Cancer is a tremendous threat to human life, and cancer metastasis accounts for the major cause of mortality in cancer patients. Circulating tumor cells (CTCs) is one of the clinical biomarkers of cancer metastasis. Current in vitro methods for detecting CTCs in blood samples are based on the assumption that the distribution of CTCs in the peripheral blood do not change significantly over time; however, the correctness of this method is being challenged by recent studies. Because it is not practical to draw blood from patients or experimental animals continuously to investigate the daily oscillation of CTC count, the ideal method is to monitor CTCs in vivo over a long period of time.

In a new paper published in Light Science & Application, a team of scientists, led by Professor Xunbin Wei from Med-X Research Institute and School of Biomedical Engineering, Shanghai Jiao Tong University, China, Biomedical Engineering Department, Peking University, and co-workers have developed a noninvasive optical method to monitor CTCs in xenograft tumor models. The optical system they developed is named as “in vivo flow cytometry” (IVFC), which is different from conventional “in vitro” flow cytometry that can only detect fluorescently labeled cells in vitro. In IVFC, the laser was adjusted to focus across the micro artery of experimental mouse ear. When a fluorescently labeled CTC went through the light sheet, fluorescence was exited and detected by a photomultiplier tube (PMT). To point out the significance of this optical construction, CTCs in blood circulation can be detected noninvasively, repeatedly and continuously.

“Our IVFC technique is different from current in-lab or clinical methods used for CTC detection. Drawing blood is not required for the purpose of operating the system.  Since the biological environment is not damaged from drawing blood repeatedly, it allows us to monitor CTCs over a long period of time periodically and noninvasively.” They said.

With this technique, they monitored GFP-expressing CTCs in an orthotopic mouse model of prostate cancer over a 24-hour period at different stages of cancer progression. For CTC counts, they observed striking daily oscillations at the onset of night, which is the active phase for rodents. The IVFC was used to detect CTCs on Day 6, 12, 18, and 24 in real time, and the result indicated that an obvious burst activity appeared at the early stage of metastatic circulation. It suggested the probability of the bursting activity was higher at early stages than at late stages.

“The findings may extend our understanding of the relation between CTCs and the time frame. CTCs are not evenly distributed in the blood all day long. They are different in the day and night. It indicates that circadian rhythm might regulate CTC release. This factor should be taken into consideration for clinical detection of CTCs,” said the researchers, adding that “CTCs seems to be more complicate than people expected. This study shows us a potential factor that affect clinical CTC detection. It would be very important to know if the CTC changes and bursts over day and night, and thus enhance the understanding of their regularity of distribution. With the technique of IVFC, it is not necessary to draw blood samples at different time points since the repetitive procedures could cause biological environment to be altered. There is no doubt that we are increasingly understanding about CTCs and cancer metastasis. The detection of CTCs is becoming more precise than ever before.”

Featured Image: A demonstration of CTC burst in animal model monitored by IVFC. b, Circadian rhythm of CTCs in mice maintained under different light conditions. The red line shows the CTC counts in a day under 12-hours light and 12-hours dark. The blue line show the CTC counts in a day under 12-hours dark and 12-hours light. Illustration by: Xi Zhu, Yuanzhen Suo, Yuting Fu, Fuli Zhang, Nan Ding, Kai Pang, Chengying Xie, Xiaofu Weng, Meilu Tian, Hao He and Xunbin Wei