The Mechanisms of Action of Tumor Treating Fields

Tumor treating fields (TTFields), a new modality of cancer treatment, are electric fields transmitted transdermally to tumors. The FDA has approved TTFields for the treatment of glioblastoma multiforme and mesothelioma, and they are currently under study in many other cancer types. While antimitotic effects were the first recognized biological anticancer activity of TTFields, data have shown that tumor treating fields achieve their anticancer effects through multiple mechanisms of action. TTFields therefore have the ability to be useful for many cancer types in combination with many different treatment modalities. Here, we review the current understanding of TTFields and their mechanisms of action.

Permeabilizing Cell Membranes with Electric Fields

The biological impact of exogenous, alternating electric fields (AEFs) and direct-current electric fields has a long history of study, ranging from effects on embryonic development to influences on wound healing. In this article, we focus on the application of electric fields for the treatment of cancers. In particular, we outline the clinical impact of tumor treating fields (TTFields), a form of AEFs, on the treatment of cancers such as glioblastoma and mesothelioma. We provide an overview of the standard mechanism of action of TTFields, namely, the capability for AEFs (e.g., TTFields) to disrupt the formation and segregation of the mitotic spindle in actively dividing cells. Though this standard mechanism explains a large part of TTFields' action, it is by no means complete. The standard theory does not account for exogenously applied AEFs' influence directly upon DNA nor upon their capacity to alter the functionality and permeability of cancer cell membranes. This review summarizes the current literature to provide a more comprehensive understanding of AEFs' actions on cell membranes. It gives an overview of three mechanistic models that may explain the more recent observations into AEFs' effects: the voltage-gated ion channel, bioelectrorheological, and electroporation models. Inconsistencies were noted in both effective frequency range and field strength between TTFields versus all three proposed models. We addressed these discrepancies through theoretical investigations into the inhomogeneities of electric fields on cellular membranes as a function of disease state, external microenvironment, and tissue or cellular organization. Lastly, future experimental strategies to validate these findings are outlined. Clinical benefits are inevitably forthcoming.

Abstract 472: What electric field strength is necessary for maximum tumor-treating fields efficacy

Why Is 4 V/cm Electric Field Strength Necessary for Maximum Tumor-Treating Fields Efficacy? INTRODUCTION: Tumor Treating Fields (TTFields) extend overall survival in glioblastoma patients. Threshold field strength for TTFields is typically cited as 2V/cm. Yet plots of cell death vs. field strength in vitro indicate a power law relationship asymptotic to 4 V/cm for 100% efficacy. The mechanisms of action (MoA) of TTFields underlying this relationship are currently under investigation. TTFields are suspected of disrupting critical mitotic processes performed by large polar cellular molecules such as microtubules (MT). METHODS: Using the power law relationship for the energy (intensity, I) carried by an electric field and its amplitude (Eq. 1), we calculated energy vs. TTFields amplitude. (1) RESULTS: We suggest Eq. 1 is a likely source of the relationship between TTFields' amplitude and cell death proportion that has been demonstrated empirically. Further, Table 1 shows energy transmitted per unit MT surface area per TTFields cycle, specifically, the energy per MT dimer band around an MT helix per TTFields period of 5 µs.

TTFields Field Strength (V/cm)Radiant Flux Density (W/nm2)Energy/(Cycle-Area) (J/nm2)Energy/MT Dimer/Cycle (J)Energy/MT Dimer Band/Cycle (J)113.3 × 10−186.63 × 10−234.01 × 10−224.17 × 10−20253.1 × 10−1826.6 × 10−231.60 × 10−211.67 × 10−193119 × 10−1859.7 × 10−233.61 × 10−213.75 × 10−194212 × 10−18106 × 10−236.42 × 10−216.67 × 10−195332 × 10−18166 × 10−231.00 × 10−201.04 × 10−186478 × 10−18239 × 10−231.44 × 10−201.50 × 10−18

A metric for disruption of cellular processes is 1-2 orders of magnitude greater than the thermal background energy in the cell, kT = 4.3 × 10−21 J. Table 1 shows that the energy absorbed by a MT dimer is insufficient to disrupt cellular processes at 2 V/cm, while due to energy increasing with square of field strength, at 5 V/cm it may be disruptive. For an entire dimer band around the MT helix, 2 V/cm appears to be sufficient, while 4 V/cm seems amply strong to disrupt MT function, in accordance with in vitro experiments. CONCLUSION: Assuming the power law of energy vs. electric field strength and the resulting energy absorbed by microtubule components, we found a correlation between typical TTFields' amplitudes and estimated disruption thresholds of MT function. TTFields MoA. 4 V/cm, not 2V/cm, should be the target field strength to realize the full efficacy of TTFields.

