What’s new in veterinary cancer?

With the increasing research in cancer immunology and genetics, veterinary oncology is changing rapidly. At the Fetch dvm360^® virtual conference, Sue Ettinger, DVM, DACVIM (Oncology)—known as Dr Sue Cancer Vet—presented results from recent clinical trials and discussed newly approved medications and molecular tests used in veterinary cancer. Among the topics Ettinger discussed are novel treatments for mast cell tumors (MCTs), lymphoma, and osteosarcoma, as well as a novel diagnostic test for urothelial carcinoma, including transitional cell carcinoma and prostatic carcinoma.

Mast cell tumors

Tigilanol tiglate (Stelfonta; Virbac) is an intratumoral therapy that received FDA approval in November 2020 for the treatment of nonmetastatic cutaneous MCTs in dogs. Tigilanol tiglate is a protein kinase C activator derived from the blushwood plant in the Australian rain forest that stimulates rapid tumor destruction within 7 days. It can be used for all grades of nonmetastatic cutaneous MCTs, as well as subcutaneous MCTs at or distal to the elbow or hock.

The dose of tigilanol tiglate was optimized based on a multi-institutional, open-label study.¹ Dosing is based on tumor volume (tumor volume = ½ [length (cm) × width (cm) × depth (cm)]) as measured on the day of administration, at a dose of 50% volume of drug per volume of tumor. The maximum treatable tumor volume is 10 cm³, with a minimum and maximum dose of 0.1 and 5 mg, respectively (the maximum dose approved in the European Union is 4 mg).

The treatment protocol includes concomitant medications (Figure 1). Prednisone is administered on a tapering course beginning 2 days prior to tigilanol tiglate injection, and is continued on a tapering course for 10 days. Beginning on the day of intratumoral tigilanol tiglate injection, a histamine-1 blocker (diphenhydramine) and a histamine-2 blocker (famotidine) are administered and continued for 7 days. The intratumoral injection does result in local swelling and tissue reaction, and the resulting tumor necrosis results in an open wound. Ettinger said that, unlike standard wounds, the tigilanol tiglate injection wound is generally not covered or protected, and pets are permitted to lick the wound site. This disparity from typical wound management requires some significant education on the use of this treatment. In most patients studied, the wound was fully healed by 4 to 6 weeks, and by 12 weeks 98% of patients were fully healed.¹

Figure 1. Tigilanol tiglate (Stelfonta; Virbac) is injected intratumorally for the treatment of nonmetastatic cutaneous MCTs in dogs.

Outcomes were evaluated in a multi-institutional, double-blinded, crossover study.² A total of 123 dogs were enrolled. Of the treated dogs, 75% had a complete response (CR) 28 days after the single-dose treatment, and 5% had a partial response, for a total response rate of 80%. By comparison, the control group had a 5% CR rate due to prednisone alone, which was statistically significantly (P < .001). The 20% of treated dogs that did not have a CR to the first injection were able to receive a second injection, after which the CR rate increased to 87%. At 12 months, 88% of dogs remained disease free. Adverse effects (AEs) were generally mild (grade 1 or 2). The majority (61%) were related to the mechanism of action of the treatment and included a wound, pain, and lameness. Additional AEs that occurred commonly included vomiting, diarrhea, and inappetence.

Lymphoma

Ettinger also updated attendees on new treatment options for lymphoma. When reflecting on why owners decline treatment for lymphoma, she noted that associated cost and AEs of therapy often play a significant role. Often, multiagent protocols require frequent veterinary visits. One agent that shows promise is rabacfosadine (Tanovea-CA1; VetDC). Rabacfosadine is conditionally approved by the FDA to treat any type of lymphoma (B-cell, T-cell, cutaneous, newly diagnosed, or relapsing cases). This agent can be used alone or in combination with doxorubicin, and requires only 5 to 6 treatments depending on the protocol used.^3,4 Although rabacfosadine is more expensive than some conventional chemotherapy drugs, the overall costs tend to be lower because of the decreased number of visits.

When used alone, rabacfosadine is administered intravenously over 30 minutes every 21 days for 5 treatments. When administered in combination with doxorubicin, rabacfosadine is alternated with doxorubicin every 3 weeks, for a total of 3 treatments with each agent.⁴ While conditionally approved, rabacfosadine must be used only as labeled; however, antiemetic agents, antidiarrheal agents, and corticosteroids can be given concurrently. Initial studies enrolled treatment-naïve patients as well as those that had failed prior therapy. Despite this, the overall response rate for rabacfosadine was 77%, with 45% of patients achieving CR.³ When combined with doxorubicin, the overall response rate was 84% (68% CR, 16% partial response).⁴

