In the last few years, we have a seen a number of publications showing the great potential that circulating tumor cells (CTCs) have with cancer therapy – and how their use may help benefit cancer patients. The biggest apparent challenge, however, is which technology will be the best one to use for detecting and isolating these highly prized cells. In this blog post, we will introduce the three main strategies used for CTC enrichment.
The first CTC enrichment method that is frequently used for CTC isolation is the immunoaffinity-based strategy. There are two types of approaches within the immunoaffinity-based strategy: 1) positive enrichment, and 2) negative enrichment. Positive enrichment targets tumor-associated antigens, and negative enrichment depletes the hematopoietic cells by targeting CD45.
Both approaches have great advantages. Positive enrichment offers a high level of purity in isolated cells; negative enrichment eliminates bias in the subpopulation isolated since it does not rely on antibody selection.
The second CTC enrichment method uses biophysical properties. This method relies on the physical characteristics of the cell – including size, form and density of the cells. Technologies such as centrifugation or microfiltration have been around for more than 50 years. Even though these technologies are considered to be inexpensive and easy to use, their biggest disadvantage is the relatively low purity levels of the cells isolated. Purity levels can vary anywhere between 1-10%, making these technologies less than ideal when used alone – but more acceptable when used as an initial enrichment step.
And last, but not least, of CTC enrichment methods is direct imaging. Direct imaging may use preenrichment that combines flow cytometry with fluorescence, resulting in improved speed. ImageStream, for example, can analyze 5000 cells/sec – a speed that is five times faster than the speed in previous instruments. Other methods, such as FASTcell, are enrichment-free. FASTcell scans use optical fibers and come with the great advantage of reducing exposure time due to the use of a laser light source and a photomultiplier detector. The disadvantage of these scans, however, is the potential for the high speed imaging to reduce resolution. Lower resolution can worsen accuracy, necessitating the confirmation of cells by microscopy.
Recent advances in the CTC field have seen an explosion of different technologies that help researchers isolate and study important and rare cells. However, only few technologies have used clinical samples to validate their system – highlighting the importance of developing performance metrics to align and compare these different technologies.
1. Ferreira, M.M., Ramani, V.C. and Jeffrey, S.S. (2016) Circulating tumor cell technologies. Science direct 10, 374–394. (Link)