Cancer cells acquire abnormalities in energy metabolism, collectively known as the Warburg effect, affecting substrate availability of thiamine-dependent enzymes. To investigate a strategy to exploit abnormal cancer-associated metabolism related to thiamine, we tested the cytotoxicity of native Bacillus thiaminolyticus thiaminase I enzyme, which digests thiamine, in the NCI60 cell line drug cytotoxicity screening program and found that leukemia cell lines were among the most sensitive to thiaminase I. We obtained additional lymphoid leukemia cell lines and confirmed that native thiaminase I and linear chain PEGylated thiaminase I enzyme (LCPTE) have cytotoxic activity in these cell lines. In addition, the IC(50) of 3 of the 5 leukemia cell lines (Reh, RS4, and Jurkat) were at least 1,000-fold more sensitive than Molt-4 cells, which in turn, were among the most sensitive in the NCI60 panel. The 3 LCPTE-sensitive leukemia cell lines were also sensitive to removal of thiamine from the medium, thus suggesting the mechanism of action of LCPTE involves extracellular thiamine starvation. Surprisingly, rapamycin showed a protective effect against LCPTE toxicity in the 3 LCPTE-sensitive cell lines but not in the other 2 cell lines, suggesting involvement of an mTOR-dependent pathway. Immunoblot analysis of the LCPTE-sensitive cell lines after LCPTE exposure revealed changes in mTOR pathway phosphorylation. Nude mice bearing RS4 leukemia xenografts showed both tumor growth delay and prolonged survival after a single dose of LCPTE. Therefore, disruption of thiamine-dependent metabolism may be a novel therapeutic approach to target altered energy metabolism in leukemia and other cancers.