Drug resistance in canine multicentric lymphoma

Abstract

Lymphoma is amongst the most common forms of cancer in the dog and is routinely treated with a multidrug chemotherapy protocol that includes (as a minimum) doxorubicin and prednisolone. Despite initial good treatment results (complete response rate ±80%), the tumor will recur in the majority of dogs. Tumor recurrence is often associated with resistance to chemotherapy and as a result less than 20% of dogs is cured. It has been found that pretreatment with glucocorticoids like prednisolone negatively affects treatment outcome with subsequent chemotherapy and this is thought to result from induction of drug resistance. In human oncology, the upregulation of efflux pumps of the ABC-transporter family, including P-gp, MRP1 and BCRP, is a common and well-studied cause for drug resistance. In this PhD-project drug resistance was studied in two ways: in vitro in a canine lymphoid leukemia cell line and in tumor samples from dogs with lymphoma.

The in vitro studies showed that resistance to doxorubicin results from the upregulation of the ABC-transporter P-gp (P-glycoprotein). This upregulation coincided with resistance to the cytostatic drug vincristine, but not to prednisolone. Furthermore it was found that, in contrast to humans, prednisolone failed to induce the expression of P-gp or other ABC-transporters. It was demonstrated that P-gp function could be inhibited, and drug resistance reversed, by using the classical P-gp inhibitor PSC833 and the veterinary registered tyrosine-kinase inhibitor masitinib.

In order to verify these in vitro results in vivo, a prospective, randomized clinical trial was performed in dogs that were diagnosed with multicentric lymphoma and treated with a standardized doxorubicin-based chemotherapy protocol with only half of these dogs receiving prednisolone. It was found that removing prednisolone from the chemotherapy protocol did not affect treatment results (response-rate, duration of response, or adverse events) and it is concluded that prednisolone can be safely removed from the current first-line treatment protocols. Assuming that the use of prednisolone would increase ABC-transporter expression and lead to drug resistance, this could explain the poorer treatment results with further chemotherapy following relapse. However, we were not able to demonstrate a poorer response to rescue chemotherapy treatment in dogs treated with prednisolone in the initial protocol compared to those dogs that did not receive prednisolone. ABC-transporter mRNA expression was assessed in the previously mentioned group of dogs prior to starting chemotherapy, at tumor relapse and at clinical therapy resistance. ABC-transporter expression was demonstrated in all samples, but pre-treatment ABC-transporter expression was not predictive for treatment response. Clinical drug resistance could not be associated with overexpression of one (or a combination of) ABC-transporter(s), but overexpression differed per immunophenotype and while drug resistant B-cell lymphomas were associated with increased expression of ABCB1 (P-gp), T-cell lymphomas showed increased ABCB5 and ABCG2 (BCRP) expression. Similar to our in vivo study, we were not able to demonstrate upregulation of ABC-transporter expression in dogs treated with prednisolone.

Since ABC-transporter expression was not able to predict treatment outcome and clinical drug resistance, it is likely that other drug resistance mechanisms play a role in canine lymphoma.