Researchers in the lab

Credit: Rhoda Baer

Researchers say they have identified a source of drug resistance in chronic lymphocytic leukemia (CLL).

In a letter to The New England Journal of Medicine, the team described how a mutation in Bruton’s tyrosine kinase (BTK) triggers resistance to ibrutinib, a drug that treats CLL by inhibiting BTK.

The researchers discovered this point mutation in a CLL patient enrolled in a clinical trial. The patient initially responded well to ibrutinib but stopped responding after almost 20 months.

“In a way, we are repeating, at a faster pace, the story of Gleevec [imatinib],” said study author Y. Lynn Wang, MD, PhD, of the University of Chicago in Illinois.

“That story began with development of an effective drug with few side effects, but, in many patients, the leukemia eventually found a way around it after long-term use. So researchers developed second-line drugs to overcome resistance.”

The ibrutinib study began in 2010 at Weill Cornell Medical College in New York, one of several sites for a phase 1 trial of ibrutinib. The researchers recruited 16 patients with CLL whose disease had progressed or relapsed despite multiple treatments.

Dr Wang arranged to track the progress of each patient’s leukemic cells before and during treatment and to correlate any cellular or molecular changes with each patient’s disease activity.

One of the 16 patients in the trial seemed to be unusual. This 61-year-old woman was diagnosed in 2000 at age 49. She had unsuccessfully received several different treatments before entering the study.

Within 18 months of starting ibrutinib, she showed significant improvement. At about 20 months, however, she started to decline, developing a respiratory infection that did not improve with treatment. By 21 months, it was clear she was having a relapse. The clinical team increased her dose, with no discernable effect.

Dr Wang’s team quickly began analyzing her blood samples, looking for changes that occurred between the period when she was responding well to ibrutinib and after she began to relapse.

Because complete gene sequencing would be time consuming, Dr Wang asked a graduate student working on the project, Menu Setty from Memorial Sloan-Kettering in New York, to first focus on 3 proteins that were likely candidates. One of the candidates was BTK.

And sure enough, Setty discovered a tiny but consistent change in BTK in about 90% of post-relapse cells. It was a thymidine-to-adenine mutation at nucleotide 1634 of the BTK complementary DNA, leading to a substitution of serine for cysteine at residue 481 (C481S).

When the researchers later analyzed the entire set of the patient’s genes, they found no other genetic changes that correlated with the patient’s clinical course. BTK made perfect sense as the cause for drug resistance, the researchers noted, as it’s the primary target of ibrutinib binding, and it plays a central role in rapid cell proliferation.

Dr Wang and her colleagues used structural and biochemical measures to confirm that the C481S mutation made CLL cells resistant to ibrutinib. The studies indicated that ibrutinib was 500 times less likely to bind to mutant BTK.

In an attempt to save the patient, the researchers tested alternative kinase inhibitors against the patient’s leukemic cells in the lab.

They found some kinase inhibitors remained effective against ibrutinib-resistant cells. (These studies are described in a separate manuscript that has been submitted for publication.) Unfortunately, despite this effort, the patient passed away a few weeks later, due to sepsis.

The researchers noted that the C481S mutation is one of several mechanisms that underlie resistance to ibrutinib, but this research highlights the mutation’s role in disease development and drug resistance.

Understanding the molecular and cellular mechanisms of resistance is the first step toward monitoring, early detection, and development of novel strategies to overcome drug resistance.

Mutation causes ibrutinib resistance in CLL