Rabacfosadine was generally well tolerated, with AEs typical of most anticancer therapies, including hyporexia, anorexia, diarrhea, vomiting, and weight loss.^3,4 To minimize these AEs, Ettinger prescribes maropitant (Cerenia; Zoetis) and capromorelin (Entyce; Elanco) for a full week after each dose. Neutropenia may occur but is typically mild to moderate. Most AEs were mild to moderate (grade 1 or 2). AEs specific to rabacfosadine include otitis, dermatologic reactions (primarily affecting the pinnae and ventral abdomen), and pulmonary fibrosis.^3,4 The pulmonary effects occur in 4% of cases and are idiosyncratic. Where they occur, they tend to be delayed, and appear about 5 months after treatment. For this reason, baseline thoracic radiographs are recommended prior to the first dose, and every 2 to 3 months thereafter. Due to the potential respiratory complications, rabacfosadine is contraindicated in West Highland white terriers.

Ettinger provided answers to some frequently asked questions regarding rabacfosadine. The efficacy and safety are optimized for the frequency and rate of administration. As such, it should not be given more frequently than every 21 days, and should not be given at a faster or slower rate than 30 minutes. Vials are single-use and any remaining drug must be disposed of after administration. Rabacfosadine does not cross the blood-brain barrier. It is not a substrate for the multidrug resistance (MDR1) mutation, and standard dosing can be administered to patients with the MDR1 mutation. Another common question, Ettinger said, is whether rabacfosadine can be repeated after the maximum of 5 doses. She confirmed that cycles cannot be repeated in succession; however, once relapse occurs, then the cycle can be repeated.

FDA approval of rabacfosadine is anticipated following completion of an additional, ongoing multi-institutional study of 100 dogs with B-cell and T-cell lymphoma, provided results confirm efficacy and safety.

Based on outcome data, Ettinger said she still prefers the CHOP (cyclophosphamide, doxorubicin hydrochloride, vincristine sulfate, and prednisone) protocol as a first-line treatment for B-cell lymphoma; however, rabacfosadine alone or in combination with doxorubicin is her second-line protocol. For T-cell lymphoma, she said there is no consensus regarding protocols, other than that T-cell lymphoma cases have a poorer prognosis. When weighing whether prednisone should be administered concomitant with therapy, Ettinger referred to 2 recent studies in which outcomes were not significantly different when prednisone was administered concurrent to a multiagent protocol. Although it is still commonly prescribed in lymphoma patients, these studies provide comfort in those cases where prednisone is contraindicated (eg, diabetes, unregulated hyperadrenocorticism) or where corticosteroid side effects are intolerable.

Urothelial cancer

The latest news in urothelial cancer (including transitional cell carcinoma of the bladder [Figure 2], urethra, and prostate) is not a treatment but rather a diagnostic test. A novel molecular test, the CADET BRAF test (Antech Diagnostics) detects urothelial carcinoma DNA in urine and avoids the need for invasive testing, which may lead to earlier diagnosis.⁵ Historically, urothelial carcinoma is often quite advanced at the time of diagnosis, because cases may be misdiagnosed as having recurrent urinary tract infections prior to performing imaging studies or surgical biopsy to achieve the diagnosis of urothelial cancer. Ettinger said 15% of cases have local lymph node metastasis at time of diagnosis, and 20% have distant metastasis. The availability of the BRAF test as a urine biomarker may lead to earlier diagnosis, because noninvasive testing may be more palatable to owners earlier in the course of disease, compared with previous tests that are often delayed because of the desire to avoid invasive testing.

Figure 2. Transitional cell carcinoma of the urinary bladder.

Using a digital droplet polymerase chain reaction assay, the test screens for cells containing a mutation in the BRAF gene, which indicates a predisposition for urothelial carcinoma.⁵ The BRAF test is performed exclusively by Antech, and requires 40 ml of urine (although the test may be performed on smaller volumes if necessary in cases where urine collection is problematic). A voided urine sample must be used, because that allows exfoliation of cells in all areas of the urinary tract, including prostatic or urethral tumors. Urine samples must be placed into a designated container with preservative within 15 minutes of collection to maintain DNA viability; however, multiple voidings can be combined in the same container over 2 to 7 days. Once in the preservative, samples are stable for weeks at both room temperature and refrigerated, and need only be protected from light. The BRAF test screens for cells containing the BRAF mutation, and in cases that are negative for the mutation, the BRAF-PLUS test is performed as a secondary screen using a proprietary algorithm to detect a second cell signature present. Results are typically available within 2 to 3 days.

