Mads Daugaard, PhD

Photo by Vivian Sum

A protein expressed by the malaria parasite Plasmodium falciparum may prove useful for treating a range of cancers, according to research published in Cancer Cell.

This protein, VAR2CSA, binds a chondroitin sulfate (CS) that is found in placenta but is also present in many different cancer cells.

So investigators combined recombinant VAR2CSA (rVAR2) with 2 different toxins to create cancer-targeting treatments.

The treatments effectively targeted cancers in vitro and in vivo, impeding tumor growth and even eradicating cancer in some mice.

An idea is born

This research was born while the investigators were exploring why pregnant women are particularly susceptible to malaria. The team found that P falciparum produces VAR2CSA, which binds to a particular CS in the placenta, and that placenta-like CS (pl-CS) is found in most cancers.

This suggested the pl-CS could be a target for anticancer drugs, and VAR2CSA could provide the tool for carrying such drugs to tumors.

“Scientists have spent decades trying to find biochemical similarities between placenta tissue and cancer, but we just didn’t have the technology to find it,” said project leader Mads Daugaard, PhD, of the University of British Columbia in Vancouver, Canada.

“When my colleagues discovered how malaria uses VAR2CSA to embed itself in the placenta, we immediately saw its potential to deliver cancer drugs in a precise, controlled way to tumors.”

Testing rVAR2

After demonstrating that rVAR2 binds only to pl-CS, the investigators tested rVAR2 in patient-derived cancer cell lines of hematopoietic, epithelial, and mesenchymal origin. rVAR2 reacted with 95% (106/111) of these cell lines.

To determine whether rVAR2 could be used as a pl-CS-specific tumor-targeting system, the investigators fused the cytotoxic domain of diphtheria toxin (DT388) to rVAR2, creating a recombinant rDT388-VAR2 (rVAR2-DT) fusion protein.

The rVAR2-DT protein killed tumor cell lines of epithelial and mesenchymal origin, but it had no effect on normal primary human endothelial cells.

The investigators also tested rVAR2-DT in mouse models of prostate cancer and found that as few as 3 doses were enough to significantly inhibit tumor growth.

However, the team noted that clinical trials with DT fusions have shown that high drug concentrations are not well-tolerated.

So they chemically conjugated a hemiasterlin analog (KT886) to rVAR2 via a protease cleavable linker. The rVAR2-KT886 drug conjugate (VDC886) carried an average of 3 toxins per rVAR2 molecule.

The investigators tested VDC886 in 33 cancer cell lines and observed cytotoxicity in all cell lines.

So the team went on to test VDC886 in mouse models of non-Hodgkin lymphoma, prostate cancer, and metastatic breast cancer. VDC886 significantly inhibited tumor growth in all 3 models.

In mice with non-Hodgkin lymphoma, treated tumors were about a quarter the size of tumors in control mice. For the mice with prostate cancer, tumors completely disappeared in 2 of the 6 treated mice a month after they received the first dose of VDC886.

In mice with metastatic breast cancer, 5 of the 6 treated mice were cured and alive after almost 8 weeks. None of the control mice with metastatic breast cancer survived that long.

The investigators said they did not observe any adverse effects in the mice, and their organs were unharmed by the therapy.

“It appears that the malaria protein attaches itself to the tumor without any significant attachment to other tissue,” said Thomas Mandel Clausen, a PhD student at the University of Copenhagen in Denmark.

“And the mice that were given doses of protein and toxin showed far higher survival rates than the untreated mice. We have seen that 3 doses can arrest growth in a tumor and even make it shrink.”

Based on these results, 2 companies—Vancouver-based Kairos Therapeutics and Copenhagen-based VAR2 Pharmaceuticals—are developing the compound for clinical trials. The investigators believe this will take a few years.

Malarial protein is basis for potential cancer therapy