A proof-of-concept trial by researchers at ETH Zurich, the University Hospitals of Basel and Zurich, and the Basel-Land Cantonal Hospital, demonstrated that treating breast cancer patients with a low, safe dose of the Na+/K+ ATPase inhibitor digoxin significantly reduced the numbers of circulating tumor cells (CTCs) in CTC clusters. The team reported on the study results in Nature Medicine, in a paper titled, “Digoxin for reduction of circulating tumor cell cluster size in metastatic breast cancer: a proof-of-concept trial,” in which they concluded “… our data provide a first-in-human proof of principle that digoxin treatment leads to a partial CTC cluster dissolution, encouraging larger follow-up studies with refined Na+/K+ ATPase inhibitors and that include clinical outcome endpoints.”

Certain tumor types do not remain at their point of origin but spread throughout the body and form metastases. This is because the primary tumor continuously releases cancer cells into the blood. These CTCs can join together into small clusters of up to a dozen cells and settle in other organs. There, the clusters grow into larger metastatic tumors. Metastatic tumors are still a major medical problem: every year, around seven million people worldwide die from them. “CTCs are living cells that are shed from both primary and metastatic lesions into the bloodstream, acting as metastatic pioneers,” the authors wrote. “The presence of CTC clusters is associated with disease progression and reduced survival in a variety of cancer types.”

One example of such a spreading tumor is breast cancer. As soon as the primary tumor forms metastases, the chances of survival decrease drastically. Tens of thousands of women worldwide still die of metastatic breast cancer. Oncologists are therefore looking for ways to weaken or destroy the clusters in order to prevent the formation of metastases. “The presence of CTCs has been firmly established to be predictive of poor prognosis in patients with breast cancer,” the team noted.

Recent studies by the researchers and other groups found that clusters of CTCs have a larger metastatic capacity and a stronger link to poor prognosis than a single CTC. “Breast cancer metastasis depends on CTC clusters,” explained principal investigator Nicola Aceto, PhD, professor of molecular oncology at ETH Zurich. “The larger they are, the more successful they are.”

The Achilles’ heel of CTC clusters are the sodium-potassium pumps (Na+/K+-ATPases) that are located in the membranes of tumor cells and are responsible for transporting sodium out of the cells and potassium into them. Digoxin blocks these ion pumps and thus suppresses the ion exchange. The cells therefore absorb more calcium from the outside of the cell membrane. This weakens the cohesion of the cancer cells in the cluster, causing them to fall apart.

The active ingredient digoxin originally comes from the foxglove plant (Digitalis sp.) and is usually used for heart conditions such as heart failure. The ETH researchers discovered in 2019 that digoxin could also be effective in the context of breast cancer. They carried out an extensive screening in which they systematically tested more than 2,400 different substances in cell cultures to find active agents against clusters of circulating tumor cells (CTCs). “A screen with 2,486 U.S. Food and Drug Administration-approved drugs demonstrated that Na+/K+ ATPase inhibitors, such as cardiac glycosides, effectively dissolve CTC clusters into single cells, leading to metastasis suppression in mouse models of breast cancer.”

However, digoxin alone does not eliminate the existing tumor. The agent must be administered in combination with other substances that kill existing cancer cells.

Aceto and colleagues established the prospective first-in-human proof-of-concept Phase I Digxoin Induced Dissolution of CTC Clusters (DICCT), as a therapeutic exploratory Phase I study to examine whether the Na+/K+ ATPase inhibitor digoxin could disrupt CTC clusters in patients with metastatic breast cancer at dose levels that are safe and well tolerated. “The primary objective of the study was to assess the effect of digoxin on CTC cluster size in patients with metastatic breast cancer,” the team commented. “Of note, the size of CTC clusters, rather than their general abundance, best reflects cluster-dissolution properties.”

The study included nine patients with locoregionally recurrent or metastatic breast cancer with progressive disease not amenable to treatments with curative intent, who were treated using digoxin, and nine patients with CTC clusters and matched clinical characteristics assigned to an untreated control group.

The study met its primary endpoint, with results showing that among patients treated with digoxin, the number of cells per cluster decreased significantly—by an average of 2.2 cells. Given that typical cluster sizes are only a handful of cells, this means a significant reduction in the risk of metastases. “… the DICCT trial successfully demonstrated that a partial dissolution of CTC clusters can be achieved through the inhibition of the Na+/K+ ATPase in patients with metastatic breast cancer,” the investigators reported. “Although clinical outcome endpoints were not assessed in this proof-of-concept study, the DICCT trial provides first-in-class evidence that supports the design of novel approaches for metastasis prevention.”

The team also noted that the effects of digoxin on a relatively low dose on cluster size were significant but mild. “We highly anticipate future studies in this context, designed for a longer treatment duration, more frequent monitoring of drug serum levels or higher dosage or using refined Na+/K+ ATPase inhibitors with stronger cluster-dissolution activity and aimed at measuring clinical endpoints related to new metastasis development.”

In the next step, the researchers want to develop new molecules based on digoxin that are even better at dissolving the CTC clusters. The ETH spin-off, Page Therapeutics is already working on this solution. Aceto also wants to expand his research to other types of cancer that spread, such as prostate, colorectal, or pancreatic cancer, as well as melanoma. Initial experiments in his laboratory have already begun.