FLT3 Mutations in Acute Myeloid Leukemia: Key Concepts and Emerging Controversies

FLT3 Mutations in Acute

Myeloid Leukemia: Key Concepts

and Emerging Controversies

Vanessa E. Kennedy

and Catherine C. Smith

Division of Hematology and Oncology, Department of Medicine, University of California San Francisco, San Francisco, CA,

United States

The FLT3 receptor is overexpressed on the majority of acute myeloid leukemia (AML)

blasts. Mutations in FLT3 are the most common genetic alteration in AML, identiﬁed in

approximately one third of newly diagnosed patients. FLT3 internal tandem duplication

mutations (FLT3-ITD) are associated with increased relapse and inferior overall survival.

Multiple small molecule inhibitors of FLT3 signaling have been identiﬁed, two of which

(midostaurin and gilteritinib) are currently approved in the United States, and many more of

which are in clinical trials. Despite signiﬁcant advances, resistance to FLT3 inhibitors

through secondary FLT3 mutations, upregulation of parallel pathways, and extracellular

signaling remains an ongoing challenge. Novel therapeutic strategies to overcome

resistance, i ncluding combining FLT3 inhibitors with other antileukemic agents,

development of new FLT3 inhibitors, and FLT3-directed immunotherapy are in active

clinical development. Multiple questions regarding FLT3-mutated AML remain. In this

review, we highlight several of the current most intriguing controversies in the ﬁeld

including the role of FLT3 inhibitors in maintenance therapy, the role of hematopoietic

cell transplantation in FLT3-mutated AML, use of FLT3 inhibitors in FLT3 wild-type

disease, signiﬁcance of non-canonical FLT3 mutations, and ﬁnally, emerging concerns

regarding clonal evolution.

Keywords: Acute Myeloid Leukemia, FLT3 inhibitor, FLT3 resistance, FLT3 inhibitor maintenance, non-canonical

FLT3 mutation

FLT3 EPIDEMIOLOGY, BIOLOGY, AND PROGNOSTIC

ASSOCIATIONS

Acute Myeloid Leukemia (AML) is an aggressive hematologic malignancy characterized by a

heterogenous genetic landscape and complex clonal evolution (1). Fms-like tyrosine kinase 3

(FLT3), a member of the receptor tyrosine kinase family, is widely expressed in hematopoietic

progenitor cells and is overexpressed on the majority of AML blasts (2). Upon binding to the FLT3

ligand, FLT3 receptors activate and dimerize, leading to conformational change, cellular

proliferation, and inhibition of apoptosis and differentiation (3). Mutations in FLT3 are the most

common genomic alteration in AML, identiﬁed in approximately one-third of newly diagnosed

adult patients (4), and are common in pediatric AML as well (5).

Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 6128801

Edited by:

Alessandro Isidori,

AORMN Hospital, Italy

Reviewed by:

Adolfo De La Fuente,

MD Anderson Cancer Center Madrid,

Spain

Hussein A. Abbas,

M D Anderson Cancer Center,

United States

*Correspondence:

Vanessa E. Kennedy

[email protected]

Specialty section:

This article was submitted to

Hematologic Malignancies,

a section of the journal

Frontiers in Oncology

Received: 01 October 2020

Accepted: 19 November 2020

Published: 23 December 2020

Citation:

Kennedy VE and Smith CC (2020)

FLT3 Mutations in Acute Myeloid

Leukemia: Key Concepts and

Emerging Controversies.

Front. Oncol. 10:612880.

doi: 10.3389/fonc.2020.612880

REVIEW

published: 23 December 2020

doi: 10.3389/fonc.2020.612880

FLT3 mutations can be subdivided into internal tandem

duplicates (ITD), present in approximately 25% of patients,

and point mutations in the tyrosine kinase domain (TKD),

present in approximately 5%. Both FLT3-ITD and FLT3-TKD

mutations are constitutively activating, leading to ligand-

independent FLT3 signaling and cellular proliferation (3).

FLT3-ITD AND FLT3-TKD

FLT3-ITD mutations a re in-frame duplications of variable

size, ranging from 3 to >1,000 nucleotides, and are located

within the receptor’s autoinhibitory juxatamembrane domain.

In wild type (WT) FLT3, the FLT3 juxtamembrane domain

inhibits receptor act ivation; the presence of ITDs disrupts t his

inhibitory effect, resulting in constitutive activation (6).

Clinically, FLT3-ITD muta ted AML is associated with higher

rates of relapse and inferior overall survival, although the full

prognostic im pact is affect ed both by mutant allele burden

and presence of co-existing mutations (7). High allele ratio

(AR; FLT3-ITD

high

), generally deﬁned as a FLT3-ITD to FLT3-

WT ratio of ≥0.5, is associated with higher disease risk.

Low AR (FLT3-ITD

low

) is associated with favorable risk in

patients with a co-occurrent nucleophosmin 1 (NPM1)

mutations and intermediate risk in patients with NPM1-WT.

These associations are reﬂected in the 2017 European

LeukemiaNet (ELN) risk stratiﬁcation of AML (8).

FLT3-TKD mutations are point mutations within the

receptor’s activation loop which stabilize the active kinase

conformation and also result in constitutive kinase activation

(3). In contrast with FLT3-ITD mutant AML, the prognostic

relevance of FLT3-TKD mutations is less clear and may be

dependent on the presence of co-occurring mutations and

cytogenetic changes (9, 10). Currently, the presence of a FLT3-

TKD mutation does not alter formal AML risk assessment (8).

TESTING CONSIDERATIONS

Given the prognostic and treatment implications, testing for

FLT3-ITD in patients with newly diagnosed AML is

recommended by both ELN and National Comprehensive

Cancer Network (NCCN) guidelines (8, 11). In addition, given

the clonal evolution observed in AML, FLT3 mutations can be

gained or lost at disease relapse and progression. In an individual

patient, the presence of FLT3 mutations will often need to be re-

assessed over time.

Currently, there remains considerable variability in FLT3

assay types, associated sensitivity and speciﬁcity, and

turnaround time among treating centers (12). There are two

main methods for determining FLT3 status are polymerase chain

reaction (PCR)-based assays and next generation sequencing

(NGS). Due to competition from the FLT3-WT allele, the

sensitivity of standard PCR-based assays may be lower than

NGS-based methods, although this can be overcome using

patient-speciﬁc PCR primers (13).

The FLT3 allelic frequency (AF) has also been used to deﬁne

FLT3-ITD positivity, primarily in research studies. Unlike the

AR, which calculates the ratio of the area under the curve (AUC)

of mutant to WT alleles (FLT3-ITD/FLT3-WT), the AF

calculates the fraction of mutant alleles as a percentage of all

FLT3 alleles (FLT3-ITD/FLT3-WT + FLT3-ITD). For example, a

FLT3 AR of 0.5 (0.5 ITD/1.0 WT) would be equal to a FLT3 AR

of 0.33 (0.5 ITD/0.5 ITD + 1.0 WT = 0.33 AF). NGS studies are

typically reported as VAF while PCR assays typically report AR.

Historically, FLT3-ITD has been inherently difﬁcult to detect

using NGS. NGS relies on the reconstruction of short (<300 base

pair) sequences, and longer length ITDs may not be detected (14,

15) although this can be overcome using novel bioinformatic

approaches (16). NGS is becoming more commonly used,

especially in monitoring for FLT3+ minimal residual disease

(MRD) following treatment. An NGS-based FLT3 assay is now

commercially available and is currently used in in an ongoing

trial of gilteritinib maintenance therapy following hematopoietic

cell transplantation (HCT) (NCT02997202).

Both PCR and NGS-based assays are supported by the current

NCCN guidelines (8); however, ELN risk stratiﬁ cation is

dependent upon FLT3 AR, which can only be determined by

PCR. There is currently no standardized methodology or

laboratory referenc e values for AR determination, and the

current cut-off of ≥0.5 has not been prospectively validated. In

retrospective analyses, higher AR is generally associated with

inferior clinical outcomes (17), although these studies are prior

to the widespread use of FLT3 inhibitors. It is likely that impact

of AR on prognosis exists on a continuum, rather than a discrete

cut-off. Finally, many patients do not receive FLT3 testing at all.

In a large survey by the American College of Pathologists in

2015, only 51% of new AML referrals received

FLT3 testing (18).

OVERVIEW OF CURRENTLY

ESTABLISHED FLT3 INHIBITORS

Given the prevalence and poor prognosis of FLT3-ITD mutated

AML, targeting FLT3 signaling through small molecule

inhibitors is a promising therapeutic strategy. FLT3 inhibitors

can be stratiﬁed using two primary schema: ﬁrst vs sec ond

generation FLT3 inhibitors and type I vs type II FLT3

inhibitors. FLT3 inhibitors currently in use or in active

development, including toxicity proﬁles, are detailed in Table 1.

Current usage and ongoing trials of established FLT3

inhibitors in newly diagnosed and relapsed/refractor (R/R)

AML are summarized in Tables 2 and 3, respectively.

Stratiﬁcation of FLT3 Inhibitors

First generation FLT3 inhibitors include sorafenib, midostaurin,

lestaurtinib, sunitinib, and tandutinib. These multi-kinase

inhibitors target not only FLT3 but other kinases as well,

including PKC, SYK, FLK-1, AKT, PKA, KIT, FGR, SRC,

PDGFRa/b, and VEGFR 1/2 (midostaurin) and RAF, VEGFR

1/2/3, PDGFRb, KIT, and RET (sorafenib). The antileukemic

effects of these non-speciﬁc inhibitors likely derive not only from

Kennedy and Smith Emerging Topics in FLT3 AML

Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 6128802

FLT3 inhibition, but from inhibition of targets in these parallel

pathways as well. Similarly, due to their mul tiple off- target

effects, ﬁrst generation FLT3 inhibitor may also be associated

with increased toxicities. In contrast, second generation FLT3

inhibitors are more potent, FLT3-speciﬁc and thus have fewer

off-target effects at clinically relevant doses. Second generation

FLT3 inhibitors include gilteritinib, quizartinib, and crenolanib.

