The future of cancer research is targeting, meaning the ability to destroy cancer cells efficiently and with few to no side-effects on normal tissues. Chimeric antigen receptor (CAR) T-cells are a step forward in this regard, and have proven to be an effective treatment for leukemia in trials. Researchers are now attempting to adapt their use to other types of cancer. In this example, the T-cells are engineered to make them more discriminating when targeted at solid tumors:
Many solid cancers have high levels of certain proteins such as ErbB2 and EGFR, which make them suitable targets for anticancer therapies. However, such proteins are also present at low levels in normal cells. Because of this, CAR T cells that are developed to target one of these proteins on tumor cells also recognize and attack normal cells that have the protein, causing severe toxicity.
To develop CAR T cells that can distinguish between cancer and normal cells, researchers first constructed a panel of CARs with the single chain variable fragments (scFv) - the part of the CAR T cell that recognizes the tumor target - using sequences from mutated 4D5 antibodies that had varying affinities to ErbB2, a protein present at high levels in some solid tumors, including breast cancer. Next, they incorporated different scFvs into the CAR backbone or "construct," such that they resulted in a range of CAR T cells - from those that had high affinity to ErbB2 to those that had low affinity to ErbB2. The newly engineered CAR T cells varied in their affinity to ErbB2 by three orders of magnitude. The researchers then conducted a series of experiments to test the functionality of the affinity-tuned CAR T cells and found that high-affinity CAR T cells did not discriminate tumor cells from normal cells and attacked all of them, whereas low-affinity CAR T cells were sensitive to tumor cells that had high levels of ErbB2 and not to normal cells that had low levels of the protein.
Next, they tested the engineered CAR T cells in mice that bore human cells with high levels of ErbB2 on one side of their bodies and human cells with normal levels of ErbB2 on the other side of their bodies. Here again, low-affinity CAR T cells selectively eliminated cells that had high levels of ErbB2 but had no effect on cells that had normal levels of the protein. In order to prove that this technology can be extended to other solid tumor targets, the researchers developed low-affinity CAR T cells targeting EGFR, a protein present in high levels in some lung and colon cancers, among others, and preliminary preclinical results showed that these CAR T cells were able to discriminate between cancer cells and normal cells.