By Elsa Edery, DVM, MSc Clinical Oncology, DipECVIM-CA(Int Med) 

Cytotoxic agents are used widely to manage cancer in dogs and cats and from their mechanisms of action we can understand their efficacy but also their toxicities. 

Mechanisms of action 

Cytotoxic agents act by interfering with the cell cycle which is the mechanism by which somatic cells divide. They can cause damages to the DNA during replication (Intercalation into DNA, single/double strand breaks, telomeric DNA damages…), inhibit topoisomerases I and I (enzymes involved in the unwinding of the DNA helix), act as fraudulent metabolites for the synthesis of nucleotides (building blocks of DNA), produce free radicals, interfere with polymerization or depolymerization of the microtubule of the mitotic spindle… As cancer cells have less efficient repair mechanisms than normal cells, these damages lead to apoptosis (cell death) 

Concepts 

• MTD (maximum tolerated dose): this is the highest dose that can be given in the absence of irreversible or unacceptable adverse effects to a limited population sample. 

• Dose intensity (DI): amount of drug delivered per unit of time and is usually standardized to body surface area as mg/m2/week. This concept is useful to compare various protocols 

• Therapeutic gain: benefit of combining 2 drugs with different mechanisms (toceranib and vinblastine for mast cell tumours, carboplatin increasing sensitivity of cancer cells to radiation therapy) : synergistic effect 

With cytotoxic agents, we aim for the maximum antitumour effect with as little toxicity as possible on normal cells. The dosage is done according to body surface area (BSA) which correlates with basal metabolic rate, blood volume, cardiac output, PK drug disposition, renal function. However; BSA tends to overdose small animals and it is recommended that in patients weighing <10-15 kg some drugs are better dosed according to the body weight. 

The formula used to convert BW to BSA is:  

BSA (m2)= (A x (BW in grams)^2/3) / 10000 (A = 10.1 (dogs) and 10.0 (cats) 

Conversion tables are available in formularies and veterinary oncology books. 

As most drugs, cytotoxic agents are metabolized and/or eliminated by liver and kidneys. However, there is no standard dose adjustment available in patients with compromised hepatic or renal function, with the exception of carboplatin in cats where there is a formula available for dosing it according to the glomerular filtration rate and a target Area Under the Curve (Suggested readings: Bailey et al, AJVR 2004; Bailey et al, JAVMA 2009), although, in practice, it is generally easier to calculate carboplatin dose with conventional dosing based on the BSA.  

Toxicities 

Toxicity arises from collateral damages on normal cells with high growth fraction (bone marrow stem cells, digestive crypt epithelial cells, hair follicle cells). The Veterinary Cooperative Oncology Group has developed a document with grading scales for toxicities in every organ (VCOG-Common terminology Criteria for Adverse Events); which is a useful document to report uniformly adverse events across studies. 

Most cytotoxic agents have in common myelosuppression and gastrointestinal toxicities but to various severity depending of the drug.  

Acute toxicity develops within 48 hours of administration: infusion hypersensitivity (anaphylaxis-like reaction due to histamine release can be observed with L-asparaginase, doxorubicin, paclitaxel), nausea/vomiting (cisplatin, streptozotocin and very emetic drugs with effects during the infusion), extravasation (reported with vesicants such as vinca-alkaloids, doxorubicin) 

Dealyed acute toxicities develop between 2 and 14 days and mainly comprises of bone marrow suppression and gastrointestinal upset 

Chronic and cumulative (after several administration) toxicities occur up to years after administration and hepatotoxicity with lomustine, cardiac toxicity (doxorubicin in dogs), renal toxicity (cisplatin in dogs, doxorubicin in cats) are well documented examples in veterinary oncology. 

There are some individual unpredictable toxicities: concurrent illness or organ insufficiency, extreme tumor burden, obesity, breed specific sensitivities.  

