New Drug Target Might Sidestep Gleevec Resistance

A drug that could turn on the gene that makes the protein C/EBP-alpha, a "transcription factor" required for cells to differentiate, then, might control or even eliminate the cancer.

According to Bruno Calabretta, M.D., Ph.D., professor of cancer biology at Jefferson Medical College of Thomas Jefferson University, transcription factors are molecular switches that turn on genes when their function is needed. C/EBP-alpha expression is low in leukemia cells such as those with the BCR-ABL protein defect, which causes chronic myeloid leukemia (CML), a disease that Gleevec treats so well.

Gleevec is normally prescribed for patients early on in CML, which is characterized by an overabundance of white blood cells. But when the disease advances to the terminal stage, or "blast crisis" phase, the cells, called blasts, remain undifferentiated and accumulate rather than becoming more mature white blood cells called granulocytes. Gleevec is much less effective in this stage, Dr. Calabretta says. Yet, in leukemia cells that respond to treatment with Gleevec, expression of C/EBP-alpha increases.

The researchers looked at what might happen if C/EBP-alpha was turned back on in blast crisis patients and in CML patients who were resistant to Gleevec. Reporting in the journal Blood, they found that in both the laboratory and in mice, immature white blood cells began to differentiate again, and leukemia development was stopped.

Gleevec is a new type of cancer drug -- the first of its kind developed to fight cancer by turning off an enzyme that causes cells to become cancerous and multiply. In CML, ABL, an enzyme, goes into overdrive because of a chromosomal mix-up that occurs during blood cell development. The genes ABL and BCR become fused and produce a hybrid BCR-ABL enzyme that is always active. The overactive BCR-ABL, in turn, drives the excessive proliferation of white blood cells.

In one study, reactivating C/EBP-alpha in Gleevec-resistant mice cured leukemia in six out of seven animals.

This isn't surprising, Dr. Calabretta says, because C/EBP-alpha isn't involved in the mechanism of action of Gleevec. "Essentially, this is a downstream effect," he explains. "The BCR-ABL protein repressed C/EBP-alpha. In bypassing BCR-ABL, which is the first target, regardless of whether BCR-ABL is normal or mutated (and Gleevec-resistant), turning on C/EBP-alpha will always suppress leukemia. If the function of C/EBP-alpha is introduced in leukemic cells that are resistant to Gleevec because of a mutation, the phenotype is reverted.

"This could be used as a new strategy," he says. "The strategy now against leukemias resistant to BCR-ABL is to develop compounds that may inactivate the mutant -- so called second generation compounds. But another way to do this is to focus on targets downstream."

In the laboratory, the researchers took a cell line in which normal C/EBP-alpha is repressed, and put back another C/EBP-alpha protein. The scientists used an inactive C/EBP-alpha protein that is fused to an estrogen receptor. The protein can only be activated by treating mice with a hormone, in this case tamoxifen, which is used in breast cancer. The receptor responds to tamoxifen. "Now we essentially have an in vivo system in which we have leukemia cells that carry a chimeric protein, C/EBP-alpha-ER," Dr. Calabretta explains. "If you activate CEBP alpha by injection of tamoxifen, this chimeric protein is switched on and suppresses tumorigenesis."

They also were able to halt leukemia in cells resistant to Gleevec. They established a leukemia cell line that expressed a Gleevec-resistant mutation, reactivated the CEBP-alpha, and stopped leukemia development.

Could Gleevec be combined with a product that could also activate C/EBP-alpha, just in case patients develop resistance to the drug?

"The question is, what is more practical to do," he says. "We did a gene transfer experiment -- we put back a gene and activated it. To use C/EBP-alpha as a drug, we would try to deliver it directly." Dr. Calabretta suggests that a better idea might be to identify and deliver a small molecule that can activate the C/EBP-alpha gene.

The discovery could also have applications for other leukemias, he notes, because there are several different mechanisms by which C/EBP-alpha is inactivated. Other scientists have shown that in various types of leukemia, reintroducing C/EBP-alpha can restart differentiation and eliminate leukemia cells.