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Anthrax Toxin-Based Cancer Therapy Targets Tumor Blood Vessels

, by NCI Staff

Anthrax toxin consists of three proteins that individually are nontoxic. The protein known as protective antigen (left) binds to two receptors on cells that have been implicated in the growth of new blood vessels in tumors.

Credit: CC BY-SA 3.0 (David S. Goodsell)

Armed with a greater understanding of the detailed structure and function of the deadly anthrax toxin, scientists have engineered components of the toxin as a potential therapy for solid cancers.

Now, NIH scientists developing the toxin-based therapy have shown that it works by selectively targeting and inhibiting proliferation of cells that line the inside wall of blood vessels that feed tumors and support their growth and spread. When given in combination with two drugs that can block the production of antibodies against the toxin, the treatment greatly suppressed tumor growth in mouse models of lung cancer and melanoma, the researchers reported June 29 in the Proceedings of the National Academy of Sciences.

“One of the more attractive things about this [treatment] strategy is that it’s not tumor-type specific,” said Christine Alewine, M.D., Ph.D., of NCI’s Center for Cancer Research (CCR), who was not involved in the research.

Many targeted cancer therapies, including other toxin-based approaches, zero in on molecular targets present on a specific tumor type, such as lung or breast cancer, Dr. Alewine said. By contrast, “this targeted approach could potentially be used to treat many different types of tumors,” because the targets are present in blood vessels.

Exploiting a Deadly Toxin to Treat Cancer

The ability to target anthrax toxin to tumors so that healthy tissue is spared depends on a unique property of the toxin, explained study leader Stephen Leppla, Ph.D., of the National Institute of Allergy and Infectious Diseases. Unlike other bacterial toxins, anthrax toxin, which binds to receptors on the cell surface, must be snipped by protein-chewing enzymes called proteases before it can enter and kill a cell.

More than 10 years ago, Dr. Leppla and study co-author Thomas Bugge, Ph.D., of the National Institute of Dental and Craniofacial Research, an expert on cell-surface proteases, engineered modified forms of the anthrax toxin that require activation by specific cell-surface proteases, called MMPs and uPA, that are found at much higher levels in tumors than in normal tissues.

However, “some normal tissues produce modest amounts of MMPs and uPA, which could cause toxicity in those tissues,” Dr. Leppla said. To minimize the risk of adverse effects on normal tissues, he continued, “we engineered the next-generation toxin so that it needs both proteases to be activated, not just a single one.”

Identifying the Engineered Toxin’s Target

Anthrax toxin, which is made by the bacterium Bacillus anthracis, consists of three proteins that individually are nontoxic. One of the three proteins, known as protective antigen (PA), normally binds to two cell-surface receptors, TEM8 and CMG2. Both TEM8 and CMG2 have been implicated in the growth of new blood vessels in tumors, a process that is stimulated directly and indirectly by chemical signals released by the tumor.

Using various genetically altered mouse models, the NIH team showed that the engineered toxin works by binding to CMG2 receptors on endothelial cells in the blood vessels feeding the tumor—that is, cells in the animal’s own tissues—and not on the tumor cells themselves.  

In other words, Dr. Leppla said, “We’re killing the blood vessels that the tumor induces the host to make.” By contrast, blood vessels in various normal mouse tissues the researchers examined were not affected.

The research team, which also included Toren Finkel, M.D., Ph.D., of the National Heart, Lung, and Blood Institute, believe that two properties of endothelial cells in tumors explain why the engineered toxin targets them without harming endothelial cells in other tissues: First, the tumor environment is rich in MMPs and uPA, while these proteases are rare in normal endothelial cells and other cell types; and second, rapidly growing endothelial cells in tumors are more dependent than normal endothelial cells on a specific type of intracellular enzyme that is targeted by the engineered anthrax toxin once it enters a cell.

Additional experiments with endothelial cells isolated from tumors in the mice and grown in culture showed that the engineered toxin does not actually kill the endothelial cells but instead strongly inhibits their proliferation.

Suppressing Antitoxin Antibodies

One challenge when using toxins to treat cancer is that the host immune system sees the toxin proteins as “foreign” and produces antibodies that neutralize the toxin.

NCI CCR researcher Raffit Hassan, M.D., and his colleagues, who also work on toxin-based cancer treatments, previously showed that a regimen of two immune-suppressing chemotherapy drugs, pentostatin and cyclophosphamide, prevents mice from developing antibodies against another investigational toxin-based cancer therapy.

In this new study, using this two-drug regimen in mouse models of highly malignant and metastatic lung cancers and melanomas allowed the researchers to give up to five courses of the modified anthrax toxin without inducing antitoxin antibodies. In those experiments, Dr. Leppla said, “we extended the treatment out to 45 or 50 days, and in the best cases the tumor was reduced to a residual scar at that point.”

To their surprise, Dr. Leppla’s team found that the antitumor effects of the immune-suppressing regimen and the engineered toxin in mice were synergistic, meaning that the combined effects were greater than the sum of effects of either treatment given alone.

In addition, the study authors noted, because the modified toxin targets blood vessels rather than tumor cells themselves, tumors are less likely to develop resistance to the toxin-based strategy.

Possible Limitations and Next Steps

“We think that these agents are ready for testing in human clinical trials,” Dr. Leppla said, noting that some of his collaborators have obtained encouraging results in veterinary clinical trials with larger animals. However, he acknowledged that “extensive tests of toxicity must be done before putting these materials into humans.”

Indeed, Dr. Alewine noted, “Anthrax toxin is obviously a nasty poison, and the question is how specific [the engineered toxin] really is when you are using it in a human.”

In mice, she continued, “there is a therapeutic window where you can give the right amount so that you’re damaging tumor cells without damaging normal cells or interfering with normal biological processes.” But a similar window must be identified in humans in order for the treatment to be given safely.

In addition, said Dr. Alewine, “it’s not clear that the immune-suppressing drug regimen would permanently suppress the immune response to anthrax toxin.”

Dr. Hassan’s research group has used the same regimen in clinical studies of another toxin-based therapy, and found that it can delay the immune response against the toxin but not prevent it entirely in most patients. So it’s still unclear, she said, whether the two-drug regimen “would buy you enough cycles of [the engineered anthrax toxin] in humans that it could still be effective against the cancer.”

Nevertheless, she said, “It’s a very interesting strategy.”

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