Metronomics: At-home Cancer Care
The purpose of metronomic therapies is to slow the growth of new blood vessels and possibly control the growth of cancer stem cells. If one or both of these objectives can be met, tumor growth can be slowed down or sometimes even stopped. Depending on the disease, tumors can also shrink or disappear on metronomic therapies.
All tissues (including tumors) need blood vessels to obtain oxygen, nutrients, and to take away waste products. The first thing a tumor must have to grow is new blood vessels. Only then can it spread out and get larger. Growth will continue until the tumor out grows its blood supply or once it reaches a physical barrier that prevents its growth. If we can control how fast and how many blood vessels the tumor cells can make, then we can slow down or stop the growth of the tumor. Without adequate nutrients and oxygen, tumor cells will begin to die.
Stem cells are cells that have the potential to become almost any cell in the body. Some stem cells have more freedom to become different types of cell than others. In normal tissues, stem cells are typically few in number and rarely divide. As a whole, they act as a reservoir of functional cells for an organ or body system. Stem cells are naturally and genetically resistant to the influences of drugs and toxins. This makes them safe from damage and allows them to act as a reservoir for cells. Many organs of the body, especially the immune system, use stem cells as a part of their normal function. Ultimately, they are also the source of an organ’s ability to heal from an injury or disease. Stem cells function by dividing when needed into two separate cells. One of these new cells will continue to act as a stem cell and no longer divide. The other new cell will continue divide, with each subsequent cell division the resulting cells become more mature and specialized. This maturation and cell division process will continue until there are enough cells of a specific cell type to perform whatever functions are needed by the body.
It is thought that tumors also have a type of stem cell. There are likely only a few cancer stem cells within a tumor; especially in comparison to the more mature tumor cells. Cancer stem cells are thought to be naturally or genetically resistant to cancer drugs. Their resistance is also likely because of their slow growing nature. Many cancer therapies are designed to kill only fast growing (dividing) cells. That means these therapies are targeting the more mature tumor cell, and not the cancer stem cells. Recurrence or progression of a cancer could therefore be due to the naturally resistant cancer stem cells repopulating the tumor. Cancer stem cells could act as a reservoir of cells for the tumor, only stimulated to divide when there is a signal that the level of mature cancer cells has been depleted, such as after being treated by a traditional therapy.
Metronomic protocols are designed to target the formation of blood vessels in tumors and possibly target the slow growing stem cell population within that tumor or cancer. How this protocol causes an effect is by providing a continual low amount of therapy over a long period of time. By delivering the drugs in this manner, you ensure that there is a constant level of drug in the blood at all times. This constant level of drug ensures that cells who divide rarely will be exposed to these drugs at the critical times during their division. These protocols do not typically provide fast ‘results’ but instead help slow growth over a period of time or maintain the results achieved during traditional therapies. In most cases, the amount of time needed for efficacy is 4 to 12 weeks. During this period of time, tumor cells can continue to grow and the tumor can progress. If there is no significant growth or progression during this time, metronomics have been shown to provide longer surgical and clinical remissions, or even stabilize tumor size, than those animals that receive no therapy or even receive other types of therapy. If there is gross disease present (an obvious tumor) at the time of starting this protocol, in rare cases, the protocol can cause shrinkage of the tumor. Most commonly, tumors will continue to grow very slowly. Lastly, there is always a chance metronomic protocols will not work or will suddenly stop working and the cancer will progress.
If your doctor recommends a metronomic protocol and you choose to pursue it, your pet will typically need an exam and blood work checked at two weeks, six weeks, and three months after starting this protocol. These appointments will allow us to assess whether there are any problems with the protocol and how the tumor is responding to the therapy. At the three month appointment, if all is going well, we will continue with the every three month appointments to recheck an exam, repeat blood work, and any other needed diagnostics to monitor the cancer or for cancer recurrence. This schedule is variable, however, and is ultimately based on what your pet’s specific needs are and how they respond/tolerate the therapy.