Early Fludarabine Treatment of TCL-1 Transgenic Mice Promotes Drug Resistance through a Mechanism Not Involving p53 Mutations: Implications for Patients with Chronic Lymphocytic Leukemia (CLL).

Abstract

Initial therapy for Chronic Lymphocytic Leukemia (CLL) typically includes nucleoside analogs such as fludarabine. Mutations of p53 are uncommon in CLL patients at diagnosis, but are found at increased frequency in patients with prior treatment and are often associated with resistance to standard therapies including fludarabine. The causality of fludarabine treatment to p53 mutation has not been clearly defined. In previous work with the TCL-1 model of CLL, we demonstrated that mice with active leukemia do not have p53 mutations and respond to fludarabine therapy with a modest survival advantage (Johnson et al, Blood 108:1334, 2006). We therefore initiated a randomized trial of fludarabine treatment (34mg/kg daily intraperitoneal injection for five days every 28 days) versus observation (treatment with volume-matched vehicle controls on the same dosing schedule) in TCL-1 transgenic mice prior to the onset of leukemia. The goals of this study were to determine if early treatment with fludarabine would 1) delay disease onset; 2) eventually yield a fludarabine-resistant phenotype; or 3) promote development of p53 mutations in mice developing progressive leukemia. Transgenic TCL-1 mice between the ages of eight and 12 weeks were continually treated with fludarabine (n=32) or saline (n=45) until death while being screened monthly for disease progression through changes in WBC count by peripheral blood differentiation smear, lymphocyte subsets by flow cytometry, and nodal and spleen exams. Despite continued treatment with fludarabine, drug resistance as manifested by leukemia and eventual death occurred. Median time to leukemia onset was 308 days (95% CI: 224, ∞) in the fludarabine group and 338 days (95% CI: 252, ∞) in the saline group. Thus, the risk of developing leukemia was not significantly different between the two groups. Of interest, p53 mutations in exons 2–11 were not found in either group (n=10 each) at the time of death. Microarray studies are currently underway to discover other genes differentially expressed between treated and untreated mice that might contribute to fludarabine resistance and will be presented. In conclusion, our data demonstrate that early fludarabine treatment in the TCL-1 transgenic model of CLL does not prevent onset of leukemia, but instead results in disease resistant to fludarabine. The source of fludarabine resistance in this mouse model was not through acquisition of p53 mutations, but rather an alternative unidentified mechanism(s).