Traditionally, chemotherapy has been delivered at maximally tolerated doses intended to maximize the destruction of cancer cells. Some normal cells in the body also divide rapidly, however, and may be killed by chemotherapy. To minimize the effects of normal cell death, chemotherapy drugs are typically stopped for a period of time to allow patients to recover. Unfortunately, this recovery period may also provide time for cancer to rebound.
Currently, a different approach to chemotherapy is being studied. This is referred to as “metronomic” or continuous, low-dose chemotherapy, which targets the blood vessels that provide oxygen and nutrients to a tumor. As tumors enlarge, they require a greater blood supply, which is delivered by new blood vessels formed in a process called angiogenesis. In angiogenesis, tumors send signals to nearby blood vessels, which induces new blood vessels to form and grow toward the tumor. Once this connection is established, the tumor receives oxygen and nutrients from the blood, enabling further tumor growth and spread. These blood vessels are sensitive to extremely low dosages of chemotherapy, however, so when chemotherapy is given chronically, the blood vessels stop growing and can be pruned away from the tumor, resulting in decreased tumor growth and ultimately, tumor death. A metronomic protocol typically involves daily administration of a chemotherapy drug (chlorambucil, cyclophosphamide) at a low dose in conjunction with a nonsteroidal medication (piroxicam, rimadyl, meloxicam, deramaxx).
The goal of metronomic chemotherapy is to stop a tumor from growing and spreading, not to eliminate it completely, while limiting side effects. These medications are given orally so they may be administered at home. Several veterinary studies have noted positive responses to metronomic chemotherapy used to tumor roots left behind following surgery.
During metronomic chemotherapy, patients require a regular physical examination and laboratory testing of blood and urine samples. Metronomic therapy can be combined with standard chemotherapy or radiation therapy and provides another therapeutic option for patients with cancer.