This blog post finally combines the two great passions of my life: antibodies and dogs. Therapeutic antibody development is a huge area and is certainly not limited to humans. In the process of developing antibodies, we often use mouse or rat antibodies, obtained by injecting the animal with the antigen of choice and then collecting the resulting antibodies. The first monoclonal antibodies (mAbs) were produced in this way, by fusing spleen B cells from an immunised mouse or rabbit with immortalised myeloma cells to form antibody-expressing hybridoma cells. However, using antibodies to treat disease in animals lags behind humans.
Unfortunately, dogs can develop cancers just like humans, particularly as they age. In the US each year, over 4.2 million dogs are diagnosed with cancer – ten times the rate of humans (500 vs 5300 per 10,000) [1]. Treatment options for dogs are similar to their owners: surgery, radiotherapy and chemotherapy, which are not always successful. Immunotherapies such as antibodies have progressed significantly in humans in recent years, and researchers are looking at how they might also be developed for dogs.
Rituximab is a therapeutic antibody for human B cell cancers which was approved for use in humans back in 1997 [2]. Rituximab targets CD20, which is expressed on the surface of all B cells and plays a key role in the development and differentiation of the B cell. When rituximab binds to CD20, cell death is triggered through mechanisms such as antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity. This is useful in cancers (and autoimmune disease) associated with B cell proliferation, such as non-Hodgkin lymphoma and chronic lymphocytic leukaemia. In July, an anti-CD20 antibody for use in dogs was published [3]. This is a rat-canine chimeric antibody and was tested in cell lines, a mouse model and in healthy beagles, and showed a reduction in B cell numbers. Another canine therapeutic antibody, published in 2017, targets PD-1 [4], also a common cancer target. T cells have a PD-1 receptor on their surface which suppresses T cell immune activity when bound, vital to prevent overstimulation of the immune system. Cancer cells can exploit this by expressing PD-L1 proteins which bind to the PD-1 receptor and suppress T cell activation. Blocking PD-1/PD-L1 binding has been demonstrated as an efficient strategy to reduce the immune suppression exhibited by cancer cells in humans. Like the anti-CD20 antibody, this is PD-1 targeting antibody is also a rat-canine chimeric antibody, and underwent a successful pilot clinical study.
These cancer-antigen targeting antibodies were initially identified in rat, so the chimerisation step is important; if a rat antibody targeting CD20 or PD-1 was injected directly into a dog, the immune system may recognise it as foreign and trigger an immune response and allergic reaction. Therefore, parts of the rat antibody not essential to the target binding are replaced with corresponding dog regions. Rituximab is also a chimeric antibody – originally a mouse antibody, the constant region of the antibody was replaced with a human constant region.
Given the similarities between the human and dog cancers [5], it is perhaps surprising that treatments have not previously been developed. Now that the chimerisation process for making a rat antibody seem ‘dog-like’ has been proven, hopefully we will see more of these treatments emerging!
References:
[1] Schiffman, Joshua D., and Matthew Breen. “Comparative oncology: what dogs and other species can teach us about humans with cancer.” Philosophical Transactions of the Royal Society B: Biological Sciences 370.1673 (2015): 20140231.
[2] FDA Rituxan (Rituximab) Label https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/103705s5367s5388lbl.pdf
[3] Mizuno, Takuya, et al. “Generation of a canine anti-canine CD20 antibody for canine lymphoma treatment.” Scientific Reports 10.1 (2020): 1-12.
[4] Maekawa, Naoya, et al. “A canine chimeric monoclonal antibody targeting PD-L1 and its clinical efficacy in canine oral malignant melanoma or undifferentiated sarcoma.” Scientific reports 7.1 (2017): 1-12.
[5] Addissie, Selamawit, and Hans Klingemann. “Cellular immunotherapy of canine cancer.” Veterinary sciences 5.4 (2018): 100.