FLT3 inhibitors can also be subdivided based upon how they

interact with the intracellular kinase domain (KD) of the FLT3

receptor. In normal physiology, FLT3 ligand binds to the

extracellular domain, causing the FLT3 receptor to dimerize,

assume an enzymatically active conformation and subsequently

activate downstream signaling. Type I FLT3 inhibitors, which

include midostaurin, gilteritinib, lestaurtinib, and crenolanib,

bind the receptor in the active conformation, thus inhibiting

both FLT3-TKD and FLT3-ITD mutated receptors. Type II

inhibitors, including quizartinib and sorafenib, bind to a region

adjacent to the ATP-binding pocket and only inhibit the receptor

TABLE 2 | Select Trials of Established FLT3 inhibitors in newly diagnosed AML.

NCT/Trial

Identiﬁer

Phase Treatment Setting Patient Population

Midostaurin

NCT01477606/

AMLSG 16-10

II Combination with induction and consolidation chemotherapy, plus maintenance FLT3-ITD; age 18−70

NCT03512197 III Midostaurin vs placebo in combination with induction and consolidation

chemotherapy

FLT3-WT, deﬁned as FLT3-ITD, D835, or I836 AR

< 0.05; age ≥ 18; no prior FLT3 inhibitor therapy

NCT03900949 I Combination with induction chemotherapy plus gemtuzumab ozogamicin FLT3-ITD or -TKD, CD33 +; age ≥ 18

NCT04385290/

MOSAIC

II Combination with induction chemotherapy plus gemtuzumab ozogamicin FLT3-ITD or -TKD, age 18−75

Gilteritinib

NCT02236013 I Combination with induction and consolidation chemotherapy All AML; age ≥ 18

NCT04027309/

HOVON 156

III Gilteritinib vs Midostaurin in combination with induction and consolidation

chemotherapy, plus maintenance

FLT3-ITD or -TKD AML; age ≥ 18

Quizartinib

NCT02668653/

QuANTUM-First

III Quizartinib vs Placebo in combination with induction and consolidation

chemotherapy, plus maintenance post-chemotherapy and post-HCT

FLT3-ITD AML; age 18−75

NCT03723681 I Combination with induction and consolidation chemotherapy All AML; age 18−75

NCT03135054 II Combination with Omacetaxine mepesuccinate FLT3-ITD AML; age ≥ 18

NCT04047641 II Combination with cladribine, idarubicin, and cytarabine All AML, MDS; age 18–65 (ﬁrst-line cohort)

NCT04107727 II Quizartinib vs placebo in combination with induction and consolidation chemotherapy FLT3-WT, deﬁned as FLT3-IT AR < 0.03; age 18–

NCT04128748 I/II Combination with liposomal cytarabine and anthracycline (CPX-351; Vyxeos) All AML; age ≥ 60

Crenolanib

NCT03258931 III Crenolanib vs Midostaurin in combination with induction and consolidation

chemotherapy, plus maintenance

FLT3-ITD or -TKD, age 18−60

TABLE 1 | Established FLT3 inhibitors and common toxicity proﬁles.

FLT3 Inhibitor Type Common or Notable Toxicities

First Generation

Midostaurin Type I • Hematologic: Cytopenias, including febrile neutropenia and abnormal bruising or bleeding; differentiation syndrome

• Constitutional: Pyrexia, ﬂu-like symptoms

• Cardiac: Cardiac failure (rare, <5%)

• GI: Abdominal pain, nausea, vomiting, diarrhea, stomatitis, AST or ALT increase (19–21)

Sorafenib Type II • Hematologic: Cytopenias, usually mild; differentiation syndrome

• Constitutional: Fatigue, can be severe in ~6% of patients

• Cardiac: Hypertension, cardiac ischemia (rare, <5%)

• GI: Diarrhea

• Dermatologic: Rash, erythema, hand-foot skin reaction (22–24)

Second Generation

Quizartinib Type II • Hematologic: Cytopenias, including febrile neutropenia and abnormal bruising or bleeding; differentiation syndrome

• Cardiac: QTc prolongation (dose-dependent, can be severe)

• GI: Abdominal pain, nausea, anorexia (25, 26)

Gilteritinib Type I • Hematologic: Cytopenias, usually mild; differentiation syndrome

• GI: Diarrhea, pancreatitis (rare, <5%, but can be severe), AST or ALT increase

• Neurologic: Peripheral neuropathy, headache (27, 28)

Crenolanib Type I • Hematologic: Differentiation syndrome

• GI: Abdominal pain, nausea, AST or ALT increase

• Other: Peripheral edema (29, 30)

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Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 6128803

in the inactive conformation. Type II inhibitors are inactive

against most FLT3-TKD mutations as these mutations bias the

active kinase conformation of FLT3. This distinction is key in

understanding mechan isms of FLT3 inhibitor resistance, as

discussed later in this review.

FIRST-GENERATION FLT3 INHIBITORS

Midostaurin

Midostaurin was one of the ﬁrst FLT3 inhibitors to be studied in

AML. Early studies of single-agent midostaurin in R/R AML

demonstrated the drug was well-tolerated, but had limited

efﬁcacy (19, 31).

Results of midostaurin in combination therapy have been

more promising. In the phase III RATIFY trial, midostaurin was

evaluated in combination with standard induction and

consolidation chemotherapy in adults <60 years with FLT3-

mutated AML. This combination demonstrated sign iﬁcant

improvement in the primary endpoint of overall survival (OS),

with a median OS of 74.7 m onths in patients receiv ing

midostaurin plus chemotherapy vs 25.6 mont hs in patients

receiving placebo plus chemotherapy (p = 0.009) (20). In 2017,

over two decades since FLT3 mutations were ﬁrst described in

AML, midostaurin gained US Food and Drug Administration

(FDA) approval, becoming ﬁrst agent to achieve FDA approval

for AML since the year 2000.

While the resul ts of RATIFY were promising, there are

some important caveats to consider. In RATIFY, 23% of the

study population had a FLT3-TKD mutation, signiﬁ cantly

larger than that seen in the general population, and perhaps

biasing the results toward this less-aggressive disease

subtype. In addition, while patients in RATIFY were younger

(median age 48), FDA approval was extended to all age groups

(20). The phase II AMLSG 16-10 trial is currently evaluating

midostaurin in combination with induction and consolidation

chemotherapy in adults up to age 70 (NCT01477606), with initial

results suggesting that older age does not signiﬁcantly impact

outcomes, despite the majority of patients requiring midostaurin

dose reduction (32).

Sorafenib

Unlike RATIFY, trials of sorafenib in combination with

chemotherapy have not been conducted in only FLT3-mutant

patients and can thus be more challenging to interpret. Early

phase I/II results of sorafenib in combination with induction

chemotherapy demonstrated prom ising results, with a 75%

complete response (CR) rate in all patients and a 93% CR rate

in the subset with FLT3-ITD mutant AML (22). These ﬁndings,

however, were not replicated in follow-up randomized studies. In

2013, sorafenib plus chemotherapy was evaluated in older adults

(61–80 years), but did not show beneﬁt in either the primary

endpoint of EFS or in OS, including in FLT3-ITD subgroup

analysis (median EFS 7 vs 5 months, p = 0.12). Furthermore,

sorafenib demonstrated marked toxicity, presumably due to off-

target effects (33).

In 2015, sorafenib plus chemotherapy was evaluated in adults

<60 years in the randomized phase II SORAML trial. While

sorafenib demonstrated improvement in the primary endpoint of

EFS compared to placebo in all patients regardless of FLT3 status

(median EFS 21 vs 9 months, p = 0.013) (23), there was

ultimately no difference in OS (5y OS 61 vs 52%, p = 0.23) (34).

Sorafenib may have efﬁcacy as a single agent. Early phase I

and retrospective studies of sorafenib monotherapy in R/R AML

irrespective of FLT3 status demonstrated acceptable toxicity

proﬁles but mixed CR rates, ranging from 10 to 48% (35, 36).

In studies of FLT3-ITD R/R AML, response rates of sorafenib

monotherapy were 23–92% (37–39) with some post-transplant

patients achieving sustained remission for multiple years (39,

40). Sorafenib does not have regulatory approval for AML but

can be used off-label in the US as it is approve d for

hepatocellular, renal cell, and thyroid cancer.

Sunitinib, Lestaurtinib, Tandutinib

Other ﬁrst-generation FLT3 inhibitors have demonstrated

limited antileukemic effect as monotherapy and mixed results

when combined with chemotherapy. As single-agent therapy for

patients with R/R disease, sunitinib (41), lestaurtinib (42), and

tandutinib (43) have all demonstrated limited and short-lived

responses. In combination with chemotherapy, a phase I/II study

of sunitinib plus frontline chemotherapy in adults >60

TABLE 3 | Select Trials of Established FLT3 inhibitors in R/R AML.

