Cancer drugs that recruit antibodies from the body's own immune system to help kill tumors have shown much promise in treating several types of cancer. However, after initial success, the tumors often return.

A new study from MIT reveals a way to combat these recurrent tumors with a drug that makes them more vulnerable to the antibody treatment. This drug, known as cyclophosphamide, is already approved by the Food and Drug Administration (FDA) to treat some cancers.

Antibody drugs work by marking tumor cells for destruction by the body's immune system, but they have little effect on tumor cells that hide out in the bone marrow. Cyclophosphamide stimulates the immune response in bone marrow, eliminating the reservoir of cancer cells that can produce new tumors after treatment.

"We're not talking about the development of a new drug, we're talking about the altered use of an existing therapy," says Michael Hemann, the Eisen and Chang Career Development Associate Professor of Biology, a member of MIT's Koch Institute for Integrative Cancer Research, and one of the senior authors of the study. "We can operate within the context of existing treatment regimens but hopefully achieve drastic improvement in the efficacy of those regimens."

Jianzhu Chen, the Ivan R. Cottrell Professor of Immunology and a member of the Koch Institute, is also a senior author of the paper, which appears in the Jan. 30 issue of the journal Cell. The lead author is former Koch Institute postdoc Christian Pallasch, now at the University of Cologne in Germany.

Finding cancer's hiding spots

Antibody-based cancer drugs are designed to bind to proteins found on the surfaces of tumor cells. Once the antibodies flag the tumor cells, immune cells called macrophages destroy them. While many antibody drugs have already been approved to treat human cancers, little is known about the best ways to deploy them, and what drugs might boost their effects, Hemann says.

Antibodies are very species-specific, so for this study, the researchers developed a strain of mice that can develop human lymphomas (cancers of white blood cells) by implanting them with human blood stem cells that are genetically programmed to become cancerous. Because these mice have a human version of cancer, they can be used to test drugs that target human tumor cells.

The researchers first studied an antibody drug called alemtuzumab, which is FDA-approved and in clinical trials for some forms of lymphoma. The drug successfully cleared most cancer cells, but some remained hidden in the bone marrow, which has previously been identified as a site of drug resistance in many types of cancer.

The study revealed that within the bone marrow, alemtuzumab successfully binds to tumor cells, but macrophages do not attack the cells due to the presence of lipid compounds called prostaglandins, which repress macrophage activity. Scientists believe the bone marrow naturally produces prostaglandins to help protect the immune cells that are maturing there. Tumor cells that reach the bone marrow can exploit this protective environment to aid their own survival.

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New weapon fights drug-resistant tumors hiding in bone marrow