4 Discussion
AML1/ETO-positive AML patients usually have a good prognosis following intensive chemotherapy, but there is still a subset of patients who are ‘unfit’ for IC. The combination of venetoclax and hypomethylating agents has been an effective strategy for ‘unfit’ AML. However, the assessment of ‘fitness’ is imperfect, and the determination of ‘fit’ or ‘unfit’ for IC is somewhat subjective and may be influenced by many factors [
19,
20]. For instance, during the COVID-19 epidemic, the emphasis on low-intensity therapies made it possible for ‘fit’ patients to receive VEN/HMA therapy [
21]. Therefore, it is necessary to understand the impact of VEN/HMA in AML patients with
AML1/ETO fusion and determine whether
AML1/ETO-positive AML prefers IC or VEN/HMA.
To our knowledge, this is the first cohort study to report the efficacy of VEN/HMA treatment in patients with AML1/ETO-positive AML. Our study suggested that patients with AML1/ETO-positive AML had a significantly lower ORR with VEN/HMA frontline therapy than those who had AML1/ETO-negative AML, and they also had a significantly shorter EFS after being balanced for ELN risk. Moreover, for AML1/ETO-positive AML, the ORR and EFS were worse in patients treated with frontline VEN/HMA than in those treated with frontline IC.
In this study, most patients who relapsed or were refractory to VEN/HMA received intensive salvage chemotherapies, and 64.3% of them reached complete remission with a prolonged duration of response. Moreover, intensive salvage chemotherapy can still achieve a high response rate in AML1/ETO-positive patients with primary resistance to VEN/HMA. These results suggested that AML1/ETO-positive AML patients who failed VEN/HMA therapies still have a good prognosis after salvage treatments with IC, which may be one of the reasons explaining the lack of significant differences in overall survival between cohorts.
KIT mutations were found in 12.8–46.8% of
AML1/ETO-positive AML [
15,
22]. In patients with
AML1/ETO-positive AML,
KIT mutations were associated with poor survival [
16]. In our study, similar to previous data from intensive therapies,
KIT mutations were associated with lower ORR and shorter EFS in
AML1/ETO-positive AML patients treated with frontline VEN/HMA. The subgroup analysis showed that in the
KIT-mutated subgroup,
AML1/ETO positivity had a significant effect on the poor response and survival of AML patients treated with frontline VEN/HMA, while the effect was not significant in the
KIT wild-type subgroup. Of the 11 patients with coexisting
AML1/ETO fusion and
KIT mutations who were treated with frontline VEN/HMA, none achieved CR, suggesting that VEN/HMA therapies should be used with great caution in this subgroup of patients. The analysis is subject to some bias because of the small sample size. Further studies with larger sample sizes are needed to confirm the results.
Resistance to venetoclax arises through various mechanisms, including dysregulation of BCL-2 family apoptotic proteins, p53 inactivation, activating kinase mutations, and altered mitochondrial structure [
23,
24]. Database analysis in our study showed that
AML1/ETO-positive patients have lower BCL2 expression and down-regulation of the mitochondrion morphogenesis pathway, which may lead to resistance to venetoclax. Previous studies have showed that AML with mature differentiation (such as monocytic AML) primarily relies on MCL-1 for survival instead of BCL-2 and is more resistant to venetoclax than primitive AML [
25,
26]. However,
AML1/ETO-positive patients presented less differentiation, indicating that the resistance is not due to the blast maturation state. Previous studies have shown that activating kinase mutations play an important role in venetoclax resistance [
24].
FLT3 mutations and mutations that activate the Ras/Raf/MEK/ERK pathway may drive expressions of MCL-1 [
27,
28]. Co-mutations (e.g.,
KIT,
FLT,
RAS) are common in
AML1/ETO-positive AML, which may possibly result in activated signal transduction pathways and lead to venetoclax resistance. Further studies of venetoclax resistance are urgently needed in patients with
AML1/ETO fusion, as well as in patients with inv(16),
MLL rearrangement, inv(3), or other more rare fusions, who are also likely to be resistant to VEN/HMA.
Hypomethylating agents exert anti-tumor effects by reversing DNA methylation. Mutations in genes involved in DNA methylation such as
DNMT3A and
TET2 may predict good prognosis with HMAs in patients with myeloid malignancies [
29‐
31]. Previous studies reported that in patients with
AML1/ETO-positive AML, the incidence of a
DNMT3A mutation is about 3–6%, and the incidence of
TET2 mutations is about 7–11% [
32,
33]. In our study, the mutation rate of
DNMT3A in
AML1/ETO-positive patients was significantly lower than that in
AML1/ETO-negative patients. In the SNV analysis of AML patients in datasets, we also found a low probability of
DNMT3A mutations in
AML1/ETO-positive patients, which may contribute to the poor response of
AML1/ETO-positive patients to HMAs.
Our study demonstrated the importance of using IC in
AML1/ETO-positive AML, even in relatively older patients, because of the vastly superior outcomes compared with VEN/HMA. However, in the truly elderly or frail patients who are unable to tolerate IC, lower intensity strategies other than VEN/HMA should be explored. Targeted therapy (e.g.,
FLT 3 inhibitors,
IDH inhibitors) is an option for patients with targetable mutations. However, treatment for patients without targeted mutations is a great challenge. Low-dose cytarabine (LDAC) has shown low CR rates, ranging from 7 to 32% [
34]. Glasdegib, a hedgehog inhibitor, was approved to be used in combination with LDAC in older or unfit patients with AML based on a phase II trial showing better efficacy than LDAC alone [
35]. However, the CR rate (17%) and OS (8.8 months) demonstrated by the combination therapy are not very satisfactory. Nucleoside analogs have been shown to improve the outcomes in older AML patients. In a phase II study of older patients with AML treated with cladribine plus LDAC alternating with decitabine, a CR rate of 58% was achieved with a median OS of 13.8 months [
36]. In another phase II study, LDAC and cladribine combined with venetoclax alternating with azacitidine demonstrated a CR rate of 93% in older patients [
37]. In previous studies, the effect of low-intensity treatment in the subgroup of
AML1/ETO-positive AML had not been described separately. Combination therapy with cladribine may be a choice for this group of patients based on the available data. Further clinical trials are urgently needed in older patients with
AML1/ETO fusion.
Our study also affirmed that ELN risk stratification, which was developed from intensively treated patients, may not be suitable for VEN/HMA-treated patients. Patients treated with VEN/HMA need their own risk stratification criteria, which is an important issue that future studies need to address.
There were a few limitations of the current study. First, because it was a retrospective study, the baseline characteristics between cohorts were not completely comparable. Although propensity score matching reduces the bias, it further reduces the number of patients included in the analysis. Then, because of the lack of prospective design, the combinations and dosages of induction therapies for patients in the same treatment subgroup varied, and the treatment strategies after achieving remission differed. Lastly, more accurate subgroup analyses were limited by the small sample size of patients with AML1/ETO-positive AML receiving VEN/HMA. Larger and better matched cohort studies are needed to validate our results.