Abstract

Tumour-treating fields (TTFields) use alternating electric fields which interfere with dividing cells, thereby reducing tumour growth. Previous reports suggest that electrical forces on cell structure proteins interfered with the chromosome separation during mitosis and induced apoptosis. In the present report we evaluate electromagnetic exposure of cells in telophase/cytokinesis in order to further analyse the mechanism of action on cells. We performed numerical electromagnetic simulations to analyse the field distribution in a cell during different mitotic phases. Based thereon, we developed an electric lumped element model of the mitotic cell. Both the electromagnetic simulation and the lumped element model predict a local increase of the specific absorption rate (SAR) as a measure of the electromagnetically induced power absorption density at the mitotic furrow which may help to explain the anti-proliferative effect. In accordance with other reports, cell culture experiments confirmed that TTFields reduce the proliferation of different glioma cell lines in a field strength- and frequency-dependent manner. Furthermore, we found an additional dependence on the commutation time of the electrical fields. The report gives new insights into TTFields' anti-proliferative effect on tumours, which could help to improve future TTFields application systems.

Figure 1

(a) Electrode setup for in-vitro TTFields exposure system. (b) Simulated electric fields in utilized setup, applied voltage V_e = 10 V_pp. Cells were cultivated within the area delineated by the red circle in the middle of the setup (d = 15 mm).

Figure 2

Calculated electric field distribution by application of a homogenous electrical field of E = 100 V_rms/m. Assumed parameters: cell diameter d_c = 20 μm, membrane thickness and permittivity chosen to realize a capacitance of C_m,ges = 29 pF, relative permittivity of external medium and cytosol: ε_r = 80, conductivity  σ = 1.3 S/m. For the logarithmic (colour) scaling in dB we calculated $\frac{E}{E_0}$, with E₀ the mean field strength in the surrounding medium.

Figure 3

Calculated local SAR in response to TTFields (E = 100 V_rms/m) applied at f = 100 kHz (s. Fig. 1). For the logarithmic scaling in dB we calculated $\frac{SAR}{SA{R}_0}$, with SAR₀ the mean SAR in the surrounding medium.

Figure 4

(a) Schematic representation of the lumped element model of a cell in telophase/cytokinesis exposed to an electric field polarised parallel to the longitudinal axis of the hourglass shape of the cell. See methods section for used element parameters. (b) Frequency-dependent currents in the cleavage furrow region as calculated with the lumped element model.

Figure 5

Simulated SAR in the cleavage furrow region.

Figure 6

Block diagram of the developed TTFields exposure setup.

Figure 7

Time- and field strength-dependent reduction of BT4Ca cell numbers after application of TTFields. Average cell numbers ± SEM from at least three replicates are shown. Significant differences compared to the control (cont., Student’s t test) are marked with asterisks: *P < 0.05, **P < 0.01.

Figure 8

Frequency-dependent reduction of BT4Ca cell numbers after application of TTFields (9 V_pp) for 72 h. Average cell numbers (% of control (cont.)) ± SEM from at least three replicates are shown. Significant differences compared to the control (Student’s t test) are marked with asterisks: *P < 0.05, **P < 0.01.

Figure 9

Commutation time-dependent reduction of BT4Ca cell numbers after application of TTFields (9 V_pp) for 72 h. Average cell numbers (% of control (cont.)) ± SEM from at least three replicates are shown. Significant differences compared to the control (Student’s t test) are marked with asterisks: *P < 0.05, **P < 0.01.

Figure 10

(a) Representative images of BT4Ca cells cultured in collagen I gels and stained for actin filaments (green) and nuclei (blue). (b) Reduction of the mean projection area of BT4Ca cell clusters cultured three-dimensionally in collagen I gels after application of TTFields (12 V_pp) for 72 h. Results are given as average ± SEM from five replicates.