Chemotherapies currently constitute one main venue of cancer treatment. For a large number of adult and elderly
patients, however, treatment options are poor. These patients may suffer from disease that is resistant to conventional
chemotherapy or may not be candidates for curative therapies because of advanced age or poor medical conditions. To
control disease in these patients, new therapies must be developed that are selectively targeted to unique characteristics
of tumor cell growth and metastasis. A reliable early evaluation and prediction of response to the chemotherapy is
critical to its success. Chemotherapies induce apoptosis in tumor cells and a portion of such apoptotic cancer cells may
be present in the circulation. However, the fate of circulating tumor cells is difficult to assess with conventional methods
that require blood sampling. We report the in situ measurement of circulating apoptotic cells in live animals using in
vivo flow cytometry, a novel method that enables real-time detection and quantification of circulating cells without blood
extraction. Apoptotic cells are rapidly cleared from the circulation with a half-life of ~10 minutes. Real-time monitoring
of circulating apoptotic cells can be useful for detecting early changes in disease processes, as well as for monitoring
response to therapeutic intervention.
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