NCT/Trial

Identiﬁer

Phase Treatment Setting Patient Population

Quizartinib

NCT03989713/

Q-HAM

II Combination with salvage chemotherapy (Ara-C, Mitoxantrone); R/R after ﬁrst-line treatment, including HCT FLT3-ITD AML; age 18

−75

NCT04047641 II Combination with cladribine, idarubicin, and cytarabine; any previous treatment All AML, MDS; age ≥ 18

(R/R cohort)

NCT04209725 II Combination with liposomal cytarabine and anthracycline (CPX-351; Vyxeos); R/R after any prior treatment, ﬁrst-line

treatment must have been standard induction chemotherapy

FLT3-ITD AML; age 18

−75

NCT04128748 I/II Combination with liposomal cytarabine and anthracycline (CPX-351; Vyxeos); R/R to ﬁrst, second, third, or fourth

line therapy

All AML, MDS; age ≥ 18

NCT04112589 I/II Combination with FLAG-Ida (ﬂudarabine, cytarabine, idarubicin, GCSF); R/R to frontline standard induction

chemotherapy

All AML; age 18−70

Crenolanib

NCT03250338 III Crenolanib vs Placebo plus salvage chemotherapy; R/R after ﬁrst or second-line treatment, including HCT FLT3-ITD and -D835;

age 18−75

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Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 6128804

demonstrated a 59% CR rate; however, multiple patients

experienced d ose-limiting toxicities (44). To contrast, a

randomized phase III trial of lestaurtinib plus frontline

induction and consolidation chemotherapy in patients with

FLT3-mutated AML demonstrated no difference in prim ary

endpoints of OS (5-year OS 46% lestaurtinib vs 45% control,

p = 0.3) or RFS (5-year RFS 40 vs 36%, p = 0.30) (45). Similarly, a

randomized phase III trial of chemotherapy with or without

lestaurtinib in patients with FLT3-mutated AML in ﬁrst relapse

demonstrat ed no di fference in the primary endpoint of CR

rates (26 vs 21%, p = 0.35) (42). Currently, none of these

agents are approved for AML and development in AML has

been abandoned.

SECOND-GENERATION FLT3 INHIBITORS

Unlike midostaurin and sorafenib, second-generation FLT3

inhibitors are more speciﬁc to the FLT3 receptor, exhibit

greater potency, and have thus been far more efﬁcacious as

single-agent therapy. Both gilteritinib and crenolanib are type I

inhibitors, active against both the active and inactive

conformation of the FLT3 receptor, while quizartinib is a type

II inhibitor, active only against the inactive form.

Quizartinib

Quizartinib has demonstrated improved potency and selectivity

against FLT3-ITD in preclinical cellular assays, and targets KIT

and PDGFR as well as FLT3 (46). Early phase II studies of

quizartinib monotherapy in R/R disease were highly promising,

with CR rates of 46–56% and median OS of 25 weeks in FLT3-

ITD mutated AML (25, 47). Quizartinib also demonstrated an

acceptable safety pro ﬁle aside from QTc prolongation, leading to

an additional phase IIb study which explored dose reduction to

60 and 90 mg daily vs up to 200 mg daily (47).

Quizartinib was subsequently evaluated in the phase III

randomized QuAN TUM-R trial, which randomized patients

with R/R FLT3-ITD AML to single-agent quizartinib vs salvage

chemotherapy (26). Quizartinib was associated with a

signiﬁcantly longer primary endpoint of overall survival (6.2 vs

4.7 months, p = 0.02), and a greater proportion of patients were

able to proceed to hematopoietic cell transplantation (HCT; 32 vs

11%). Total treatment-associated toxicities were similar between

the two arms, although 2% of patients receiving quizartinib

experienced Grade 3 QTc prolongation. Based on these results,

the authors of QuANTUM-R concluded that quizarti nib

monotherapy should be considered a standard of care option

in R/R FLT3-ITD mutated AML (26).

Despite these positive results, in 2019, both the FDA and the

European Medicines Agency (EMA) rejected approval for

quizartinib, although regulatory approval was granted in Japan.

Although the FDA Oncologic Drug Advisory Committee

(ODAC) raised concerns that up to four deaths in the

quizartinib arm were attributed to cardiotoxicity, the decision

to decline was ultimately due to concerns regarding trial design

(48). These concerns included (1) an imbalance in patients who

were randomized but not treated, with 23% of patients

randomized to chemotherapy not receiving treatment vs 11%

of patients randomized to quizartinib and (2) in stratiﬁed

analysis, a signiﬁcant survival beneﬁt only when quizartinib

was compared against low-intensity therapy (low-dose

cytarabine) and not against high-intensity therapy (MEC or

FLAG-Ida). The ODAC concluded that, while a modest

survival beneﬁt of 6 weeks was still meaningful, especially if

more patients were bridged to HCT, ultimately additional data

would be needed for quizartinib to be approved in this setting

(48) . The phase II Q-HAM tri al, which has not yet begun

recruiting, will evaluate quizartinib in combination with

salvage chemotherapy in R/R FLT3 -ITD AML (NCT03989713).

In newly di agnos ed AML, quizartinib is currently being

evaluated in the randomized phase III QuANTUM-First trial,

which will compare quizartinib vs placebo in combination with

induction and consolidation chemot herapy (NCT02668653).

Additional phase I/II studies evaluating quizartinib in

combination with frontline cytotoxic chemotherapy are

ongoing as well (NCT03723681, NCT03135054, NCT04047641).

Gilteritinib

Gilteritinib is a selective and potent type I FLT3 inhibitor which

also has activity against AXL, ALT, and ALK (49). Gilteritinib is a

particularly promising agent due to its ability to target KD

mutation s, including the D835 residue, the developm ent of

which is a key mechanism of both quizartinib (50)and

sorafenib (51) resistance. In the phase I/II CHRYSALIS trial,

gilteritinib demonstrated promising results as monotherapy in

R/R FLT3-mutant AML with an overall response rate (ORR) of

40% and a median OS of 25 weeks (52). Notably, patients with

FLT3-D835 mutations also responded to gilteritinib, albeit at

lower rates than patients with FLT3-ITD mutations, with an

ORR of 55% in FLT-ITD-mutated patients, 17% in FLT3-D835-

mutated patients, and 62% in patients with both FLT3-ITD and

FLT-D835 mutations (52).

Following these results, the randomized phase III ADMIRAL

trial compared single-agent gilteritinib vs salvage chemotherapy

in R/R FLT3-mutated AML (27) with co-primary endpoints of

CR rate and OS. Compared to standard salvage chemotherapy,

gilteritinib demonstrated signiﬁcantly greater CR rate (34 vs 15%,

p = 0.0001) and improvement in OS (9.3 vs 5.6 months, p <

0.001). While prior use of midostaurin or sorafenib was allowed,

as the trial enrolled prior to midostaurin’s approval and

widespread use, the majority (88%) of patients had not

received a prior FLT3 inhibitor, limiting the ability of this trial

to evaluate the ability of gilteritinib to overcome midostaurin

resistance; however, results were similar in patients with FLT3-

ITD and FLT3- TKD mutated disease (27). Based on a pre-

planned interim analysis, in 2018, the FDA approved

gilteritinib as single-agent therapy for R/R FLT3-mutated AML.

It is unknown whether gilteritinib is similarly beneﬁcial in

newly diagnosed AML. Gilteritinib is currently being studied in a

phase I study in combination with induction and consolidation

chemotherapy in newly diagnosed AML (NCT02236013); interim

results indicate this approach is safe and feasible (53). The phase

III HOVON 156 trial, which is actively accruing, will compare

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gilteritinib vs midostaurin in combination with chemotherapy

followed by FLT3 inhibitor maintenance (NCT04027309).

Crenolanib

Crenolanib is a potent type I inhibitor with activity against

PDGFRb, FLT3-ITD, and FLT3-TKD mutations, including at

D835 (54). Two smaller phase II studies have demonstrated

efﬁcacy of single-agent crenolanib in R/R FLT3-mutated AML,

with a CR rate of 23−39% in patients naïve to FLT3 inhibitors,

and 5% patients with prior FLT3 exposure (29, 30).

Crenolanib also demonstrates promising results in

combination with chemotherapy. A phase II trial of crenolanib

plus chemotherapy in newly diagnosed FLT3-mutated AML

demonstrated a high CR rate of 85%; notably, 70% of patients

remained alive and disease free with a median follow-up of 29.3

months (55). Crenolanib plus chemotherapy is also efﬁcacious in

older adults. In a phase II trial of adults 61–75 years with newly

diagnosed FLT3-mutated AML, crenolanib plus chemotherapy

demonstrated a CR rate of 86% and was relatively well-tolerated,

although four of 14 patients did require dose-reductions due to

toxicity (56). A phase III trial of crenolanib vs midostaurin plus

chemotherapy in newly diagnosed FLT3-mutated AML is

currently recruiting (NCT03258931).

RESISTANCE TO FLT3 INHIBITORS

Despite the relative success of established FLT3 inhibitors,

responses are frequently short-lived and therapeutic resistance

poses an ongoing challenge. Mechanisms of FLT3 inhibitor

resistance differ based on drug type, but broadly can be

subdivided into cell intrinsic and extrinsic mechanisms.

Intrinsic resistance can be further sub-divided into on-target

secondary mutations within FLT3 and off-target mutations in

downstream or parallel signaling pathways. These mutations

may develop de novo or via expansion of pre-existing

subclones (57). Extrinsic resistance can occur through altered

expression or metabolism of the FLT3 ligand or changes in the

interactions between the leukemic blast and the bone marrow

microenvir onment. Figure 1 illustrates common resistance

pathways and mechanisms.

On-Target Secondary Mutations

One common mechanism of FLT3 inhibitor resistance is

development of a secondary FLT3 mutation, most commonly

in the KD (58). These mutations commonly occur at gatekeep

F691 and activation loop (AL) D835 residues, but can involve

other KD residues I836, D839, and Y842, among others (59).

This mechanism is most relevant for type II FLT3 inhibitors,

which interact weakly with the active receptor formation caused

by some KD mutations. Type I inhibitors gilteritinib and

crenolanib are both active against D835 mutations (60, 61).

