Chemotherapy

Chemotherapeutics can be given orally (as tablets or capsules), intravenously, or directly into the tumour. In general, lower dosages of chemotherapeutics are given to dogs than humans and therefore dogs tend to tolerate chemotherapy better than humans. The typical symptoms of vomiting, nausea and hair loss are only seen in a small percentage of patients (less than 20%).

Several forms of chemotherapy exist, with different mechanisms of action and different types of side effects.

1. Classic chemotherapy (high dose, MTD ‘Maximum Tolerated Dose’)

The most common form of chemotherapy is the intermittent administration of high doses of chemotherapeutics. These high doses mostly target fast dividing cells such as cancer cells. Unfortunately, the high doses can cause side effects for other fast dividing cells (i.e.cells of the gastro-intestinal tract, immune system). Because the body needs time to recover from this type of treatment, there are breaks of 1, 2 or 3 weeks in between treatments depending on the individual patient and the chemotherapeutic agent used.

Chemotherapeutics can kill or damage cancer cells and can be used as a sole treatment or in combination with another.

  • As a sole treatment: this is mostly used for blood cancers such as lymphoma and leukemia, as well as metastasized tumours or tumours with a high risk for metastasis (such as osteosarcoma, hemangiosarcoma, high-grade mast cell tumours etc.).
  • As a complementary treatment before (to shrink the tumour and decrease the complexity of the surgical procedure) or after surgery or radiation (to delay progression /recurrence).

The side effects of high-dose chemotherapy are in general far less pronounced in animals than in humans. In contrary to human medicine, veterinary medicine focuses primarily on quality of life rather than extension of life. As a result, some animals demonstrate an increased quality of life and improved appetite during and after the course of chemotherapy.

Although a better quality of life is pursued through chemotherapy, high-dose chemotherapy can be accompanied by side effects, such as vomiting, nausea, refusal to eat, diarrhea, hair loss and bone marrow suppression (depending on the compounds used).

2. Metronomic chemotherapy (low dose)

A less well-known method of chemotherapy is the continuous administration of low doses of chemotherapeutics. The term “metronomic” is derived from the metronome (the device used in music to designate the rhythm of the music), to indicate its regular administration.

The goal of metronomic chemotherapy is to interrupt the food supply of the tumour by disturbing the formation of new blood vessels in the tumour. The less the tumour is fed, the less it can grow. Additionally, this form of chemotherapy will also stimulate the immune system of the body against the tumour. The goal of this form of chemotherapy therefore is to halt tumour growth rather than to reduce the amount of tumour tissue.

This treatment type is promising, but currently the optimal dosing schedule is unknown as are the most suitable treatments to be combined with metronomic chemotherapy. In general, this treatment type causes less side effects than high dose chemotherapy, is easier to administer and has a lower cost, but it can only be used for a limited number of slow-growing tumours.

Metronomic chemotherapy is always given orally (e.g. tablets or capsules) and is administered at home by the owner. This may increase the risk of exposure for the owner and their family to chemotherapeutic drugs. Please find more information below about the precautions when dealing with chemotherapeutics.

3. Interference treatment

A newer treatment modality is interference treatment, such as tyrosine kinase inhibitors (TKI). Tyrosine kinases are enzymes that stimulate the growth of cancer cells by disturbing certain signaling pathways (meaning: the signal for tumour growth is continuously ‘ON’). By inactivating these enzymes, cellular growth can be suppressed.

This treatment type can also cause side effects to cells that are dividing in a normal way, such as cells of the gastro-intestinal tract or immune system. Currently, two tyrosine kinase inhibitors (Masivet®, Palladia®) are registered for the treatment of dogs with mast cell tumours. They are also used for other tumour types, but this use is off-label.

Tyrosine kinase inhibitors are given orally (e.g. tablets or capsules) and administered at home by the owner. This may increase the risk of exposure for the owner and their family to chemotherapeutic drugs. Please find more information about the precautions when dealing with chemotherapeutics below.

Important: precautions for dealing with chemotherapeutics:
  • Keep the chemotherapeutic in tablet or capsule form in a safe place at home, out of reach of children and pets.
  • Wear disposable gloves when administering the pills to your dog.
  • Avoid as much as possible contact with urine, stool, vomit and saliva of your dog during the excretion period (differs from agent to agent). So, don’t let your dog lick you during this period. If it does happen, wash it off with water and soap. Should your dog urinate or defecate inside, clean it with disposable gloves and use absorbant disposable material (tissues or paper towels).
  • Take extra care that children or pregnant women do not get in contact with these medicines.
  • All this should be discussed thoroughly during a conversation with your veterinarian prior to treatment initiation.
References
  1. Gustafson DL, Rodney LP. Cancer Chemotherapy. Withrow and MacEwen's Small Animal Clinical Oncology, 5th edition, Chapter 11 (p 157-163).
  2. Mutsaers JA. Molecular/Targeted Therapy of Cancer: Antiangiogenic and Metronomic Therapy. Withrow and MacEwen's Small Animal Clinical Oncology, 5th edition, Chapter 14 (p 231-232).
  3. London CA. Molecular/Targeted Therapy of Cancer: Signal Transduction and Cancer. Withrow and MacEwen's Small Animal Clinical Oncology, 5th edition, Chapter 14 (p 226-227).