PREDICTIVE TEST FOR CML RESISTANCE TO IMATINIB

July 29, 2010 — A highly sensitive and specific test in the works may be able to predict which patients with chronic myeloid leukemia (CML) will develop resistance to imatinib (Gleevec; Novartis) and may also predict responses to alternative agents, the test's developers claim.

The test, which is still under development, uses a biosensor to characterize the activity of the BCR-ABL kinase, the target of imatinib, in cells from patients with CML. It works with relatively few cells and can spot early signs of drug resistance by detecting resistant cell populations constituting less than 1% of a sample, the investigators write in the August 1 issue of Clinical Cancer Research.

The test uses fluorescence resonance energy transfer (FRET), a technique for measuring the interaction of 2 proteins, write Yusuke Ohba, MD, PhD, associate professor at the Hokkaido University Graduate School of Medicine in Sapporo, Japan, and colleagues.

"Using this test, we are now able to identify and predict the most suitable treatment option for individual [patients with CML]," Dr. Ohba says in a press release. "This technique is both sensitive and practical to use; it is especially useful for patients who are in relapse, a case in which the clinician's important decision regarding the next step in treatment must be made quickly and accurately."

The study has important implications for clinical practice beyond CML, write Shaoying Lu, PhD, and Yingxiao Wang, PhD, from the University of Illinois Urbana-Champaign, in an accompanying editorial.

"[T]he authors showed that the FRET biosensor in combination with flow cytometry can be used to detect a small percentage of drug-resistant cancer cells mixed in a large cell population. This finding is exciting because these drug-resistant cells may likely constitute the main reason for CML relapse and therapeutic failure," they write.

"Therefore, the FRET biosensor developed here can provide a powerful tool to assess the biopsy samples from a particular patient and to predict the future probability of his or her resistance to specific drugs. This tool should provide invaluable information for clinicians and physicians to identify and design alternative therapeutic approaches with better efficiency and higher chances of success," they add.

Protein Sandwich With Pickles

Dr. Ohba and colleagues dubbed their test "Pickles" (for phosphorylation indicator of CrkL en substrate). They created it by sandwiching the adapter protein CrkL, which is known to bind to BCR-ABL, between 2 layers of fluorescent proteins. They saw a roughly 80% increase in FRET efficiency — a measure of fluorescence intensity indicative of activity in the presence of BCR-ABL.

After tweaking the biosensor for optimal response, they tested its ability to identify drug-resistant cells within cultures and to determine which tyrosine kinase inhibitor would be most effective against a given mutation. For example, they showed that small populations of cells resistant to imatinib and to nilotinib (Tasigna; Novartis) were sensitive to dasatinib (Sprycel; Bristol-Myers Squibb).

They then tested the technique for its ability to evaluate drug efficacy in cells obtained from 11 patients with CML and found that the test results both accorded with current patient status and were predictive of future outcomes.

"In view of the results we are obtaining, we feel confident that following our study using a large patient cohort, this Pickles-based method will be used successfully to provide reliable information about drug sensitivity in CML, both at the time of initial diagnosis and during the course of the disease, as well as to predict potential future recrudescence during/after imatinib therapy," the investigators write.

The study was supported by grants from the Japanese government, Japan Society for the Promotion of Science, Japan Health Sciences Foundation, and Yasuda Medical Foundation. The investigators and the editorialists disclosed no potential conflicts of interest.

Clin Cancer Res. 2010:16;3964-3975, 3822-3824.