FAQs - Metronomic Chemotherapy in Pets

What is metronomic chemotherapy? What cancers can I try it on? What are the pros and cons? How frequent are the visits? What are the side effects? Vets, here are some answers to some ‘Frequently Asked Questions’ about metronomic chemotherapy in dogs & cats. 

What is metronomic chemotherapy?

Metronomic chemotherapy is the long-term daily or every second daily administration of lower than maximum tolerated dosages of chemotherapy. In contrast to the administration of standard dosages of chemotherapy (i.e. conventional chemotherapy), which directly results in cancer cell kill, metronomic dosing of chemotherapy suppresses angiogenesis (i.e. suppresses blood vessel growth) and stimulates the immune response against cancer. If cancer loses their blood vessel supply, it will eventually starve itself of oxygen and nutrients, and thus die. 

How is it different from conventional chemotherapy?

Conventional chemotherapy is the systemic administration of drugs at the highest maximum tolerated dosages to kill cancer cells. Conventional chemotherapy aims to disrupt cancer cell division by targeting rapidly dividing cells. However, conventional chemotherapy will not know the difference between rapidly dividing cancer cells and normal tissues. Thus, side effects may occur to the normal, rapidly dividing gastrointestinal tract and bone marrow. These side effects are usually temporary and require regular breaks to allow for the normal cells to recover. Conventional chemotherapy is typically administered every one to three weeks. 

What drugs are usually involved?

Cyclophosphamide is the chemotherapy drug most commonly used in metronomic protocols in people and pets. Metronomic therapy typically consists of a combination of three medications that are administered orally at home: cyclophosphamide, piroxicam and frusemide. Piroxicam is a non-steroidal anti-inflammatory drug that has anti-cancer properties. Frusemide is a diuretic that is administered concurrently with cyclophosphamide to facilitate urination and thus, decrease the risk of sterile haemorrhagic cystitis to <5%. 

Other chemotherapy drugs that can be administered at low doses and continuously include chlorambucil, melphalan and lomustine (CCNU).

What are the pros and cons of metronomic chemotherapy, compared to conventional chemotherapy?

Metronomic therapy is an attractive treatment option for many owners because they can administer the drug orally to their pets at home, the risk of chemotherapy toxicity is low (around 5%), and the cost is relatively low, compared with other treatment options. Most of the side effects are usually attributable to piroxicam, and not to the actual chemotherapy drug itself.

However, the cons are that it is still chemotherapy and needs to be administered by pet owners safely at home. Once started, it is considered life-long therapy for as long as it keeps pets in remission, and because it does not necessarily directly result in cancer cell killing, it can take an average of 6-12 weeks before a response occurs. Therefore, in pets with rapidly diving tumours, there may not be enough time to see a response to therapy. Lastly, long-term usage may be associated with bone marrow suppression and sterile haemorrhagic cystitis, although these side effects are uncommon. 

It is important to be aware that stable disease is considered a success if the pet has a good quality of life.

The Pet Oncologist has a client handout entitled 'Handling Chemotherapy Medications at Home'. Please refer to this handout on tips on how to administer oral chemotherapy at home. This handout is also relevant to staff who administer chemotherapy and handle cytotoxic pets in the hospital.

When can I consider metronomic chemotherapy in pets?

Many cancers have the potential to respond to metronomic chemotherapy. It can be considered in a situation where pets have unfavourable prognostic factors (such as metastasis [i.e. cancer spread]), when surgery or other forms of therapy are not possible, when pets have failed standard of care, or if owners wish to try an anti-cancer therapy that has a low toxicity profile and is affordable. Often, metronomic therapy is not considered the standard of care, but it can be helpful and as a ‘trial and see’ approach.

In dogs, some indications include:

·       Low-grade soft tissue sarcomas that have been incompletely removed with surgery, to delay recurrence.

·       Splenic haemangiosarcoma.

·       Metastatic pulmonary carcinoma.

·       Transitional cell carcinoma of the bladder.

·       Indolent lymphoma.

·       Chronic lymphocytic leukaemia.

Cats with cancer can also respond to metronomic chemotherapy, predominately sarcoma, carcinomas, multiple myeloma and low-grade lymphoma. 

Metronomic chemotherapy can be safely combined with other anti-cancer therapies, such as Palladia® (toceranib), radiation therapy and some conventional chemotherapy drugs.  

How frequent are the visits?

When I start with metronomic therapy, I typically recommend a recheck two weeks later, four weeks later, six weeks later, then every 2-3 months after that. Each visit will consist of a physical examination, cancer measurement, complete blood count, urinalysis. Now and then, a serum biochemistry panel is performed.

What are the side effects?

Most pets that receive metronomic therapy (95%) experience no side effects.

Approximately 5% of dogs experiencing a side effect (usually mild and gastrointestinal signs). Most side effects (if picked up early) are managed with discontinuation of therapy, dose reduction or addition of supportive medications. 

It is uncommon for a side effect to result in sterile haemorrhagic cystitis, long-term bone-marrow suppression or hospitalization (<5%).

If any gastrointestinal signs occur (such as diarrhoea, melaena, vomiting or inappetence), it is usually due to piroxicam. These side effects typically resolve with temporarily discontinuing piroxicam and administering supportive care medications.

Vets, I hope this information helps you understand a bit more about the use of metronomic chemotherapy in pets. If you have a question about metronomic chemotherapy or have a pet that you think may benefit from metronomic therapy, please do not hesitate to get in touch by clicking here.