To contrast, the gatekeeper F691L mutation confers

resistance not only to quizartinib and sorafenib, but also to

gilteritinib and crenolanib as well (51, 60–62). In practice, the

impact of F691L mutations on type I inhibitor resistance may be

relatively minor. In studies of both single-agent gilteritinib (62,

63) and single-agent crenolanib (64) resistance in patients with

FIGURE 1 | Proposed Intrinsic and Extrinsic Mechanisms of FLT3 Inhibitor Resistance. Schematic of FLT3 inhibitor resistance mechanisms, including on-target

secondary FLT3 mutations, off-target mutations in parallel and/or downstream signaling pathways, and extrinsic alterations in drug metabolism and the bone marrow

microenvironment.

Kennedy and Smith Emerging Topics in FLT3 AML

Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 6128806

R/R FLT3-mutated AML, only 12% of patients receiving

gilteritinib, and 11% of patients receiving crenolanib developed

F691L mutations at the time of resistance.

In most cases, on-target mutations are not detected prior to

FLT3 inhibitor treatment (65). In a recent study of 11 patients

treated with quizartinib monotherapy, single-cell sequencing

revealed 7/11 patients developed KD mutations at relapse and

no patients had these mutations prior to FLT3 inhibition,

suggesting TK mutations typically evolve de novo (66) or exist

at levels undetectable by current sequencing methods.

Off-Target Resistance in Parallel

or Downstream Pathways

KD mutations only partly explain FLT3 inhibitor resistance, and

expansion or emergence of non-FLT3 mutant clones is a key

resistance mechanism. In patients treated with gilteritinib and

crenolanib, sequencing of paired patient samples pre- and post-

therapy demonstrated a wide variety of mutations at resistance,

including genes involved in the RAS pathways (NRAS, KRAS,

PTPN11), ASXL1, TP53, TET2, and IDH1/2. These mutations

occurred not only in cells expressing the FLT3-mutant allele, but

in FLT3-WT cells as well (63, 64). These off-target mutations are

not limited to type I inhibitors. In a study using single-cell

sequencing to analyze 11 patients treated with quizartinib, 3/11

demonstrated off-target mutations at relapse, all of which were

present in small clonal populations prior to FLT3 inhibitor

therapy (66).

Of these off-target pathways, activation of the downstream

Ras and associated PI3K/Akt/mTOR and MAPK/Erk pathways

are particularly common in clinical gilteritinib and crenolanib

resistance (62–64), and in vitro studies have demonstrated

mutations in these pathways can confer resistance to FLT3

inhibitors in FLT3-mutant cell lines (67, 68). Development of

the BCR-ABL1 fusion gene has been describe in patients with

gilteritinib resistance (63, 69).

Upregulation of parallel AXL tyrosine kinase signaling is

another mechanism of FLT3 inhibitor resistance. In one study,

blasts from a patient with FLT3-ITD AML were exposed to both

midostaurin and quizartinib and were found to have increased

AXL phosphorylation up on development of FLT3 inhibitor

resistance. This resistance could be overcome with use of the

AXL inhibitor TP-0903 (70, 71) and a phase I trial of TP-0903

with or without azacitidine in FLT3-mutated AML has recently

opened (NCT04518345).

Together, these studies suggest off-target resistance

mechanisms are common to all FLT3 inhibitors and frequently

arise via selection of pre-existing sub-clones harboring survival

advantages under the selective pressure of FLT3 inhibition.

Extrinsic Mechanisms of Resistance

The majority of leukemic blasts, regardless of mutational status,

express the FLT3 receptor and proliferate in response to FLT3

ligand. Increased levels of FLT3 ligand in the bone marrow

microenvironment have been demonstrated during induction

and consolidation chemotherapy, and can lead to increased

signaling via the native FLT3 receptor, even in the presence of

FLT3 inhibitors (72, 73). FLT3 inhib itors may also have

decreased efﬁcacy due to decreased drug availability, either due

to enhanced CYP34A expression on BM stromal cells (74)or

iatrogenic drug-drug interactions (75).

The bone marrow microenvironment c an also directly

contribute to FLT3 inhibitor resistance. In addition to directly

secreting FLT3 ligand, bone marrow stro mal cells can also

upregulate Ras/MAPK signaling independent of the FLT3

receptor via secretion of FGF2 (76). In one study of patients

treated with quizartinib, stromal cell expression of FGF2 conferred

FLT3 inhibitor resistance, and could be overcame by FGFR

inhibition (76, 77). Multiple dual FLT3/FGFR inhibitors are in

pre-clinical development (78, 79) with MAX-40279 currently in

phase I clinical trials (NCT03412292, NCT04187495). In addition,

ponatinib, a multikinase inhibitor approved in CML, has activity

against both FLT3 and FGFR (80).

NOVEL FLT3 INHIBITOR COMBINATIONS

One strategy to overcome resistance and provide durable

remissions is to use FLT3 inhibitors in novel combinations

with other antileukemic agents (Tables 4, 5; Figure 2).

Hypomethylating Agents

Aside from conventional cytotoxic chemotherapy, the most well-

studied FLT3 inhibitor combination is with the hypomethylating

agents (HMA) azacitidine or decitabine. This combination is

particularly attractive, both due to the synergistic cytotoxicity in

preclinical studies as well as the established tolerability and

durable responses of HMAs in older adults (81, 82).

Multiple phase I/II trials have demonstrated the combination

of midostaurin and HMA to be feasible in adults with FLT-

mutated AML who are unﬁt for traditional chemotherapy in the

frontline setting (83) and in R/R disease (82, 84 ). Additional

trials of midost aurin plus HMA for older/unﬁtadultshave

opened, but have terminated due to inability to accrue

(NCT01846624. NCT02634827). There is an ongoing trial of

midostaurin plus azacitidine for newly diagnosed AML

regardless of FLT3 mutational status (NCT01093573), w ith

primary endpoints of tolerability and ORR.

Sorafenib plus HMA have also been shown to be safe and

efﬁcacious in single-arm and retrospective studies in both the

R/R (85, 86) and frontline settings (87). In the frontline setting,

sorafenib plus HMA demonstrated an ORR of 78%, the study’s

primary endpoint, and a and median response duration of 14.5

(87). Similarly, a phase I/II study of sorfenib plus azacitidine

demonstrated an ORR of 46%, the secondary endpoint for the

trial’s phase II portion, and median response duration of 2.3

months (85). Notably, FLT3 ligand levels remained low with this

combination, which is intriguing as increase in ligand expression

has been suggested as a possible mechanism of FLT3 inhibitor

resistance (72, 85).

Of the second generation FLT3 inhibitors, an interim analysis

of a phase I/II trial of quizartinib plus azacitidine in unﬁt patients

with newly diagnosed or in R/R FLT3-ITD AML demonstrated

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an ORR of 75%, the secondary endpoint for the trial’s phase II

portion, including in four/ﬁve patients with prior FLT3 inhibitor

exposure (88). A phase II study of gilteritinib plus azacitidine in

unﬁt patients with newly diagnosed FLT3-ITD AML with

primary endpoint of OS is currently accruing (NCT02752035)

with interim results of secondary endpoints demonstrating CR

and ORR rates of 67 and 80%, respectively (89).

Venetoclax

Venetoclax, an inhibitor of the anti-apoptotic protein Bcl-2, is

particularly intriguing in combinatio n with FLT3 inhibitors.

Preclinical studies have shown FLT3-ITD mutated blasts have

higher Bcl-2 expression compared to FLT3-WT blasts (90) and

upregulation of antiapoptotic proteins is a mechanism of FLT3

inhibitor resistance (91). In preclinical mouse models, venetoclax

has demonstrated s ynergistic antileukemic activity with

midostaurin (92), gilteritinib (92), and quizartinib (93). In

addition, FLT3-ITD mutations were frequently observed at

progression in trials of venetoclax monotherapy in AML (94, 95).

Gilteritinib is currently being studied in combination with

venetoclax in R/R AML (NCT03625505). Quizartinib is being

evaluated in combination with venetoclax and decitabine in

poor-risk newly diagnosed or R/R FLT3- mutated AML

(NCT03661307) and in combination with venetoclax alone in

R/R FLT3-ITD AML (NCT03735875).

Proteosome Inhibitors

Proteosome inhibitors, including bortezomib, have

demonstrated cytotoxicity toward FLT3-ITD mutant cells in

preclinical studies (96) and a phase I study of midostaurin in

combination with bortezomib and chemotherapy in R/R AML

demonstrated activity, albeit with marked toxicity ( 97). Sorafenib

TABLE 4 | Select Trials of Established FLT3 Inhibitors in Combination Therapy.

