New drug fights a chronic leukemia

A genetically engineered drug known as BL22 has sent a type of cancer called hairy cell leukemia into full remission in 11 of 16 patients, a study shows. Two other patients experienced partial remission.

The findings, reported in the July 26 New England Journal of Medicine, suggest that BL22 may become a treatment for this type of leukemia. The name hairy cell stems from the microscopic appearance of malignant white blood cells, which in this disease have irregular hairlike projections. This leukemia accounts for roughly 2 percent of leukemia cases, affecting thousands of people in the United States each year.

The chemotherapy compounds cladribine and pentostatin can control the cancer in as many as 85 percent of patients. However, when hairy cell leukemia patients become resistant to these drugs–as had all 16 people in this study–they face a dire prognosis. Some of the patients in the study already were getting weekly blood transfusions to survive.

Even so, “some only had a few months to live,” says study coauthor Robert J. Kreitman, a medical oncologist at the National Cancer Institute in Bethesda, Md. On average, these patients were diagnosed with their cancer 8 years prior to the study.

Eight of the 11 patients who experienced full remission had no recurrence of cancer during the study. The other three had their cancer recur within 16 months of being treated with BL22 but then were successfully treated again, Kreitman says. Full remission means the scientists could no longer detect cancer cells in the blood or bone marrow.

Regardless of their response to the treatment, all 16 patients, average age 54, received at least three intravenous doses at 3-week intervals, and some subsequently got several more.

BL22 singles out the molecule CD22, which appears on the surface of both hairy cell leukemia cells and a type of white blood cell called a B cell, Kreitman says. He and his colleagues designed BL22 as a so-called fusion protein that combines a toxin with an antibody that attaches to the CD22 molecule. This enables the drug to dock with the cancerous cells and dump the toxin into them. This fusion protein may also latch onto healthy B cells, killing some, but it proves more consistently lethal to the hairy cells, which have more CD22 receptors, says Kreitman.

He adds that the drug may also impede other cancers, such as B cell lymphoma and chronic lymphocytic leukemia.

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