NEW MEDICAL THERAPEUTIC APPROACHES TO CANCER MANAGEMENT Joyce Obradovich, DVM, Diplomate, ACVIM (Oncology) Animal Cancer & Imaging Center, Canton, MI

Treatments such as chemotherapy and radiation therapy, although improving with newer drugs and treatment regimens, have been in existence for many years. For diseases such as lymphoma, we have not made significant improvements on median survival times with cytotoxic chemotherapy for over two decades. For this reason, newer approaches to ways to attack and kill cancer cells are emerging which are taking advantage of unique cellular targets and biochemical pathways. Three of the most exciting developments in recent years include the tyrosine kinase inhibitors, a canine melanoma vaccine, and the metronomic approach to chemotherapy.

Receptor tyrosine kinase inhibitors:

Although the use of tyrosine kinase inhibitors is not new, the introduction of the receptor tyrosine kinase (RTK) inhibitors to the veterinary market is. Protein kinases play a critical role in normal cellular signal transduction acting to regulate cell growth and differentiation. External signals, such as various growth factors, initiate kinase phosphorylation (binding ATP and using it to add phosphate groups to key residues resulting in a downstream signal within the cell that ultimately leads to alternations in gene transcription that have an impact on cell proliferation and survival). Protein kinases can be on the cell surface, within the cytoplasm or within the nucleus and are typically divided into 3 main categories:

Tyrosine kinases Serine-threonine kinases Mixed kinases (phosphorylated on all 3 of these amino acids)

Tyrosine kinases on the cell surface that bind growth factors are known as ―receptor tyrosine kinases (RTKs). Examples of RTKs include Kit, Met, Axl, and epidermal growth factor receptor (EGFR). All of these RTKs have been shown to play a role in certain human cancers. In addition to the RTKs being responsible for regulating normal cell function, certain ones play an important role in tumor angiogenesis including vascular endothelial growth factor and it’s receptor (VEGF and VEGFR), platelet derived growth factor and it’s receptor (PDGF and PDGFR), fibroblast growth factor and it’s receptor (FGF and FGFR) and Tie 1 and Tie 2. For example, VEGFRs expressed on vascular endothelium and the VEGF – VEGFR interaction is critical for endothelial migration and proliferation. Dysfunction/dysregulation of the protein kinases can lead to uncontrolled growth and survival and ultimately cancer. These specific dysregulated kinases serve as a target for what are called ―small molecule inhibitors‖. These work by blocking the ATP binding sites of kinases acting as competitive inhibitors. In human medicine, the most successful small molecule kinase inhibitor is Gleevec (imatinib mesyhlate – Novartis) which blocks the activity of a cytoplasmic kinase called Abl. A constitutively active Bcr-Abl fusion protein is found in 90% of people with acute myelogenous leukemia (AML). In chronic phases of AML, Gleevec induces remission rates close to 95% for longer than 1 year. Gleevec also blocks ATP binding of Kit, and is therefore effective in treating people with systemic mastocytosis and gastrointestinal stromal tumors (GIST). In GIST, there is a 50-70% response rate vs. only a 5% response rate to chemotherapy. Gleevec inhibits Kit signaling in dogs and cats, but in dogs problems with hepatotoxicity exist. In addition, this drug is extremely costly, precluding it’s use in many veterinary patients. In cats it appears to be very well tolerated.1 The FDA approval of a new small molecule inhibitor has made an exciting impact on the practice of veterinary oncology. The following are 2 currently used RTK inhibitors in veterinary practice.

1. Palladia (toceranib phosphate) is the first RTK inhibitor approved by the FDA for veterinary use for dogs with Grade II or III recurrent mast cell tumors with or without lymph node metastasis. This drug inhibits the receptor tyrosine kinase, Kit, which is mutated in up to 30% of canine mast cell tumors, and causes an anti-proliferative effect. In addition to it’s effect on Kit, Palladia also targets VEGFR-2 on endothelial cells and PDGFR-B on pericytes to provide an antiangiogenic effect. In the initial studies, the overall response rate with Palladia (complete and partial remission) was 42.8%. Palladia is administered orally. It is initially given every other day, usually at night. The most common side effects were GI which were managed by stopping the drug temporarily and resuming at a lower dosage. If GI signs develop and the drug is stopped, it is typically resumed on a Monday-Wednesday-Friday schedule. Monitoring of blood counts is important as neutropenia is a potential, but less common, side effect.2 2. Kinavet (masitinib) is also a tyrosine kinase inhibitor, which is currently approved for use in Europe but is only available through compassionate use in the United States. Kinavet targets KIT, PDGFRα and PDGFRβ, and FGFR-3. In a prospective clinical trial, dogs with measurable Grade II or III MCT without metastasis that were randomized to receive Kinavet as first-line therapy had a significantly longer time to tumor progression (TTP = 253 days) than dogs receiving placebo, regardless of Kit mutation status. The response was more pronounced if it was used as the first line of therapy rather than as a rescue, 75 days (placebo) vs. 253 days. There was an increase in overall survival if patients had mutated Kit, 182 days (placebo) vs. 417 days.3

Canine Oral Melanoma Vaccine

Canine oral malignant melanoma (OMM) is the most common oral malignancy in dogs and carries an extremely poor prognosis. With a metastatic rate of greater than 80%, even when local control can be achieved with surgery or radiation, systemic disease is likely to be the cause of death in patients. Response rates to systemic agents including carboplatin, cisplatin, and piroxicam have been noted, but unfortunately in less than 28% of patients treated.