Combination

Agent

FLT3

inhibitor

NCT/Trial

Identiﬁed

Phase Treatment Setting Patient Population

Hypomethylating Agents

Azacitidine Midostaurin NCT01093573 I/II Newly diagnosed All AML; age ≥ 18 and unﬁt for

chemotherapy

Decitabine Midostaurin NCT04097470;

HO-155

II Midostaurin plus decitabine vs decitabine alone, newly

diagnosed

All AML, age 18–100 and unﬁt for

chemotherapy

Azacitidine Sorafenib NCT02196857 II Newly diagnosed FLT3-ITD, TKD AML, MDS; age ≥ 60 or 18

−60 and unﬁt for chemotherapy

Azacitidine Gilteritinib NCT02752035;

LACEWING

II/III Gilteritinib monotherapy vs azacitidine monotherapy vs

gilteritinib plus azacitidine; newly diagnosed AML

FLT3-ITD, TKD; age ≥ 65 or 18–65 and

unﬁt for chemotherapy

Azacitidine or

Low-Dose AraC

Quizartinib NCT01892371 I/II Quizartinib plus azacitidine or cytarabine; Newly diagnosed or

R/R after ﬁrst or second-line treatment, including HCT

All AML, MDS, CMML; age ≥ 60 (all

settings) or age ≥ 18 (R/R only)

Venetoclax +/- HMA

Venetoclax Gilteritinib NCT03625505 I R/R to at least one prior therapy All AML; age ≥ 18

Venetoclax +

Azacitidine

Gilteritinib NCT04140487 I/II Newly diagnosed or R/R FLT3-ITD or D835 AML; age ≥ 18

Venetoclax +

Decitabine

Quizartinib NCT03661307 I/II Newly diagnosed; R/R and not eligible for salvage

chemotherapy or HCT

FLT3-ITD or FLT3-ITD/TKD co-mutations;

age ≥ 60 or age ≥ 18 and unﬁt for

chemotherapy

Venetoclax Quizartinib NCT03735875 Ib/II R/R up to 4 prior therapies FLT3-ITD; age ≥ 18

Proteosome Inhibitors

Bortezomib Sorafenib NCT01371981 III Bortezomib plus sorafenib plus chemotherapy vs sorafenib

plus chemotherapy; Newly diagnosed

FLT3-ITD; age < 29

Bortezomib,

then decitabine

Sorafenib NCT01861314 I Newly diagnosed, R/R to at least one prior therapy All AML; age ≥ 60 or ≥ 18 and unﬁt for

chemotherapy

Bortezomib,

Vorinostat

Sorafenib NCT01534260 I/II R/R to at least one prior therapy FLT3-mutated or poor-risk cytogenetics;

age ≥ 18

Targeted Agents

Pim kinase

inhibitor

(LGH447)

Midostaurin NCT02078609 I R/R after ﬁrst or second-line treatment All AML, MDS; age ≥ 18

mTOR inhibitor

(Everolimus)

Midostaurin NCT00819546 I R/R to at least one prior therapy All AML, MDS; age ≥ 18

HDM2 inhibitor

(HDM201)

Midostaurin NCT04496999 I R/R to at least one prior therapy FLT3-ITD or FLT3-TKD and TP53-WT; age

≥ 18

CDK 4/6

inhibitor

(Palbociclib)

Sorafenib NCT03132454 I Palbociclib in combination with sorafenib vs decitabine vs

dexamethasone, newly diagnosed

All AML, ALL; age ≥ 15

MDM2 inhibitor

(Milademetan)

Quizartinib NCT03552029 I Newly diagnosed and ineligible for intensive therapy; R/R to at

least one prior therapy

FLT3-ITD; age ≥ 18

Immunotherapy

Atezolizumab Gilteritinib NCT03730012 I/II R/R to at least one prior therapy FLT3-mutated AML; age ≥ 18

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plus bortezomib was also studi ed in a combination with

vorinostat, a histone deacetylase inhibitor, in a phase I/II trial

of R/R AML and demonstrated a modest ORR of 28% in all

patients and 60% in patients with FLT3-ITD AML (98). Ongoing

studies of this combination include a phase III trial of sorafenib

plus bortezomib in younger adults (up to age 29) with newly

diagnosed FLT3-ITD AML (NCT01371981) and sorafenib plus

bortezomib followed by decitabine in newly diagnosed or R/R

AML regardless of FLT3 status (NCT01861314).

Additional Targeted Agents

Multiple agents targeting signaling pathways downstream of

FLT3 signaling, including those frequently implicated in FLT3

inhibitor resistance, have been studied in combination therapies.

TABLE 5 | Select Trials of Novel FLT3-Directed Therapies.

Combination Agent NCT Phase Treatment Setting Study Population

Multikinase Inhibitors

Ponatinib NCT02428543 I/II AML in ﬁrst CR, following induction and consolidation

with standard cytotoxic chemotherapy

FLT3-ITD with AR > 10%; age 18−70

Ponatinib NCT03690115;

PONALLO

II AML in CR, following allo-HCT FLT3-ITD; age 18 - 70

Novel Dual Agents

SEL24/MEN1703: Dual FLT3/Pim

kinase inhibitor

NCT03008187 I R/R, no standard treatment options available All AML; age ≥ 18

MAX-040279: Dual FGFR/FLT3

inhibitor

NCT03412292 I R/R, no standard treatment options available All AML; age ≥ 18

MAX-040279: Dual FGFR/FLT3

inhibitor

NCT04187495 I R/R, no standard treatment options available All AML; age ≥ 18

CG-806: Dual BTK/FLT3 inhibitor NCT04477291 Ia/b R/R to at least one prior therapy All AML; age ≥ 18

Novel FLT3 Inhibitors

FF-10101 NCT03194685 I/IIa R/R to at least one prior therapy All AML; age ≥ 18

HM43239 NCT03850574 I/II R/R to at least one prior therapy All AML; age ≥ 18

Biologic Agents

FLYNSYN: Anti-FLT3 IgG1

Antibody

NCT02789254;

FLYSYN-101

I/II AML in and hematologic CR but MRD+ after

chemotherapy but not HCT

All AML, but leukemic cells must express

FLT3 by ﬂow cytometry; age ≥ 18

ASP1235 (AGS62P1): anti-FL3

antibody-drug-conjugate

NCT02864290 I R/R to ﬁrst, second, or third therapy All AML; age ≥ 18 and not candidate for

salvage chemotherapy

AMG 553: FLT3 CART NCT03904069 I R/R, no standard treatment options available All AML, but leukemic cells must express

FLT3 by ﬂow cytometry; age ≥ 12

FIGURE 2 | Established and In-Development FLT3 inhibitors, dual inhibitors, and combination agents. Schematic detailing mechanisms of action of established and

in-development FLT3 inhibitors, dual and multikinase inhibitors, and combination agents.

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JAK/STAT5/Pim-1 is a key signaling pathway parallel and

downstream of FLT3. Pim kinase inhibitors inhibit Pim-1, a

kinase which promotes FLT3 signaling via positive feedback (99,

100). In preclinical models, Pim kinase and FLT3 inhibitors

demonstrate synergistic cytotoxicity (100, 101). The Pim kinase

inhibitor L GH447 is b eing studie d in com bination wit h

midostaurin in a phase I trial (NCT02078609), as is the novel

dual Pim kinase/FLT3 inhibitor SEL24 (NCT03008187). Other

agents, including the dual JAK/FLT3 inhibitor pacritinib, have

demonstrated efﬁcacy in phase I trials as well (102).

mTOR inhibitors, such as everolimus, target the PI3K/AKT/

mTOR pathways, which is similarly downstream of FLT3 signaling.

Concomitant inhibition of both mTOR and FLT3 demonstrate

synergistic cytotoxicity (103)andmTORisupregulatedinAML

blasts resistant to FLT3 inhibitors (68). A phase I study of

midostaurinpluseverolimusisactive, but not currently recruiting

(NCT00819546). In addition, metformin, a drug long approved in

diabetes, can also down-regulate the P31K/Akt/mTO R pathways,

and has shown to act synergistically with sorafenib in FLT3-

mutated cell lines (104).

Cyclin-dependent kinase 6 (CKD6) is a key regulator of cell cycle

progression, part of a transcriptional complex that promotes

leukemogenesis, and is found at the promoter of both FLT3 and

PIM1 genes (105, 106). A phase I study of the CDK4/6 inhibitor

palbociclib, which is approved in breast cancer, in combination with

sorafenib is actively recruiting (NCT03132454). In add ition,

multiple novel dual FLT3/CDK4 inhibitors are in active

preclinical development, including AMS-925 (107) (Keegan),

ETH-155008 (108), and FLX925 (109), which recently completed

a phase I dose-escalation trial (NCT02335814).

Finally, the tumor suppressor p53 is increasingly recognized as a

mechanism of FLT3 inhibitor resistance, particularly to crenolanib

(64). Milademetan, an inhibitor of the p53-regulatory protein

MDM2, has demonstrated synergistic anti-leukemic activity with

quizartinib in FLT3-mutant cell lines (110)andaphaseItrialis

actively recruiting (NCT03552029). Similarly, HDM-201, an

inhibitor of the p53-regulatory protein HDM2,isactively

investigated in combination with midostaurin (NCT04496999).

Immunotherapy Combinations

Compared to lymphoid and many solid malignancies, AML has

thus far demonstrated limited response to immunotherapies.

Exploratory studies have indicated that elevated programmed

cell death 1 (PD-1) and PD-1 ligand (PD-L1) are associated with

inferior OS in AML, including in patients with FLT3- mutated

disease (111). A phase I/II trial of gilteritinib in combination with

the checkpoint inhibitor atezolizumab in R/R FLT-mutated AML

is ongoing (NCT03730012).

NOVEL FLT3 INHIBITORS

IN DEVELOPMENT

In addition to novel FLT3 inhibitor combinations, multiple novel

FLT3 inhibitors are in active preclinical and clinical development

(Table 5; Figure 2). These include multikinase inhibitors

approved for non-AML malignancies as well as speciﬁc, next-

generation agents. In addition to overcoming pre-established

mechanisms of resistance, including F691 mutations, these novel

agents also offer alternative and potentially more desirable

toxicity proﬁles.

Multikinase Inhibitors

Ponatinib is approved to target BCR-ABL in chronic myelogenous

leukemia (CML), but is also a type II FLT3 inhibitor with

activity against F691L (112). Ponatinib demonstrated modest

efﬁcacy in a phase I trial of heavily-pretreated R/R AML patients

(113), and is being actively investigated in combination with

chemotherapy (NCT02428543).

Cabozantinib is a multikinase inhibitor currently approved

for medullary thyroid and renal cell carcinomas. Cabozantinib is

selectively cytotoxic to FLT3-ITD mutant cells in culture (114)

and a phase I trial demonstrated sustained inhibition of FLT3-

ITD and -F691 mutant cells, although no treated patients had a

formal disease response (115).