Citation Format: Kristen W. Carlson, Jack A. Tuszynski, Socrates Dokos, Nirmal Paudel, Zeev Bomzon. What electric field strength is necessary for maximum tumor-treating fields efficacy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 472.

Fiber Threshold Accommodation as a Mechanism of Burst and High-Frequency Spinal Cord Stimulation

OBJECTIVES

Burst and high-frequency spinal cord stimulation (SCS), in contrast to low-frequency stimulation (LFS, < 200 Hz), reduce neuropathic pain without the side effect of paresthesia, yet it is unknown whether these methods' mechanisms of action (MoA) overlap. We used empirically based computational models of fiber threshold accommodation to examine the three MoA.

MATERIALS AND METHODS

Waveforms used in SCS are composed of cathodic, anodic, and rest phases. Empirical studies of human peripheral sensory nerve fibers show different accommodation effects occurring in each phase. Notably, larger diameter fibers accommodate more than smaller fibers. We augmented our computational axon model to replicate fiber threshold accommodation behavior for diameters from 5 to 15 μm in each phase. We used the model to predict threshold change in variations of burst, high frequency, and LFS.

RESULTS

The accommodation model showed that 1) inversion of larger and smaller diameter fiber thresholds produce a therapeutic window in which smaller fibers fire while larger ones do not and 2) the anodic pulses increase accommodation and perpetuate threshold inversion from burst to burst and between cathodic pulses in burst, high frequency, and variations, resulting in an amplitude "window" in which larger fibers are inactivated while smaller fibers fire. No threshold inversion was found for traditional LFS.

CONCLUSIONS

The model, based on empirical data, predicts that, at clinical amplitudes, burst and high-frequency SCS do not activate large-diameter fibers that produce paresthesia while driving medium-diameter fibers, likely different from LFS, which produce analgesia via different populations of dorsal horn neural circuits.

Abstract 3725: Numerical simulation of tumor treating fields effects on cell structures: Mechanism and signaling pathway candidates

Tumor Treating Fields (TTFields) have become a fourth modality for cancer treatment. Mild electric fields (~1-4 V/cm) produce few side effects and significantly extend overall survival of glioblastoma patients, and TTFields are in clinical trials for a variety of tumor cell types. Our goal is to uncover TTFields’ mechanism and cell signaling pathways by numerically modeling their effects on sub-cellular structures, such as microtubules (MTs) and their interactions with motor proteins. METHODS: We have built finite element models in COMSOL Multiphysics (tm) of the MT and its micro-environment to test hypotheses on TTFields’ mechanism of action by predicting effects on sub-cellular structures. RESULTS: One model prediction is that current density induced in the MT counter-ion layer by TTFields essentially shunts electric current within them. The strongest current flows through the counter-ion layer surrounding the MT’s C-termini and energy density in this layer likely exceeds the level to disrupt motor protein ‘walk’ along the MT. The energy density is predicted at 10-20 Joules when both the field and the MTs are aligned with the cell axis. A second mechanism examined by our model is disruption of the ‘foot’ of kinesin, released from its C-terminus contact by ATP (10-19 Joules). The final phase of the walk is driven by thermal buffeting of the forward foot randomly positioning it near enough to the C-terminus for electrostatic forces to bind it. A stall force ~10-19 - 10-16 N from TTFields would prevent diffusion and disrupt the kinesin walk. A recent clinical study segregating patient cohorts treated vs. not treated with dexamethasone found overall survival indefinitely for the non-dexamethosone cohort, leading us to hypothesize that TTFields activate the intrinsic Bcl2-mediated apoptotic signaling pathway. Future modeling will seek to tie disruption of motor protein transport along MTs to activating intrinsic apoptosis, e.g. via failure to silence the G2 cell cycle checkpoint. CONCLUSION: Our modeling predicts that TTFields in cytosol induce electric currents along MTs that are strong enough to disrupt key cellular functions such as the kinesin walk and C-termini transitions, both of which are crucial for motor protein transport. Hence, TTFields disrupt the most delicate mechanisms involved in the carefully-orchestrated succession of steps in mitosis.

Citation Format: Kristen W. Carlson, Nirmal Paudel, Jack A. Tuszynski, Zeev Bomzon. Numerical simulation of tumor treating fields effects on cell structures: Mechanism and signaling pathway candidates [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3725.

