Phenotyping

On a very basic level, lymphoma is broken down into two main phenotypes, B and T cell. Traditionally 2/3 of dogs with lymphoma are classified as B cell and 1/3 are T cell. A minor percentage (<2%) are deemed “null cell.” Phenotyping of lymphoma patients can be achieved through a variety of tests including immunohistochemistry, immunocytochemistry, PARR, and flow cytometry. T cell lymphoma is more commonly associated with certain breeds including the boxer, golden retriever, Australian shepherd, Asian lap dogs, and Siberian husky. T cell LSA is also associated with certain anatomic forms including cutaneous (epitheliotropic, a.k.a. ELSA), mediastinal, hepatic and gastrointestinal.

Why Phenotype?

Many studies have documented a worse prognosis for dogs with T-cell lymphoma and for this reason, many oncologists have begun modifying protocols based upon phenotype. Further support of this was based upon a several studies: Beaver et al. assessed the response of T cell LSA to a single dose of doxorubicin was ∼50% vs. ∼100% for dogs with B cell LSA.

The idea of using more alkylating agents into T cell protocols was partially based on the high response rates noted in dogs with ELSA treated with lomustine (CCNU). Although only 17% experienced a complete remission (CR), 61% experienced a partial remission (PR). Furthermore, the combination of L-asparaginase, mechlorethamine, vincristine, procarbazine, and prednisone (L-MOPP) was investigated in dogs T-cell lymphoma and was associated with a complete remission rate of 78%, and overall survival 270 days. However, >20% were alive at >900 days. The challenge with this protocol is cost, the difficulty of administration and toxicity. Currently protocols with substitutions of CCNU for doxorubicin and Elspar in each cycle are underway, however, data is lacking. There does exist one report evaluating only T cell phenotype treated with a standard CHOP-based protocol and its associated survival is similar to that noted in other studies.

Additional support for the importance of phenotyping comes from human experience with monoclonal antibodies, specifically Rituximab, which are standard of care in CHOP-based protocols (R-CHOP). Monoclonal antibody (mAb) can be used to specifically bind to target cells or proteins. This may then stimulate the patient’s immune system to attack those targeted cells and remove them from the body. In human oncology, monoclonal antibodies have been developed for T and B cell and these have now become standard of care therapy. The first series in Veterinary Oncology were granted full licensure by the USDA but are no longer commercially available due to specificity issues.

Merial has gained conditional licensure for its Oncept LSA vaccine, which uses the same concept but is designed to induce immunity to CD20 (present on the surface of B cell LSA). Data from a recently completed trial in which patients were administered the LSA vaccine during the second cycle of chemotherapy, noted an improvement in remission duration of 4 months.

Feline New Classification System

In feline lymphoma, a new classification system has been developed. The current vernacular of “low, intermediate, and high grade” are being replaced with more informative descriptions, such as diffuse large B-cell lymphoma (DLBCL), and enteropathy associated T-cell lymphoma (EATL). The rationale is that these tumors are as biologically different from one another as is comparing a mast cell tumor and a plasma cell tumor. Histologically, nodal lymphomas are first classified based on their growth pattern (i.e., diffuse vs. follicular), nuclear size next (i.e., small, intermediate, and large cell lymphomas) that are further classified using mitotic counts, phenotype, etc. For gastrointestinal (GI) LSA, tumors are differentiated as primary “extranodal” GI LSA or LSA with extension to the GI tract. Based upon immunohistochemistry and clonal analysis, a significantly higher incidence of T-cell lymphomas than B-cell lymphomas occurs in the intestines of cats. Furthermore, the majority of feline intestinal T-cell lymphomas are small cell, mucosal T-cell lymphomas, consistent with type 2 EATL.

The second form, consisting of large cell transmural T-cell lymphomas, is classified as type 1 EATL. There is an uncommon and very aggressive subtype of EATL type I called large granular lymphoma (LGL), that is comprised of either cytotoxic T-cells or NK cells. LGL is characterized by multiple masses within the GI tract and other organ systems, including the blood. In cats, B-cell lymphomas are less common and are usually diffuse large B-cell (DLBCL). They commonly occur primarily in the stomach and the ileo-ceco-colic junction or in both locations.

Applying the Terminology to the Real World

Cats with EATL type I often present with a palpable mass effect and more acute and severe clinical signs while cats with EATL type II, present with more chronic signs, similar to that with IBD.