The multikinase inhibitor pexidartinib (PLX3397) has been

studied in multiple solid tumors and also demonstrates activity

against FLT3, including F691L (50). Recently, a phase I/II trial of

single-agent pexidartinib in R/R FLT3-ITD AML demonstrated

an ORR of 21% and CR rate of 11%; furthermore, several patients

were successfully bridged to transplant ( 116 ).

Finally, the BTK inhibitor ibrutinib, currently approved for

lymphoid malignancies, also demonstrates type II FLT3

inhibitory effects (117). A phase I trial of CG-806, a dual BTK/

FLT3 inhibitor, recently opened for patients with R/R

AML (NCT04477291).

Novel FLT3 Inhibitors

One of the most promising novel agents is FF-10101, the ﬁrst

covalently-binding FLT3 inhibitor. In pre-clinical studies, FF-

10101 demonstrated potent activity against quizartinib-resistant

AL and gatekeeper F691 muta tions (118).Otheragentsin

development include TTT-3002, G-749, MZH-29, and

HM43239, all highly se lective FLT3 inhibitors with activity

against D835 and F691 residues (119–122). In preclinical

studies, these agents have demonstrated activity against AML

blasts resistant to sorafenib and quizartinib (119) or midostaurin

and quizartinib (120), and may represent options for refractory

disease. Phase I/II trials of both FF-10101 and HM43239 are

ongoing (NCT03194685, NCT03850574).

Biologic Agents

To date, pharmacologic targeting of FLT3-mutant AML has

primarily focused on signaling inhibition via small molecules;

however, multiple immunotherapeutic approaches are

in development.

Ongoing trials are investigating targeting FLT3 through an

IgG1 antibody (FLYSYN; NCT02789254), with promising

preliminary efﬁcacy data (123), as well as via an anti-FLT3

antibody drug conjugate (124) (NCT02864290). In addition, an

anti-FLT3/anti-CD3 bi-speciﬁc antibody has shown promise in

preclinical models as well (125). Finally, FLT3 may represent a

potential target for chimeric antigen receptor T cell (CART)

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therapy (126), including in combination with established FLT3

inhibitors (127). A phase I trial of a FLT3-directed CART has

recently opened (NCT03904069).

ONGOING QUESTIONS

AND CONTROVERSIES

Despite the considerable advances in trea ting FLT3-mutant

AML, many outstanding questions and controversies remain.

We have summarized a few of the most intriguing and timely

questions below.

What is the Beneﬁt of Maintenance

Therapy

in FLT3-Mutant AML?

Of all the questions regarding the clinical use of FLT3 inhibitors,

one of the most pressing is the role of maintenance therapy. Use

of FLT3 inhibitor maintenanc e, either during remission for

patients who do not undergo HCT or during post-HCT

remission, is currently not standard of care; however, there are

data to suggest this may be a promising approach. Table 6

describes ongoing clinical trials of maintenance therapy.

Post-Chemotherapy Maintenance

Perhaps most notably, in the experimental arm of the

phase III RATIFY study, patients could receive up to one year

of midostaurin maintenance following induction and

consolidation chemotherapy plus midostaurin (20). In

unplanned post-hoc analysis of the subset of p atients who

received either midostaurin or placebo maintenance, there was

no beneﬁt seen with midostaurin, although it was well-tolerated

(128). Importantly, the m edian duration of midostaurin

exposure was 3 months, as the majority (59%) of p atients

randomized to midostaurin received allogeneic HCT and thus

received only two to three cycles of therapy, limiting the ability to

draw conclusions from midostau rin maintenance data (20).

Maintenance midostaurin after chemotherapy did not receive

US FDA approval; however, regulatory approval for

maintenance midostaurin was granted by the EMA.

Currently, both the single-arm AMLSG 16–10 trials, which

will also evaluate midostaurin in combination with

chemotherapy in older adults, and the phase III HOVON 156

AML trial, which will compare midostaurin vs gilteritinib plus

chemotherapy in newly diagnosed AML, will each evaluate up to

one year of midostaurin maintenance following chemotherapy

(NCT01477606, NCT04027309). Of note, neither of these trials

will directly compare maintenance midostaurin against placebo,

which will make it difﬁcult to isolate the true beneﬁt of post-

chemotherapy midostaurin maintenance.

Multiple ongoing trials are also evaluating second generation

FLT3 inhibitors as post-chemotherapy maintenance. In the

HOVON 156 AML trial, patients may receive up to one year

of gilteritinib maintenance, although again, this study will not

compare gilteritinib against a placebo control. A separate

randomized phase III trial will compare gilteritinib vs placebo

maintenance for up to two years following induction and

consolidation therapy in patients who are not proceeding to

HCT accrual (NCT02927262). Similarly, the QuANTUM-First

trial will include up to three years of post-chemotherapy

quizartinib maintenance and will be placebo-controlled.

Post-HCT Maintenance

In the post-HCT setting, midostaurin was evaluated in a phase II

trial as single-agent maintenance following midostaurin plus

chemotherapy and subsequent HCT. Of the 56% of patients

who ultimately received HCT and subsequent maintenance,

mido staurin maintenance was associated with improved OS

compared to a historical control group; however, given this

comparison was relative to historic controls it must be

interpreted with caution (129). Midostaurin maintenance was

also associated with signiﬁcant toxicities, particularly in

older adults.

In the phase II RADIUS trial, patients were ran domized

to receive up to one year of maintenanc e midostaurin vs

TABLE 6 | Select Trials of FLT3 Inhibitors as Post-Chemotherapy or Post-HCT Maintenance Therapy.

FLT3 inhibitor NCT/Trial

Identiﬁer

Phase Treatment Setting Patient Population

Midostaurin NCT01477606/

AMLSG 16−10

II Combination with induction and consolidation chemotherapy, plus maintenance FLT3-ITD; age <70

Midostaurin/

Gilteritinib

NCT04027309/

HOVON 156

III Gilteritinib vs Midostaurin in combination with induction and consolidation chemotherapy,

plus maintenance

FLT3-ITD or -TKD AML;

age ≥18

Midostaurin NCT03951961/

MAURITUS

II Midostaurin maintenance post-HCT; MRD + disease FLT3-mutated; age ≥18

Midostaurin/

Crenolanib

NCT03258931 III Midostaurin vs Crenolanib post-HCT; MRD+ disease FLT3-mutated; age 18

−60

Gilteritinib NCT02927262 III Gilteritinib vs Placebo for up to 2 years following induction and consolidation

chemotherapy

FLT3-ITD; age ≥18 with

no plan for HCT

Gilteritinib NCT02997202/BMT

CTN 1506

III Gilteritinib vs. Placebo following HCT; AML in CR1 or CRi1 FLT3-ITDi; age ≥ 18

Quizartinib NCT02668653/

QuANTUM-First

III Quizartinib vs Placebo in combination with induction and consolidation chemotherapy,

plus maintenance post-chemotherapy and post-HCT

FLT3-ITD AML; age 18

−75

Crenolanib NCT02400255 II Crenolanib following HCT; AML in any CR by morphologic assessment FLT3-ITD and -D835; age

≥18

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standard care following HCT. Although preliminary reports

suggested that addition of midostaurin could reduce risk of

relapse at 18 months post-HCT by 46%, 63% of patients

receiving midostaurin required dose modiﬁcations and 25%

discontinued midostaurin due to toxicity. RADIUS ultimately

had inadequate enrollment to detect a statistically signiﬁcant

difference in 18-month relapse-free survival (RFS), the study’s

primary endpoint (estimated 18-month RFS 89% in midostaurin

arm vs 76% in standard-of-care arm, p = 0.27) (130).

Similarly, post-HCT sorafenib maintenance was shown to be

tolerable and potentially efﬁcacious in both retrospective and

prospective phase I/II studies, although frequent dose

adjustments were needed ( 131, 132 ). More recently, the phase

II SORMAIN trial randomized patients with FLT3-ITD AML in

remission following HCT to maintenance with two years of

sorafenib vs placebo (133). The HR for RFS, the primary

endpoint, demonstrated a signiﬁcant beneﬁt with sorafenib vs

placebo (HR 0.39, 95% CI 0.18–0.85, p = 0.013); however, only 9/

43 (21%) of patients receiving sorafenib had received a frontline

FLT3 inhibitor, so it remains unclear whether the same beneﬁt

would be seen in patients who received a FLT3 inhibitor in the

frontline setting. Similar to the RADIUS trial, study drug

discontinuation was more common in the sorafenib arm (22 vs

5%), although this difference was not signiﬁcant. Finally, and

again similar to the RADIUS trail, SORMAIN did not reach is

target accrual and was terminated prematurely.

Building on the results of RADIUS and SORMAIN, there the

multiple ongoing trails evaluating post-HCT maintenance

therapy. The phase II, single-arm MAURITUS trial will

evaluate midostaurin maintenance following HCT in MRD-

positive FLT3-mutated AML (NCT03951961) and a phase III

trial with compare post-HCT midostaurin vs cren olanib

maintenance (NCT03258931).

For the second generation FLT3 inhibitors, BMT CTN 1506, a

randomized phase III trial of gilteritinib vs placebo for FLT3-ITD

mutated AML following HCT is ongoing (NCT02997202).

Importantly, although prior FLT3 inhibitor treatment is not an

inclusion criterion, since enrollment occurred after the

widespread use of midostaurin in the frontline setting, the

majority of patients enrolled on BMT CTN 1506 will likely

have received prior FLT3 inhibitor therapy, answering a key

question raised by SORMAIN. In addition, quizartinib was

shown to b e well-tolerated as single-agent maintenance

following HCT in a phase I study (134) and this strategy will

be further explored in the QuANTUM-First trial. Finally, a non-

randomized trial is evaluating the efﬁcacy of crenolanib post-

HCT in FLT3-mutated AML (NCT02400255).

