Methods to detect the presence of MRD by flow cytometry reflect work by the European Research Initiative on CLL (commonly referred to as ERIC), whereby the standardization to achieve a sensitivity of 1 in 10,000 (10⁻⁴) cells was achieved and was validated. This has been our primary gold standard for MRD analysis in clinical trials. Some trials have used allele-specific oligonucleotide polymerase chain reaction, but baseline samples are involved, which is a bit more complicated. 

The US Food and Drug Administration (FDA) has expressed concerns regarding the potential use of flow cytometry–based MRD as a surrogate end point in clinical trials, raising questions about the extent of standardization. In addition, access in the community has been limited in that most local hospitals do not routinely perform MRD-level flow cytometry.

So, for a variety of reasons, there has been interest in other methods of MRD analysis, including methods to achieve deeper sensitivity. The next-generation sequencing methods are increasingly of interest, and the clonoSEQ assay by Adaptive Biotechnologies has been licensed by the FDA for use in CLL with blood or bone marrow. In many of our trials, we are using this assay in parallel with flow cytometry, and it does seem to be more sensitive. Under ideal conditions, there is the potential to achieve a sensitivity of 1 in 10⁻⁶ cells with clonoSEQ, but that requires a very large amount of input DNA, which may not be achievable across a broad number of patients in practice or in clinical trials; a level of sensitivity of 1 in 10⁻⁵ may be more broadly feasible. 

As a prognostic marker, undetectable MRD (U-MRD) is predictive of longer progression-free survival (PFS) and overall survival after certain time-limited regimens for CLL (ie, venetoclax plus obinutuzumab, fludarabine plus cyclophosphamide and rituximab [FCR]). The CLL14 trial found that U-MRD was associated with longer PFS and overall survival than MRD positivity in patients receiving the combination of venetoclax and obinutuzumab. Data on FCR from The University of Texas MD Anderson Cancer Center and data on venetoclax plus obinutuzumab from the CLL14 trial suggest that deeper MRD levels (ie, down to 10⁻⁵ and 10⁻⁶) than 10⁻⁴ may be associated with better PFS. 

Where MRD analysis is not as useful is in the setting of continuous Bruton's tyrosine kinase (BTK) inhibitor monotherapy, where the rate of complete remission in relapse is less than 10%; in the front line, the rate of complete remission might be 25% to 30% after many years of therapy. Moreover, U-MRD with BTK inhibitor monotherapy is extremely rare. Nonetheless, patients who stay on BTK inhibitor monotherapy continuously with some low level of residual disease can do quite well for many years. Deeper remissions on BTK inhibitors might be advantageous in some scenarios, but, in general, the data in this area are limited. 

Thus, the current main potential application for MRD is to predict the duration of response and PFS after certain time-limited regimens. We are not truly making decisions in the clinic based on MRD analyses quite yet, in the sense that we do not have data showing that changing therapy based on the patient's MRD status should be standard practice in CLL. There have only been a few trials that have looked into this possibility.