Pharmacogenetics polymorphisms 

Herding breeds (Border collies, Autralian Shepherd dog…) carrying the ABCB1-1delta (previously named “MDR1”) mutation have increased sensitivity to drugs that are known substrate for the P-glycoprotein: vincristine/vinblastine, doxorubicin/epirubicin,  

ABCB1 (Adenosine triphosphate Binding Cassette) genes code for the P-glycoprotein which is a membrane transporter on the intestinal luminal epithelial cells, renal tubular cells, biliary canalicular cells, that actively pumps substrate drugs into bile and urine for excretion, also present on the blood brain barrier (pump drugs back into capillary lumen out of the endothelial capillary cells) 

When the transporter is non functional because of mutation, delayed clearance of the cytotoxic agents results in increased concentration and increased toxicity. Vincristine associated myelosuppression and neurotoxicity have been described in both homo and heterozygous for the ABCB1-1delta allele dogs 

(Suggested reading: Mealey et al JVIM 2015) 

Myelosuppression  

Myelosuppresion is the major dose limiting toxicity (DLY) for most cytotoxic agents and manifests with the development of neutropenia (as they have a short half life compared to other blood cells) and less commonly thrombocytopenia and anemia. 

Before chemotherapy, a complete blood count is done to check the absolute neutrophil count (ANC) and although different centres use different cut-offs, a recent prospective study (Fournier et al, JVIM 2018) established that using a cut-off value of 1500/microL minimized delays in treatment without increasing toxicity. 

Chemotherapy-induced neutropenia (CIN) is defined as a neutrophil count < 1000/microL and tends to occur at 7-10 days after administration for most agents (exception : Vinblastine, paclitaxel: 4-5 days 

Carboplatin : 2-3 weeks) 

Febrile neutropenia is CIN complicated by fever (sepsis) and is a medical emergency which requires hospitalization for IV fluid, and IV antibiotics with a broad spectrum 

Neutropenia is considered of clinical significance when the absolute neutrophil count is <1 x10^9/L (1000/microL) 

The risk of febrile neutropenia is estimated at <10% with mortality rates of 8%  

Small size dogs, dogs with lymphoma, treatment with doxorubicin and vincristine are associated with  increased risk of developing sepsis  

(Suggested reading : Bisson et al, Vet Comp Oncology 2015) 

The prophylactic use of antimicrobial is indicated if: 

• ANC <1000/microL in patients with risk factors (hematological malignancies, concomitant disease, dogs less than 14kg)   

• ANC<750/microL  

In these situations, oral broad spectrum anaerobes sparing antibiotics (tripethoprim-sulfa, fluoroquinolones) are indicated 

On the other end, clinically ill patients +/- febrile or signs of sepsis: hospitalization, IV fluid, IV antibiotics targeting Gram+ and Gram-.  

Investigations to locate the focus of infection are optional and the IV antibiotics must be started immediately. 

For the future treatments, a dose reduction of 10-20% is to be considered  

Gastrointestinal toxicity 

Vomiting results from damages to crypt cells and direct stimulation of the vomiting center and the CRTZ (chemoreceptor trigger zone) 

Diarrhoea result from damages to crypt cells and alterations in intestinal enzyme balance 

Symptomatic treatments are recommended: antiemetics, antidiarrhoeals (kaolin,loperamide), diet adjustment. Hospitalization for parenteral treatment and iv fluid may be required. The use of antibiotics should be considered carefully due to risk of adverse effects on the intestinal microbiome with increase in pathogenic bacteria and some evidence is arising that this disturbance of the intestinal flora might decrease the response to chemotherapy agents. 

Extravasation: 

Phlebitis, cellulitis, necrosis, sclerosis within 10 days and tissue damages progress until 3 weeks 

The injection/infusion should be stopped immediately and attempt to reaspirate as much as possible should be done.  

