FLT3 as a marker of minimal residual disease: Time to re‐think?
Authors: Mark Levis
Journal: American Journal of Hematology (2017)

With new information about the genetic underpinnings of AML emerging almost daily, clinicians, of course, hope to leverage these discoveries into improving outcomes for their patients. Even if a particular mutation can't be targeted immediately, at the very least its presence can be used to provide a more accurate prognosis, or be used to track the presence of the disease after therapy. In this regard, it is now well-established that in AML patients undergoing allogeneic transplant, minimal residual disease (MRD) detected by flow cytometry or by detection of some AML-associated genetic lesion can have a profoundly detrimental impact on outcome.1 A reliable assay of MRD for AML patients in remission has been somewhat of a holy grail in this field. Which mutations can be used for this purpose? Because AML pathogenesis proceeds in a stepwise fashion from clonal hematopoiesis to full transformation,2 tracking the presence of a so-called founding mutation such as DNMT3a may not be helpful because it is present in pre-leukemic clones as well as in the transformed disease. Such a mutation is present in every leukemia cell, but also in non-leukemic cells, so the specificity for the disease is not particularly high. FLT3 mutations are the most common genetic lesions found in AML, and yet they are deemed of little value as markers of MRD because they are probably almost always occur relatively late in leukemogenesis, and because of their apparent instability.3 However, in this issue of the American Journal of Hematology, Gaballa and colleagues present data that casts some doubt on the position that FLT3 mutations have no role as a marker of MRD.4 Their study is derived from a review of the records of 1255 patients transplanted over a 15-year period at the MD Anderson Cancer Center. Two hundred of these patients had FLT3 mutations detected at diagnosis, most with an ITD mutation. The study analyzed outcomes for patients classified as being in CR1, CR2, high-risk CR, or as having morphologically detectable AML, and pre-transplant marrow samples were screened for detectable FLT3 mutations by the conventional PCR (polymerase chain reaction) assay used in most AML cases in the US today. Of the 150 patients in CR (ie, no morphologically detectable disease), 27 (18%) had a FLT3 mutation detectable by PCR pre-transplant. Not surprisingly, morphologically detectable disease was highly associated with relapse risk, but FLT3 positivity ran a close second. A patient in CR1 or CR2 who had a detectable FLT3 mutation was almost twice as likely to relapse after transplant compared to a patient lacking a detectable FLT3 mutation. Broadly, this study confirms previous findings that MRD of any sort prior to allogeneic transplant predicts for a bad outcome post-transplant.5 More importantly, however, it offers evidence that a routine assay can be used for detection of MRD. Historically, it has proven devilishly difficult to develop assays for MRD that can be standardized for general use. Flow cytometry is a wonderful technique,6 but standardization remains a problem, and it often seems as if only a very skillful master of multi-parameter flow can pick out the leukemia-associated phenotype with consistency. Likewise, detection of NPM1 mutations in remission marrows seems like a worthwhile MRD assay,7 but again, no standardized method is routinely available outside of academic centers. The assay for FLT3 mutations stands in contrast to this. The PCR technique used to detect FLT3 mutations (and used in the study by Gaballa et al.) was developed over a decade ago and is the technique used in the commercial labs running this assay worldwide today.8 Therefore, at a minimum, this study tells us that an assay routinely available in a commercial lab can detect a key prognostic factor in 18% of patients proceeding to transplant. Perhaps the important point about this is the specificity of such a finding. Because such mutations occur as a final step in transformation, if we detect a FLT3-ITD mutation in a patient, we are most likely detecting leukemia—particularly if it is the exact length insertion that was present in the leukemia at diagnosis. That unique length is a signature in its own way. A next step is to improve the sensitivity of the FLT3 mutation assay. Because of competition from the wild type allele (“PCR bias”9) the sensitivity of standard PCR for FLT3-ITD mutations is low. However, next-generation sequencing (NGS) techniques are being developed that will likely overcome this problem. Perhaps a more important issue is what we do about the MRD that we detect. At the present time, there are several small molecule FLT3 inhibitors that are being studied in advanced clinical trials, including trials examining the use of these agents as post-transplant maintenance.10 It isn't too outlandish to predict that in the not-too-distant future, a highly sensitive NGS-based assay will be used to guide maintenance therapy with a FLT3 inhibitor in these patients. Nothing to report.