Abstract: ASP2215 (Gilteritinib) is a highly selective, second-generation, type I tyrosine kinase inhibitor targeting both FMS-like tyrosine kinase 3 (FLT3) and AXL. It has emerged as a transformative targeted therapy for patients with relapsed or refractory (R/R) acute myeloid leukemia (AML) harboring FLT3 mutations. Unlike first-generation and type II inhibitors, gilteritinib effectively inhibits both internal tandem duplication (ITD) and tyrosine kinase domain (TKD) mutations, including the D835 resistance mutation. Based on the pivotal phase III ADMIRAL trial, which demonstrated a significant overall survival benefit compared to salvage chemotherapy, gilteritinib received FDA and EMA approval as a monotherapy for R/R FLT3-mutated AML. This review comprehensively examines the pharmacological activity, molecular mechanisms, structure-activity relationships, current clinical limitations, and future perspectives of gilteritinib in the management of AML.
1. Introduction
Acute myeloid leukemia (AML) is a heterogeneous hematologic malignancy characterized by the rapid proliferation of immature myeloid blasts. Among the most frequent genomic alterations in AML are mutations in the FMS-like tyrosine kinase 3 (FLT3) gene, occurring in approximately one-third of newly diagnosed adult patients [8][9]. FLT3 mutations primarily present as internal tandem duplications (ITD) in the juxtamembrane domain or point mutations in the tyrosine kinase domain (TKD) [8]. These mutations drive constitutive kinase activation, leading to highly proliferative disease, shorter remission durations, and increased relapse rates [8][9].
To address the poor prognosis associated with FLT3-mutated AML, targeted tyrosine kinase inhibitors (TKIs) have been developed. ASP2215, generically known as gilteritinib, is a potent, rationally designed, second-generation FLT3 inhibitor [6][8]. It was specifically developed to overcome the limitations of earlier FLT3 inhibitors, particularly their inability to target certain resistance-conferring mutations [2][8]. Gilteritinib is currently approved by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the treatment of adult patients with relapsed or refractory (R/R) FLT3-mutated AML [1][9].
2. Pharmacological Activity
Gilteritinib has demonstrated robust antileukemic activity in both preclinical models and clinical trials. In the phase I/II CHRYSALIS trial, gilteritinib was evaluated in heavily pretreated R/R AML patients. The maximum tolerated dose was determined to be 300 mg/day, with 120 mg/day selected as the recommended dose due to its potent FLT3 inhibition and tolerable safety profile [4][8]. The overall response rate (ORR) was 40%, with a median overall survival (OS) of 25 weeks [2][8].
The efficacy of gilteritinib was definitively established in the randomized phase III ADMIRAL trial, which compared gilteritinib monotherapy (120 mg/day) to the investigator's choice of salvage chemotherapy in 371 patients with R/R FLT3-mutated AML [4][18]. Gilteritinib significantly prolonged median OS (9.3 months vs. 5.6 months) and achieved higher rates of complete remission with full or partial hematologic recovery (CR/CRh: 34.0% vs. 15.3%) compared to chemotherapy [4][9]. The survival benefit was maintained across subgroups, including patients receiving high- or low-intensity chemotherapy and those with high FLT3-ITD allelic ratios [1][9].
Beyond medullary disease, gilteritinib has shown promising efficacy in treating extramedullary relapses, such as myeloid sarcoma (MS), inducing complete responses in rare extramedullary localizations including meningeal and skin involvements [47].
3. Molecular Mechanism of Action
Gilteritinib functions as a dual inhibitor of FLT3 and AXL receptor tyrosine kinases [6][8]. Upon binding, it decreases the phosphorylation of FLT3 and its downstream signaling targets, effectively halting the proliferation of leukemic blasts [6]. The inhibition of AXL is particularly noteworthy; AXL is an oncogenic tyrosine kinase frequently overexpressed in AML that facilitates FLT3 activation and is implicated in resistance to other FLT3 inhibitors [8].
