(A) Fold changes of pyrimidine nucleotide abundances, as measured by LC-MS/MS, in vehicle treated MDA-MB-231 xenograft tumors versus MDA-MB-231 xenograft tumors treated with 1 mg/kg doxorubicin for 24 hours. (B) MDA-MB-231 xenografts were treated with leflunomide (Lef), doxorubicin (Dox) or a combination of leflunomide and doxorubicin (5 mice per group). Tumors were measured with calipers. (C) Waterfall plot depicting relative tumor volume between the treatment groups 28 days after treatment with vehicle (blue), leflunomide (orange), doxorubicin (purple) or leflunomide and doxorubicin (green). Each bar represents an individual mouse. (D) The body weight of mice treated with vehicle, leflunomide, doxorubicin or leflunomide and doxorubicin was monitored every 7 days for 28 days. No significant changes in body weight were observed during the course of the experiment. All error bars represent SEM. N.S. not significant, ** P < 0.01, *** P < 0.001 by a Student’s t-test.