High levels of choline kinase (ChoK) expression and choline phospholipid metabolites are often associated with malignant transformation, invasion, and metastasis, particularly in breast cancer. These findings have led to the development of novel pharmacologic or gene therapeutic interventions for ChoK-targeted inhibition. To identify pharmacodynamic markers for the therapeutic evaluation of ChoK down-regulation, we investigated the uptake and efflux of [(3)H]choline, a natural substrate of ChoK, and two other important metabolic indicators of malignancy, namely, [(3)H]thymidine and [(3)H]fluorodeoxyglucose, which measure proliferation and glucose metabolic changes, respectively, in ChoK-downregulated cells. Choline uptake in nonmalignant and malignant breast epithelial cell lines expressing graded levels of ChoK showed a ChoK-dependent uptake, retention, and efflux of [(3)H]choline. Reduced proliferation observed because of ChoK down-regulation resulted in reduced [(3)H]thymidine uptake and incorporation into DNA within 48 hours of treatment. Reduced [(3)H]thymidine incorporation levels were consistent with a decreased cell cycle S-phase fraction. No change in [(3)H]fluorodeoxyglucose uptake was observed between ChoK-downregulated and control cells in any of the three cell lines tested. These results demonstrate the utility of radiolabeled choline or choline analogs and proliferation imaging agents as pharmacodynamic markers for ChoK-targeted therapies and suggest a ChoK-mediated mechanism for tumor sequestration of choline-based imaging agents.