"Cancer" is a scary word that is often equated with death. There is often a visceral fear of cancer, and pet owners think cancer equals pain and suffering. There are many myths and misconceptions about chemotherapy in pets. Owners think cancer treatment will just make the patient sicker.

But cancer is not a death sentence. With treatment, many cancer patients are not only living longer, but living well. Chemotherapy is well tolerated in the majority of dogs and cats undergoing treatment. Whether you are directly managing chemotherapy patients or sharing cases with an oncologist, there are simple tips and tricks to improve quality of life and minimize gastrointestinal side effects in chemotherapy patients.

Conventional chemotherapy is typically given at high dosages, known as maximum tolerated dose, or MTD. The goal is to kill the rapidly dividing cancer cells. But some normal cells that also have high turnover often can be temporarily damaged by MTD chemotherapy. The normal tissues that typically are most sensitive to chemotherapy are the bone marrow, hair follicles (alopecia), and the gastrointestinal lining. This is often referred to as "BAG." As a result there is a break period to allow these cell populations to recover. MTD is typically given weekly to every 3 weeks.

The overall toxicity rate is very low in veterinary chemotherapy patients. In my experience, only 15–20% experience side effects, and this is even less common in cats than dogs. The primary goal is to provide the best quality of life possible for as long as possible. As I say, live longer, live well. Most side effects are mild and medically manageable.

Gastrointestinal (GI) Toxicity

Gastrointestinal (GI) toxicity includes vomiting, diarrhea, decreased appetite, nausea. It typically occurs 1 to 5 days after chemotherapy and is self-limiting - lasting on average 2–3 days. These side effects are less common in feline chemotherapy patients than dogs. I recommend being very proactive with nausea/antiemetic drugs.

I often will use Cerenia or mirtazapine preventatively and as needed. I recommend giving Cerenia at administration with the following drugs: doxorubicin, vincristine, vinblastine, carboplatin, mitoxantrone, dacarbazine, and the MOPP protocol. If the pet has nausea/vomiting event within 24 hours of administration, I will add Cerenia SQ or IV at the time of administration at the subsequent treatment. For oral chemotherapy being given at home, I advise the owner give oral Cerenia 1 hour before chemotherapy pill dosing.

I always recommend oral Cerenia for 4 days after doxorubicin in dogs to prevent nausea and vomiting. If there are side effects with other chemotherapeutics, I also typically will add prophylactic medications to prevent side effects like nausea, vomiting or diarrhea as indicated. If the GI side effects are more severe in a patient, the drug type or dosage may be adjusted at subsequent treatments to minimize the chance of side effects recurring.

Unlike dogs, I do not routinely use GI medications unless the cat had issues with a prior treatment or had GI clinical signs prior to treatment (i.e., GI lymphoma).

For diarrhea, I typically send my patients home with metronidazole and a probiotic. Metronidazole is a synthetic nitroimidazole with antibacterial, antiprotozoal and anti-inflammatory properties and is commonly prescribed for acute and chronic diarrhea. It is metabolized and excreted by the liver, so take care with patients with impaired liver function. Neurotoxicity is associated with higher doses and chronic use, so I do not recommend chronic use. Dose: 15 mg/kg PO BID for 5 days

Rx Clay is a good option for chronic diarrhea and patients needing multiple courses of metronidazole. Rx Clay is a calcium aluminosilicate (CAS), which is geological nanomaterial that adsorbs bacterial enterotoxins and increases reabsorption of intraluminal water in GIT.

Acute vomiting is typically associated with cisplatin, doxorubicin (Adriamycin), dacarbazine (DTIC), cyclophosphamide, actinomycin, 5-FU streptozotocin. This can typically be prevented with pre-treatment.

Delayed vomiting is more common in our patients. This is due to direct damage to rapidly dividing GIT cells (crypt cells) or via the centrally mediated CRTZ stimulated via gut vagal efferents. Delayed vomiting is most commonly 2 to 5 days post-chemo and seen with doxorubicin and the vinca alkaloids. Clinical signs include vomiting, diarrhea, anorexia, lethargy, weakness, + dehydration.

For workup, I recommend CBC, chemistry panel, UA, +/- fecal floatation and cultures. If abdominal pain is present, consider AXR or AUS to rule out foreign body, obstruction, and intussusception. For patients with GI neoplasia, it can be challenging to differentiate chemotherapy side effects vs. disease, and a good history can be key.

For outpatient treatment, I recommend NPO, food & water trial, bland diet, antiemetics, antibiotics with severe diarrhea, and a probiotic. Do not forget to discontinue oral chemotherapy or delay chemotherapy treatment. In addition, I recommend prophylactic therapy with the next chemotherapy.

For inpatient, I add injectable antiemetics, IV fluid therapy, and IV antibiotics. An important note, I strongly encourage owners to not euthanize at this time. It is amazing with 1 to 2 days of good supportive care how quickly these patients improve. And with prophylactic therapy and a dose reduction, these patients can tolerate the same chemotherapy drug.

References

1.  Gustafson DL, Page RL. Cancer chemotherapy. In: Small Animal Clinical Oncology. 5 ed. St. Louis, MO; Elsevier Saunders; 2013:157–179.

2.  Rodriquez CO. Chemotherapy. In: Cancer Management in Small Animal Practice. Saunders; 2010:101–121.

3.  Dhaliwal RS. Managing oncologic emergencies. In: Cancer Management in Small Animal Practice. Saunders; 2010:122–136.

4.  Chretin JD, et al. Prophylactic trimethoprim-sulfadiazine during chemotherapy in dogs with lymphoma and osteosarcoma: a double-blind, placebo-controlled study. J Vet Intern Med. 2007;21(1):141–8.

5.  Rau SE, et al. Efficacy of maropitant in the prevention of delayed vomiting associated with administration of doxorubicin to dogs. J Vet Intern Med. 2010;24(6):1452–7.