The USDA conditional approval of a xenogeneic plasmid DNA encoding human tyrosinase for use against canine oral melanoma in 2007 provided a new option by which to address micrometastatic disease.4 The vaccine ―Oncept‖ is intended for use in dogs with oral melanoma that is locally controlled via surgery or radiation therapy. Dogs are vaccinated every 2 weeks for 4 doses, and then administered a booster vaccine every 6 months. The vaccine uses a DNA plasmid containing a gene for the human version of tyrosinase, a protein present on melanoma cancer cells in humans and dogs. Following a dog’s vaccination with Oncept, some of the dog’s cells will produce the human version of tyrosinase. The dog’s immune system reacts by attacking not only the foreign human version of tyrosinase but also the homologous canine version—and thus the melanoma.5

Metronomic Chemotherapy

The traditional way to use chemotherapy is to give the ―maximum tolerated dose‖ (MTD) to a patient in order to kill cancer cells. There is a break following MTD chemotherapy which gives time for normal noncancerous tissues, such as bone marrow and gastrointestinal tissues to repair. In the initial phases of using a new chemotherapeutic, phase I toxicity attempt to determine the maximum dosage that can be given to a patient while keeping the overall toxicity to an acceptable level. The concept of metronomic chemotherapy is based on the premise that using chemotherapy in a different manner than MTD can alter the tumor microenvironment. While the MTD approach aims to maximize cancer cell kill with as high a dosage as is possible (until dose limiting toxicity is reached), the metronomic approach uses low chemotherapeutic dosages (often 10 times less than the MTD) used frequently (daily to every other day) to avoid the breaks in therapy required by MTD regimens. Rather than targeting the cancer cell directly, metronomic chemotherapy aims to alter the microenvironment rendering essential processes like angiogenesis ineffective. Angiogenesis (new blood vessel formation) is necessary for tumor cell growth and metastasis. Chemotherapy affects endothelial cells in a much more direct and selective manner, endothelial progenitor cells derived from the bone marrow seem to be directly targeted by metronomic chemotherapy scheduling, and metronomic chemotherapy reduces the levels of angiogenic growth factors. Upregulation of the inhibitor thrombospondin-1 (TSP-1) appears to be a key molecule in this process, but downregulation of other stimulatory factors such as VEGF also plays a role to inhibit angiogenesis.6 Side effects of metronomic chemotherapy appear to be far less intense than for MTD making this an attractive approach for older patients with concurrent disease.

In dogs undergoing surgery for soft tissue sarcomas (STS), the ability to achieve a microscopically tumor free margin is often difficult. Radiation therapy is a very effective adjunct to an incomplete surgical excision. However, in the absence of radiation, metronomic chemotherapy appears to delay the recurrence of incompletely excised STS. Using a well-tolerated daily dosing of cytoxan (10 mg/m2) and piroxicam (0.3 mg/kg), disease free intervals were significantly improved compared to patients undergoing incomplete surgical excision alone.7 In another study, metronomic chemotherapy was found to be an acceptable alternative to MTD for dogs with hemangiosarcoma.8 We have been using a combination of masitinib and low dose cytoxan as metronomic chemotherapy with positive results.

1. London CA. The role of small molecule inhibitors for veterinary patients. Vet Clin Small Anim 2007; 37:1121-1136. 2. London CA, et al. Phase I dose-escalating study of SU11654, a small molecule receptor tyrosine kinase inhibitor, in dogs with spontaneous malignancies. Clin Cancer Res 2003:9(7):2755-68. 3. Hahn KA, et al. Masitinib is safe and effective for the treatment of canine mast cell tumors. J Vet Intern Med 2008;22:1301-1309. 4. USDA licenses DNA vaccine for treatment of melanoma in dogs. J Am Vet Med Assoc 2010;236(5):495. 5. Bergman PJ, et al. Development of a xenogeneic DNA vaccine program for canine malignant melanoma at the Animal Medical Center. Vaccine 2006;24(21):4582-85. 6. Mutsaers, AJ. Chemotherapy: New uses for old drugs. Vet Clin Small Anim 2007;37:1079-1090. 7. Elmslie RE, et al. Metronomic therapy with cyclophosphamide and piroxicam effectively delays tumor recurrence in dogs with incompletely resected soft tissue sarcomas. J Vet Intern Med 2008;22:1373-1379. 8. Lana S, et al. Continuous low-dose oral chemotherapy for adjuvant therapy of splenic hemangiosarcoma in dogs. J Vet Intern Med 2007;21:764-769.