A new screening test will allow clinicians to identify spontaneous mutations that produce resistance to imatinib both before and during drug treatment, according to an article in the March 21st issue of Cell. Chronic myelogenous leukemia is caused by mutation within the gene called BCR/ABL.

Molecular biologists developed the screen to identify patients with chronic myelogenous leukemia whose cancer cells contained mutant alleles that would make them resistant to imatinib, the antibody that targets the protein encoded by BCR/ABL.

"In the leukemia cells of a [chronic myelogenous leukemia] patient, BCR/ABL is constantly mutating," said George Daley, leader of the research team that developed the screening strategy. "Cells carrying certain mutations can resist the drug and continue to grow while the remainder of leukemia cells are suppressed. We have found a way to discover those mutations experimentally."

The American group used recombinant DNA methods to induce random mutations within BCR/ABL. The mutant alleles were then transferred into millions of mouse blood cells and exposed to imatinib. Although most cells were destroyed by the drug, some of the cells with specific mutations thrived.

"We looked for those cells that continued to grow, extracted them and sequenced their genes," said Mohammad Azam, Ph.D., lead author of the new study. That examination revealed 15 mutations that researchers elsewhere had previously linked to imatinib resistance -- plus 97 additional mutations. As research on the mutations continues and more patients are surveyed, more such mutations are likely to be found.

With the catalogue of mutations in hand, Daley said, physicians could one day examine patients who have relapsed because of drug resistance and pinpoint which mutation is responsible. It would be even better clinically if mutations resulting in resistance were identified before treatment began or during the course of treatment. "Doctors could monitor the patient's leukemia at the molecular level for signs of impending resistance," he said, "and decide when it is best to switch medications."

The screening technique could also be used by pharmaceutical scientists involved in drug development. Scientists could detect the location of the variations that cause drug resistance to first-generation compounds and design new drugs that remain effective despite those mutations. Physicians could then prescribe a combination of drugs that complement one another, much like the drug cocktails given to patients with HIV or antibiotic-resistant bacterial infections.

"We understand this leukemia with molecular precision, so its significance outweighs its incidence," Daley said. "Everything that we're learning about Gleevec [imatinib] and BCR/ABL will be a recurring theme for every new target-directed agent being developed against cancer."

Imatinib binds to a specific pocket within the BCR/ABL kinase domain -- the part of the protein that regulates enzyme function. All mutations previously found in leukemia patients who relapsed while on imatinib therapy have been found within that domain.

The identification during the current study of mutations in other parts of the enzyme may enable researchers to improve their understanding of how interaction among different parts of the protein affects its overall function and to develop new targeted agents.