Monoclonal antibody therapy for canine lymphoma: Promoting the fight from within
The standard treatment for canine lymphoma is vincristine, cyclophosphamide, doxorubicin and prednisone, known as a CHOP-based protocol, with or without L-asparaginase.1-3 Remission rates for canine lymphomas are greater than 85 percent, with survival times ranging from eight to 12 months, but there has been no substantial improvement in patient outcome in nearly two decades.1-4 Canine monoclonal antibodies (mAbs) against lymphoma may be a breakthrough in the treatment of canine lymphoma.
What is a monoclonal antibody?
Antibodies are made by B cells. Antibodies ambush foreign antigens circulating in the blood stream. When a B cell encounters the kind of antigen that triggers it to become active, it gives rise to plasma cells, which produce antibodies. A mAb is a type of antibody that is more uniform than a natural antibody and binds specifically to its target protein.
Originally, mAbs were produced by fusing B cells from the spleen of an animal that had been immunized with the target protein with a myeloma cell line that was selected for the inability to produce immunoglobulin. Köhler and Milstein developed this hybridoma technology, which made it possible to produce large quantities of antibodies with high purity and monospecificity for a single binding region (epitope) on an antigen.5 Newer technologies have been developed since to generate humanized and human antibodies.
An antibody is divided into three domains consisting of two identical antigen-binding (Fab) domains connected to an effector, or Fc region, by a flexible hinge sequence. IgG antibodies are composed of two identical light chains and two identical heavy chains, with the chains joined by disulfide bonds, resulting in a bilaterally symmetrical complex.6 The Fab domains mediate the binding of IgG molecules to their cognate antigens and are further divided into variable (Fv) and constant (Fc) regions.6 Canine mAbs that have been recently developed by Aratana Therapeutics are caninized antibodies, where the hypervariable regions of the variable antigen-binding domain (Fv) are derived from a mouse antibody and the rest of the Fv and the entire Fc region are derived from canine sequences.
How mAbs work
Two types of mAbs are available in human medicine—unconjugated mAbs and conjugated mAbs. Conjugated mAbs indirectly exhibit antitumor effects by delivering cytotoxic payloads. Conjugated mAbs have been used to deliver a wide variety of agents, including chemotherapy, toxins, radioisotopes and cytokines.7 Unconjugated mAbs display direct antitumor effects that are mediated by the following mechanisms6,8:
1. Antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP)—When antibodies engage the tumor antigen on the surface of tumor cells, Fc-gamma receptors that are expressed on the cell surface of effector cells, such as natural killer cells and monocytes or macrophages, bind to the Fc domain of the IgG molecules. This bridging induces effector cell activation, resulting in natural killer cell cytotoxicity or phagocytosis by neutrophils, monocytes or macrophages.
2. Complement-dependent cytotoxicity (CDC)–mAbs can recruit the complement cascade to kill cells via CDC. Antibodies activate complement through the classical pathway, which kills the antibody-bound cells.
mAb therapy for human lymphoma
In human medicine, chemoimmunotherapy regimens incorporating rituximab, a chimeric mAb targeting the CD20 receptor, were the first strategies in decades to prolong the survival of patients with diffuse large B-cell non-Hodgkin’s lymphoma, follicular lymphoma and chronic lymphocytic leukemia.9 The first study to show the benefit of rituximab involved patients more than 60 years of age who were randomly assigned to receive CHOP plus rituximab (R-CHOP) or CHOP alone.10 In this study, the complete remission rate (76% versus 63%, P = 0.005) and two-year event-free survival rate (57% versus 39%, P < 0.001) were improved with R-CHOP, ultimately translating to an improvement of overall survival at 10 years (43.5% versus 27.6%, P = 0.005).10,11
In a randomized phase III trial investigating the efficacy of R-CHOP versus CHOP in untreated, younger (less than 60 years of age), good-prognosis patients with diffuse large B-cell lymphoma, patients treated with R-CHOP had significantly higher rates of three-year event-free survival (79% versus 59%, P < 0.0001) and three-year overall survival (93% versus 84%, P < 0.0001) compared with CHOP alone.12
In most studies, the combination of rituximab and standard chemotherapy did not result in an increase in toxicity, so rituximab became a common part of many lymphoma treatment regimens. Rituximab has been used both in combination with chemotherapy or alone as an induction, maintenance and rescue agent. Other types of mAbs have been developed since and used in combination with chemotherapy for different types of lymphoma.
mAb therapy for canine lymphoma
Because of the success in human medicine, mAb therapy could potentially be effective in treating canine lymphomas. Rituximab was investigated for potential therapeutic efficacy in treating B cell lymphoma in dogs. An ex vivo study showed that rituximab does not bind to canine CD20, likely because of lack of conservation of the rituximab epitope in the canine protein.13 Recently, mAbs for both T-cell and B-cell lymphoma have been developed and are under clinical investigation.
1. Canine CD52 antibody. CD52 is a glycoprotein highly expressed on both B and T cells. The USDA granted a conditional license in January 2014 and is anticipated to grant full licensure this year. It is available only in a limited number of sites nation wide. In a poster presentation at the Veterinary Cancer Society 2014 meeting regarding bioavailability and safety of intravenously administered CD52 antibody in dogs with high grade T-cell lymphoma (WHO stage stage IV to V), the canine CD52 antibody was detectable in plasma after a single dose and accumulated after multiple doses.14 No meaningful changes in hematology or serum chemistry values were observed. Clinically relevant hypotension was infrequent, and anaphylactoid reactions were rare. A multicenter, randomized, placebo-controlled study of CD52 antibody in combination with lomustine (CCNU) chemotherapy in the treatment of canine T-cell lymphoma is currently underway.
2. Canine CD20 antibody. CD20 is a glycoprotein expressed exclusively on mature B cells. This mAb was granted full approval for licensure on Jan. 1, 2015, but is currently only being manufactured for certain institutions; the release date to other institutions is unknown at this time. Two pilot studies were presented at the Veterinary Cancer Society 2014 meeting:
- In a prospective, double-blind, randomized, placebo-controlled study of CD20 antibody in combination with L-CHOP chemotherapy, 26 of 27 dogs that received L-CHOP with mAb achieved complete remission. The median-progression-free survival and overall survival times for the canine mAb arm of the study were 167 days and 325 days, respectively, compared with 93.5 days and 177 days for the placebo arm.15 Adverse events were restricted to the L-CHOP cycle.
- In an open-label pilot study of CD20 antibody in combination with doxorubicin chemotherapy, nine of 12 dogs treated with doxorubicin and mAb achieved complete remission. The median-progression-free survival time for the canine mAb arm of the study was 98 days compared with 57 days with doxorubicin alone.16
The efficacy of these mAbs against canine lymphomas is still under investigation, but using combinations of mAbs with current treatments is likely to help prolong the life of dogs with lymphoma, while also providing them a good quality of life.
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