Dynamic Computational Model of the Human Spinal Cord Connectome

Connectomes abound, but few for the human spinal cord. Using anatomical data in the literature, we constructed a draft connectivity map of the human spinal cord connectome, providing a template for the many calibrations of specialized behavior to be overlaid on it and the basis for an initial computational model. A thorough literature review gleaned cell types, connectivity, and connection strength indications. Where human data were not available, we selected species that have been studied. Cadaveric spinal cord measurements, cross-sectional histology images, and cytoarchitectural data regarding cell size and density served as the starting point for estimating numbers of neurons. Simulations were run using neural circuitry simulation software. The model contains the neural circuitry in all ten Rexed laminae with intralaminar, interlaminar, and intersegmental connections, as well as ascending and descending brain connections and estimated neuron counts for various cell types in every lamina of all 31 segments. We noted the presence of highly interconnected complex networks exhibiting several orders of recurrence. The model was used to perform a detailed study of spinal cord stimulation for analgesia. This model is a starting point for workers to develop and test hypotheses across an array of biomedical applications focused on the spinal cord. Each such model requires additional calibrations to constrain its output to verifiable predictions. Future work will include simulating additional segments and expanding the research uses of the model.

Abstract 3209: Numerical modeling of intracellular mechanisms in tumor-treating fields

Tumor Treating Fields (TTFields) are 100-500 kHz electric fields with intensities of about 1-4 V/cm. They are known to exert an antimitotic effect on cancer cells, most likely by exerting forces on highly polar tubulin dimers, thereby disrupting spindle formation. Calculations show that TTFields-tubulin interaction energy is negligible compared to the thermal energy in the cell (1). Therefore, this interaction is unlikely to disrupt cellular function. Conductivity of polymerized tubulin, microtubules (MTs), was measured to be 20 S/m, which is 400 times higher than that of the ambient cytosol (0.05 S/m) (2). Thus when TTFields penetrate the cytosol, they may induce electric currents along MTs that are strong enough to disrupt key cellular functions. In particular, if the power (energy per unit time) deposited by these currents is on par with that the power consumed by the molecular motor kinesin, then TTFields may disrupt the forces needed for cell division, thereby disrupting mitosis. To test this hypothesis, we performed numerical simulations evaluating the magnitude of the electric current along MTs exposed to TTFields at 200 kHz. Based on studies of MTs and their microenvironment, we model the MT as a layered cylindrical structure (1): Innermost is the lumen (15 nm in thickness), surrounded by 13 strands of alpha-beta tubulin dimers linked in a helix (4.5 nm). C-termini extend out from the helix with a thickness of 3.5 nm. MTs carry net negative charge; thus they are surrounded by a counter-ion layer (2 nm), and an outer nonconductive Bjerrum layer (3 nm). We built a finite element model in COMSOL Multiphysics (tm) incorporating these layers and examined the current density induced in each layer by TTFields for MTs varying in length from 1-10 µm within an ambient 200 kHz AC electric field of 1-4 V/cm. Our model shows that MTs act as electrical "shunts" that conduct electric current within them. The highest current flows through the counter-ion layer surrounding the C-termini. The current density in this layer exceeds the level likely to disrupt the motor protein kinesin "walk" along the C-termini. The current density is highest when both the field and the MTs are aligned with the cell axis, in accord with in vitro experiments (3). Our model is consistent with the hypothesis that when cells are exposed to TTFields, MTs act as cables carrying high-density electric currents strong enough to disrupt the function of molecular motors, ultimately disrupting mitosis.

References:

1. Tuszynski JA et al. An overview of sub-cellular mechanisms involved in the action of TTFields. Int J Environ Res Public Health 2016.

2. Santelices IB et al. Response to alternating electric fields of tubulin dimers and microtubule ensembles in electrolytic solutions. Sci Rep 2017.

3. Kirson ED et al. Alternating electric fields arrest cell proliferation in animal tumor models and human brain tumors. Proc Natl Acad Sci U S A 2007.

Citation Format: Kristen W. Carlson, Jack A. Tuszynski, Socrates Dokos. Numerical modeling of intracellular mechanisms in tumor-treating fields [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3209.

Mechanism of dorsal column stimulation to treat neuropathic but not nociceptive pain: analysis with a computational model

OBJECTIVE

Stimulation of axons within the dorsal columns of the human spinal cord has become a widely used therapy to treat refractory neuropathic pain. The mechanisms have yet to be fully elucidated and may even be contrary to standard "gate control theory." Our hypothesis is that a computational model provides a plausible description of the mechanism by which dorsal column stimulation (DCS) inhibits wide dynamic range (WDR) cell output in a neuropathic model but not in a nociceptive pain model.

MATERIALS AND METHODS

We created a computational model of the human spinal cord involving approximately 360,000 individual neurons and dendritic processing of some 60 million synapses--the most elaborate dynamic computational model of the human spinal cord to date. Neuropathic and nociceptive "pain" signals were created by activating topographically isolated regions of excitatory interneurons and high-threshold nociceptive fiber inputs, driving analogous regions of WDR neurons. Dorsal column fiber activity was then added at clinically relevant levels (e.g., Aβ firing rate between 0 and 110 Hz by using a 210-μsec pulse width, 50-150 Hz frequency, at 1-3 V amplitude).