Maintenance Therapy: Where Do We Go From Here?

The role of FLT3 inhibitor maintenance, while promising,

remains unknown, and placebo-controlled randomized trials

are necessary to establish the efﬁcacy of this approach.

Multiple questions remain, including: How can we identify the

patients for whic h FLT3 inhibitor mainten ance is most

beneﬁcial? In SORMAIN, the strongest beneﬁt from sorafenib

maintenance was in patients with undetectable MRD pre-HCT

and detectable MRD post-HCT (133). Notably, the ongoing

BMT 1506 trial will include correlative MRD analysis to

determine if the presence of MRD is predictive of beneﬁt from

FLT3 inhibitor maintenance. What is the optimal duration of

maintenance the rapy ? In a correlative analysis of RADIUS,

decreased levels of phosphorylated FLT3 were associated with

improved OS (135), suggesting this may be one potential

biomarker for determining maintenance duration. What type

of FLT3 inhibitor is most efﬁcacious as a maintenance agent, a

multikinase, ﬁrst-generation agent, or a more targeted, second

generation one? In the post-chemotherapy setting, do es

mainten ance serve as a bridge to transplant, or obv iate the

need? Finally, what is the effect of maintenance therapy on

health-associat ed quality of life? Given that t his patient

population may not have active disease, studies sp eciﬁcally

investigating health-associated quality of life are ne eded to

fully inform the beneﬁt of prolonged maintenance therapy.

What Is the Role of Transplant in FLT3-

Mutant AML?

Historically, FLT3-mutated AML has been considered adverse

risk disease, and patients with FLT3-ITD have been

recommen ded to undergo HCT in the ﬁrst CR (136, 137).

More recently, the ELN has re-classiﬁed FLT3-mutated AML

such that patients with FLT3-ITD

low

(AR < 0.5), normal

cytogenetics, and mutated NPM1 are considered low risk,

suggesting these patients may have good prognosis without

HCT (138). This is not widely accepted, however, and

retrospective data have demonstrated that ‘ low risk’ FLT3-ITD

AML, with FLT3- IT low AR and positive NPM1 mutational

status, still conveys poor prognosis, with a 5-year OS of 41.3%,

and OS is improved by HCT (139). A similar retrospective study

found that HCT in ﬁrst CR improves OS in all FLT3-ITD AML,

regardless of AR or NPM1 status (140). Importantly, both these

studies and the ELN guidelines were written prior to widespread

FLT3 inhibitor use, and the role of HCT in FLT3-mutated AML

today remains an open question.

In a retrospective analysis of the RATIFY trial, the beneﬁcial

effect of midostaurin was seen across all ELN subgroups;

however, the beneﬁt of HCT was only seen in patients with

adverse risk disease (138). This should be interprete d with

caution as RATIFY was not powered for these sub group

analyses. Nonetheless, this study provides support that in low

or even intermediate risk FLT3-ITD AML, HCT could

potentially be delayed until ﬁrst relapse or MRD positivity. Of

note, in RATIFY, patients that did not receive HCT did receive

post-consolidation midostaurin maintenance, an indication that

was not approved as discussed above.

More recently, in a phase II study of crenolanib plus

chemotherapy followed by crenolanib maintenance in newly

diagnosed FLT3-mutated AML, 85% of patients achieved CR.

Of the 27 patients on trial, 7/27 received consolidation but not

HCT; of those, 6/7 remained in long-term remission. OS was

similar between patients who underwent HCT vs those who did

not (55). While the number of patients is small and the ELN risk

category not speciﬁed, this again raises the question of whether

HCT is needed in all patients with FLT3-mutated AML.

Kennedy and Smith Emerging Topics in FLT3 AML

Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 61288012

In a recent position statement by the European Society for

Blood and Marrow Transplantation (EBMT), HCT in ﬁrst CR

was recommended for patients with intermediate or adverse risk

FLT3-ITD AML and could be considered in low risk disease.

Furthermore, in the absence of GVHD, post-HCT F LT3

inhibitor maintenance was recommended, ideally on a clinical

trial (141). Randomized trials are needed to further clarify these

algorithms. Multiple trials of FLT3 inhibitors plus chemotherapy

in newly diagnosed AML are actively accruing, and it will be

interesting to see the role of HCT in these studies.

Given New FLT3-Directed Therapies, Will

We Need to Re-Think Risk Stratiﬁcation?

The current ELN criteria were developed before FLT3 inhibitors

were routinely used. Prior to widespread FLT3 inhibitor use, the

prognosis for FLT3-ITD AML was dismal. Historically, while

patients with FLT3-ITD responded to induction chemotherapy

with similar remission rates as other AML subtypes, patients

were more likely to relapse and had inferior OS (7, 142).

In a retrospective analysis of the RATIFY trial, for all enrolled

patients, OS differed signiﬁcantly among ELN risk groups. In all

risk groups, midostaurin signiﬁcantly improved OS, with 5-year

OS probabilities for the midostaurin arm of 0.53 and 0.52 for

intermediate- and adverse-risk, respectively (138). More recently

crenolanib plus chemotherapy in newly diagnosed AML has

demonstrated a 3-year OS of 0.76 in adults ≤60 ( 55) and a 1-year

OS of 0.67 in ad ults 61–75 (56). As a comparison, in at

retrospective validation of ELN risk stratiﬁcation in newly-

diagnosed patients receiving conventional chemotherapy, 5-year

OS probabilities were 0.36 for FLT3-ITD

low

/NPM1

and 0.29 for

FLT3-ITD

high

/NPM1

mutated

(intermediate risk) and 0.09 for FLT3-

ITDhigh/NPM-WT (adverse risk) (143). While historic comparisons

must be interpreted with caution, this may suggest FLT3 inhibitors

have shifted the risk associated with FLT3-mutated AML.

Given the multiple FLT3-directed therapies both approved

and in development, the prognosis of FLT3-mutated AML may

be changing. Will FLT3 mutations in AML e ventually be

analogous to HER2 amp liﬁcatio n in breast cancer or BCR-

ABL1 fusions in acute lymphoblastic leukemia? In both of

these cases, the development of targeted therapies has

dramatically improved the prognosis of a previously poor-risk

subtype, and a similar pattern may evolve with FLT3 as well.

What Is the Prognostic and Therapeutic

Impact of Non-Canonical FLT3 Mutations?

As sequencing technology improves, FLT3 mutations outside of

the ITD and D835 regions have been described. These non-

canonical (NC) mutations are frequently in exon 14 of the

juxtamembrane (JM) domain, where ITDs occur, or in the KD

domain adjacent to D835; however NC mutations in other sites,

including the extracellular (EC) domain, have been described as

well. Select NC mutations identiﬁed in the clinical literature are

summarized in Table 7.

In the pediatric literature, whole genome sequencing of 799

pediatric AML patients revealed a 7.6% prevalence of NC FLT3

point mutations and insertions-deletions compared to a 23%

total prevalence of all FLT3 mutations (including FLT-ITD and

D835). This included 9 JM mutations with activating potential

(E598D, E573D/G, L576R, T582N, D586Y, Y589H, E596K/G,

Y599C, D600G), many which occurred as co-mutations with

FLT3-ITD (145). Similarly, in a large study of 1,540 adult AML

patients, targeting genetic sequencing identiﬁed four NC FLT3

driver mutation, including two EC (S451F and V491L) and two

JM (V592A, E598D) mutations (4).

TABLE 7 | Non-Canonical Mutations Identiﬁed in Clinical Studies.

Mutation Exon Domain Clinical Activity

D200N 5 EC Maintained through crenolanib treatment (64)

T227M 6 EC Confers resistance to sorafenib (68)

K429E 10 EC Maintained through crenolanib treatment (64)

S451F 11 EC Pediatric AML (5); Adult AML (4); minimal midostaurin sensitivity (144)

V491L 12 EC Pediatric AML (5); Adult AML (4)

Y572C 14 JM High midostaurin sensitivity (144); maintained through crenolanib treatment (64)

E573D/G 14 JM Pediatric AML (145)

L576R 14 JM Pediatric AML (145)

T582N 14 JM Pediatric AML (145)

D586Y 14 JM Pediatric AML (145)

Y589H 14 JM Pediatric AML (145)

V592A/G 14 JM Pediatric AML (5); Adult AML (4); high midostaurin sensitivity (144); clinical sorafenib response (146)

E596K/G 14 JM Pediatric AML (145)

E598D 14 JM Adult AML (4); Pediatric AML (5, 145); found after relapse on Giltertininb (62)

Y599C 14 JM Pediatric AML (145)

D600G 14 JM Pediatric AML (145)

L601F 14 JM Mutation maintained through crenolanib treatment (64)

N676D/K 16 KD Clinical sorafenib response (146), Resistance to midostaurin (147); resistance to quizartinib (148)

G697R 16 KD Resistance to quizartinib (148)

A833S 20 AL Eliminated with crenolanib treatment (64)

R834Q 20 AL High Midostaurin sensitivity (144)

D839Y/G 20 AL Eliminated with crenolanib treatment (64)

N841K 20 AL Eliminated with crenolanib treatment (64)

Y842C 20 AL Eliminated with crenolanib treatment (

64)

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Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 61288013

It is unclear to whether NC mutations confer FLT3 inhibitor

susceptibility, resistance, or are simply passenger mutations

unimportant to disease biology. In one study of 222 AML

patients without FLT3-ITD or -D835 mutations, four NC

driver mutations were identiﬁed which had variable sensitivity

to midostaurin (144 ). Similarly, in a study of 18 patients treated

with crenolanib, pre- and post-treatment sequencing identiﬁed

small populations of four NC mutations at baseline which were

eliminated over the course of treatment (A833S, D839Y/G,

N841K, Y842C) (64). In a recent in vitro study, gilteritinib was

active against multiple NC mutations, including mutations like

N676, which are associated with resistance to midostaurin and

quizartinib (149). Finally, in a recent case series, two patients

with non-FLT3-ITD or D835 AML were found to have JM FLT3-

V592G and KD FLT3-N676K mutations, both of which clinically

responded to sorafenib (146).

