An improved approach to determining the stage of a cancer patient’s disease has been developed.
Early diagnosis of cancer and information on the stage of a cancer patient’s disease can both have a huge impact on the success of treatment. Circulating tumor cells (CTCs), which are released by tumors into the bloodstream and eventually deposited in other parts of the body during metastasis, are a good indicator of the stage of disease. They are, however, difficult to detect in the presence of healthy blood cells given their very low concentration.
That challenge may soon be overcome with technology developed by researchers at Harvard University’s Wyss Institute (). The group has modified a broad-spectrum pathogen capture agent, the engineered human blood opsonin protein known as FcMBL, so that it targets CTCs.
Diagnostic approaches for CTCs currently rely on detection of the cancer cell marker epithelial cell adhesion molecule (EpCAM), which is expressed on the surfaces of tumor cells. This approach is problematic, however, because expression of EpCAM is significantly decreased on CTCs compared to regular tumor cells.
For its key agent, the Harvard researchers used mannose-binding lectin (MBL), a naturally occurring protein found in the body that binds to carbohydrates on the surfaces of different pathogens, thus tagging them for destruction by the white blood cells. The researchers realized that while healthy cells are immune to MBL, cancer cells have unusual carbohydrates on their surfaces that are similar to the carbohydrates present on the surfaces of pathogens.
Previously, the group had found that MBL can be stabilized by binding it to the Fc fragment of an antibody to generate FcMBL. They had also demonstrated that magnetic beads coated with FcMBL bound strongly to pathogens and under an applied magnetic field carried the cells toward the magnet.
This approach was then applied to targeting CTCs. Magnetic beads were coated with FcMBL and then mixed with blood taken from mice with implanted, grown fluorescently labeled human breast cancer cells. The beads were then pulled out of suspension using a magnet and evaluated for CTCs. The researchers found that more than 90% of seven types of cancer cells were captured and the concentration of CTCs in the blood was reduced by more than 93%, indicating that the FcMBL was able to capture the CTCs even with their reduced EpCAM expression levels.
“The FcMBL-coated beads are unlikely to be bound to normal cells, and so when we measured the movement of cancer cells versus normal cells, the cancer cells moved much faster because they were being dragged to the magnet by the beads,” explained Joo Kang, Ph.D., who was a Technology Development Fellow at the Wyss Institute while completing this study and is now an Assistant Professor at the Ulsan National Institute of Science and Technology.
Next, the coated beads were tested using six other types of cancer cells (human non-small cell lung cancer, lung carcinoma, and glioblastoma). Again, greater than 90% capture was observed. These results are similar to those obtained using EpCAM-targeting methods, however, two additional cancer cell types (lung carcinoma and glioblastoma) were captured that are not captured using the EpCAM technology.
“We were able to rapidly isolate CTCs both in vitro and from blood, including some which are not bound by today’s standard CTC-targeting technologies,” says Michael Super, Ph.D., Lead Senior Staff Scientist at the Wyss Institute and co-author of the paper. “This new technique could become useful in cancer diagnostics.” He also added that “Our results suggest that while the EpCAM marker can be useful for some tumors, it becomes less and less useful over time as EpCAM expression decreases and the cell becomes metastatic. Our FcMBL system can either be used as an alternative to EpCAM-based diagnostics, or as a follow-up method once EpCAM ceases to be expressed.”
Further work is being conducted to identify the specific carbohydrates targeted by FcMBL in order to improve capture specificity and efficacy.