Alternating electric fields (TTFields) inhibit metastatic spread of solid tumors to the lungs

Abstract

Tumor treating fields (TTFields) are low intensity, intermediate frequency, alternating electric fields used to treat cancerous tumors. This novel treatment modality effectively inhibits the growth of solid tumors in vivo and has shown promise in pilot clinical trials in patients with advanced stage solid tumors. TTFields were tested for their potential to inhibit metastatic spread of solid tumors to the lungs in two animal models: (1) Mice injected with malignant melanoma cells (B16F10) into the tail vein, (2) New Zealand White rabbits implanted with VX-2 tumors within the kidney capsule. Mice and rabbits were treated using two-directional TTFields at 100-200 kHz. Animals were either monitored for survival, or sacrificed for pathological and histological analysis of the lungs. The total number of lung surface metastases and the absolute weight of the lungs were both significantly lower in TTFields treated mice then in sham control mice. TTFields treated rabbits survived longer than sham control animals. This extension in survival was found to be due to an inhibition of metastatic spread, seeding or growth in the lungs of TTFields treated rabbits compared to controls. Histologically, extensive peri- and intra-tumoral immune cell infiltration was seen in TTFields treated rabbits only. These results raise the possibility that in addition to their proven inhibitory effect on the growth of solid tumors, TTFields may also have clinical benefit in the prevention of metastatic spread from primary tumors.

Figures

Fig. 1

Placement of electrodes on mice (a) and rabbits (b). The four insulated electrodes were attached to the skin using hydrogel after depilation. The electrodes were wrapped with leucoplast and electrodes wires were connected to the TTFields generating system or to the sham control system. The four electrodes were functionally divided into two pairs each generating one field direction through the animal. The electrode pairs were placed so as to create two perpendicular field directions at the center of body. Sham electrodes were placed in the same configuration

Fig. 2

Malignant melanoma metastases as seen on the surface of the lungs of mice treated with TTFields. Exemplary photos of lungs of mice treated with TTFields (a) or sham control (b) are shown after removal of the pulmonary blood by perfusion with saline. Average number of surface metastases (±SD) in treated and control mice (c). Average lung weight (±SD) of treated and control mice

Fig. 3

Metastases number and size distribution in mice treated with TTFields (openbars) and sham control (closedbars). The mice were treated for 7 days followed by 7 days of recovery

Fig. 4

T1 weighted MRI images (post gadolinium) of the maximal cross-sectional area of a TTFields treated (a) versus sham control tumor (b) in rabbit kidneys. Arrows indicate tumor location. Scalebar 1 cm

Fig. 5

Kaplan–Meier survival curve of rabbits treated with TTFields (n = 23; redline) versus sham controls (n = 20; blackline). The median survivals of 70 vs. 57 days, respectively, are indicated by dottedlines

Fig. 6

Exemplary photos of surface lung metastases in TTFields treated (a) versus sham control rabbits (b). Treatment was initiated on day 12 from implantation of the kidney tumor. The average total number (±SD) of surface metastases (c) and the average number of large metastases (±SD) (d) in control versus treated rabbits

Fig. 7

Discrete intra-tumoral infiltration of CD45 positive T cells in control tumours (a) and abundant intra-tumoral CD45 positive T cells in TTFields treated tumours (b). Scale bar 100 μm