The antibodies made by leukemia cells of patients with chronic lymphocytic leukemia (CLL) are highly restricted, suggesting that these antibodies play a role in the development and/or progression of this disease. One model is that these antibodies bind to an antigen, which could be from something in the environment or from bacteria or viruses that cause an infection. The binding of these antibodies to this antigen could stimulate the cell to grow and/or to resist the normal cell turnover that ordinarily helps to clear the body of unwanted or excessive numbers of cells. Such stimulation over time could result in the cell transforming into a cancer cell. Patients with aggressive disease typically have leukemia cells that have antibodies that do not have mutations. Through recombinant engineering, we are isolating the genes that make such antibodies and using these genes to make antibodies in the laboratory that can be examined. Through this work we hope to learn what these antibodies bind, hoping to identify the antigens that could contribute to the development or progression of this disease.

Patients with aggressive disease typically have leukemia cells that express the ZAP-70, a protein that originally was identified in T cells. ZAP-70 that plays a critical role in T cell receptor signaling, meaning that this protein serves to wire the T cell receptor to the inner workings of the cell. T cells that do not have ZAP-70 cannot receive signals from their receptors for antigen and therefore cannot function normally. We found that CLL cells that have ZAP-70 are able to receive signals from the antibodies that they have on their surface much more efficiently than CLL cells that do not have this protein. As such, the CLL cells that have ZAP-70 may be stimulated more efficiently by their surface antibodies, which serve as receptors for the antigens that we hope to identify and that we feel may be driving development of this disease.

Targeting ZAP-70 for treatment of CLL:
We have found that small amounts of 17-allyl-amino-demethoxy geldanamycin (17-AAG) (in the nanomolar concentration range) can lower the amount of ZAP-70 in CLL cells, but not in normal T cells. 17-AAG is an inhibitor of heat shock protein 90 (Hsp90), a protein that complexes with other proteins to protect certain proteins in the cells from being degraded. Research has found that Hsp90 can complex with other proteins to protect ZAP-70 in CLL cells. This does not occur in normal T cells. When we inhibit Hsp90, the level of ZAP-70 in the CLL cells, but not in the T cells, goes down. This reduces the signals that the leukemia cells can receive from their surface antibodies, thereby lowering the stimulation that these cells can receive. More importantly, the selective lowering of ZAP-70 in CLL cells results in the CLL cells undergoing “apoptosis”, a term given to a process of “programmed cell death”. CLL cells that undergo apoptosis are rapidly cleared from the body. As such, the use of selective inhibitors of Hsp90 may serve as drugs that may be effective in treating CLL, particularly CLL cells that have high levels of the ZAP-70 protein.

We currently are investigating for other CLL proteins that also may be protected by Hsp90. Some of these other proteins may become altered during the course of the disease through mutation in the genes that make such proteins. Although these altered proteins may not account for the initial development of the leukemia, they may be responsible for the more aggressive growth and/or drug-resistance of CLL cells that some patients may experience over time. The questions we are asking is whether Hsp90 also may protect these proteins and if so whether we can lower the amounts of such proteins in leukemia cells by using drugs such as 17-AAG. If so, then such drugs also may be effective in treating even patients who do not have leukemia cells that make the ZAP-70 protein, but still have aggressive or drug-refractory disease.
Finally, we are developing clinical trials to test whether drugs such as 17-AAG can kill leukemia cells. Some of these drugs can be taken orally. If they are found in clinical trials to affect the same activity as we are seeing in the laboratory against CLL cells, then we may have a new treatment for patients with leukemia cells that have ZAP-70 or other altered proteins that are protected by Hsp90.