The BRAF test has a sensitivity of 85%, and with the BRAF-PLUS has a sensitivity greater than 95%.⁵ Specificity is greater than 99.5%, meaning that false positives essentially do not occur.⁵ This test has been validated in more than 7000 dogs, and the mutation was not detected in healthy dogs, dogs with other types of cancer, or dogs with other noncancerous urinary disease, including polyps, inflammation, infection, or chronic cystitis.^5,6

Once a positive test result is detected, imaging studies are still required to determine the specific location of the carcinoma (bladder, urethra, prostate), although Ettinger said BRAF tests may be positive weeks before a mass is visible on ultrasound evaluation. Histologic samples are still required for tumor grading. However, having such a simple and noninvasive test is more palatable to both owners and veterinarians, and may promote earlier diagnosis.

Ettinger also finds the BRAF test useful for monitoring treatment response. Test results are reported not only as positive or negative, but with a fractional abundance (FA) score. If the FA score is increasing, then the tumor cells do not appear to be responding to treatment, and another protocol can be attempted. A decreasing FA score is an objective indication of treatment response.

Osteosarcoma

For canine osteosarcoma, a novel recombinant Listeria vaccine initially showed great promise when data indicated a median disease-free interval of 615 days and median survival of 956 days (double that of historical controls) in treated patients.⁷ However, this vaccine was discontinued in January 2020 after conditional licensure was granted, when several patients developed complications from Listeria-positive infections.⁸

In its stead, another immunotherapy protocol is under investigation and showing promise. The ELIAS cancer immunotherapy (ECI) is a personalized cancer vaccine derived from a patient’s tumor cells.⁹ The vaccine is administered intradermally in a series of 3. After vaccination, leukapheresis is performed to harvest mononuclear cells and plasma, so that the antigen-specific T cells that are primed by the vaccine are reinfused, followed by interleukin-2 injections that further modulate the immune system and stimulate T-cell multiplication in vivo. The entire treatment protocol is completed within 8 weeks, unlike conventional chemotherapy, which continues for many months. In a recent published study of 14 dogs that underwent ECI the median survival time was 415 days and 5 dogs survived longer than 730 days.⁹ This is very exciting as these dogs had amputation and immunotherapy but no chemotherapy

Rebecca A. Packer, DVM, MS, DACVIM (Neurology/Neurosurgery), is an associate professor at Colorado State University College of Veterinary Medicine and Biomedical Sciences in Fort Collins, and is board certified in neurology by the American College of Veterinary Internal Medicine. She is active in clinical and didactic training of veterinary students and residents, and has developed a comparative neuro-oncology research program at Colorado State University.

References

1. Miller J, Campbell J, Blum A, et al. Dose characterization of the investigational anticancer drug tigilanol tiglate (EBC-46) in the local treatment of canine mast cell tumors. Front Vet Sci. 2019;6:106. doi:10.3389/fvets.2019.00106

2. De Ridder TR, Campbell JE, Burke-Schwarz C, et al. Randomized controlled clinical study evaluating the efficacy and safety of intratumoral treatment of canine mast cell tumors with tigilanol tiglate (EBC-46). J Vet Intern Med. Published online June 16, 2020. doi:10.1111/jvim.15806

3. TANOVEA-CA1 package insert. Fort Collins, CO: VetDC Inc., 2016.

4. Thamm DH, Vail DM, Post GS, et al. Alternating rabacfosadine/doxorubicin: efficacy and tolerability in naïve canine multicentric lymphoma. J Vet Intern Med. 2017;31(3):872-878. doi:10.1111/jvim.14700

5. Mochizuki H, Shapiro SG, Breen M. Detection of BRAF mutation in urine DNA as a molecular diagnostic for canine urothelial and prostatic carcinoma. PLoS One. 2015;10(12):e0144170. doi:10.1371/journal.pone.0144170

6. Breen M, Vaden S. Canine transitional cell carcinoma (TCC)/urothelial carcinoma (uc)/bladder cancer in dogs—new research provides an opportunity for early detection. Breen Lab at NCSU. Accessed November 20, 2020. https://www.breenlab.org/bladder%20cancer.html

7. Mason NJ, Gnanandarajah JS, Engiles JB, et al. Immunotherapy with a HER2-targeting Listeria induces HER2-specific immunity and demonstrates potential therapeutic effects in a phase I trial in canine osteosarcoma. Clin Cancer Res. 2016;22(17):4380-4390. doi:10.1158/1078-0432.CCR-16-0088

8. Musser ML, Berger EP, Tripp CD, Clifford CA, Bergman PJ, Johannes CM. Safety evaluation of the canine osteosarcoma vaccine, live Listeria vector. Vet Comp Oncol. Published online July 30, 2020. doi:10.1111/vco.12642

9. Flesner BK, Wood GW, Gayheart-Walsten P, et al. Autologous cancer cell vaccination, adoptive T-cell transfer, and interleukin-2 administration results in long-term survival for companion dogs with osteosarcoma. J Vet Intern Med. 2020;34(5):2056-2067. doi:10.1111/jvim.15852