RESULTS

Analysis of the nociceptive pain, neuropathic pain, and modulated circuits shows that, in contradiction to gate control theory, 1) nociceptive and neuropathic pain signaling must be distinct, and 2) DCS neuromodulation predominantly affects the neuropathic signal only, inhibiting centrally sensitized pathological neuron groups and ultimately the WDR pain transmission cells.

CONCLUSION

We offer a different set of necessary premises than gate control theory to explain neuropathic pain inhibition and the relative lack of nociceptive pain inhibition by using retrograde DCS. Hypotheses regarding not only the pain relief mechanisms of DCS were made but also regarding the circuitry of pain itself, both nociceptive and neuropathic. These hypotheses and further use of the model may lead to novel stimulation paradigms.

Inhibition of mouse melanoma cell proliferation by corticotropin-releasing hormone and its analogs

BACKGROUND

Observations that epidermal cells release both corticotropin-releasing hormone (CRH) and proopiome lanocortin (POMC) peptides has raised questions about the physiological relevance of this hypothalamo-pituitary-like system in mammalian skin. As CRH has shown anti-proliferative effects on cultured keratinocytes, we tested whether CRH can also regulate growth of melanoma cells.

MATERIALS AND METHODS

CRH, [D-Glu20]-CRH, [D-Pro5]-CRH, acetyl-cyclo(30-33)[D-Phe12,D-Glu20,Nle21,D-His32,Lys33,D-Nle38]-CRH(4-41), acetyl-cyclo(30-33)[D-Phe12,Nle18,D-Glu20,Nle21,D-Ala32]-urotensin I(4-41), urocortin, and sauvagine were tested on Cloudman melanoma cell proliferation in culture and B16 melanoma tumor growth in C57B1/6 mice. Calcium-sensitive fluorescence measurements were used to examine the effect of CRH on intracellular Ca2+ signaling. The effects of CRH and [D-Glu20]-CRH on blood pressure were compared by measuring mean arterial pressure in anesthetized rats.

RESULTS

CRH and six analogs were tested, and all demonstrated exceptional potency in inhibiting Cloudman cell proliferation in culture, with half-maximal effective concentrations ranging between 0.2 and 100 pM. The amplitude of ionomycin-induced Ca2+ influx into Cloudman cells grown in suspension was reduced by 50% after 48-hr exposure to CRH. Daily injections of CRH or [D-Glu20]-CRH, 100 micrograms/kg.day s.c., for 5 days, reduced net B16 tumor volume in mice by 30-60% compared to control animals. [D-Glu20]-CRH was less hypotensive compared to CRH, despite having similar anti-proliferative potency.

CONCLUSION

CRH, and various analogs thereof, inhibit proliferation of Cloudman cells in culture, and inhibit B16 tumor growth rate in vivo, most likely by activation of endogenous CRH1 receptors and subsequent altered intracellular Ca2+ signaling. CRH analogs, such as [D-Glu20]-CRH, with less hypotensive activity may provide new directions of therapy for melanoma.

Nuclear Ca2+/calmodulin translocation activated by mu-opioid (OP3) receptor

Previous evidence has suggested a role for calmodulin (CaM) in opioid receptor signaling. We demonstrate here that morphine stimulation of the mu-opioid (OP3) receptor causes rapid CaM translocation to the nucleus in OP3-transfected human embryonic kidney (HEK)-293 cells and in SH-SY5Y human neuroblastoma cells. Ca2+ influx into the cells resulting from OP3 receptor activation was required for nuclear CaM translocation. Moreover, in HEK-OP3 and SH-SY5Y cells, increased nuclear CaM content was associated with enhanced phosphorylation of the nuclear transcription factor cyclic AMP-responsive element-binding protein. This appeared to be mediated by Ca2+/CaM kinases and also by a pathway involving protein kinase C. CaM was previously shown to bind directly to the OP3 receptor and to be released from the plasma membrane on agonist stimulation. To test whether OP3-mediated CaM release contributes to nuclear CaM signaling, we used a mutant OP3 receptor (K273A) with reduced affinity for CaM that fails to release CaM from the plasma membrane. K273A-OP3 activated Ca2+ influx to a similar extent as wild-type OP3; however, CaM translocation to the nucleus was attenuated. These results indicate that OP3-stimulated Ca2+ influx results in nuclear CaM translocation, which appears to be enhanced by simultaneous CaM release by OP3 wild-type receptor from plasma membranes. These results suggest a novel Ca2+/CaM signaling pathway of opioid receptors in the regulation of transcriptional activity.