Chemotherapeutic treatment efficacy and sensitivity are increased by adjuvant alternating electric fields (TTFields)

Abstract

Background: The present study explores the efficacy and toxicity of combining a new, non-toxic, cancer treatment modality, termed Tumor Treating Fields (TTFields), with chemotherapeutic treatment in-vitro, in-vivo and in a pilot clinical trial.

Methods: Cell proliferation in culture was studied in human breast carcinoma (MDA-MB-231) and human glioma (U-118) cell lines, exposed to TTFields, paclitaxel, doxorubicin, cyclophosphamide and dacarbazine (DTIC) separately and in combinations. In addition, we studied the effects of combining chemotherapy with TTFields in an animal tumor model and in a pilot clinical trial in recurrent and newly diagnosed GBM patients.

Results: The efficacy of TTFields-chemotherapy combination in-vitro was found to be additive with a tendency towards synergism for all drugs and cell lines tested (combination index <or= 1). The sensitivity to chemotherapeutic treatment was increased by 1-3 orders of magnitude by adjuvant TTFields therapy (dose reduction indexes 23 - 1316). Similar findings were seen in an animal tumor model. Finally, 20 GBM patients were treated with TTFields for a median duration of 1 year. No TTFields related systemic toxicity was observed in any of these patients, nor was an increase in Temozolomide toxicity seen in patients receiving combined treatment. In newly diagnosed GBM patients, combining TTFields with Temozolomide treatment led to a progression free survival of 155 weeks and overall survival of 39+ months.

Conclusion: These results indicate that combining chemotherapeutic cancer treatment with TTFields may increase chemotherapeutic efficacy and sensitivity without increasing treatment related toxicity.

Figure 1

Effect of 72 hour continuous application of TTFields and chemotherapeutic agents, separately and in combination on the cell proliferation of ER-negative MDA-MB-231 cells (presented as percent viable cells compared to control). (A) Percent viable cells vs. TTFields intensity. Effect of different concentrations of paclitaxel (B), doxorubicin (C) and cyclophosphamide (D), alone and in combination with TTFields of 1.75 V/cm. In B, C and D Filled Circles – represent drug alone; Filled Squares – drug in combination with TTFields. Each point represents mean values ± SEM of 18 to 36 replicate measurements. Dotted lines demarcate the IC₅₀ values for each curve.

Figure 2

Time course of the effects of 72 hour exposure of MDA cells to Paclitaxel (A), Doxorubicin (B) and Cyclophosphamide (C) alone and in combination with 1.75 V/cm TTFields. Each graph shows the number of viable cells in culture over time in control cells (interrupted lines), drug alone (open squares), TTFields alone (open circles) and drug-TTFields combination (closed squares). Data are presented as mean ± SEM. Each experimental condition included 18–36 samples.

Figure 3

Time course of recovery from 24 hour exposure to Paclitaxel (A), Doxorubicin (B) and Cyclophosphamide (C) alone and in combination with 1.75 V/cm TTFields. Each graph shows the number of viable cells in culture over time in control cells (interrupted lines), drug alone (open squares), TTFields alone (open circles) and drug-TTFields combination (closed squares). Data are presented as mean ± SEM. Each experimental condition included 18–36 samples.

Figure 4

Effect of light activated DTIC and TTFields (1.75 V/cm) on cell proliferation of U-118 glioma cells, presented as percent of viable cells compared to control. Open Circles – 72 hours of DTIC treatment alone. Filled Circles – 72 h of Combined DTIC – TTFields treatment.

Figure 5

Effect of combined Paclitaxel/TTFields on VX2 tumors in Rabbits. A VX-2 Kidney tumor volumes were normalized to pre-treatment tumor volume (day 7) and are presented over time for; control (diamonds), 5 mg Paclitaxel (circles), TTFields (squares) and combined TTFields-Paclitaxel (triangles). The effect of combined TTFields and Paclitaxel is equal to the sum of the effects of either treatment alone at both time points measured during the study (2 and 3 weeks from treatment start; n = 23; bars are standard errors of means). B Exemplary MRIs of the maximal contrast enhancing tumor area (demarcated by orange boarders) in the kidneys of rabbits in each of the experimental groups (sham control, Paclitaxel 5 mg, TTFields 2 V/cm, combined Paclitaxel and TTFields).

Figure 6

Kaplan Meier curves for A – progression free survival (PFS) and B – overall survival (OS) of newly diagnosed GBM patients receiving either combined TTFields – Temozolomide treatment or Temozolomide treatment alone. Red line – patients receiving combined TTFields – Temozolomide treatment (n = 10). Black line – concurrent/historical control patients that received Temozolomide treatment alone. A – The difference between the PFS curves is highly significant – Log-Rank Test (P = 0.0002), Hazard Ratio 3.32 (95%CI 1.9–5.9). B – The difference between the OS curves is highly significant – (Log-Rank Test; P = 0.0018). Dashed lines mark the median values for each curve.

Figure 7

Mechanisms of potentiation of chemotherapeutic efficacy by TTFields. A Tubulin chains are elongated by Paclitaxel, leading to an increase in the average dipole moment of free tubulin chains (d – length of an individual tubulin dimmer; f – force between the microtubule chain and the dimmer; F-force acting on the tubulin dimmers by TTFields; Arrow length is proportional to the intensity of these forces). The forces TTFields exert on these larger dipoles, F, are enhanced leading to an increase in the disruption of the mitotic spindle by TTFields. B TTFields act as an M-phase inhibitor, while alkylating agents act at the G and S phases of the cell cycle. This separation between cell cycle phases affected explains the additivity seen experimentally.