A drug called STI-571, now being tested in clinics to treat a rare form of leukemia, selectively blocks a mutant enzyme that causes the disease without harming its molecular cousins. Reporting in the Sept. 15 issue of Science, a team of researchers from the Howard Hughes Medical Institute at The Rockefeller University, the Memorial Sloan-Kettering Cancer Center and the State University of New York at Stony Brook has shown how STI-571 accomplishes this feat, suggesting new avenues for the structure-based design of cancer drugs with reduced side effects.
Chronic myelogenous leukemia (CML), one of the four forms of leukemia, arises from the accidental activation of a single molecule called the Abelson kinase (Abl). Abl is a member of a family of proteins called tyrosine kinases, a type of enzyme that activates other proteins by adding a phosphate to them in a process called phosphorylation. Kinases act like biological switches and play important roles in regulating cell growth and division. Scientists estimate that each human cell contains about 2000 tyrosine kinases, all of which are virtually identical except for small differences in their amino acid sequences. STI-571 inactivated Abl by binding to it but not other, normally functioning kinases.
“Tyrosine kinases are usually switched off until they are activated by signals from outside the cell. Generally, when a tyrosine kinase–which is a signaling switch– is on, the cell goes into hyperactivity,” says author John Kuriyan, Ph.D., professor and head of a Laboratory of Molecular Biophysics at Rockefeller and an investigator at the Howard Hughes Medical Institute. “We’ve shown that STI-571 functions by recognizing not the ‘on’ state, but the ‘off’ state of the protein kinase. This offers insights into the development of future drugs to treat other diseases of protein kinase malfunction, particularly other cancers.”
The researchers used a technique called X-ray crystallography to visualize how STI-571 binds to the “off” state of Abl. Protein kinases have a small, hairpin loop that folds in and out depending on whether the protein is phosphorylated. When the protein is switched on, it is phosphorylated and the loop is out. When the protein kinase is switched off, it is unphosphorylated and the loop is tucked in.
The researchers showed that STI-571 blocks Abl by partially mimicking a molecule called ATP, which provides the phosphate for the protein in the phosphorylation reaction, and binding tightly to the area of the kinase normally occupied by ATP.
The researchers also showed why STI-571 binds to Abl, but not to many other protein kinases, including a well-studied cancer-causing protein called Src. Every amino acid that touches STI-571 in the Abl structure is essentially the same in Src. However, Src and Abl differ in the way in which the hairpin loop that swings in and out switches each kinase on and off, and STI-571 recognizes the difference.
“A key property of STI-571 is that it interferes with the mechanism underlying the proper switching of Abl between the active and inactive states,” says Kuriyan. “Our discovery that STI-571 exploits a conformational variability that is proving to be a hallmark of the protein kinases is likely to have broad consequences for the further development of specific protein kinase inhibitors.”
Kuriyan’s co-authors are Thomas Schindler, Ph.D., at Rockefeller, Bayard Clarkson, M.D.,and William Bornmann, Ph.D., at Memorial Sloan-Kettering Cancer Center, and Patricia Pellicena, Ph.D., and Todd Miller, Ph.D., at SUNY Stony Brook School of Medicine.