The past 30 years have witnessed the thriving development of molecular related fundamental researches on hematopoietic cells, which in turn greatly enriched therapeutic strategies for acute myeloid leukemia and led the priority treatment of leukemia entered the era of molecularly guided precision therapy[11–13]. Take the treatment of acute promyelocytic leukemia (APL) as an example, application of all-trans retinoic acid and arsenous cured about 90% of APL patients. Undoubtedly, therapies based on genetics and epigenetics changes played an increasing role in the treatment of non-M3 AML. In combination with standard 3 + 7 regimen, molecular targeted agents were supposed to improve remission rate of induction therapy effectively and provide more opportunities for subsequent HSCT.
Mutant FLT3 contained two kinds of mutation forms, internal tandem duplication (ITD) in juxtamembrane domain and point mutation within the activation loop of tyrosine kinase domain (TKD). FLT3 mutation occurred in 30% of de novo AML patients and accounted for the most predominant mutation type in AML. Notably, FLT3-ITD was considered as an independent unfavorable risk factor in AML, for patients with FLT3-ITD always be resistant to chemotherapy and present a higher relapse rate and more early death cases. Though the clinical significance of FLT3-TKD on AML remains to be confirmed due to relative rare incidence, meta-analysis showed that FLT3-TKD exerted similar unfavorable effect on AML as FLT3-ITD.
Generally, patients with mutant FLT3 frequently accompanied with hyperleukocytosis. Nevertheless, the effect of each mutation form, particularly Y842D, on clinical phenotype remains to be clarified. Back to the case we reported, in just 40 days, patient’s white blood cell count raised from 7.63×109/L to 203.39×109/L. The doubling time was as short as 8.45 days, which was very similar to the clinical characterization of patients with FLT3-ITD. It was generally known that consecutive activation of FLT3 and downstream signaling pathways resulted in uncontrolled proliferation of hematopoietic cells. According to Kindler T research, point mutation at Y842 altered the conformation of FLT3 receptor tyrosine kinase domain and drived the formation of activation loop. Mediated by downstream signaling pathway, including STAT5, myeloid cells with Y842C acquired the ability to proliferate indefinitely. Murine 32D cells transfected with human FLT3-Y842C showed activated FTL3 and STAT5 signaling pathway which can be inhibited by FLT3 inhibitor (N-benzoyl staurosporine, PKC412). The above results demonstrated that mutation of FLT3-Y842 played a critical role in the development of AML. Comprehensively, it was reasonable to deduced that hyperleukocytosis of the patient might attribute to continuous activation of FLT3 caused by Y842D.
Another important biological characteristic of FLT3-Y842D mutant AML cells was that they may be resistant to certain FLT3 inhibitors. Through cultivating cells with different gradient concentrations of FLT3 inhibitors, researchers successfully screened out FLT3 inhibitors-resistant FTL3 mutant cells. Base on this screening experiment, von Bubnoff N and his colleagues found that, at the same oncentration, myeloid cells with FLT3-Y842D was resistant to sorafenib instead of midostaurin . This may be the reason why sorafenib was only used in the treatment of FLT3-ITD mutant AML patients.In a phase III, randomized control clinical trial, researchers compared induction therapy effect of daunorubicin (60mg/m2, d3-5), cytarabine (20mg/m2, d1-7) with or without midostaurin in 717 AML patients with mutant FLT3, including 162 patients with FLT3-TKD. Statistically, the median overall survival time in midostaurin group was 74.7 months (95%CI:31.5-not achieved), significantly better than 25.6 months (95%CI:18.6–42.9) in placebo group. Meanwhile, the incidence of adverse events between the above two groups revealed no significant difference. That was also a critical reason for clinical application of midostaurin in AML patients with FLT3 mutation and why we chose midostaurin, cytarabine, and idarubicin as re-induction regimen in this case. 23 days after re-induction, morphological remission was achieved, which might indicate that patient with FLT3-Y842D potentially be sensitive to midostaurin.
The third characteristic of this case was that the patient presented with differentiation syndrome on the 10th day of midostaurin treatment. Red nodular rash firstly appeared in symmetrical parts of both lower limbs, accompanied by pruritus and fever. This type of rash often occured in patients treated with selective FLT3 inhibitors, such as cuzatinib, gliptinib[17, 18]. Pathologically, FLT3 inhibitors induced differentiation of primordial cells into neutrophils, this rash was exactly caused by the infiltration of neutrophils in deep skin and subcutaneous tissues. Take AML patients who received cuzatinib monotherapy as an example, though the proliferative degree of nucleated cells did not alter significantly 29 days after therapy, the proportions of cells with different degrees of differentiation changed a lot. The proportion of primordial cells decreased from 77–6%, the proportion of myelocyte, metamyelocyte, and neutrophils raised from 9–57%. Since the mutation frequency of FLT3-ITD did not changed, it was sufficiently to conclude that the mature neutrophils derived from differentiation of primordial cells.Mc Mahon cm and his colleague evaluated the validity of gliptinib in 21 AML patients with FLT3 mutation, 10 patients suffered from differentiation syndrome during treatment. The overall survival time and morphological remission rate of these 10 patients was much more worse than that of the other 11 ones. Similar with the prevention of retinoic acid-induced differentiation syndrome, it was confirmed that, combination of glucocorticoid with FLT3 inhibitor not only upregulated the expression of proapoptotic protein Bim, but also induced leukemia cell apoptosis by promoting the degradation of antiapoptotic protein Mcl-1. Therefore, FLT3 inhibitor combined with glucocorticoid in the prevention of FLT3 inhibitor induced differentiation syndrome needed further study. In addition, patient in this report did not concurrent with rearrangement of NPM1 or CBF genes, which were also regarded as important unfavorable prognostic factors in AML. To sum up, it was sensible to treat this patient with allogeneic-HSCT as soon as possible, and maintenance therapy after transplantation with midostaurin would further improve the clinical outcome.
In conclusion, this case showed that FLT3-Y842D mutation was a new activating mutation form of FLT3 in AML. Patients with this mutation tend to be sensitive to midostaurin. In the process of midostaurin treatment, we should be alert to the existence of differentiation syndrome, once observed, glucocorticoid should be used as soon as possible. It is notable that allogeneic hematopoietic stem cell transplantation should be conducted since patients achieved complete remission as soon as possible.