Development of prednisone resistance in naïve canine lymphoma: Longitudinal evaluation of NR3C1α, ABCB1, and 11β-HSD mRNA expression.

Prednisone resistance develops rapidly and essentially universally when dogs with lymphoma are treated with corticosteroids. We investigated naturally occurring mechanisms of prednisone resistance in seven dogs with naïve multicentric lymphoma, treated with oral prednisone; four dogs were administered concurrent cytotoxic chemotherapy. Expression of NR3C1α, ABCB1 (formerly MDR1), 11β-HSD1, and 11β-HSD2 mRNA was evaluated in neoplastic lymph nodes by real-time RT-PCR. Changes of expression levels at diagnosis and at time of clinical resistance to prednisone were compared longitudinally using a Wilcoxon signed-rank test. Clinical resistance to prednisone was observed after a median of 68 days (range: 7-348 days) after initiation of treatment. Relative to pretreatment samples, prednisone resistance was associated with decreased NR3C1α expression in biopsies of all dogs with high-grade lymphoma (six dogs, p=.031); one dog with indolent T-zone lymphoma had increased expression of NR3C1α. Resistance was not consistently associated with changes in ABCB1, 11β-HSD1, or 11β-HSD2 expression. Decreased expression of the glucocorticoid receptor (NR3C1α) may play a role in conferring resistance to prednisone in dogs with lymphoma. Results do not indicate a broad role for changes in expression of ABCB1, 11β-HSD1, and 11β-HSD2 in the emergence of prednisone resistance in lymphoma-bearing dogs.