Biology session

Although recent advance in tumor therapies have had a significant impact on patient survival, treatment of malignancies is still a major challenge in the human and canine cancer clinic. The efficacy of new, promising drugs (signaling pathway and angiogenesis inhibitors, targeted therapy) is often hampered by the emergence of multidrug resistance (MDR) of tumors. As a result of MDR, the initial hypersensitivity of cancer is lost as the cells evolve to survive in the highly toxic environment. One of the main contributors to the mechanisms responsible for MDR is the overexpression of a member of the ABC (ATP-binding cassette) transporter family, namely P-glycoprotein (Pgp, ABCB1, MDR1). Pgp extrudes a wide spectrum of cytotoxic compounds (from chemotherapeutics to anti-HIV drugs) from the cell, resulting in the decreased intracellular concentration of the given drugs. The best understood mechanism leading to elevated transporter expression is based on the higher mutation rate of cancer cells. Recently, an alternative mechanism has been proposed with an emphasis on epigenetic regulation.

My goal was to examine the influence of Pgp function on the treatment outcome of canine lymphoma. Using flow cytometry I analyzed Pgp expression and function as well as the immunphenotype of 16 canine lymphoma samples. Here I present two case studies with a long term follow up (25 weeks). Dogs diagnosed with B cell lymphoma were treated according to the Wisconsin-Madison protocol 16 times over the course 25 weeks. Pgp function and the immunphenotype of isolated tumor cells were determined at diagnosis and relapse (4 measurements).

In both cases, initial Pgp function significantly increased during treatment. Surprisingly, resistance proved to be reversible, as a break in the therapy (“drug holiday”) resulted in the loss of resistance. To study mechanisms underlying the rapid emergence and loss of the drug resistance observed in the canine clinic, I performed in vitro experiments using P388 B cell type mouse lymphoblastic leukemia cells. The contribution of epigenetic regulatory mechanisms was delineated using a panel of epigenetic inhibitors. The results will be discussed in the context of emerging views on the relation of epigenetic regulation and MDR.