At the cellular level, gilteritinib induces apoptosis in FLT3-ITD positive cells. Preclinical studies indicate that gilteritinib downregulates the expression of anti-apoptotic proteins, including induced myeloid leukemia cell differentiation protein (MCL-1), B-cell lymphoma 2-like protein 10 (BCL2L10), and survivin [8]. The downregulation of MCL-1 is a critical mechanism that synergistically enhances apoptotic activity when gilteritinib is combined with other agents, such as the BCL-2 inhibitor venetoclax or conventional chemotherapy [8].
4. Structure-Activity Relationship (SAR)
As a type I FLT3 inhibitor, gilteritinib binds to the active conformation of the kinase domain [2][8]. This structural interaction allows it to potently inhibit both FLT3-ITD and FLT3-TKD mutations. Crucially, it retains activity against mutations at the D835 codon, which is the most common acquired point mutation conferring resistance to type II FLT3 inhibitors like sorafenib and quizartinib [2][8].
In addition to FLT3 and AXL, kinase profiling demonstrates that gilteritinib strongly inhibits anaplastic lymphoma kinase (ALK) and leukocyte receptor tyrosine kinase (LTK) [8]. Importantly, unlike several other multi-targeted FLT3 inhibitors (such as midostaurin and quizartinib), gilteritinib exhibits weak or no significant inhibition of c-KIT (approximately 800-fold weaker than its activity against FLT3) [8]. This specific SAR profile is clinically advantageous, as the lack of c-KIT inhibition translates to a lower incidence of severe myelosuppression [8][13].
5. Current Limitations
Despite its efficacy, gilteritinib therapy is associated with several limitations, including adverse events and the emergence of secondary resistance.
Adverse Events: Gilteritinib is generally well-tolerated, but common treatment-related adverse events include diarrhea, fatigue, and elevated liver transaminases (AST/ALT) [4][8]. Grade 3 or higher toxicities include febrile neutropenia, anemia, and thrombocytopenia [8][9]. Rare but notable side effects include QTc prolongation, posterior reversible encephalopathy syndrome (PRES), and pancreatitis [4]. Furthermore, because gilteritinib is metabolized by CYP3A4, co-administration with strong CYP3A4 inhibitors (e.g., azole antifungals) or inducers requires careful monitoring [14].
Resistance Mechanisms: Relapse on gilteritinib is frequently driven by clonal evolution. Off-target resistance mechanisms include the acquisition of mutations in the Ras/MAPK pathway (e.g., NRAS and KRAS), which mediate continued signaling despite FLT3 inhibition [8][77]. On-target resistance can occur via the emergence of the FLT3-F691L "gatekeeper" mutation, which reduces drug binding affinity, although preclinical data suggest higher doses of gilteritinib might partially overcome this [8]. Additionally, BCR-ABL1 fusions have been identified as a targetable mechanism of resistance at relapse [8][77].
6. Future Perspectives
To maximize the clinical utility of gilteritinib and overcome resistance, future strategies are heavily focused on combination therapies and expanding its use into earlier treatment lines.
Combination Therapies: Preclinical and early clinical data show high synergy between gilteritinib and venetoclax. A phase Ib trial of this combination in R/R AML demonstrated an encouraging ORR of 90% in FLT3-mutated patients, including those previously treated with other TKIs [8]. Combinations with hypomethylating agents (azacitidine) and intensive chemotherapy (7+3 regimen) are also under active investigation [1][9].
Frontline and Maintenance Settings: Ongoing phase III trials are evaluating gilteritinib in newly diagnosed FLT3-mutated AML, comparing it against or combining it with standard induction chemotherapy (e.g., HOVON 156 trial) [2]. Furthermore, gilteritinib is being investigated as a maintenance therapy following consolidation or allogeneic HSCT to eradicate minimal residual disease and prevent relapse [1][9].