Depending on the extravasated drug, either adopt a “spread and dilute”(vinca-alkaloids/warm compresses, saline flushing, hyaluronidase injection in the extravasation site to facilitate drug detachment from the tissue, topical DMSO) or a “localize and neutralize” (doxorubicine/ cold compresses, administration of Dexrazorane, a free radical scavenger, developed to reduce anthracycline-induced cardiotoxicity which should be administered within 6 hrs, IV bolus, at 10 times the dose of doxorubicin then 24 and 48 hours after the infusion) 

Specificity of cytotoxic agents used commonly in veterinary medicine 

Cyclophosphamide:  

Sterile hemorragic cystitis (SHC) is a known complication and presents as painful, microscopic or gross hematuria, stranguria, dysuria, pollakiuria, sometimes irreversible with fibrosis. 

It can develop several months after start of treatment (in metronomic cyclophosphamide the median time to development of SHC is 511 days) 

Prevention of SHC with co-administration of frusemide PO or IV. The exact mechanism for prevention is suspected to be diuretic effect, with decreased contact time of urotoxic metabolites (acrolein) with the epithelium. 

2 cases of bladder transitional cell carcinomas in dogs have been reportedly associated with cyclophosphamide toxicity. 

Lomustine 

The dose limiting toxicity is the myelosuppression: neutropenia, irreversible thrombocytopenia 

Hepatotoxicity is a delayed cumulative dose dependent toxicity with increase in the activity of ALT in 84% of dogs 

Prevention: Co-administration of SAMe-Sylibin reduced the severity of elevation of ALT.  

Anthracyclines (Doxorubicin, Epirubicin, Mitoxantrone) 

Acute toxicity: head shaking, urticaria, blushing of skin, collapse (histamine release with rapid IV) 

Acute DLT: Myelosuppression, GI toxicity 

Acute cardiac toxicity (Dogs): asymptomatic arrhythmia 

Chronic cumulative myocardial toxicity: DLT (250 mg/m2 dogs), dose dependent, DCM and CHF 

Chronic cumulative nephrotoxicity: cats 

Antimicrotubules 

Vinca alkaloids (vincristine, vinblastine): bind to tubulin and prevents polymerization to microtubule 

• Vincristine reported to have neurotoxicity (peripheral neuropathy), ileus 

• Vinblastine is more myelosuppressive than vincristine 

Taxanes (paclitaxel): stabilize microtubule and prevent depolymerization 

Platinum compounds 

Cisplatin: 

• CI in cats (fatal pulmonary vasculitis and edema) 

• DLTs: Nephrotoxicity (dose dependent, cumulative, irreversible), intractable nausea and vomiting  

Carboplatin: 

• DLT: Myelosuppression 

• Easier to use, less GI toxicity 

 L-asparaginase 

L-asparaginase depletes L-asparagine causing inhibition of protein synthesis in tumour cells leading to apoptosis 

Reported adverse effects include hypersensitivity, pancreatitis, diabetes and coagulation abnormalities.  

Development of neutralizing antibodies limits its effect after repeated dosing. 

Health and safety (suggested reading: “Preventing occupational and environmental exposure to cytotoxic drugs in veterinary medicine”. ECVIM 2007 second version) 

Cytotoxic agents are mutagenic, carcinogenic, teratogenic, abortifacient, and poses risk of stillbirth. They pose occupational hazards involved in the preparation, administration, handling of chemotherapeutic drug/patient wastes. 

Owners should be informed about those risks, particularly when the pet lives with breastfeeding and pregnant women and young children) 

Training for all healthcare staff should be provided in the practice. 

The following are strongly recommended when handling cytotoxic agents: 

• Designated areas for preparation, administration 

• Class II type 2 biologic safety cabinet 

• PPE: gown, gloves, eye/face protection, mask 

• Closed System Transfer Devices  

• Spill kit 

• Decontamination with bleach 

• Waste management 

Conclusion: 

It is critical to be or become familiar with all aspects of use of cytotoxic agents before using them to anticipate their adverse effects 

Their use poses occupational hazards which are a challenge for the entire healthcare team.  

(Suggested reading: Withrow and McEwen’s Small Animal Clinical Oncology 6th ed)