Cytology can often provide an expedited diagnosis depending upon the form. For large cell lymphoma, whether EATL I, DLBCL or LGL lymphoma, the predominant cell type is a large lymphocyte, cytology is an excellent tool for diagnosis. With EATL Type II, which is primarily comprised of small lymphocytes, cytology is often unrewarding.

EATL-Type II is previously known as small-cell lymphoma/lymphocytic/indolent LSA is considered a less aggressive variant of lymphoma. Although unproven, there is certainly evidence to suggest a possible link to inflammatory bowel disease and the potential that the two syndromes represent a continuum of the same disease process, as both are often present in the same tissue concurrently. In general, clinical signs occur over a prolonged timeframe (several months to even years) and typically include weight, diarrhea, intermittent vomiting and loss of appetite. Unlike the aggressive EATL Type I form which is composed of large “blastic” lymphocytes, this form is comprised of small lymphocytes that invade the intestines, lymph nodes, stomach and liver. Bloodwork is generally unremarkable; however, mild anemia and leukocytosis may be noted. Imaging findings with ultrasound often yield thickened intestines and rarely a mass effect. Aspirates can be unrewarding in providing a definitive diagnosis, as lymph nodes normally contain mostly small lymphocytes. Definitive diagnosis requires histopathology and, in some cases more advanced testing to include clonality testing and immunohistochemistry. In general, this requires either an endoscopy, or more preferred by this author, an exploratory surgery, whereby multiple organs can be biopsied (full thickness), in an effort to determine the true extent of the disease.

Treatment for EATL Type II does not involve multidrug protocols, only daily prednisolone and chlorambucil (Leukeran), which may be q 21 days, every other day or m/w/f depending upon protocol chosen. Resolution of gastrointestinal signs and weight gain, and in some cases abdominal ultrasound are used to monitor efficacy. The overall response rate is very high >90% and evidence suggest that treatment may be discontinued after a year. If a relapse occurs, rescue therapy with cyclophosphamide can be considered, as this protocol has been associated with a high response rate. The overall median survival time is nearly 2 years for EATL Type II. Since many cases have concurrent.

Tanovea™ VetDC

Tanovea™ was discovered by Gilead Sciences, Inc., and licensed to VetDC for use in animal cancer, (previously known as VDC-1101). This agent was designed to preferentially target and attack cancer cells implicated in lymphoma.^15-17 The data from studies totaling well over 330 patients have shown Tanovea™ to be highly effective against LSA with a 60–80% overall response rate. Not surprisingly, responses are higher in naïve LSA vs relapse and in dogs with a B cell phenotype.

Data suggests Tanovea™ is well-tolerated with a similar side effect profile as other commonly used agents. The administration is via the intravenous route with a 1 mg/kg dosing every 3 weeks. The FDA is in the final staging to approve Tanovea.

Side Effects

Although the majority of side effects with this agent are similar to most chemotherapeutics, two unique side effects are present that clinicians need to be educated in order to better recognize and treat them.

Pulmonary fibrosis: This appears to be recorded in a small percentage of the patients treated and the mechanism is unknown. In some cases, this was fatal, thus clearly screening with thoracic films and exclusion of patients with pre-existing pulmonary issues or particular breeds at risk of pulmonary fibrosis is warranted.

Dermatopathy: Also occurring in a minority of patients and often appears along the pinna and chest. Per VetDC, resolution of the side effects occurs once discontinuing the protocol.

References

1.  Thalheim L, et al. J Vet Intern Med. 2013;27:1509–16.

2.  Modiano JF, et al. Cancer Res. 2005;65:5654.

3.  Beaver LM, et al. J Am Vet Med Assoc. 2010;237:1052–5.

4.  Brodsky EM, et al. J Vet Intern Med. 2009;23:578–84.

5.  Rebhun RB, et al. VCO. 2013;9:38–44.

6.  Taylor SS, et al. J Feline Med Surg. 2012;15:142–147.

7.  Kiupel M, et al. Vet Pathol. 2011;48:212–222.

8.  Kiselow MA, et al. J Am Vet Med Assoc. 2008;232:405–410.

9.  Pope KV, et al. J Vet Med Sci. 2015;15162.

10.  Thamm DM, et al. JVIM. 2017;31:872–878.