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The Use of Chemotherapy in the Fight Against Cancer

How various agents attack mutating cancer cells

Cancer has been referred to as “the immortal cell” because cancer cells have mutated and developed six “hallmarks of cancer.” This mutation allows cancer cells to continually divide, invade tissues and spread throughout the body. When critical cell receptors are changed by cancer cells, the body’s normal immune system is no longer able to remove these wayward cells.

Chemotherapy is one of the main strategies used to fight back with many types of cancer. Different drugs are designed to interfere with cancer cells during different points of the cell cycle, such as DNA replication, cell division, and cell signaling pathways. By studying the way cancer cells mutate and originate, more targeted chemotherapeutics are being developed to help in the fight against cancer.

Depending on the type of cancer your pet has, chemotherapy may be used alone or in combination with surgery or radiation. Chemotherapy acts to target rapidly dividing cells within the body; however, this may also result in normal cells being targeted. There are seven general groups of chemotherapy, each of which targets specific parts of a cell’s life cycle. The groups of chemotherapy agents include:

  • Alkylating agents – Alkylating agents bind to DNA and are the broadest spectrum form of chemotherapy. They bind to DNA at any phase of the cell cycle, triggering cell death.
  • Antimetabolites – Antimetabolites are more specific to stages of the cell cycle and prevent cells from using metabolites when they grow and divide.
  • Antitumor antibiotics or topoisomerase inhibitors – Topoisomerase inhibitors and antitumor antibiotics are specific to certain parts of the cell cycle. They affect cells by breaking DNA.
  • Plant alkaloids – Plant alkaloids are derived from plants such as the periwinkle flower. They are generally more specific to certain parts of the cell cycle.
  • Hormones – Hormones such as steroids can be used for palliative care or in combination with other forms of chemotherapy.
  • Platinum agents – Platinum agents function similarly to alkylating agents and are generally non-specific to what part of the cell cycle is affected.
  • Tyrosine Kinase Inhibitors –  Tyrosine kinase inhibitors are more targeted therapy that specifically targets receptors on certain cancer cells, particularly mast cells.

The following are common chemotherapy agents that you might encounter in your pet’s treatment plan:

Carboplatin

Carboplatin is commonly used to treat a variety of cancers. Side effects may include bone marrow suppression and gastrointestinal symptoms, such as vomiting and diarrhea. Administration is often intravenous injection. However, if fluid is present in the chest, the chemo may be given ‘intracavitary’, or directly into the chest.

Chlorambucil

Chlorambucil is commonly used to treat small-cell lymphoma in cats. It can also be used as a maintenance chemotherapy for many types of cancer. When used as ‘metronomic’ chemotherapy, it is given in a lower dose with the intent to slow down cancer spread and minimize side effects. Side effects can include bone marrow suppression and gastrointestinal symptoms, such as vomiting and diarrhea. It is usually given as a tablet by mouth at home. 

Cyclophosphamide

Cyclophosphamide is commonly used to treat lymphoma. One possible side effect of this chemotherapy agent is blood in the urine. In order to minimize the chance of this side effect, this chemotherapy can be given with a diuretic to reduce the concentration in the bladder. It is usually given by mouth in a hospital setting.

Doxorubicin (Adriamycin)

Doxorubicin is used for a variety of cancers like lymphoma. Common side effects in dogs include bone marrow suppression and gastrointestinal symptoms, such as vomiting and diarrhea. An EKG may be performed prior to giving doxorubicin to ensure the dog has good heart function, as this chemotherapy can cause changes to the heart in dogs. Due to the risk of changes to the heart, doses are limited to five in dogs. In cats, kidneys can be affected. Doxorubicin is usually given as an intravenous injection over 15-30 minutes.

L-Asparaginase (Elspar)

L-Asparaginase is unique among chemotherapy agents. It is an enzyme that takes away an essential nutrient that some cancer cells, particularly lymphoma cells, need and are unable to make on their own. Since lymphoma cells are unable to make that essential nutrient, Elspar specifically targets lymphoma cells and generally does not harm ‘good’ cells within the body.

Palladia (Toceranib)

Palladia is commonly used for mast cell tumors but may be used for other types of cancer when there is evidence that the cancer has spread to the lungs. An oral chemotherapy, it is usually given at home on Mondays, Wednesdays, and Fridays. It does not stay in the body for more than 48 hours. Common potential side effects include bone marrow suppression and gastrointestinal symptoms, such as vomiting and diarrhea. In rare cases, it may cause difficulties with walking, hypertension, and kidney and liver damage.

Prednisone

Prednisone/prednisolone is a hormone – classified as a corticosteroid that the body naturally produces. In higher concentrations, it has a potent anti-inflammatory effect and can be very effective at killing cancer cells. Prednisone, an oral medication given at home, is used as an adjunctive therapy for many types of cancer with common side effects including excess drinking and urination, hunger, and panting.

Vinblastine

Vinblastine, which is given as an intravenous injection, is used for a variety of types of cancer, such as mast cell tumors. The main side effects include gastrointestinal signs, such as vomiting and diarrhea, as well as bone marrow suppression.

Vincristine

Similar to Vinblastine and also administered as an intravenous injection, Vincristine is commonly used as part of a lymphoma treatment protocol. The main side effects include gastrointestinal signs, such as vomiting and diarrhea, as well as bone marrow suppression. You’ll of course want to consult a veterinary oncology specialist to determine the best treatment protocol for your pet’s specific cancer type.