Development of multidrug resistance in a canine lymphoma cell line.

New multidrug resistant cell lines developed from the canine B cell lymphoma cell line (GL-1) were characterized in terms of chemosensitivity to some antineoplastics and P-glycoprotein (Pgp) expression. GL-1 was continuously exposed to a culture medium containing gradually increasing levels of doxorubicin and the cells that could grow in the presence of doxorubicin were obtained. Chemosensitivity of these cells to various antineoplastics were investigated with or without verapamil, which reversed Pgp-mediated drug resistance. The expression of Pgp on the cells was also examined by Western blot analysis. As a result, three kinds of resistant cell lines, designated as GL-DOX60, 300, and 4000 were obtained. These cell lines showed stable proliferation in the medium containing 60, 300, and 4000 ng/ml, respectively. These cells were much more resistant to vincristine than doxorubicin. This resistance was strongly reversed by the presence of verapamil. On the other hand, cisplatin was effective enough in killing these derived cells. In the Western Blot analysis, some bands that reacted to the anti-human Pgp monoclonal antibodies were observed in GL-DOX4000. The cells derived from GL-1 have multidrug resistance potential mediated by canine Pgp. The cells produced in this experimental trial are considered to be useful models for various investigations on canine multidrug resistance.