To contrast, maintenance or development of NC mutations

have also been observed in conjunction with FLT3 inhibitor

resistance. In patients treated with crenolanib, four NC FLT3

mutations (D200N, K429E, Y572C, L601F) were maintained at

time of relapse (64). In an analysis of 40 patients with FLT3-

mutated AML treatment with gilteritinib on the ADMIRAL trial,

six patients had new FLT3 mutations at time of relapse, ﬁve of

which were in gatekeeper F691L and two at the NC site JM

E598D (62).

Many questions remain regarding the nature of NC

mutations, including their true prevalence (as they are not

detected outside of whole-exome or genome sequencing),

prognostic impact, and role in the FLT3 inhibitor susceptibility

and resistance.

Is There a Role for FLT3 Inhibitors in FLT3-

WT Disease?

Although only one-quarter of AML patients harbor FLT3

mutations, the FLT3 receptor is over-expressed on leukemic

blast regardless of mutational status (2), and early studies of

FLT3 inhibitors observed blast reduction in patients with FLT3-

WT disease (19).

Early trials of FLT3 inhibitors were not limited to patients

with FLT3-mutated AML, and results from these studies may

indicate beneﬁt in targeting FLT3-WT. In the RATIFY trial,

using an arbitrary AR cut-off of ≥0.7, post-hoc analysis noted a

similar OS beneﬁt withFLT3-ITD

low

, FLT3-ITD

high

, and FLT3-

TKD AML (20). Similarly, in the SORAML trial, EFS beneﬁt and

trend toward OS beneﬁt were demonstrated irrespective of FLT3

mutation status (23, 34).

Given that both midostaurin and sorafenib are multikinase

inhibitors, it is possible these results are due to inhibition of

alternative kinase-dependent pathways. For example, both

sorafenib and midostaurin also inhibit KIT, and KIT mutations

areseenin30–46% of core binding factor (CBF) AML and may

impact prognosis (150, 151). The use of midostaurin in CBF AML is

currently being explored in a phase II study (NCT03686345), and a

trial of midostaurin in c-KIT or FLT3-ITD mutated t(8,21) AML

recently completed accrual (NCT01830361).

Efﬁcacy in FLT3-WT disease has also been demonstrated in

second generation FLT3 inhibitors, which are more speciﬁcto

the FLT3 receptor. While the initial phase I/II study of gilteritinib

demonstrated strongest antileukemic effect in patients with

FLT3-mutated disease, a 12% ORR was see n in FLT3-WT

AML (52). Similarly, quizartinib monotherapy demonstrated

54 and 32% ORR in patients with FLT3-ITD and FLT3-WT

disease, respectively (152).

Trials of FLT3 inhibitors in FLT3 -WT AML are ongoing,

including a randomized phase III trial of chemotherapy +/−

midostaurin (NCT03512197) and chemotherapy +/− quizartinib

(NCT04107727), both in patients with newly-diagnosed FLT3-

WT AML. In addition, many phase I trials of novel combination

therapies, dual FLT3 inhibitors, and biologic agents are not

restricted to FLT3-mutant disease.

How Should We Approach Tumor

Heterogeneity in FLT3-Mutant AML?

Increasingly, AML is understood as a heterogenous disease with

multiple genetically distinct subclones, dynamically evolving

under pressure of therapy. Clonal evolution is particularly

relevant in FLT3-mutated AML as FLT3 mutations can be

gained with disease progression, and development of a new

FLT3-ITD is an independent negative prognostic factor (153).

Developm ent of FLT3-mutated clones can arise under

targeted therapy. In patients with R/R AML treated with

venetoclax, analy sis of pre- and post-treatment sam ples

demonstrated 4/20 patients developed new FLT3-ITD clones

following therapy (94). In a larger study of 81 patients treated

with frontline venetoclax-based combinations, 5/25 patients

showed increased FLT3-ITD clonal burden at relapse, two of

which had newly acquired FLT3-ITD clones (95). Similarly, in

patients with IDH1-mutated AML treated with ivosidenib, bulk

NGS at time of progression identiﬁed multiple patients with new

FLT3-ITD or -TKD mutations not present at therapy initiation

(154), and in patients with IDH2-mutated AML treated with

enasidenib, analysis of paired pre- and post-treatment samples

described several cases with increased FLT3 variant allele

frequency at relapse (155).

Complex clonal evolution has also been observed following

FLT3 inhibition. In an analysis of patients treated with single

agent gilteritinib on the phase III ADMIRAL or phase II

CHRYSALIS trial, targeted NGS identiﬁed emerging clones with

mutations activating RAS/MAPK signaling, including NRAS and

KRAS. Serial single-cell sequencing conﬁrmed early selection for

RAS-mutant subclones under gilteritinib pressure (63). In an

analysis of paired pre- and post-treatment samples of patients

treated with quizartinib, NRAS development was similarly noted

at relapse; furthermore, single cell sequencing conﬁrmed these

distinct subclones existed in small populations prior to therapy

and expanded under FLT3 inhibition (66).

It is unclear how to best address clonal evolution and

associated adaptive resistance in AML. Is the treatment of

multiple clones best appro ached through blunt approaches,

such as cytotoxic chemotherapy or broad, triple-therapy

combinations, such as HMA/Venetoclax/Gilteritinib?

Alternatively, would it be more beneﬁcial to target individual

subclones sequentially, perhaps focusing on the highest-risk or

fastest-growing subclones ﬁrst? As genomic technologies such as

Kennedy and Smith Emerging Topics in FLT3 AML

Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 61288014

single cell sequencing become more widely adopted, available

genomic information will increase dramatically. L ooking

forward, innovative bioinformatic and machine learning-based

approaches may be employed to rationally treat the complex

clonal architecture in FLT3-mutated AML.

TREATING FLT3-MUTATED AML TODAY

The current standard of care for an adult with newly diagnosed

FLT3-mutated AML with AR ≥0.05 who is eligible for treatment

is induction and consolidation chemotherapy in combination

with midostaurin. As FLT3 AR is not universally reported, if not

available the presence of a FLT3-ITD or TKD mutation should

warrant treatment with a FLT3 inhi bitor. This is generally

followed by HCT, as the largest survival beneﬁt in the RATIFY

trial was observed in patients who underwent HCT in ﬁrst

remission (20), although whether this approach holds in

patients with low-AR FLT3-ITD, FLT3-TKD, or concomitant

NPM1 mutations is debatable.

While not standard of care, use of single-agent maintenance

FLT3 inhibitors, either in remission for patients not going to HCT

or in post-HCT remission can be considered. This would ideally be

done in a clinical trial setting, although off-label use of sorafenib or

midostaurin maintenance is routinely practiced in the US (156,

157). We await the results of ongoing clinical trials to help

determine in which settings FLT3 inhibitor maintenance is

most useful.

Should a patient with FLT3-ITD AML relapse or a patient with

FLT3-WT AML develop a new FLT3-mutated clone, then single-agent

gilteritinib is standard salvage therapy world-wide and single-agent

quizartinib could be considered in certain practice locations. If response

if achieved, HCT would be recommended in all ﬁtpatients.Ifapatient

remains refractory or develops relapse, then multiple clinical trials could

be considered, including FLT3 inhibitor combinations, novel FLT3

inhibitors, or FLT3-directed biologics (Figure 3). Although both

gilteritinib and venetoclax are FDA-approved, and there is promising

pre-clinical data on this combination (92), th is regimen can be

associated with marked myelosuppression and we would not

recommend it outside of a clinical trial. Similarly, the triplet

combination of HMA/Gilteritinib/Venetoclax should only be used as

part of clinical trial until the toxicities associated with this novel

combination are better understood.

Finally, special consideration should be given to older adults,

who may be intolerant of intensive induction chemotherapy as well

as the side effects associated with multikinase inhibitors. As

described above, multiple studies are actively investigating FLT3

inhibitors in combination with lower-intensity therapies currently

approved as frontline therapy for older adults, including HMAs as

well as liposomal cytarabine and anthracycline (CPX-351, Vyxeos).

CONCLUDING REMARKS

While this provides a baseline treatment paradig m, survi val

remains poor in FLT3-mutated AML, and additional treatment

options are needed. This includes increasing the diversity of

approved FLT3, investigating FLT3 inhibitors in new

combinations and treatment settings, and development of

novel agents. Given the diversity in FLT3 inhibitor potency,

FIGURE 3 | Current Standard of Care for Treating FLT3-mutated AML in the Fit Patient.

Kennedy and Smith Emerging Topics in FLT3 AML

Frontiers in Oncology | www.frontiersin.org December 2020 | Volume 10 | Article 61288015

resistance patterns, and toxicity proﬁles, ideally the oncologist

will have a range of FLT3-directed therapies to choose from,

similar to TKI selection in chronic myelogenous leukemia, so

that a particular FLT3 inhibitor could be matched to an

individual patient’s needs.

AUTHOR CONTRIBUTIONS

VK and CS both contributed to the writing of this manuscript.

All authors contributed to the article and approved the

submitted version.

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Conﬂict of Interest: The authors declare that the research was conducted in the

absence of any commercial or ﬁnancial relationships that could be construed as a

potential conﬂict of interest.

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